Linux IP Masquerade HOWTO - The Linux Documentation Project

Linux IP Masquerade HOWTO 

David A. Ranch 

 

November 13, 2005 

November 13, 2005 

This document describes how to enable the Linux IP Masquerade feature on a given Linux host. IP 

Masquerade is a form of Network Address Translation or NAT which NAT allows internally connected 

computers that do not have one or more registered Internet IP addresses to communicate to the Internet via the 

Linux server's Internet IP address.


Table of Contents 

Chapter 1. Introduction......................................................................................................................................1 

1.1. Introduction to IP Masquerading or IP MASQ.................................................................................1 

1.2. Foreword, Feedback & Credits.........................................................................................................1 

1.3. Copyright & Disclaimer....................................................................................................................2 

Chapter 2. Background Knowledge..................................................................................................................3 

2.1. What is IP Masquerade?...................................................................................................................3 

2.2. Current Status...................................................................................................................................3 

2.3. Who Can Benefit From IP Masquerade?..........................................................................................4 

2.4. Who Doesn't Need IP Masquerade?.................................................................................................4 

2.5. How does IP Masquerade Work?.....................................................................................................4 

2.6. Requirements for IP Masquerade on Linux 2.4.x.............................................................................6 

2.7. Requirements for IP Masquerade on Linux 2.2.x.............................................................................9 

2.8. Requirements for IP Masquerade on Linux 2.0.x...........................................................................11 

Chapter 3. Setting Up IP Masquerade............................................................................................................13 

3.1. Compiling a new kernel if needed..................................................................................................13 

3.2. Checking your existing kernel for MASQ functionality.................................................................13 

3.2.1. Compiling Linux 2.4.x Kernels.............................................................................................15 



3.3. Assigning Private Network IP Addresses to the Internal LAN......................................................35 

3.4. Configuring IP Forwarding Policies...............................................................................................36 

3.4.1. Configuring IP Masquerade on Linux 2.6.x and 2.4.x Kernels.............................................36 

3.4.2. Configuring IP Masquerade on Linux 2.2.x Kernels............................................................45 

3.4.3. Configuring IP Masquerade on Linux 2.0.x Kernels............................................................53 

Chapter 4. Configuring the other internal to−be MASQed machines.........................................................60 

4.1. Configuring Microsoft Windows 95 and OSR2.............................................................................60 

4.2. Configuring Windows NT..............................................................................................................62 

4.3. Configuring Windows for Workgroup 3.11....................................................................................62 

4.4. Configuring UNIX Based Systems.................................................................................................63 

4.5. Configuring DOS using NCSA Telnet package.............................................................................63 

4.6. Configuring MacOS Based System Running MacTCP..................................................................64 

4.7. Configuring MacOS Based System Running Open Transport.......................................................64 

4.8. Configuring Novell network using DNS........................................................................................65 

4.9. Configuring OS/2 Warp..................................................................................................................66 

4.10. Configuring OS/400 on a IBM AS/400........................................................................................67 

4.11. Configuring Other Systems..........................................................................................................67 

Chapter 5. Testing IP Masquerade.................................................................................................................68 

5.1. Loading up the rc.firewall ruleset...................................................................................................68 

5.2. Testing internal MASQ client PC connectivity..............................................................................69 

5.3. Testing internal MASQ client to MASQ server connectivity.........................................................69 

5.4. Testing internal MASQ server connectivity...................................................................................70 

5.5. Testing internal MASQ server to MASQ client connectivity.........................................................70 

5.6. Testing External MASQ server Internet connectivity....................................................................71 

5.7. Testing internal MASQ client to external MASQ server connectivity...........................................72 

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Chapter 5. Testing IP Masquerade 

5.8. Testing external MASQ ICMP forwarding.....................................................................................73 

5.9. Testing MASQ functionality without DNS....................................................................................74 

5.10. Testing MASQ functionality with DNS resolution......................................................................75 

5.11. Testing more MASQ functionality with DNS..............................................................................75 

5.12. Any remaining functional, performance, etc. issues....................................................................76 

Chapter 6. Other IP Masquerade Issues and Software Support..................................................................77 

6.1. Problems with IP Masquerade........................................................................................................77 

6.2. Incoming services...........................................................................................................................77 

6.3. Supported Client Software and Other Setup Notes.........................................................................77 

6.3.1. Network Clients that −Work− with IP Masquerade..............................................................77 

6.3.2. Clients that do not have full support in IP MASQ:...............................................................80 

6.4. Stronger firewall rulesets to run after initial testing.......................................................................80 

6.4.1. Stronger IP Firewall (IPTABLES) rulesets...........................................................................80 

6.4.2. Stronger IP Firewall (IPCHAINS) rulesets...........................................................................90 

6.4.3. Stronger IP Firewall (IPFWADM) Rulesets.........................................................................97 

6.5. IP Masquerading multiple internal networks................................................................................103 

6.5.1. iptables support for multiple internal lans...........................................................................103 

6.5.2. ipchains support for multiple internal lans..........................................................................103 

6.5.3. ipfwadm support for multiple internal lans.........................................................................103 

6.6. IP Masquerade and Dial−on−Demand Connections.....................................................................104 

6.7. Port Forwarding with IPTABLES or external tools like IPPORTFW, IPMASQADM, 

IPAUTOFW, REDIR, UDPRED, and other Port Forwarding tools...................................................104 

6.7.1. IPTABLES−based PORTFWD'ing: Using IPTABLES's PREROUTING option for 

2.6.x and 2.4.x kernels...........................................................................................................106 

6.7.2. IPMASQADM−based PORTFWD'ing: Using IPMASQADM with 2.2.x kernels............108 

6.7.3. IPPORTFW−based PORTFWD'ing: Using IPPORTFW on 2.0.x kernels.........................110 

6.8. CU−SeeMe and Linux IP−Masquerade........................................................................................113 

6.9. Mirabilis ICQ................................................................................................................................113 

6.10. Gamers: The LooseUDP patch...................................................................................................116 

Chapter 7. Frequently Asked Questions.......................................................................................................118 

7.1. ( Distro ) − What Linux Distributions support IP Masquerading?...............................................118 

7.2. ( Requirements ) − What are the minimum hardware requirements and any limitations for IP 

Masquerade? How well does it perform?...........................................................................................119 

7.3. ( Errors ) − When I run my specific rc.firewall−* ruleset, I get "command not found" errors. 

Why?...................................................................................................................................................120 

7.4. ( Still wont work ) − I've checked all my configurations, I still can't get IP Masquerade to 

work. What should I do?.....................................................................................................................120 

7.5. ( Email list ) − How do I join or view the IP Masquerade and/or IP Masqurade Developers 

mailing lists and archives?..................................................................................................................120 

7.6. ( NAT vs. Proxy ) − How does IP Masquerade differ from Proxy or NAT services?..................121 

7.7. ( GUI ) − Are there any GUI firewall creation/management tools?.............................................123 

7.8. ( MASQ and Dynamic IPs ) − Does IP Masquerade work with dynamically assigned IP 

addresses?...........................................................................................................................................123 

7.9. ( MASQ and various networks ) − Can I use a cable modem (both bi−directional and with 

modem returns), DSL, satellite link, etc. to connect to the Internet and use IP Masquerade?...........124 

ii



Chapter 7. Frequently Asked Questions 

7.10. ( Dial on Demand ) − Can I use Diald or the Dial−on−Demand feature of PPPd with IP 

MASQ?...............................................................................................................................................124 

7.11. ( Apps ) − What applications are supported with IP Masquerade?............................................124 

7.12. ( Distro Setup ) − How can I get IP Masquerade running on Redhat, Debian, Slackware, 

etc.?.....................................................................................................................................................125 

7.13. ( Timeouts ) − Connections seem to break if I don't use them often. Why is that?....................125 

7.14. ( Odd Behavior ) − When my Internet connection first comes up, nothing works. If I try 

again, everything then works fine. Why is this?.................................................................................125 

7.15. ( MTU ) − IP MASQ seems to be working fine but some sites don't work. This usually 

happens with WWW and some FTP sites..........................................................................................126 

7.15.1. Enabling PMTU Clamping for PPPoE and some PPP Users:...........................................127 

7.15.2. Clamping the MSS via IPTABLES:..................................................................................127 

7.15.3. Changing the External MTU of the MASQ server:..........................................................128 

7.15.4. Changing the MTU of various operating systems:............................................................128 

7.16. ( FTP ) − MASQed FTP clients don't work................................................................................132 

7.17. ( Performance ) − IP Masquerading seems slow........................................................................132 

7.18. ( PORTFW ) − IP Masquerading with PORTFWing seems to break when my line is idle 

for long periods...................................................................................................................................134 

7.19. ( PORTFW − Locally ) − I can't reach my PORTFWed server from the INTERNAL lan........134 

7.20. ( Logs ) − Now that I have IP Masquerading up, I'm getting all sorts of weird notices and 

errors in the SYSLOG log files. How do I read the IPTABLES/IPCHAINS/IPFWADM firewall 

errors?.................................................................................................................................................135 

7.21. ( Log Reduction ) − My logs are filling up with packet hits due to the new "stronger" 

rulesets. How can I fix this?................................................................................................................140 

7.22. ( MASQ Security ) − Can I configure IP MASQ to allow Internet users to directly contact 

internal MASQed servers?..................................................................................................................140 

7.23. ( Free Ports ) − I'm getting "kernel: ip_masq_new(proto=UDP): no free ports." in my 

SYSLOG files. Whats up?..................................................................................................................140 

7.24. ( SETSOCKOPT ) − I'm getting "ipfwadm: setsockopt failed: Protocol not available" 

when I try to use IPPORTFW!............................................................................................................141 

7.25. ( SAMBA ) − Microsoft File and Print Sharing and Microsoft Domain clients don't work 

through IP Masq!................................................................................................................................141 

7.26. ( IDENT ) − IRC won't work properly for MASQed IRC users. Why?.....................................142 

7.27. ( IRC DCC ) − mIRC doesn't work with DCC Sends.................................................................142 

7.28. ( IP Aliasing ) − Can IP Masquerade work with only ONE Ethernet network card?.................142 

7.29. ( Multiple−LANs ) − I have two MASQed LANs but they cannot communicate with each 

other!...................................................................................................................................................143 

7.30. ( SHAPING ) − I want to be able to limit the speed of specific types of traffic.........................143 

7.31. ( ACCOUNTING ) − I need to do accounting on who is using the network.............................143 

7.32. ( MULTIPLE IPs − DMZ segments) − I have several EXTERNAL IP addresses that I 

want to PORTFW to several internal machines. How do I do this?...................................................144 

7.33. ( 1:1 NAT ) − I'd like to do 1:1 NAT but I can't figure out how to do it....................................145 

7.34. ( Netstat ) − I'm trying to use the NETSTAT command to show my Masqueraded 

connections but its not working..........................................................................................................146 

7.35. ( VPNs ) − I would like to get Microsoft PPTP (GRE tunnels) and/or IPSEC (Linux 

SWAN) tunnels running through IP MASQ.......................................................................................146 

7.36. ( Games ) − I want to get the XYZ network game to work through IP MASQ but it won't 

iii




work. Help!.........................................................................................................................................146 

7.37. ( Stops working ) − IP MASQ works fine for a while but then it stops working. A reboot 

seems to fix this. Why?.......................................................................................................................146 

7.38. ( SMTP Relay ) − Internal MASQed computers cannot send SMTP or POP−3 mail!...............146 

7.39. ( Source Routing ) − I need different internal MASQed networks to exit on different 

external IP addresses...........................................................................................................................147 

7.40. ( IPCHAINS rulesets on 2.4.x kernels ) − What the ipchains.o module can do on 2.4.x 

kernels.................................................................................................................................................148 

7.41. ( IPTABLES vs. IPCHAINS vs. IPFWADM ) − Why do the 2.4.x, 2.2.x, and 2.0.x kernels 

use different firewall systems?............................................................................................................149 

7.42. ( Upgrades ) − I've just upgraded to the x.y.z kernel, why isn't IP Masquerade working?........149 

7.43. ( EQL ) − I need help with EQL connections and IP Masq........................................................150 

7.44. ( Wussing out ) − I can't get IP Masquerade to work! What options do I have for Windows 

Platforms?...........................................................................................................................................150 

7.45. ( Developers ) − I want to help with IP Masquerade development. What can I do?..................151 

7.46. ( More INFO ) − Where can I find more information on IP Masquerade?.................................151 

7.47. ( Translators ) − I want to translate this HOWTO to another language, what should I do?.......151 

7.48. ( Updates ) − This HOWTO seems out of date, are you still maintaining it? Can you 

include more information on ...? Are there any plans for making this better?...................................152 

7.49. ( Thanks ) − I got IP Masquerade working, it's great! I want to thank you guys, what can I 

do?.......................................................................................................................................................152 

Chapter 8. Miscellaneous...............................................................................................................................153 

8.1. Useful Resources..........................................................................................................................153 

8.2. Linux IP Masquerade Resource....................................................................................................153 

8.3. Thanks to the following supporters..............................................................................................154 

8.4. Reference......................................................................................................................................155 

8.5. ChangeLOG..................................................................................................................................155 

iv

Chapter 1. Introduction 

1.1. Introduction to IP Masquerading or IP MASQ 

This document describes how to enable the Linux IP Masquerade feature on a given Linux host. IP 

Masquerade, called "IPMASQ" or "MASQ" for short, is a form of Network Address Translation (NAT) which 

allows internally connected computers that do not have one or more registered Internet IP addresses to 

communicate to the Internet via the Linux server's Internet IP address. Since IPMASQ is a generic 

technology, you can connect the Linux server's internal and external to other computers through LAN 

technologies like Ethernet, TokenRing, and FDDI, as well as dialup connections line PPP or SLIP links. This 

document primarily uses Ethernet and PPP connections in examples because it is most commonly used with 

DSL / Cablemodems and dialup connections. 

"This document is intended for systems running stable Linux kernels like 2.4.x, 2.2.x, and 2.0.x preferably 

on an IBM−compatible PC. IP Masquerade does work on other Linux−supported platforms like Sparc, 

Alpha, PowerPC, etc. but this HOWTO doesn't cover them in as much detail. Beta kernels such as 2.5.x, 

2.3.x, 2.1.x, and ANY kernels less than 2.0.x are NOT covered in this document. The primary reason for 

this is because many of the older kernels are considered broken. If you are using an older kernel version, it 

is highly advisable to upgrade to one of the stable Linux kernels before using IP Masquerading. " 

1.2. Foreword, Feedback & Credits 

From the original IPMASQ HOWTO author: 

"As a new user, I found it very confusing to setup IP masquerade on the Linux kernel, (back then, its was a 

1.2.x kernel). Although there was a FAQ and a mailing list, there was no documentation dedicated to this. 

There was also some requests on the mailing list for a HOWTO manual. So, I decided to write this HOWTO 

as a starting point for new users and possibly create a building block for other knowledgeable users. If you, 

the reader, have any additional ideas, corrections, or questions about this document, please feel free to contact 

us. " 

This document was originally written by Ambrose Au back in August, 1996, based on the 1.x kernel IPMASQ 

FAQ written by Ken Eves and numerous helpful messages from the original IP Masquerade mailing list. In 

particular, a mailing list message from Matthew Driver inspired Ambrose to set up IP Masquerade and 

eventually write version 0.80 of this HOWTO. In April 1997, Ambrose created the Linux IP Masquerade 

Resource Web site at http://ipmasq.webhop.net which has provided up−to−date information on Linux IP 

Masquerading ever since. In February 1999, David Ranch took over maintenance of the HOWTO. David then 

re−wrote the HOWTO and added a substantial number of sections to the document. Today, the HOWTO is 

still maintained by David where he constantly updates it and fixes any reported bugs, etc. 

Please feel free to send any feedback or comments regarding this HOWTO to dranch@trinnet.net if you have 

any corrections or if any information/URLs/etc. is missing. Your invaluable feedback will certainly influence 

the future of this HOWTO! 

This HOWTO is meant to be a fairly comprehensive guide to getting your Linux IP Masquerading system 

working in the shortest time possible. David only plays a technical writer on T.V. so you might find the 

information in this document not as general and/or objective as it could be. If you think a section could be 

clearer, etc.. please let David know. The latest version of the MASQ HOWTO can be found at Dranch's Linux 

Page. Additional news, mirrors of the HOWTO, and information regarding IPMASQ can be found at the IP 

Chapter 1. Introduction 1

Masquerade Resource web page. If you have any technical questions on IP Masquerade, please join the IP 

Masquerade Mailing List instead of sending email to David or Ambrose. Most MASQ problems are 

−common− for ALL MASQ users and can be easily solved by users on the list. In addition to this, the 

response time of the IP MASQ email list will be much faster than a reply from either David or Ambrose. 

The latest version of this document can be found at the following sites which also contains HTML, Postscript, 

PDF, etc. versions 

• Dranch's Linux page 

• http://ipmasq.webhop.net/: The IP Masquerade Resources 

• http://ipmasq2.webhop.net/: The IP Masquerade Resources MIRROR 

• The Linux Documentation Project 

• Also refer to IP Masquerade Resource Mirror Sites Listing for other local mirrored sites. 

1.3. Copyright & Disclaimer 

This document is copyrighted(c) 2003,2002,2001,2000 for David A. Ranch and it is a 

FREE document. You may redistribute it under the terms of the GNU General Public License (GPL). 

The information herein this document is, to the best of David's knowledge, correct. However, the Linux IP 

Masquerade feature is written by humans and thus, the chance of mistakes, bugs, etc. might occur from time 

to time. 

No person, group, or other body is responsible for any damage on your computer(s) and any other losses by 

using the information on this document. i.e. 

"THE AUTHORS AND ALL MAINTAINERS ARE NOT RESPONSIBLE FOR ANY DAMAGES INCURRED 

DUE TO ACTIONS TAKEN BASED ON THE INFORMATION IN THIS DOCUMENT. " 

Ok, with all this behind us... On with the show.. 


Chapter 1. Introduction 2

Chapter 2. Background Knowledge 

2.1. What is IP Masquerade? 

IP Masquerade is a networking function in Linux similar to the one−to−many (1:Many) NAT (Network 

Address Translation) servers found in many commercial firewalls and network routers. For example, if a 

Linux host is connected to the Internet via PPP, Ethernet, etc., the IP Masquerade feature allows other 

"internal" computers connected to this Linux box (via PPP, Ethernet, etc.) to also reach the Internet as well. 

Linux IP Masquerading allows for this functionality even though these internal machines don't have an 

officially assigned IP address. 

MASQ allows a set of machines to invisibly access the Internet via the MASQ gateway. To other machines on 

the Internet, the outgoing traffic will appear to be from the IP MASQ Linux server itself. In addition to the 

added functionality, IP Masquerade provides the foundation to create a HEAVILY secured networking 

environment. With a well built firewall, breaking the security of a well configured masquerading system and 

internal LAN should be considerably difficult to accomplish. 

If you would like to know more on how MASQ (1:Many) differs from 1:1 (true) NAT and Proxy solutions, 

please see the Section 7.6 FAQ entry. 

2.2. Current Status 

IP Masquerade has been in the Linux kernels for several years now and is quite mature as the kernel enters the 

2.4.x stage. Kernels since Linux 1.3.x have had MASQ support built−in. Today, many individuals and 

commercial businesses are using it with excellent results. 

2.4.x kernel users: 

• The 2.4.x kernel hosts an entirely re−written set of NAT code which is both far superior, faster, and 

more secure than any previous versions written for Linux. Unfortunately, several kernel modules that 

were written for the 2.2.x kernel to support things like UDP−based RealAudio, etc. have not been 

ported to 2.4.x yet. Because of this, some people should consider NOT upgrading if these network 

applications are critical to them. But, at the same time, some of these programs have been updated and 

now use different, NAT−friendly protocols. Thus special NAT treatment is no longer required. As 

always, please see the http://ipmasq.webhop.net/: The IP Masquerade Resources site for updated 

news, etc. 

Common network functionalities like Web browsing, telnet, ssh, ping, traceroute, etc. work well over stock IP 

Masquerade setups. Other network applications such as ftp, irc, and Real Audio work well with the 

appropriate additional IP MASQ modules loaded into the kernel as modules. Other network−specific 

programs like streaming audio (MP3s, True Speech, etc) should work too without any special module. Some 

users on the mailing list also had good results with video conferencing software. 

It should be noted that running IP Masquerade with only ONE network card (NIC) to MASQ between internal 

and external Ethernet networks is NOT recommended. For more details, please see Section 7.28 FAQ section. 

Anyways, please refer to Section 6.3 for a more complete listing of software supported by IP Maquerade all 

kernel versions. 

Chapter 2. Background Knowledge 3


IP Masquerade works well as a server to other 'client machines' running various operating systems and 

hardware platforms. Here is a sampling of successful reports with internal MASQed systems running : 

• UNIX: Sun Solaris, [Net,Free,Open,*i]−BSD, Hp−UX, Linux, IBM AIX, Digital UNIX, Ultrix, etc. 

• Microsoft Windows 2000, NT (3.x and 4.x), 95/98/ME, Windows for Workgroups (with the TCP/IP 

package) 

• IBM OS/2 

• Apple Macintosh MacOS machines running either MacTCP or Open Transport 

• DOS−based systems with packet drivers and the NCSA Telnet package 

• VAXen 

• Compaq/Digital Alpha running Linux and NT 

• Amiga computers with AmiTCP or AS225−stack. 

The list goes on and on but the point is, if your OS platform talks TCP/IP, it should work with Linux's IP 

Masquerade! 

2.3. Who Can Benefit From IP Masquerade? 

• If you have a Linux host connected to the Internet and.. 

• if you have internal computers running TCP/IP connected that are connected to this Linux box via on 

a network, and/or 

• if your Linux host has more than one modem and acts as a PPP or SLIP server connected to other 

computers, and these machines do not have official or public assigned IP addresses (i.e. addressed 

with private TCP/IP numbers). 

• If you want those OTHER machines to communicate to the Internet without spending extra money to 

acquire additional Public / Official TCP/IP addresses from your ISP, then you should either configure 

Linux to be a router or purchase an external router. 

2.4. Who Doesn't Need IP Masquerade? 

• If your machine is a stand−alone Linux host connected to the Internet (setting up a firewall is a good 

idea though), or 

• if you already have multiple assigned public addresses for your OTHER machines, and 

• if you don't like the idea of a 'free ride' using Linux and feel more comfortable using expensive 

commercial tools to perform the exact same functionalities. 

2.5. How does IP Masquerade Work? 

Based from the original IP Masquerade FAQ by Ken Eves: Here is a drawing of the most simplistic setup: 

PPP/ETH/etc. +−−−−−−−−−−−−+ +−−−−−−−−−−−−−+ 

to ISP provider | Linux #1 | PPP/ETH/etc. | Anybox | 

| | | | 

Ethernet connection, etc. 

The second system (which does not need to be Linux) connects into the Linux #1 box and starts its network 

traffic to the Internet. This second machine does NOT have a publicly assigned IP address from the Internet, 

so it uses an RFC1918 private address, say 192.168.0.100. (see below for more info) 

With IP Masquerade and the routing configured properly, this second machine "Anybox" can interact with the 

Internet as if it was directly connected to the Internet with a few small exceptions [noted later]. 

Quoting Pauline Middelink (the founder of Linux's IPMASQ): 

"Do not forget to mention that the "ANYBOX" machine should have the Linux #1 box configured as its 

default gateway (whether it be the default route or just a subnet is no matter). If the "ANYBOX" machine is 

connected via a PPP or SLIP connection, the Linux #1 machine should be configured to support proxy arp for 

all routed addresses. But, the setup and configuration of proxy arp is beyond the scope of this document. 

Please see the PPP−HOWTO for more details." 

The following is an excerpt on how IPMASQ briefly works though this will be explained in more detail later. 

This short text is based from a previous post on comp.os.linux.networking which has been edited to match the 

names used in the above example: 

o I tell machine ANYBOX that my PPP or Ethernet connected Linux box is its 

gateway. 

o When a packet comes into the Linux box from ANYBOX, it will assign the 

packet to a new TCP/IP source port number and insert its own IP address 

inside the packet header, saving the originals. The MASQ server will 

then send the modified packet over the PPP/ETH interface onto the 

Internet. 

o When a packet returns from the Internet into the Linux box, Linux 

examines if the port number is one of those ports that was assigned 

above. If so, the MASQ server will then take the original port and 

IP address, put them back in the returned packet header, and send 

the packet to ANYBOX. 

o The host that sent the packet will never know the difference. 

Another IP Masquerading Example: 

A typical example is given in the diagram below: 


Ethernet 

192.168.0.x 

+−−−−−−−−−−+ 

| | 

| A−box |:::::: 

| |.2 : 

+−−−−−−−−−−+ : 

: +−−−−−−−−−−+ PPP/ETH 

+−−−−−−−−−−+ : .1 | Linux | link 

| | :::::::| Masq−Gate|:::::::::::::::::::>> Internet 

| B−box |:::::: | | 111.222.121.212 

| |.3 : +−−−−−−−−−−+ 

+−−−−−−−−−−+ : 

: 

+−−−−−−−−−−+ : 


| | : 

| C−box |:::::: 

| |.4 

+−−−−−−−−−−+ 

| | | > 

| | | > 

| connected via an | | Connected from the > 

| Ethernet hub or | | Linux server to your > 

| switch | | Internet connection > 

In this example, there are (4) computer systems that we are concerned about. There is also presumably 

something on the far right that your PPP/ETH connection to the Internet comes through (modem server, DSL 

DSLAM, Cablemodem router, etc.). Out on the Internet, there exists some remote host (very far off to the 

right of the page) that you are interested in communicating with). The Linux system named Masq−Gate is 

the IP Masquerading gateway for ALL internal networked machines. In this example, the machines A−box, 

B−box, and C−box would have to go through the Masq−Gate to reach the Internet. The internal network uses 

one of several RFC−1918 assigned private network addresses, where in this case, would be the Class−C 

network 192.168.0.0. If you aren't familiar with RFC1918, it is encouraged to read the first few chapters of the 

RFC but the jist of it is that the TCP/IP addresses 10.0.0.0/8, 172.16−31.0.0/12, and 192.168.0.0/16 are 

reserved. When we say "reserved", we mean that anyone can use these addresses as long as they aren't routed 

over the Internet. ISPs are even allowed to use this private addressing space as long as they keep these 

addresses within their own networks and NOT advertise them to other ISPs. Unfortunately, this isn't always 

the case but thats beyond the scope of this HOWTO. 

Anyway, the Linux box in the diagram above has the TCP/IP address 192.168.0.1 while the other systems has 

the addresses: 

• A−Box: 192.168.0.2 

• B−Box: 192.168.0.3 

• C−Box: 192.168.0.4 


The three machines, A−box, B−box and C−box, can have any one of several operating systems, just as long 

as they can speak TCP/IP. Some such as Windows 95, Macintosh MacTCP or OpenTransport , or even 

another Linux box have the ability to connect to other machines on the Internet. When running the IP 

Masquerade, the masquerading system or MASQ−gate converts all of these internal connections so that they 

appear to originate from the masq−gate itself. MASQ then arranges so that the data coming back to a 

masqueraded connection is relayed to the proper originating system. Therefore, the systems on the internal 

network are only able to see a direct route to the internet and are unaware that their data is being 

masqueraded. This is called a "Transparent" connection. 

NOTE: Please see Chapter 7 for more details on topics such as: 

• The differences between NAT, MASQ, and Proxy servers. 

• How packet firewalls work 

2.6. Requirements for IP Masquerade on Linux 2.4.x 

" ** Please refer to IP Masquerade Resource for the latest information. ** " 

• The newest 2.4.x kernels are now using both a completely new TCP/IP network stack as well as a new 

NAT sub−system called NetFilter. Within this NetFilter suite of tools, we now have a tool called 


IPTABLES for the 2.4.x kernels much like there was IPCHAINS for the 2.2.x kernels and 

IPFWADM for the 2.0.x kernels. The new IPTABLES system is far more powerful (combines several 

functions into one place like true NAT functionality), offers better security (stateful inspection), and 

better performance with the new 2.4.x TCP/IP stack. But this new suite of tools can be a bit 

complicated in comparison to older generation kernels. Hopefully, if you follow along with this 

HOWTO carefully, setting up IPMASQ won't be too bad. If you find anything unclear, downright 

wrong, etc. please email David about it. 

Unlike the migration to IPCHAINS from IPFWADM, the new NetFilter tool has kernel modules that 

can actually support older IPCHAINS and IPFWADM rulesets with minimal changes. So re−writing 

your old MASQ or firewall ruleset scripts is not longer required. BUT.. with the 2.4.x kernels, you 

cannot use the old 2.2.x MASQ modules like ip_masq_ftp, ip_masq_irc, etc. AND IPCHAINS is 

incompatible with the new IPTABLES modules like ip_conntrack_ftp, etc. So, what does this mean? 

It basically means that if you want to use IPMASQ or PORTFW functionality under a 2.4.x kernel, 

you shouldn't use IPCHAINS rules but IPTABLES ones instead. Please also keep in mind that there 

might be several benefits in performing a full ruleset re−write to take advantage of the newer 

IPTABLES features like stateful tracking, etc. but that is dependant upon how much time you have to 

migrate your old rulesets. Please see Section 7.40 for additional details. 

Some new 2.4.x functionalities include the following: 

PROs: 

CONs: 


• Lots of new protocols modules like: amanda, eggdrop, ipsec, ipv6, portscan, pptp, quota, rsh, talk, and 

tftp 

• TRUE 1:1 NAT functionality for those who have TCP/IP addresses and subnets to use (no more 

iproute2 commands) 

• Stateful application level (FTP, IRC, etc.) and stateful protocol level (TCP/UDP/ICMP) network 

traffic inspection 

• Built−in PORT Forwarding (no more ipmasqadm or ipportfw commands) 

• The built−in PORTFW'ing support works for both external and internal traffic. This means that users 

that have PORTFW for external traffic and REDIR for internal port redirection do not need to use two 

tools any more! 

• PORT Forwarding of FTP traffic to internal hosts is now completely supported and is handled in the 

conn_trak_ftp module 

• Full Policy−Based routing features (source−based TCP/IP address routing) 

• Compatibility with Linux's FastRoute feature for significantly faster packet forwarding (a.k.a Linux 

network switching). 

Note that this feature is still not compatible with packet filtering for strong firewall rulesets. 

• Fully supports TCP/IP v4, v6, and even DECnet (ack!) 

• Supports wildcard interface names like "ppp*" for serial interfaces like ppp0, ppp1, etc 

• Supports filtering on both input and output INTERFACES (not just IP addresses) 

• Source Ethernet MAC filtering 

• Denial of Service (DoS) packet rate limiting 

• Packet REJECTs now have user−selectable return ICMP messages 

• Variable levels of logging (different packets can go to different SYSLOG levels) 

• Other features like traffic mirroring, securing traffic per login, etc. 


• 

Netfilter is an entirely new architechure thus most of the older 2.2.x MASQ kernel modules written to 

make non−NAT friendly network applications work through IPMASQ need to be re−written for the 

2.4.x kernels. Because of this, if you specifically need functionality from some of these modules (see 

below), you should stay with a 2.2.x kernel until these modules have been either ported or the 

application has been updated to use NAT−friendly protocols. If you are curious on the porting status 

of a given module, please email the author of the module and NOT David or Ambrose. We don't 

code.. we just document. :−) 

Here is the status of the known IP Masq kernel modules or patches as found on the IPMASQ WWW 

site's Application Support Matrix. In addition, you should also setup out the Netfilter Patch−o−Matic 

URL as well. If you have the time and knowledge to help in the porting of code, your efforts would be 

highly appreciated: 

Status = Module name = Description and notes 

−−−−−−−−− −−−−−−−−−−− −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

Ported CuSeeme Used for Video conferencing 

NotPorted DirectPlay Used for online Microsoft−based games 

Ported FTP Used for file transfers 

− NOTEs: Built into the kernel and 

fully supports PORTFWed FTP 

ReWritten H.323 Used for Video conferencing 

NotPorted ICQ Used for Instant messaging 

* No longer required for modern ICQ clients 

Ported Irc Used for Online chat rooms 

Ported Quake Used for online Quake games 

Ported PPTP Allow for multiple clients to the same server 

NotPorted Real Audio Used for Streaming video / audio 

* No longer required for modern RealVideo clients 

NotPorted VDO Live Used for Streaming audio? 

Documentation on how to perform MASQ module porting is available at 

http://www.netfilter.org/documentation/HOWTO/netfilter−hacking−HOWTO.html. If you have the 

time and knowledge, your talent would highly be appreciated in porting these modules. 

If you'd like to read up more on NetFilter and IPTables, please see: 

http://www.netfilter.org/documentation/index.html#HOWTO and more specifically 

http://www.netfilter.org/documentation/HOWTO//NAT−HOWTO.html 

Linux 2.4.x IP Masquerade requirements include: 


• Any decent computer hardware. See Section 7.2 for more details. 

• The 2.4.x kernel source is available from http://www.kernel.org/. 

NOTE: Most modern Linux distributions, Section 7.1, that natively come with 2.4.x kernels are 

typically modular kernels and have all the IP Masquerade functionality already included. In such 

cases, there is no need to compile a new Linux kernel. If you are UPGRADING your kernel, you 

should be aware of other programs that might be required and/or need to be upgraded as well 


• 

(mentioned later in this HOWTO). 

The program "iptables" version 1.2.4 or newer ( 1.2.7a or newer is highly recommended ) archive 

available from http://www.netfilter.org/ 

♦ 

NOTE #1: All versions of IPTABLES less than 1.2.3 have a FTP module issue that can 

bypass any existing firewall rulesets. ALL IPTABLES users are highly recommended to 

upgrade to the newest version. The URL is above. 

NOTE #2: All versions of IPTABLES less than 1.2.2 have a FTP "port" security vulnerability 

in the ip_conntrack_ftp module. All IPTABLES users are highly recommended to upgrade to 

the newest version. The URL is above. 

♦ This tool, much like the older IPCHAINS and IPFWADM tools enables the various 

Masquerding code, more advanced forms of NAT, packet filtering, etc. It also makes use of 

additional MASQ modules like the FTP and IRC modules. Additional information on version 

requirements for the newest IPTABLES howto, etc. is located at the Unreliable IPTABLES 

HOWTOs page. 

• Loadable kernel modules, preferably 2.1.121 or higher, are available from 

http://home.pi.se/blox/modutils/index.html or ftp://ftp.kernel.org/pub/linux/utils/kernel/modutils 

• A properly configured and running TCP/IP network running on the Linux machine as covered in 

Linux NET HOWTO and the Network Administrator's Guide . Also check out the TrinityOS 

document which is also authored by David Ranch. TrinityOS is a very comprehensive guide for Linux 

networking. Some topics include IP MASQ, security, DNS, DHCP, Sendmail, PPP, Diald, NFS, 

IPSEC−based VPNs, and performance sections, to name a few. There are over Fifty sections in all! 

• Connectivity to the Internet for your Linux host covered in Linux ISP Hookup HOWTO, Linux PPP 

HOWTO, and TrinityOS. Other helpful HOWTOs could include: Linux DHCP mini−HOWTO, Linux 

Cable Modem mini−HOWTO and http://www.tldp.org/HOWTO/DSL−HOWTO/index.html 

• Know how to configure, compile, and install a new Linux kernel as described in the Linux Kernel 

HOWTO. This HOWTO does cover kernel compiling but only for IP Masquerade related options. 





• 

NOTE: Most modern Linux distributions, Section 7.1, that natively come with 2.2.x kernels are 

typically modular kernels and have all the IP Masquerade functionality already included. In such 

cases, there is no need to compile a new Linux kernel. If you are UPGRADING your kernel, you 

should be aware of other programs that might be required and/or need to be upgraded as well 

(mentioned later in this HOWTO). 

♦ 


NOTE #1: −−− UPDATE YOUR KERNEL −−− Linux 2.2.x kernels less than version 2.2.20 

contain several different security vulnerabilities (some were MASQ specific). Kernels less 

than 2.2.20 have a few local vulnerabilities. Kernel versions less than 2.2.16 have a TCP root 

exploit vulnerability and versions less than 2.2.11 have a IPCHAINS fragmentation bug. 

Because of these issues, users running a firewall with strong IPCHAINS rulesets are open to 

possible instrusion. Please upgrade your kernel to a fixed version. 

NOTE #2: Some newer Section 7.1 such as Redhat 5.2 might not be Linux 2.2.x ready (upgradable). 

Tools like DHCP, NetUtils, etc. will need to be upgraded. More details can be found later in the 


HOWTO. 

• Loadable kernel modules, preferably 2.1.121 or higher, are available from 



Linux NET HOWTO and the Network Administrator's Guide . Also check out the TrinityOS 

document which is also authored by David Ranch. TrinityOS is a very comprehensive guide for Linux 

networking. Some topics include IP MASQ, security, DNS, DHCP, Sendmail, PPP, Diald, NFS, 

IPSEC−based VPNs, and performance sections, to name a few. There are over Fifty sections in all! 

• Connectivity to the Internet for your Linux host covered in Linux ISP Hookup HOWTO, Linux PPP 

HOWTO, and TrinityOS. Other helpful HOWTOs could include: Linux DHCP mini−HOWTO, Linux 

Cable Modem mini−HOWTO and http://www.tldp.org/HOWTO/DSL−HOWTO/index.html 

• IP Chains 1.3.10 or newer are available from http://www.netfilter.org/ipchains/. Additional 

information on version requirements for the newest IPCHAINS HOWTO, etc is located at the Linux 

IP Chains page (mirror at Samba.org) 



Other optional patches and tools for 2.2.x kernels 

• TCP/IP port−forwarding or re−directing: 

♦ IP PortForwarding (IPMASQADM) − RECOMMENDED − mirror 

• PORTFW FTP Solutions: 

• 

♦ 


There are 2.2.x and 2.0.x kernel MASQ Module solutions for PORTFWed FTP to a MASQed 

machine (put an FTP server behind a MASQ server). Please see the Application Page on the 

IPMASQ WWW site for full details. Please note that this is not required for 2.4.x kernels. 

There is a full FTP proxy application from SuSe that will also allow PORTFWed−like 

functionality to reach an internal FTP server. For more details, please refer to the SuSe Proxy 

URL. 

IPROUTE2 for True 1:1 NAT, Policy−based (source) routing, and Traffic Shaping: 

♦ ftp://ftp.inr.ac.ru/ip−routing 

♦ Documentation can be found at http://www.compendium.com.ar/policy−routing.txt 

♦ The Advanced Routing HOWTO 

♦ Some source code mirrors are at: 

ftp://ftp.funet.fi/pub/mirrors/ftp.inr.ac.ru/ip−routing/ (STM1 to USA) −−− 

ftp://sunsite.icm.edu.pl/pub/Linux/iproute/ 

ftp://ftp.sunet.se/pub/Linux/ip−routing/ −−− ftp://ftp.nvg.ntnu.no/pub/linux/ip−routing/ 

ftp://ftp.crc.ca/pub/systems/linux/ip−routing/ −−− ftp://ftp.paname.org (France) 

Please see the IP Masquerade Resource page for more information available on these patches and possibly 

others as well. 






NOTE: Most modern Linux Section 7.1 that natively come with 2.0.x kernels are typically modular 

kernels and have all the IP Masquerade functionality already included. In such cases, there is no need 

to compile a new Linux kernel. If you are UPGRADING your kernel, you should be aware of other 

programs that might be required and/or need to be upgraded as well (mentioned later in this 

HOWTO). 

• Loadable kernel modules, preferably 2.1.85 or newer is available from 


(modules−1.3.57 is the minimal requirement) 


Linux NET HOWTO and the Network Administrator's GuideAlso check out the TrinityOS document 

which is also authored by David Ranch. TrinityOS is a very comprehensive guide to Linux 

networking. Topics include IP MASQ, security, DNS, DHCP, Sendmail, PPP, Diald, NFS, 

IPSEC−based VPNs, performance issues, and many more. There exists over fifty sections in all! 

• Connectivity to the Internet for your Linux host is covered in Linux ISP Hookup HOWTO, Linux 

PPP HOWTO, and TrinityOS. Other helpful HOWTOs could include: Linux DHCP mini−HOWTO, 

Linux Cable Modem mini−HOWTO and Linux DSL HOWTO 

• Ipfwadm 2.3.0 or newer is available from http://www.xos.nl/linux/ipfwadm/download.html 

• More information on version requirements are on the Linux IPFWADM page 

• If you are interested in running IPCHAINS on a 2.0.x+ kernel, see Willy Tarreau's IPCHAINS 

enabler for 2.0.36+ or Rusty's IPCHAINS for 2.0.x kernels. Please note that these patches are NOT 

compatible with the IPPORTFW patches for the 2.0.x kernels. Unfortunately, its an either/or deal. 



Here is a list of IP Masquerading patches for 2.0.x kernels: 

• Steven Clarke's IP PortForwarding (IPPORTFW) − RECOMMENDED 

• IP AutoForward − NOT Recommended 

• REDIR for TCP (REDIR) − NOT Recommended unless required for internal PORTFW 

• UDP redirector (UDPRED) − NOT Recommended 

• PORTFWed FTP: 

• 


♦ If you are going to port forward FTP traffic to an internal FTP server, you might need to 

download Fred Viles's FTP server patch The reason for "might" is that some users have had 

success without the use of these pathches, while others need it. Explicit details on this topic 

can be found in Section 6.7 of this HOWTO. 

X−Windows display forwarders: 

♦ X−windows forwarding (DXCP) 

• PPTP (GRE) and SWAN (IPSEC) VPNs tunneling forwarders: 

♦ If you plan connecting an internal MASQed PC to a remote PPTP server, you MUST 

INSTALL the PPTP−Masquerade kernel patch available from the URLsbelow. If you plan on 


• 


having external PPTP users connect to an internal masqueraded PPTP server, not only do you 

need the kernel patch installed but you also need PORTFW support enabled in the kernel. 

Please see the following URLs for the patches and more information: 

John Hardin's VPN Masquerade forwarders or the old patch for just PPTP Support. 

Game specific patches: 

♦ Glenn Lamb's LooseUDP for 2.0.36+ patch. 


Chapter 3. Setting Up IP Masquerade 

3.1. Compiling a new kernel if needed 

If your private network contains any vital information, think carefully in terms of SECURITY before 

implementing IP Masquerade. By default, IP MASQ becomes a GATEWAY for you to get onto the Internet, 

but it also can allow someone from the Internet to possibly get into your internal network. 

Once you have IP MASQ functioning, it is HIGHLY recommended for the user to implement a STRONG 

IPFWADM/IPCHAINS firewall ruleset. Please see Section 6.4.1, Section 6.4.2 and Section 6.4.3 located 

below for more details. 

3.2. Checking your existing kernel for MASQ functionality 

Almost ALL modern Linux distributions come MASQ−Ready these days but its always good to check your 

system before you try to set things up. Follow these few steps for your kernel to see if your kernel is MASQ 

ready. 

To see which kernel your system is running, run the following command: 

uname −a 

• Just for clarity: 2.4.x kernels run IPTABLES :: 2.2.x kernels run IPCHAINS :: 2.0.x kernels run 

IPFWADM 

• In general, you must have kernel support for: 

♦ IP forwarding 

♦ IP masquerading 

♦ IP Firewalling 

♦ etc. 

You will also need to have most MASQ−related modules compiled (most modular kernels will already have 

all you need already done. Then you will NOT need to re−compile the kernel. If you AREN'T SURE if your 

Linux distribution is MASQ ready, do the following: 

• 2.4.x kernels (look for most of the following entries out of the much longer list): 

♦ 

Run the command "ls /proc/sys/net/ipv4" while logged into the Linux box. These 

items are required and should be present regardless if your kernel built IPMASQ as modules 

or statically. 

◊ ip_dynaddr 

ip_forward 

♦ To check if IPMASQ was compiled statically into the kernel, run the command 

"/sbin/lsmod" and see if and modules like the ones shown below for the MODULE 

section are loaded. No? Ok, now run the command "ls /proc/net/" and see if you see 

additional /proc files such as: 

Chapter 3. Setting Up IP Masquerade 13

◊ ip_masquerade 

ip_conntrack 

ip_tables_names 

If you see these /proc entries and there WEREN'T any kernel modules loaded (shown via the 

"lsmod" command mentioned above), then your kernel has the IPTABLES subsystem 

statically compiled into it and is ready to go to use IPMASQ on this system. 

♦ If your kernel uses IPTABLES via modules, most of the stuff listed above should have been 

missing (because the modules probably aren't loaded). Run the command "ls 

/lib/modules/ùname −r`/kernel/net/ipv4/netfilter/" where you 

should see files like: 

◊ 

ip_conntrack.o, ip_conntrack_ftp.o, ip_conntrack_irc.o, 

ip_nat_ftp.o, ip_nat_irc.o 

ip_tables.o, ipt_MASQUERADE.o, iptable_nat.o, 

iptable_mangle.o, iptable_filter.o 

And some optional ones like: ipchains.o, ipt_REJECT.o, and 

ipt_tcpmss.o 

If you see those kernel files, IPTABLES was compiled using modules and things look ready 

to go to use IPMASQ on this system. 

• 2.2.x kernels (look for most of the following entries out of the much longer list): list): 

♦ 




◊ ip_always_defrag 

ip_dynaddr 

ip_forward 

ip_masq_debug 

ip_masq_udp_dloose (some distros don't support this −− ignore it for now 

Other 2.2.x options can be checked by running "ls /proc/net/" 

⋅ ip_fwchains 

ip_fwnames 

ip_masquerade 

Even more 2.2.x options can be checked by running "ls /proc/net/" 

⋅ app 

icmp 



• 

icq 

mfw 

portfw 

tcp 

udp/ 

2.0.x kernels (look for most of the following entries out of the much longer list): 

♦ 




◊ ip_dynaddr 

ip_forward 

running "ls /proc/net" 

⋅ ip_forward 

ip_masq_app 

ip_masquerade 

ip_portfw 

Ultimately, it comes down to the fact if you see /proc files such as "iip_forward", "ip_masq_debug", 

"ip_masq_udp_dloose"(optional), and "ip_always_defrag" (optional) exist. 

So. Do most of the above /proc entries or kernel modules show up for your respective kernel? If so, thats 

good! If you cannot find any of the above entries or if you aren't sure if your distribution supports IP 

Masquerading by default, ASSUME IT DOESN'T SUPPORT MASQ. You can do one last check by looking 

at the Section 7.1 section and see if your Linux Distribution is listed. Still not there? Sounds like you'll need to 

compile a kernel but don't worry.. it isn't hard. 

Regardless if your current kernel has MASQ support or not, reading the remainder of this section is still 

highly recommended as it contains other useful information. 

3.2.1. Compiling Linux 2.4.x Kernels 

• 


First, you'll need to get some 2.4.x kernel sources (preferably the latest kernel version − NEWER 

*IS* BETTER IN LINUX LAND) 

♦ NOTE #1: As both the 2.4.x kernel train and the iptables program development progresses, 

the compile configurion options will change over time. As of this version of the IPMASQ 

howto, this section reflects the settings for IPTABLES 1.2.7a and the 2.4.20 kernel. If you are 

compiling against a newer or previous kernel or IPTABLES version, the dialogs and even 


commands might look different. It is recommended that you update to the newest versions of 

both the kernel and IPTABLES for added capability, performance, and stability of the kernel. 

• Next, depending on the version of the Linux kernel and IPTABLES archive you downloaded, you 

might want to apply some IPTABLES "patch−o−matic" patches against the kernel. These 

OPTIONAL patches might fix some known problems, add additional functionality you might need 

(H.323 protocol, specific issues with network games), etc. It should be noted that the Patch−O−Matic 

patches used to come with the IPTABLES archive. This is no longer the case and you have to 

download them (if any) seperately. You can find the the various URLs for downloading IPTABLES, 

the Patch−o−matic system, etc. Section 2.6. 

• If this is your first time compiling the kernel, don't be scared. In fact, it's rather easy and it's covered 

in several URLs found in Section 2.6. Please note that the instructions included here is just one way to 

do build a kernel. Please see the Kernel HOWTO for full details. 

NOTE: Please notice that it IS NOT recommended to put the new kernel sources into the 

/usr/src/linux directory. You should leave the original kernel sources that came with your Linux 

distribution in /usr/src/linux. For more details on this topic, please read the "README" file in the top 

level directory of the kernel sources. 

• For this HOWTO example, create a directory called /usr/src/kernel. Next, "cd" into this 

directory and download the newest 2.4.x kernel sources into it. Once downloaded, issue the following 

command (if the file ends in a .tar.gz): tar xvzf linux−2.4.x.tar.gz or (if the file ends in a 

.tar.bzip2): tar xyvf linux−2.4.x.tar.bz2. Please substitute the "x" in the 2.4.x filename 

with the Linux 2.4 kernel version you downloaded. 

BZ2 Note: Some Linux distributions use the "I" option instead of the "y" option to decompress bzip2 

archives. 

Once uncompressed, I recommend that you rename the directory from the stock "linux" name to 

"linux−2.4.x" (replace the "x" with the specific version of your newly installed kernel) for clarity. To 

do this, run the command "mv linux linux−2.4.x". Next, make sure there is a directory or 

symbolic link pointing to "/usr/src/kernel/linux" ie. run the command: 

ln −s /usr/src/kernel/linux−2.4.x /usr/src/kernel/linux 

again subsituting the "x" for your proper kernel version. 

• As mentioned above, you might consider applying any appropriate or optional patches to the kernel's 

MASQ code BEFORE you compile the final kernel. The IP MASQ code found in the stock kernels is 

already very useful and does not require any specific patching in order for the system to work for 

NAT−friendly network applications. Many of these patches are only to fix possible known bugs, add 

new features (some are /very/ cool), etc. Please refer to Section 2.6 for URLs and the IP Masquerade 

Resources for up−to−date information and patch URLs. 

• Applying IPTABLES and Patch−o−Matic kernel patches 

Download the iptables package and optional Patch−O−matics from the Section 2.6 and put it into a 

directory, say "/usr/src/archive/netfilter". Next, go into this new netfilter directory and 

uncompress the iptables archive with the command: 

tar xyvf iptables−x.y.z.tar.bz2 

tar xyvf patch−o−matic−x.tar.bz2 

Now, go into the new iptables−x.y.x directory (/usr/src/archive/netfilter/iptables−x.y.z) and run the 

command 

#For iptables v1.2.7a: 



• 

make KERNEL_DIR=/usr/src/kernel/linux 

#For iptables v1.2.4 (when Patch−o−matic was built−in): 

make pending−patches KERNEL_DIR=/usr/src/kernel/linux 

NOTE: this assumes that your 2.4.x kernel sources are in the /usr/src/kernel/linux 

directory. 

NOTE #2: If you append a "/" to the end of the above command line, you will get an error stating: 

"make: *** [/usr/src/kernel/linux/include/asm/socket.h] Error 1". 

Remove the trailing "/" and try again. 

Here is an example of compiling IPTABLES v1.2.7a. Your output might look different depending on 

what version you are trying to use. 

# make KERNEL_DIR=/usr/src/kernel/linux 

Extensions found: 


cc −O2 −Wall −Wunused −I/usr/src/kernel/linux/include −Iinclude/ 

−DIPTABLES_VERSION=\"1.2.7a\" −fPIC −o extensions/libipt_ah_sh.o −c 

extensions/libipt_ah.c 

ld −shared −o extensions/libipt_ah.so extensions/libipt_ah_sh.o 


−DIPTABLES_VERSION=\"1.2.7a\" −fPIC −o extensions/libipt_conntrack_sh.o −c 

extensions/libipt_conntrack.c 

ld −shared −o extensions/libipt_conntrack.so extensions/libipt_conntrack_sh.o 


−DIPTABLES_VERSION=\"1.2.7a\" −fPIC −o extensions/libipt_dscp_sh.o −c 

extensions/libipt_dscp.c 

extensions/libipt_dscp_helper.c:69: warning: `dscp_to_name' defined but not 

used 

ld −shared −o extensions/libipt_dscp.so extensions/libipt_dscp_sh.o 

. 

. 

. 


−DIPTABLES_VERSION=\"1.2.7a\" −c −o libipulog/libipulog.o 

libipulog/libipulog.c 

ar rv libipulog/libipulog.a libipulog/libipulog.o 

a − libipulog/libipulog.o 

rm libiptc/libip6tc.o libiptc/libip4tc.o libipulog/libipulog.o libipq/libipq.o 

• Ok, hopefully the IPTABLES program compiled up for you. Now, you need to install it. To do this, 

directory and run the command 

make install KERNEL_DIR=/usr/src/kernel/linux 

• Here is an example of installing IPTABLES v1.2.7a. Your output might look different depending on 

what version you are trying to use. 

• # make install KERNEL_DIR=/usr/src/kernel/linux 

cp iptables /usr/local/sbin/iptables 

cp iptables−save /usr/local/sbin/iptables−save 

cp iptables−restore /usr/local/sbin/iptables−restore 

cp ip6tables /usr/local/sbin/ip6tables 

cp extensions/libipt_ah.so /usr/local/lib/iptables/libipt_ah.so 

cp extensions/libipt_conntrack.so /usr/local/lib/iptables/libipt_conntrack.so 

cp extensions/libipt_dscp.so /usr/local/lib/iptables/libipt_dscp.so 

cp extensions/libipt_ecn.so /usr/local/lib/iptables/libipt_ecn.so 


cp extensions/libipt_esp.so /usr/local/lib/iptables/libipt_esp.so 

cp extensions/libipt_helper.so /usr/local/lib/iptables/libipt_helper.so 

. 

. 

. 

cp extensions/libip6t_udp.so /usr/local/lib/iptables/libip6t_udp.so 

cp extensions/libip6t_LOG.so /usr/local/lib/iptables/libip6t_LOG.so 

cp extensions/libip6t_MARK.so /usr/local/lib/iptables/libip6t_MARK.so 

Next, if you are interested in applying a Patch−O−Matic patch set, go into the patch−o−matic−X directory 

(/usr/src/archive/netfilter/patch−o−matic−X) and run the command 

• 

#For Patch−O−Matic later than the release of iptables v1.2.7a: 

KERNEL_DIR=/usr/src/kernel/linux 

./runme pending 

NOTE #1: The use of the "pending" batch is the most common for IPMASQ functionality but there 

are several others. See below. 

NOTE #2: this assumes that your 2.4.x kernel sources are in the /usr/src/kernel/linux 

directory. 

NOTE #3: If you append a "/" to the end of the command line, you will get an error stating: 

"make: *** [/usr/src/kernel/linux/include/asm/socket.h] Error 1". 

Remove the trailing "/" and try again. 

Here is an example of the Patch−O−Matic prompts you might receive for a 2.4.20 kernel with the 

"20030107" Patch−O−Matic set. You can also run the "runme" program in a batch mode to speed 

things up, add experimental patches, etc. if you'd like. To better understand your options, simply run 

the "./runme" command by itself. Please note that these prompts WILL CHANGE over time. 

• Welcome to Rusty's Patch−o−matic! 

Each patch is a new feature: many have minimal impact, some do not. 

Almost every one has bugs, so I don't recommend applying them all! 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

Already applied: submitted/01_2.4.19 

submitted/02_2.4.20 

submitted/ipt_ULOG−mac_len−fix 

submitted/ipt_multiport−invfix 

pending/01_ip_conntrack_proto_tcp−lockfix 

pending/02_newnat−udp−helper 

pending/03_REJECT−fwspotting−phrack60−fix 

pending/04_ftp−conntrack−msg−fix 

Testing... 05_ECN−tcpchecksum−littleendian−fix.patch NOT APPLIED (1 rejects out 

of 1 hunks) 

The pending/05_ECN−tcpchecksum−littleendian−fix patch: 

Author: Patrick McHardy 

Status: Pending for kernel inclusion 

The 2.4.20 kernel included the new iptables 'ECN' target, enabling a 

selective 

ECN disable mechanism. Unfortunately there was a bug in the incremental 

TCP 

checksum update, resulting in broken TCP checksums on little endian 

machines. 

This patch fixes the Bug. 



• 

Testing patch pending/05_ECN−tcpchecksum−littleendian−fix.patch... 

Patch pending/05_ECN−tcpchecksum−littleendian−fix.patch applied cleanly. 

Applying patch pending/05_ECN−tcpchecksum−littleendian−fix.patch... 

Patch pending/05_ECN−tcpchecksum−littleendian−fix.patch applied cleanly. 

Excellent! Kernel is now ready for compilation. 

If everything patches fine, you should see something like the text 

Excellent! Kernel is now ready for compilation. 

towards the bottom of the screen. Beyond that, you don't have to install anything at this point. The 

next step is to compile the new PATCHED kernel. 

• Ok, now the new kernel is ready to be compiled but you should make sure that you also have the 

proper matching iptables program on your machine too (just to make sure). Run the command: 

♦ whereis iptables 

and make sure its installed on the machine (the default place is in 

/usr/local/sbin/iptables. If you cannot find it or patched up your kernel sources as shown 

above, I recommend you just re−compile it up as shown above. 

Now that the kernel sources are patched up, you need to configure it to know what kinds of features you need 

(HD support, Networking support, MASQ support, etc.). Here are the MINIMUM kernel configuration 

options required to enable IP Masquerade functionality. Please understand that this HOWTO illustrates just 

ONE way to configure and compile a kernel (modules vs static). The main difference from this example vs. an 

example given by a different MASQ guide is that some people might wish to compile kernel components 

either as modules OR monolithically into the kernel. Basically, compiling things as modules gives you added 

flexibility to what is or isn't installed into the kernel (reduces unneeded memory use for things you aren't / 

won't use and modules also allow for drop−in software upgrades [usually no need to reboot the machine]). On 

the flip side, kernel modules add more complexity to your configuration and sometimes the kernel 

auto−loader might make mistakes (not that I've ever seen this happen). Compiling things directly into the 

kernel makes things simpler BUT you loose a huge level of flexibility. The following kernel configuration 

example is a mixture of both a selection of kernel modules and building them in monolithically (you probably 

will ALWAYS need MASQ functionality ready to go). 

• Side Note: It is assumed that you will also configure the kernel to use your other installed hardware 

such as USB printers, Ethernet network interfaces, SCSI and IDE HD controllers, etc. as well. Please 

refer to the Linux Kernel HOWTO and the kernel source's "README" file and "Documentation/" 

directory for detailed help on compiling a kernel. 

You will need to answer either YES, NO, or MODULE to the following program. Not all options will be 

available without the proper kernel patches described later in this HOWTO. This shouldn't be an issue as most 

3rd party patches are only needed for a very select group of users. 

Run the following commands to configure your kernel: 

• cd /usr/src/kernel/linux 

• make menuconfig 


Please note the following kernel prompts reflect a 2.4.14 kernel (with some of the optional Patch−O−Matic 

additions. Please read the following carefully for recommendations: 


[ Code maturity level options ] 

* Prompt for development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL) [Y/n/?] 

− YES: though not required for IP MASQ, this option allows the kernel to create 

the MASQ modules and enable the option for port forwarding 

* Enable loadable module support (CONFIG_MODULES) [Y/n/?] 

− YES: allows you to load kernel IP MASQ modules 

* Set version information on all module symbols (CONFIG_MODVERSIONS) [Y/n/?] 

− YES: allows newer kernels to load older modules if possible 

* Kernel module loader (CONFIG_KMOD) [Y/n/?] 

− OPTIONAL: Recommended : allows the kernel to load various kernel modules as it needs them 

== Non−MASQ options skipped 

== (CPU type, memory, SMP, FPU, specific stuff) 

[ General setup ] 

* Networking support (CONFIG_NET) [Y/n/?] 

− YES: Enables the network subsystem 


== (specific hardware, PCI, kernel binaries, PCMCIA, etc.) 

* Sysctl support (CONFIG_SYSCTL) [Y/n/?] 

− YES: Enables the ability to enable disable options such as forwarding, 

dynamic IPs, etc. via the /proc interface 

[ Block devices ] 


== (kernel binaries, power management, PnP, RAID, etc.) 

== Don't forget to compile in support for hardware that you might need: 

== IDE controllers, HDs, CDROMs, etc. 

[ Networking options ] 


* Packet socket (CONFIG_PACKET) [Y/m/n/?] 

− YES: Though this is OPTIONAL, this recommended feature will allow you 

to use TCPDUMP to debug any problems with IP MASQ 

* Packet socket: mmapped IO (CONFIG_PACKET_MMAP) [N/y/?] y 

− YES: Speed up the packet protocol 

* Kernel/User netlink socket (CONFIG_NETLINK) [Y/n/?] 

− OPTIONAL: Recommended : this feature will allow the logging of 

advanced firewall issues such as routing messages, etc 

* Routing messages (CONFIG_RTNETLINK) [N/y/?] (NEW) y 

− OPTIONAL: Allows for support of advanced kernel routing messages 

if you enabled the CONFIG_NETLINK option 

* Netlink device emulation (CONFIG_NETLINK_DEV) [N/y/m/?] (NEW) 

− NO: This option does not have anything to do with packet firewall 

logging 


* Network packet filtering (replaces ipchains) (CONFIG_NETFILTER) [N/y/?] y 

− YES: Enable this option to let IPTABLES configure the TCP/IP subsection 

of the kernel. By enabling this, then you can turn on advanced 

routing mechanisms like IP Masq, packet filtering, etc. 

* Network packet filtering debugging (CONFIG_NETFILTER_DEBUG) [N/y/?] (NEW) n 

− NO: Not required for Masquerading functionality though it may help 

for troubleshooting. There might be a performance penalty when 

enabling this. 

* Socket Filtering (CONFIG_FILTER) [Y/n/?] 

− OPTIONAL: Recommended : Though this doesn't have anything do with IPMASQ, 

if you plan on implimenting a DHCP server on the internal network, you WILL 

need to enable this option. 

* Unix domain sockets (CONFIG_UNIX) [Y/m/n/?] 

− YES: This enables the UNIX TCP/IP sockets mechanisms 

* TCP/IP networking (CONFIG_INET) [Y/n/?] 

− YES: Enables the TCP/IP protocol 

* IP: multicasting (CONFIG_IP_MULTICAST) [N/y/?] 

− OPTIONAL: You can enable this if you want to be able to receive 

Multicast traffic. Please note that your ISP must 

support Multicast as well for this all to work at all 

* IP: advanced router (CONFIG_IP_ADVANCED_ROUTER) [Y/n/?] 

− OPTIONAL: Though there is nothing in this section mandatory for 

Masquerade, some specific options might be useful 


== ( autoconf, tunneling ) 

* IP: multicast routing (CONFIG_IP_MROUTE) [N/y/?] n 

− OPTIONAL: Though not needed for IPMASQ, enabling this feature will 

let you route multicast traffic through your Linux box. 

Please note that this requires that your ISP be multicast 

enabled as well. 


== (ARPd) 


* IP: TCP Explicit Congestion Notification support (CONFIG_INET_ECN) [N/y/?] n 

− NO: Though enabling this option would be great, there are many Internet 

sites out there that will block this. Hit the "?" when configuring 

the kernel to learn more about it but it is recommended to say NO for 

now. 

* IP: TCP syncookie support (disabled per default) (CONFIG_SYN_COOKIES) [Y/n/?] 

− YES: Recommended : for basic TCP/IP network security 

[ Networking options −−> IP: Netfilter Configuration ] 

* Connection tracking (required for masq/NAT) (CONFIG_IP_NF_CONNTRACK) [N/y/m/?] (NEW) m 

− YES: (Module) This enables the kernel to track various network connections. 

This option is required for Masquerading support as well as to enable 

Stateful tracking for various filewall mechanisms. Please note that 

if you compile this directly into the kernel, you cannot enable 

the legacy IPCHAINS or IPFWADM compatibility modules. 



* FTP protocol support (CONFIG_IP_NF_FTP) [M/n/?] (NEW) m 

− YES: (Module) This enables the proper Masquerading of FTP connections if 

CONFIG_IP_NF_CONNTRACK was enabled above 

* IRC protocol support (CONFIG_IP_NF_IRC) [M/n/?] (NEW) m 

− YES: (Module) This enables the proper Masquerading of IRC connections if 

CONFIG_IP_NF_CONNTRACK was enabled above 

* Userspace queueing via NETLINK (EXPERIMENTAL) (CONFIG_IP_NF_QUEUE) [N/y/m/?] (NEW) m 

− OPTIONAL: Though this is OPTIONAL, this feature will allow IPTABLES to 

copy specific packets to UserSpace tools for additional checks 

* IP tables support (required for filtering/masq/NAT) (CONFIG_IP_NF_IPTABLES) [N/y/m/?] (NEW) m 

− YES: (Module) Enables IPTABLES support 

* limit match support (CONFIG_IP_NF_MATCH_LIMIT) [N/y/m/?] (NEW) y 

− OPTIONAL: (Module) Recommended : Though not required, this option can used to 

enable rate limiting of both traffic and loggin messages help slow down denial 

of service (DoS) attacks. 

* MAC address match support (CONFIG_IP_NF_MATCH_MAC) [N/y/m/?] (NEW) m 

− OPTIONAL: Though not required, the option can allow you to 

filter traffic based upon the SOURCE Ethernet MAC address. 

* netfilter MARK match support (CONFIG_IP_NF_MATCH_MARK) [N/y/m/?] (NEW) y 

− YES: (Module) Recommended : This enables IPTABLES to take action upon marked packets. 

This mechanism can allow for PORTFW functionality, TOS marking, etc. 

* Multiple port match support (CONFIG_IP_NF_MATCH_MULTIPORT) [N/y/m/?] (NEW) y 

− YES: (Module) Recommended : This enables IPTABLES to accept mutliple SRC/DST port 

ranges (non−contiguous) instead of one port range per IPTABLES 

statement. 

* TOS match support (CONFIG_IP_NF_MATCH_TOS) [Y/m/n/?] n 

− OPTIONAL: This allows IPTABLES to match packets based upon their 

DIFFSERV settings. 

* LENGTH match support (CONFIG_IP_NF_MATCH_LENGTH) [N/m/?] (NEW) n 


packet length. 

* TTL match support (CONFIG_IP_NF_MATCH_TTL) [N/m/?] (NEW) ? n 


TTL settings. 

* tcpmss match support (CONFIG_IP_NF_MATCH_TCPMSS) [N/y/m/?] m 

− OPTIONAL: (Module) Recommended : This option allows users to examine the MSS value in 

TCP SYN packets. This is an advanced knob but can be very valuable in 

troubleshooting MTU problems. 

* Connection state match support (CONFIG_IP_NF_MATCH_STATE) [M/n/?] m 

− YES: (Module) Recommended : This option allows for Stateful tracking of network 

connections. 

* Unclean match support (EXPERIMENTAL) (CONFIG_IP_NF_MATCH_UNCLEAN) [N/y/m/?] y 

− YES: (Module) Recommended : This option allows for connection tracking on odd packets. 

It cal also help in the detection of possibly malicious packets. 

This can be a valuable tool in tracking hostile people on the network. 

* Owner match support (EXPERIMENTAL) (CONFIG_IP_NF_MATCH_OWNER) [N/y/m/?] n 

− OPTIONAL: This option allows IPTABLES to match traffic based upon the 

user login, group, etc. who created the traffic. 



* Packet filtering (CONFIG_IP_NF_FILTER) [N/y/m/?] ? y 

− YES: (Module) This option allows for the kernel to be able filter traffic at 

the INPUT, FORWARDING, and OUTPUT traffic points. 

* REJECT target support (CONFIG_IP_NF_TARGET_REJECT) [N/y/m/?] (NEW) y 

− YES: (Module) With this option, a packet firewall can send an ICMP Reject packet 

back to the originator when a packet is blocked. 

* MIRROR target support (EXPERIMENTAL) (CONFIG_IP_NF_TARGET_MIRROR) [N/y/m/?] (NEW) n 

− OPTIONAL: This option allows the packet firewall to mirror the exact same 

network packet back to the originator when it is supposed to be 

blocked. This is similar to the REJECT option above but it actually 

sends the original packet back to the originator. i.e. a 

hostile user could actually portscan themselves. 

* Full NAT (CONFIG_IP_NF_NAT) [M/n/?] m 

− YES: (Module) This option enables the future menus to enable Masquerading, 

PORTFWing, Full (1:1) NAT, etc. 

* MASQUERADE target support (CONFIG_IP_NF_TARGET_MASQUERADE) [M/n/?] (NEW) m 

− YES: (Module) This option specifically enables Masquerade into the 

kernel 

* REDIRECT target support (CONFIG_IP_NF_TARGET_REDIRECT) [N/y/m/?] n 

− OPTIONAL: Not needed for normal MASQ functionality though people who 

want to do transparent proxy via Squid will want this. 

* Basic SNMP−ALG support (EXPERIMENTAL) (CONFIG_IP_NF_NAT_SNMP_BASIC) [N/m/?] n 

− OPTIONAL: This enables IPTABLES to properly NAT internal SNMP packets so 

that machines with duplicate addressing ranges can be properly 

managed. 

* Packet mangling (CONFIG_IP_NF_MANGLE) [N/y/m/?] y 

− YES: (Module) This option allows for advanced IPTABLES packet manipulation 

options. 

* TOS target support (CONFIG_IP_NF_TARGET_TOS) [N/y/m/?] (NEW) n 

− OPTIONAL: Enables the kernel to modify the TOS field in a packet 

before routing it on 

* MARK target support (CONFIG_IP_NF_TARGET_MARK) [N/y/m/?] (NEW) m 

− OPTIONAL: (Module) Recommended : This enables the kernel to manipulate 

packets based upon the MARK field. This can be used for PORTFW 

as well as many other things. 

* LOG target support (CONFIG_IP_NF_TARGET_LOG) [N/y/m/?] m 

− YES: (Module) This allows for the logging of packets before they are accepted, 

denied, rejected, etc. 

* TCPMSS target support (CONFIG_IP_NF_TARGET_TCPMSS) [N/y/m/?] ? m 

− YES: (Module) This option help some people with MTU problems. Typically, 

most users have to set their Internet connection's MTU to 

1500 as well as ALL internal machines to 1500. With this 

option, this whole MTU issue might be finally solved. 

* ipchains (2.2−style) support (CONFIG_IP_NF_COMPAT_IPCHAINS) [N/y/m/?] m 

− OPTIONAL: (Module) Recommended : If you have an existing IPCHAINS ruleset 


(2.2.x kernels) and enable this option, you can continue to use the 

IPCHAINS program and the majority of your old ruleset except for the 

use of any 2.2.x kernel−specific modules. Please note that if this 

IPCHAINS module is loaded, ALL IPTABLES modules will be non− 

operational. This is an either/or deal only intended for legacy 

rulesets. 

* ipfwadm (2.0−style) support (CONFIG_IP_NF_COMPAT_IPFWADM) [N/y/m/?] n 

− OPTIONAL: If you have an existing IPFWADM ruleset (2.0.x kernels) and 

enable this option, you can continue to use the IPFWADM program and 

the majority of your old ruleset except for the use of any 2.0.x 

kernel−specific modules. Please note that if this IPFWADM module 

is loaded, ALL IPTABLES modules will be non operational. This is 

an either/or deal only intended to support legacy rulesets. 


== (IPv6, khttpd, ATM, IPX, AppleTalk, etc.) −− 

* Fast switching (read help!) (CONFIG_NET_FASTROUTE) [N/y/?] n 

− NO: This performance optimization is NOT compatible with IP MASQ and/or 

packet filtering 


== (QoS, Telephony, IDE, SCSI, 1394FW, I2O, etc) 


== IDE: HDs, CDROMs, etc. 

== SCSI: HDs, CDROMs, etc. 

[ Network device support ] 

* Network device support (CONFIG_NETDEVICES) [Y/n/?] 

− YES: Enables the Linux Network device sublayer 


== (Arcnet) 

* Dummy net driver support (CONFIG_DUMMY) [M/n/y/?] 

− YES: Though OPTIONAL, this option can help when debugging problems 


== (EQL, etc..) 


== NICs: eth, tr, etc. 

== MODEMs: ppp (ppp async) and/or slip 

== WANs: T1, T3, ISDN, etc. 

== ISDN: for internal ISDN modems 


== (Amateur Radio, IrDA, ISDN, USB, etc.) 

[ Character devices ] 


== Don't forget to compile in serial port support if you are a modem user 

== Don't forget to compile in mouse support 



== (I2C, Watchdog cards, Ftape, Video for Linux, etc. ) 

[ File systems ] 


== (Quota, ISO9660, NTFS, etc ) 

* /proc filesystem support (CONFIG_PROC_FS) [Y/n/?] 

− YES: Required to dynamically configure the Linux forwarding 

and NATing systems 


== (Console drivers, Sound, USB, Kernel Hacking) 

So go ahead and select "exit" and you should be prompted to save your config. 

NOTE: These are just the kernel components you need for IP Masquerade networking support. You will need 

to select whatever other options needed for your specific setup. If you want more information on what each 

one of these kernel modules does, please see the FAQ section of this HOWTO for details. 

• Now compile the kernel (make dep; make clean; make bzImage; make modules; make 

modules_install) , etc. Again, it is beyond the scope of this HOWTO if you have problems compiling 

your kernel. Please see Section 2.6 for URLs to the KERNEL howto, etc. 

• You will then have move over the kernel binary, update your bootloader (LILO, Grub, etc.), and 

reboot. If you have questions about kernel compiling, I highly recommend to consult some of the 

URLs mentioned above in this section. 


Please see Section 2.7 for any required software, patches, etc. 

• First of all, you need the kernel source for 2.2.x (preferably the latest kernel version) 

• 

♦ NOTE #1: −−− UPDATE YOUR KERNEL −−− Linux 2.2.x kernels less than version 2.2.20 

contain several different security vulnerabilities (some were MASQ specific). Kernels less 

than 2.2.20 have a few local vulnerabilities. Kernel versions less than 2.2.16 have a TCP root 

exploit vulnerability and versions less than 2.2.11 have a IPCHAINS fragmentation bug. 

Because of these issues, users running a firewall with strong IPCHAINS rulesets are open to 

possible instrusion. Please upgrade your kernel to a fixed version. 

♦ NOTE #2: As the 2.2.x train progressed, the compile−time options keep on changing. As of 

this version, this section reflects the settings for a 2.2.20 kernel. 

If you are running either a newer or older kernel version, the dialogs will look different. It is 

recommended that you update to the newest kernel for added capability and stability of the 

system. 


If this is your first time compiling the kernel, don't be scared. In fact, it's rather easy and it's covered 




• 

NOTE: Please notice that it isn't recommended to put the new kernel sources into /usr/src/linux. You 

should leave the original kernel sources that came with your Linux distribution in /usr/src/linux. For 

more details on this topic, please read the "README" file in the top level directory of your kernel 

sources. 

For this HOWTO example, create a directory called /usr/src/kernel. Next, "cd" into this 


command (if the file ends in a .tar.gz): tar xvzf linux−2.2.x.tar.gz or (if the file ends in a 

.tar.bzip2): tar xyvf linux−2.2.x.tar.bz2. Please substitute the "x" in the 2.2.x filename 


NOTE: Some Linux distributions use the "I" option instead of the "y" option to decompress bzip2 

archives. 

Once uncompressed, I recommend that you rename the directory from "linux" to "linux−2.2.x" for 

clarity. To do this, run the command mv linux linux−2.2.x. Next, make sure there is a 

directory or symbolic link pointing to /usr/src/kernel/linux ie. run the command: ln −s 

/usr/src/kernel/linux−2.2.x /usr/src/kernel/linuxo again subsituting the "x" 

for your proper kernel version. 

• Apply any appropriate or optional patches to the kernel source code. By default, stock Linux kernels 

do not require any specific patching in order for the system to work. Features like PPTP/IPSEC 

masqurading are already built−in in the newest kernels but other tools like Xwindows forwarders are 

optional. Please refer to Section 2.7 for URLs and the IP Masquerade Resources for up−to−date 

information and patch URLs. 

• Now that the kernel is patched up (if required), here are the MINIMUM kernel configuration options 

required to enable IP Masquerade functionality. Please understand that this HOWTO illustrates just 

ONE way to compile a kernel. The main difference from this method vs. a different one is some 

people wish to compile things either as modules OR monolithically right into the kernel. Basically, 

compiling things as modules gives you added flexibility to what is or isn't installed into the kernel 

(reduces unneeded memory use and allow for drop−in upgrades [no need to reboot]) BUT they add 

more complexity to your configuration. On the flip side, compiling things directly into the kernel 

makes things simpler BUT you loose a level of flexibility. The following example is a mixture of both 

built−in AND modules. 

Side Note: It is assumed that you will also configure the kernel to use your other installed hardware 

such as network interfaces, optional SCSI controllers, etc. as well. Please refer to the Linux Kernel 

HOWTO and the kernel source's README file and Documentation/ directory for detailed help on 

compiling a kernel. 

Please note the YES or NO ANSWERS to the following. Not all options will be available without the proper 

kernel patches described later in this HOWTO. 




The following kernel prompts reflect a 2.2.20 kernel: 





− YES: though not entirely required for IP MASQ, this option allows the kernel 

to create possible additional MASQ modules such as PORTFW, etc. 


== (CPU, memory, MTRR, SMP, etc.) 

[ Loadable module support ] 

* Enable loadable module support (CONFIG_MODULES) [Y/n/?] y 


* Set version information on all symbols for modules (CONFIG_MODVERSIONS) [N/y/?] y 


* Kernel module loader (CONFIG_KMOD) [Y/n/?] y 

− OPTIONAL: Recommended : allows the kernel to load various kernel modules as 

it needs them 


* Networking support (CONFIG_NET) [Y/n/?] 

− YES: This enables the network subsystem 


== (PCI, kernel binaries, specific hardware options, etc.) 

* Sysctl support (CONFIG_SYSCTL) [Y/n/?] 

− YES: Enables the ability to enable disable options such as forwarding, 

dynamic IPs, etc. via the /proc interface 

[ Block devices ] 


== (kernel binaries, power management, PnP, IDE, SCSI, etc.) 





* Packet socket (CONFIG_PACKET) [Y/m/n/?] y 

− YES: Though this is OPTIONAL, this recommended feature will allow you 

to use TCPDUMP to debug any problems with IP MASQ 

* Kernel/User netlink socket (CONFIG_NETLINK) [Y/n/?] y 

− OPTIONAL: Recommended : This feature will allow the logging of 

advanced firewall issues such as routing messages, etc 

* Routing messages (CONFIG_RTNETLINK) [Y/n/?] y 

− OPTIONAL: If you enabled the CONFIG_NETLINK option above, this option 

will send routing messages and other information to SYSLOG. 

* Netlink device emulation (CONFIG_NETLINK_DEV) [N/y/m/?] (NEW) n 

− NO: This option does not have anything to do with packet firewall 

logging 



* Network firewalls (CONFIG_FIREWALL) [Y/n/?] y 

− YES: Enables the kernel to be comfigured by the IPCHAINS firewall tool 

* Socket Filtering (CONFIG_FILTER) [Y/n/?] y 

− OPTIONAL: Though this doesn't have anything do with IPMASQ, if you 

plan on implimenting a DHCP server on the internal network, you 

WILL need this option. 

* Unix domain sockets (CONFIG_UNIX) [Y/m/n/?] y 

− YES: This enables the UNIX TCP/IP sockets mechanisms 

* TCP/IP networking (CONFIG_INET) [Y/n/?] y 


* IP: multicasting (CONFIG_IP_MULTICAST) [N/y/?] y 



support Multicast as well for this all to work 

* IP: advanced router (CONFIG_IP_ADVANCED_ROUTER) [Y/n/?] n 

− OPTIONAL: Though there is nothing in this section mandatory for 

Masquerade, some specific options might be useful 

* IP: kernel level autoconfiguration (CONFIG_IP_PNP) [N/y/?] ? 

− NO: Not needed for normal MASQ functionality 

* IP: firewalling (CONFIG_IP_FIREWALL) [Y/n/?] y 

− YES: This enables the kernel to support packet filtering, NAT, etc. 

* IP: firewall packet netlink device (CONFIG_IP_FIREWALL_NETLINK) [Y/n/?] n 

− OPTIONAL: Though this is OPTIONAL, this feature will allow IPCHAINS to 

copy some packets to UserSpace tools for additional checks 

* IP: transparent proxy support (CONFIG_IP_TRANSPARENT_PROXY) [N/y/?] n 


want to do transparent proxy via Squid will want this. Please note 

that there is a PERFORMANCE PENALTY enabling this feature. 

* IP: masquerading (CONFIG_IP_MASQUERADE) [Y/n/?] y 

− YES: Enable IP Masquerade to re−address specific internal to external 

TCP/IP packets 

* IP: ICMP masquerading (CONFIG_IP_MASQUERADE_ICMP) [Y/n/?] y 

− YES: Enable support for masquerading ICMP ping packets (ICMP error 

codes will be MASQed regardless). This is an important feature 

for troubleshooting connections. 

* IP: masquerading special modules support (CONFIG_IP_MASQUERADE_MOD) [Y/n/?] y 

− YES: Though OPTIONAL, this enables the option to later enable other 

modules like the PORTFW to give external computers a directly 

connection to specified internal MASQed machines. 

* IP: ipautofw masq support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_IPAUTOFW) [N/y/m/?] n 

− NO: NOT recommended : IPautofw is a legacy method of port forwarding. It 

is mainly old code and has been found to have some issues. 

* IP: ipportfw masq support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_IPPORTFW) [Y/m/n/?] y 

− OPTIONAL: Recommended : This enables PORTFW which allows external computers 

on the Internet to directly communicate to specified internal MASQed 

machines. This feature is typically used to allow access to internal 

SMTP, TELNET, and WWW servers. Please note that FTP port forwarding 

needs an additional patch, as described in the FAQ section of the MASQ 


HOWTO. Please see the this FAQ section in the HOWTO for additional 

information. 

* IP: ip fwmark masq−forwarding support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_MFW) [Y/m/n/?] y 

− OPTIONAL: This is a NEW method of performing PORTFW−like functionality which is 

similar to how the new 2.4.x kernels do things. With this option, IPCHAINS 

can mark packets that should have additional work done upon it. Using a 

UserSpace tool, much like IPMASQADM or IPPORFW, IPCHAINS would then 

do things like re−address the packets, change their TOS value, etc. 

Currently, this code is less tested than PORTFW but it looks promising. 

For now, this HOWTO recommends to use IPMASQADM and IPPORTFW. If you 

have specific thoughts or comments on MFW, please email dranch. 

* IP: optimize as a router not host (CONFIG_IP_ROUTER) [Y/n/?] y 

− YES: This optimizes the kernel for the network subsystem, though it 

isn't well known if this makes a siginificant performance difference 

or not. 


== ( autoconf, tunneling, GRE ) 

* IP: multicast routing (CONFIG_IP_MROUTE) [N/y/?] n 

− OPTIONAL: Though not needed for IPMASQ, enabling this feature will 

let you route multicast traffic through your Linux box. 

Please note that this requires that your ISP be multicast 

enabled as well. 


== (Aliasing, ARPd) 

* IP: TCP syncookie support (disabled per default) (CONFIG_SYN_COOKIES) [Y/n/?] 

− YES: Recommended : for basic TCP/IP network security 

* IP: GRE tunnels over IP (CONFIG_NET_IPGRE) [N/y/m/?] 

− NO: This OPTIONAL selection is to enable PPTP and GRE tunnels through 

the IP MASQ box 


== (aliasing, ARPd) 

* IP: TCP syncookie support (not enabled per default) (CONFIG_SYN_COOKIES) [Y/n/?] 

− YES: HIGHLY recommended for basic TCP/IP network security 


== (RARP) 

* IP: Allow large windows (not recommended if

== (Slow CPU, Telephony, SCSI, I2O, etc. ) 


== SCSI: HDs, CDROMs, etc. 





== (Arcnet) 

* Dummy net driver support (CONFIG_DUMMY) [M/n/y/?] 

− YES: Though OPTIONAL, this option can help when debugging problems 


== (EQL, NICs, Wireless, IrDA, ISDN, etc..) 



== MODEMs: ppp and/or slip 




== Don't forget to compile in serial port support for modem users 



== (I2C, Watchdog cards, Ftape, Video for Linux, USB, etc. ) 



== (Quota, ISO9660, NTFS, etc ) 






== (network fs, NLS, video section, sound, kernel hacking) 

So go ahead and "exit" and you should be prompted to save your config. 

NOTE: These are just the components you need for IP Masquerade. You will need to select whatever other 

options needed for your specific setup. 







URLs above in this section. 



Please see Section 2.8 for any required software, patches, etc. 

• First of all, you need the kernel source for 2.0.x (preferably the latest kernel version) 

♦ As the 2.0.x train progress, the compile−time options keep on changing. As of this version, 

this section reflects the settings for a 2.0.39 kernel. 

• If this is your first time compiling the kernel, don't be scared. In fact, it's rather easy and it's covered 



NOTE: Please notice that it isn't recommended to put the new kernel sources into /usr/src/linux. You 

should leave the original kernel sources that came with your Linux distribution in /usr/src/linux. For 

more details on this topic, please read the "README" file in the top level directory of your kernel 

sources. 

• For this HOWTO example, create a directory called /usr/src/kernel. Next, "cd" into this 


command: tar xvzf linux−2.0.x.tar.gz . Please substitute the "x" in the 2.0.x filename 


Once uncompressed, I recommend that you rename the directory from "linux" to "linux−2.0.x" for 

clarity. To do this, run the command mv linux linux−2.0.x. Next, make sure there is a 

directory or symbolic link pointing to /usr/src/kernel/linux ie. run the command: ln −s 

/usr/src/kernel/linux−2.0.x /usr/src/kernel/linuxo again subsituting the "x" 

for your proper kernel version. 

• Apply any appropriate or optional patches to the kernel source code. By default, stock Linux kernels 

do not require any specific patching in order for the system to work. Features like IPPORTFW, PPTP, 

and Xwindows forwarders are optional but very useful. Please refer to Section 2.8 for URLs and the 

IP Masquerade Resources for up−to−date information and patch URLs. 

• Now that the kernel is patched up (if required), here are the MINIMUM kernel configuration options 

required to enable IP Masquerade functionality. Please understand that this HOWTO illustrates just 

ONE way to compile a kernel. The main difference from this method vs. a different one is some 

people wish to compile things either as modules OR monolithically right into the kernel. Basically, 

compiling things as modules gives you added flexibility to what is or isn't installed into the kernel 

(reduces unneeded memory use and allow for drop−in upgrades [no need to reboot]) BUT they add 

more complexity to your configuration. On the flip side, compiling things directly into the kernel 

makes things simpler BUT you loose a level of flexibility. The following example is a mixture of both 

built−in AND modules. 

Side Note: It is assumed that you will also configure the kernel to use your other installed hardware 

such as network interfaces, optional SCSI controllers, etc. as well. Please refer to the Linux Kernel 

HOWTO and the kernel source's "README" file and "Documentation/" directory for detailed 

help on compiling a kernel. 


Please note the YES or NO ANSWERS to the following options. Not all options will be available without the 

proper kernel patches described later in this HOWTO: 




The following kernel prompts reflect a 2.0.39 kernel: 



− YES: this will allow you to later select the IP Masquerade feature code 

[ Loadable module support ] 

* Enable loadable module support (CONFIG_MODULES) [Y/n/?] y 


* Set version information on all module symbols (CONFIG_MODVERSIONS) [N/y/?] y 


* Kernel daemon support (e.g. autoload of modules) (CONFIG_KERNELD) [N/y/?] y 

− OPTIONAL: Recommended : allows the kernel to load various kernel modules as 

it needs them 



== (FPU, memory) 

* Networking support (CONFIG_NET) [Y/n/?] y 

− YES: Enables the network subsystem 


== (memory, PCI, binary format, APM, etc.) 




* Network firewalls (CONFIG_FIREWALL) [Y/n/?] y 

− YES: Enables the IPFWADM firewall tool 


== (Aliasing) 


* TCP/IP networking (CONFIG_INET) [Y/n/?] y 


* IP: forwarding/gatewaying (CONFIG_IP_FORWARD) [N/y/?] y 

− YES: Enables Linux network packet forwarding and routing 

− Controlled by IPFWADM 



* IP: multicasting (CONFIG_IP_MULTICAST) [N/y/?] y 



support Multicast as well for this all to work 

* IP: syn cookies (CONFIG_SYN_COOKIES) [Y/n/?] y 

− YES: HIGHLY recommended for basic network security 

* IP: firewalling (CONFIG_IP_FIREWALL) [Y/n/?] y 

− YES: Enable the packet firewall features 

* IP: firewall packet logging (CONFIG_IP_FIREWALL_VERBOSE) [Y/n/?] y 

− YES: Allows the kernel to report back on various packets traversing 

the firewall. 

* IP: masquerading (CONFIG_IP_MASQUERADE [Y/n/?] y 

− YES: Enable the kernel to perform IP MASQ NAT functionality 

* IP: ipautofw masquerade support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_IPAUTOFW) [Y/n/?] n 

− NO: NOT Recommended : IPautofw is a legacy method of TCP/IP port forwarding. 

Though IPautofw works, IPPORTFW is a better choice. 

* IP: ipportfw masq support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_IPPORTFW) [Y/n/?] y 

− YES: This option is ONLY AVAILABLE VIA A PATCH for the 2.0.x kernels. 

With this option, external computers on the Internet can directly 

communicate to specified internal MASQed machines. This feature is 

typically used to access internal SMTP, TELNET, and WWW servers. 

FTP port forwarding sometimes might require an additional patch as 

described in the FAQ section. Additional information on port 

forwarding is available in the Forwards section of this HOWTO. 

* IP: MS PPTP masq support (EXPERIMENTAL) (CONFIG_IP_MASQUERADE_PPTP) [N/y/?] (NEW) n 

− OPTIONAL: Enabling this feature will allow internal MASQ clients to 

properly connect to PPTP servers on the Internet. 

* IP: MS PPTP Call ID masq support (CONFIG_IP_MASQUERADE_PPTP_MULTICLIENT) [N/y/?] (NEW) n 

− OPTIONAL: If you enabled the CONFIG_IP_MASQUERADE_PPTP above, this 

option will allow for multiple internal PPTP clients behind the MASQ 

server to communicate to the same PPTP server. 

* IP: MS PPTP masq debugging (DEBUG_IP_MASQUERADE_PPTP) [N/y/?] n 

− OPTIONAL: NOT recommended : This is not required for IP MASQ or MASQing PPTP 

connections unless you need additional troubleshooting help. If enabled, 

this can fill up your logs quickly. 

* IP: MS PPTP masq verbose debugging (DEBUG_IP_MASQUERADE_PPTP_VERBOSE) [N/y/?] (NEW) n 

− OPTIONAL: NOT Recommended : If you enabled the DEBUG_IP_MASQUERADE_PPTP 

option above, this will make the logging even more verbose. 

* IP: IPSEC ESP & ISAKMP masq support (EXPERIMENTAL) * (CONFIG_IP_MASQUERADE_IPSEC) [N/y/?] m 

− OPTIONAL: This option allows for some forms of IPSEC tunnels to be 

masquraded 

* IP: IPSEC masq table lifetime (minutes) (CONFIG_IP_MASQUERADE_IPSEC_EXPIRE) * [30] (NEW) 

− OPTIONAL: This feature allows to change the MASQ table timeouts so that 

idle IPSEC tunnels won't be prematurely disconnected. 

* IP: Disable inbound ESP destination guessing * (CONFIG_IP_MASQUERADE_IPSEC_NOGUESS) [N/y/?] n 

− OPTIONAL: This feature allows the kernel to guess where the fully encrypted IPSEC VPN 

might be going and add it to the MASQ table. 


* IP: IPSEC masq debugging (DEBUG_IP_MASQUERADE_IPSEC) [N/y/?] ? n 

− OPTIONAL: NOT recommended : This is not required for IP MASQ or MASQing IPSEC 

connections unless you need additional troubleshooting help. If enabled, 

this can fill up your logs quickly. 

* IP: IPSEC masq verbose debugging (DEBUG_IP_MASQUERADE_IPSEC_VERBOSE) [N/y/?] (NEW) n 

− OPTIONAL: NOT Recommended : If you enabled the DEBUG_IP_MASQUERADE_IPSEC 

option above, this will make the logging even more verbose. 

* IP: ICMP masquerading (CONFIG_IP_MASQUERADE_ICMP) [Y/n/?] 

− YES: Enable support for masquerading ICMP packets. Though thought of as 

optional, many programs will NOT function properly with out ICMP 

support. 

* IP: transparent proxy support (EXPERIMENTAL) (CONFIG_IP_TRANSPARENT_PROXY) [N/y/?] n 


want to do transparent proxy via Squid will want this. Please note 

that there is a PERFORMANCE PENALTY enabling this feature. 

* IP: loose UDP port managing (EXPERIMENTAL) (CONFIG_IP_MASQ_LOOSE_UDP) [Y/n/?] 

− YES: This option is ONLY AVAILABLE VIA A PATCH for the 2.0.x kernels. 

With this option, internally masqueraded computers can play 

NAT−friendly games over the Internet. Explicit details are given 

in the FAQ section of this HOWTO. 

* IP: always defragment (CONFIG_IP_ALWAYS_DEFRAG) [Y/n/?] 

− YES: This feature optimizes IP MASQ connections 


== (Accounting) 

* IP: optimize as router not host (CONFIG_IP_ROUTER) [Y/n/?] 

− YES: This optimizes the kernel for the network subsystem 


== (Tunneling, Mcast routing, RARP, PMTU, etc.) 

* IP: Drop source routed frames (CONFIG_IP_NOSR) [Y/n/?] 

− YES: HIGHLY recommended for basic network security 


== (IPX, Bridging, SCSI, etc.) 


== SCSI controllers, HDs, CDROMs, etc. 






== (Dummy, EQL, PPP, SLIP, NICs, Wireless, etc.) 




== MODEMs: ppp and/or slip 





== (Quota, ISO9660, Codepages, NTFS, etc ) 






== (multi−port serial, parallel, mice, Ftape, Sound, etc. ) 

== Don't forget to compile in serial port support for modem users 


So go ahead and "exit" and you should be prompted to save your config. 

NOTE: These are only components for IP Masquerade functionality. You may need to also select additional 

options to match your specific network and hardware setup. 






URLs above in this section. 

3.3. Assigning Private Network IP Addresses to the Internal 

LAN 

Since all INTERNAL MASQed machines should NOT have official Internet assigned addressees, there must 

be a specific and accepted way to allocate addresses to those machines without conflicting with anyone else's 

Internet address. 

From the original IP Masquerade FAQ: 

RFC 1918 is the official document on which IP addresses are to be used in a non−connected or "private" 

network. There are 3 blocks of numbers set aside specifically for this purpose. 

Section 3: Private Address Space 


The Internet Assigned Numbers Authority (IANA) has reserved the 

following three blocks of the IP address space for private networks: 

10.0.0.0 − 10.255.255.255 

172.16.0.0 − 172.31.255.255 

192.168.0.0 − 192.168.255.255 



We will refer to the first block as "24−bit block", the second as "20−bit 

block", and the third as "16−bit" block". Note that the first block is 

nothing but a single class A network number, while the second block is a set 

of 16 continuous class B network numbers, and the third block is a set of 255 

continuous class C network numbers. 

For the record, my preference is to use the 192.168.0.0 network with a 255.255.255.0 Class−C subnet mask 

and thus this HOWTO reflects this. Any of the above private networks are valid, but just be SURE to use the 

correct subnet−mask. 

So, if you're using a Class−C network, you should number your TCP/IP enabled machines as 192.168.0.1, 

192.168.0.2, 192.168.0.3, .., 192.168.0.x 

192.168.0.1 is usually set as the internal gateway or Linux MASQ machine which reaches the external 

network. Please note that 192.168.0.0 and 192.168.0.255 are the Network and Broadcast address respectively 

(these addresses are RESERVED). Avoid using these addresses on your machines or your network will not 

function properly. 

3.4. Configuring IP Forwarding Policies 

At this point, you should have your kernel and other required packages installed. All network IP addresses, 

gateway, and DNS addresses should be configured on your Linux MASQ server. If you don't know how to 

configure your Linux network cards, please consult the HOWTOs listed in either the 2.4.x Section 2.6, the 

2.2.x Section 2.7, or the 2.0.x Section 2.8. 

Now, the only thing left to do is to configure the IP firewalling tools to both FORWARD and 

MASQUERADE the appropriate packets to the correct machine. 

** This section ONLY provides the user with the bare minimum firewall ruleset to get IP Masquerading 

working. 

Once IP MASQ has been successfully tested (as described later in this HOWTO), please refer to the Stronger 

IPTABLES ruleset for 2.4.x kernels in Section 6.4.1, the Stronger IPCHAINS ruleset for 2.2.x kernels in 

Section 6.4.2, and the Stronger IPFWADM ruleset for 2.0.x kernels in Section 6.4.3. Please note that these 

stronger firewall rulesets are more of a template than anything else. For truly secure firewall rulesets, check 

out the the requirements section of the HOWTO ( 2.4.x − Section 2.6, 2.2.x − Section 2.7, 2.0.x − Section 2.8 

). 

Instead of manually typing one of these files by hand, I recommend to simply browse the Example directory 

or download an archive of all of these rc.firewall−* files. 

3.4.1. Configuring IP Masquerade on Linux 2.6.x and 2.4.x Kernels 

Please note that IPCHAINS is no longer the primary firewall configuration tool for the 2.6.x and 2.4.x 

kernels. The new kernels now use the IPTABLES toolkit though the new 2.4.x kernels CAN still run most old 

IPCHAINS or IPFWADM rulesets via a compatiblity module. It should also be noted that when running in 

this compatibility mode, NO IPTABLES modules can be loaded. The reason for this is that none of the 2.2.x 

IPMASQ modules are compatible with 2.4.x kernels. For a more detailes for these changes, please see the 

Section 7.40 section. 


Ok, as mentioned before, the /etc/rc.d/rc.local−* script can be loaded once after every reboot. The 

mechanism to load the script varies between different Linux distros (please see below for some exampels). 

The rc.firewall−iptables script will load all required IPMASQ modules as well as enable the final IPMASQ 

functionality. For advanced setups, this same file would contain very secure firewall rulesets as well. 

Anyway, create the file /etc/rc.d/rc.firewall−iptables with the following initial SIMPLE ruleset: 

 


#!/bin/sh 

# 

# rc.firewall−iptables 

FWVER=0.76 

# 

# Initial SIMPLE IP Masquerade test for 2.6 / 2.4 kernels 

# using IPTABLES. 

# 

# Once IP Masquerading has been tested, with this simple 

# ruleset, it is highly recommended to use a stronger 

# IPTABLES ruleset either given later in this HOWTO or 

# from another reputable resource. 

# 

# 

# 

# Log: 

# 0.76 − Added comments on why the default policy is ACCEPT 

# 0.75 − Added more kernel modules to the comments section 

# 0.74 − the ruleset now uses modprobe vs. insmod 

# 0.73 − REJECT is not a legal policy yet; back to DROP 

# 0.72 − Changed the default block behavior to REJECT not DROP 

# 0.71 − Added clarification that PPPoE users need to use 

# "ppp0" instead of "eth0" for their external interface 

# 0.70 − Added commented option for IRC nat module 

# − Added additional use of environment variables 

# − Added additional formatting 

# 0.63 − Added support for the IRC IPTABLES module 

# 0.62 − Fixed a typo on the MASQ enable line that used eth0 

# instead of $EXTIF 

# 0.61 − Changed the firewall to use variables for the internal 

# and external interfaces. 

# 0.60 − 0.50 had a mistake where the ruleset had a rule to DROP 

# all forwarded packets but it didn't have a rule to ACCEPT 

# any packets to be forwarded either 

# − Load the ip_nat_ftp and ip_conntrack_ftp modules by default 

# 0.50 − Initial draft 

# 

echo −e "\n\nLoading simple rc.firewall−iptables version $FWVER..\n" 

# The location of the iptables and kernel module programs 

# 

# If your Linux distribution came with a copy of iptables, 

# most likely all the programs will be located in /sbin. If 

# you manually compiled iptables, the default location will 

# be in /usr/local/sbin 

# 

# ** Please use the "whereis iptables" command to figure out 

# ** where your copy is and change the path below to reflect 

# ** your setup 


# 

#IPTABLES=/sbin/iptables 

IPTABLES=/usr/local/sbin/iptables 

DEPMOD=/sbin/depmod 

MODPROBE=/sbin/modprobe 

#Setting the EXTERNAL and INTERNAL interfaces for the network 

# 

# Each IP Masquerade network needs to have at least one 

# external and one internal network. The external network 

# is where the natting will occur and the internal network 

# should preferably be addressed with a RFC1918 private address 

# scheme. 

# 

# For this example, "eth0" is external and "eth1" is internal" 

# 

# 

# NOTE: If this doesnt EXACTLY fit your configuration, you must 

# change the EXTIF or INTIF variables above. For example: 

# 

# If you are a PPPoE or analog modem user: 

# 

# EXTIF="ppp0" 

# 

# 

EXTIF="eth0" 

INTIF="eth1" 

echo " External Interface: $EXTIF" 

echo " Internal Interface: $INTIF" 

#====================================================================== 

#== No editing beyond this line is required for initial MASQ testing == 

echo −en " loading modules: " 


# Need to verify that all modules have all required dependencies 

# 

echo " − Verifying that all kernel modules are ok" 

$DEPMOD −a 

# With the new IPTABLES code, the core MASQ functionality is now either 

# modular or compiled into the kernel. This HOWTO shows ALL IPTABLES 

# options as MODULES. If your kernel is compiled correctly, there is 

# NO need to load the kernel modules manually. 

# 

# NOTE: The following items are listed ONLY for informational reasons. 

# There is no reason to manual load these modules unless your 

# kernel is either mis−configured or you intentionally disabled 

# the kernel module autoloader. 

# 

# Upon the commands of starting up IP Masq on the server, the 

# following kernel modules will be automatically loaded: 

# 

# NOTE: Only load the IP MASQ modules you need. All current IP MASQ 

# modules are shown below but are commented out from loading. 

# =============================================================== 

echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−" 



#Load the main body of the IPTABLES module − "iptable" 

# − Loaded automatically when the "iptables" command is invoked 

# 

# − Loaded manually to clean up kernel auto−loading timing issues 

# 

echo −en "ip_tables, " 

$MODPROBE ip_tables 

#Load the IPTABLES filtering module − "iptable_filter" 

# − Loaded automatically when filter policies are activated 

#Load the stateful connection tracking framework − "ip_conntrack" 

# 

# The conntrack module in itself does nothing without other specific 

# conntrack modules being loaded afterwards such as the "ip_conntrack_ftp" 

# module 

# 

# − This module is loaded automatically when MASQ functionality is 

# enabled 

# 


# 

echo −en "ip_conntrack, " 

$MODPROBE ip_conntrack 

#Load the FTP tracking mechanism for full FTP tracking 

# 

# Enabled by default −− insert a "#" on the next line to deactivate 

# 

echo −en "ip_conntrack_ftp, " 

$MODPROBE ip_conntrack_ftp 

#Load the IRC tracking mechanism for full IRC tracking 

# 


# 

echo −en "ip_conntrack_irc, " 

$MODPROBE ip_conntrack_irc 

#Load the general IPTABLES NAT code − "iptable_nat" 

# − Loaded automatically when MASQ functionality is turned on 

# 


# 

echo −en "iptable_nat, " 

$MODPROBE iptable_nat 

#Loads the FTP NAT functionality into the core IPTABLES code 

# Required to support non−PASV FTP. 

# 


# 

echo −en "ip_nat_ftp, " 

$MODPROBE ip_nat_ftp 


#Loads the IRC NAT functionality into the core IPTABLES code 

# Required to support NAT of IRC DCC requests 

# 

# Disabled by default −− remove the "#" on the next line to activate 

# 

#echo −e "ip_nat_irc" 

#$MODPROBE ip_nat_irc 

echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−" 

# Just to be complete, here is a partial list of some of the other 

# IPTABLES kernel modules and their function. Please note that most 

# of these modules (the ipt ones) are automatically loaded by the 

# master kernel module for proper operation and don't need to be 

# manually loaded. 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

# 

# ip_nat_snmp_basic − this module allows for proper NATing of some 

# SNMP traffic 

# 

# iptable_mangle − this target allows for packets to be 

# manipulated for things like the TCPMSS 

# option, etc. 

# 

# −− 

# 

# ipt_mark − this target marks a given packet for future action. 

# This automatically loads the ipt_MARK module 

# 

# ipt_tcpmss − this target allows to manipulate the TCP MSS 

# option for braindead remote firewalls. 

# This automatically loads the ipt_TCPMSS module 

# 

# ipt_limit − this target allows for packets to be limited to 

# to many hits per sec/min/hr 

# 

# ipt_multiport − this match allows for targets within a range 

# of port numbers vs. listing each port individually 

# 

# ipt_state − this match allows to catch packets with various 

# IP and TCP flags set/unset 

# 

# ipt_unclean − this match allows to catch packets that have invalid 

# IP/TCP flags set 

# 

# iptable_filter − this module allows for packets to be DROPped, 

# REJECTed, or LOGged. This module automatically 

# loads the following modules: 

# 

# ipt_LOG − this target allows for packets to be 

# logged 

# 

# ipt_REJECT − this target DROPs the packet and returns 

# a configurable ICMP packet back to the 

# sender. 

# 

echo −e " Done loading modules.\n" 




#CRITICAL: Enable IP forwarding since it is disabled by default since 

# 

# Redhat Users: you may try changing the options in 

# /etc/sysconfig/network from: 

# 

# FORWARD_IPV4=false 

# to 

# FORWARD_IPV4=true 

# 

echo " Enabling forwarding.." 

echo "1" > /proc/sys/net/ipv4/ip_forward 

# Dynamic IP users: 

# 

# If you get your IP address dynamically from SLIP, PPP, or DHCP, 

# enable this following option. This enables dynamic−address hacking 

# which makes the life with Diald and similar programs much easier. 

# 

echo " Enabling DynamicAddr.." 

echo "1" > /proc/sys/net/ipv4/ip_dynaddr 

# Enable simple IP forwarding and Masquerading 

# 

# NOTE: In IPTABLES speak, IP Masquerading is a form of SourceNAT or SNAT. 

# 

# NOTE #2: The following is an example for an internal LAN address in the 

# 192.168.0.x network with a 255.255.255.0 or a "24" bit subnet mask 

# connecting to the Internet on external interface "eth0". This 

# example will MASQ internal traffic out to the Internet but not 

# allow non−initiated traffic into your internal network. 

# 

# 

# ** Please change the above network numbers, subnet mask, and your 

# *** Internet connection interface name to match your setup 

# 

#Clearing any previous configuration 

# 

# Unless specified, the defaults for INPUT and OUTPUT is ACCEPT 

# The default for FORWARD is DROP (REJECT is not a valid policy) 

# 

# Isn't ACCEPT insecure? To some degree, YES, but this is our testing 

# phase. Once we know that IPMASQ is working well, I recommend you run 

# the rc.firewall−*−stronger rulesets which set the defaults to DROP but 

# also include the critical additional rulesets to still let you connect to 

# the IPMASQ server, etc. 

# 

echo " Clearing any existing rules and setting default policy.." 

$IPTABLES −P INPUT ACCEPT 

$IPTABLES −F INPUT 

$IPTABLES −P OUTPUT ACCEPT 

$IPTABLES −F OUTPUT 

$IPTABLES −P FORWARD DROP 

$IPTABLES −F FORWARD 

$IPTABLES −t nat −F 

echo " FWD: Allow all connections OUT and only existing and related ones IN" 

$IPTABLES −A FORWARD −i $EXTIF −o $INTIF −m state −−state ESTABLISHED,RELATED −j ACCEPT 

$IPTABLES −A FORWARD −i $INTIF −o $EXTIF −j ACCEPT 


$IPTABLES −A FORWARD −j LOG 

echo " Enabling SNAT (MASQUERADE) functionality on $EXTIF" 

$IPTABLES −t nat −A POSTROUTING −o $EXTIF −j MASQUERADE 

echo −e "\nrc.firewall−iptables v$FWVER done.\n" 

 

Once you are finished with editing this /etc/rc.d/rc.firewall−iptables ruleset, make it executable by typing in 

chmod 700 /etc/rc.d/rc.firewall−iptables 

Now that the firewall ruleset is ready, you need to let it run after every reboot. You could either do this by 

running it by hand everytime (such a pain) or add it to the boot scripts. We have covered two methods below: 

Redhat (SyS−V style) and Slackware (BSD style) 

1. Redhat and Redhat−derived distros: 

• 

There are two ways to automatically load things in Redhat: /etc/rc.d/rc.local or a init script in 

/etc/rc.d/init.d/. The first method is the easiest but isn't doing things the SYS−V way. All you have to 

do is add the line: 

echo "Loading the rc.firewall−iptables ruleset.. " 

/etc/rc.d/rc.firewall−iptables 

to the end of the /etc/rc.d/rc.local file and thats it (as described earlier in the HOWTO). 

The problem with this approach is that the firewall isn't executed until the last stages of booting. 

• The preferred approach is to have the firewall loaded just after the networking subsystem is loaded. 

To do this, copy the following file into the /etc/rc.d/init.d directory: 

 


#!/bin/sh 

# 

# chkconfig: 2345 11 89 

# 

# description: Loads the rc.firewall−iptables ruleset. 

# 

# processname: firewall−iptables 

# pidfile: /var/run/firewall.pid 

# config: /etc/rc.d/rc.firewall−iptables 

# probe: true 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

# v05/24/03 

# 

# Part of the copyrighted and trademarked TrinityOS document. 

# http://www.ecst.csuchico.edu/~dranch 

# 

# Written and Maintained by David A. Ranch 

# dranch@trinnet.net 

# 

# Updates 

# −−−−−−− 

# 05/24/03 − removed a old networking up check that had some 

# improper SGML ampersand conversions. 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 


# Source function library. 

. /etc/rc.d/init.d/functions 

# Check that networking is up. 

[ "XXXX${NETWORKING}" = "XXXXno" ] && exit 0 

[ −x /sbin/ifconfig ] || exit 0 

# The location of various iptables and other shell programs 

# 

# If your Linux distribution came with a copy of iptables, most 

# likely it is located in /sbin. If you manually compiled 

# iptables, the default location is in /usr/local/sbin 

# 




# 


# See how we were called. 

case "$1" in 

start) 


;; 

stop) 

echo −e "\nFlushing firewall and setting default policies to DROP\n" 

$IPTABLES −P INPUT DROP 


$IPTABLES −P OUTPUT DROP 




$IPTABLES −F −t nat 

# Delete all User−specified chains 

$IPTABLES −X 

# 

# Reset all IPTABLES counters 

$IPTABLES −Z 

;; 

restart) 

$0 stop 

$0 start 

;; 

status) 

$IPTABLES −L 

;; 

mlist) 

cat /proc/net/ip_conntrack 

;; 

*) 


echo "Usage: firewall−iptables {start|stop|status|mlist}" 


esac 

2. Slackware: 

• 

exit 1 

exit 0 

 


With this script in place, all you need to do now is make it executable and then make it load upon 

reboot. First, make it executable by running: 

#Redhat−style 

# 

chmod 700 /etc/rc.d/init.d/firewall−iptables 

Now, enable the ruleset load upon reboot: 

#Redhat style 

# 

/sbin/chkconfig −−level=345 firewall−iptables on 

That's it! Now upon reboot, the firewall will be loaded automatically. Just to make sure, run the 

following command to see that the firewall should start upon reboot by running the command: 

#Redhat style 

# 

chkconfig −−list firewall−iptables 

#The output should look like: 

# 

firewall−iptables 0:off 1:off 2:off 3:on 4:on 5:on 6:off 

There are two ways to load things in Slackware: /etc/rc.d/rc.local or editing the /etc/rc.d/rc.inet2 file. 

The first method is the easiest but isn't the most secure (see below). All you have to do is append the 

following lines to the /etc/rc.d/rc.local file: 

echo "Loading the rc.firewall−iptables ruleset.." 


The problem with this approach is that if you are running a STRONG firewall ruleset, the firewall 

isn't executed until the last stages of booting. The preferred approach is to have the firewall loaded 

just after the networking subsystem is loaded. For now, the HOWTO only covers how to do so using 

/etc/rc.d/rc.local but if you know what you're doing (it's easy), go ahead and and modify the inet2 

startup script to load the /etc/rc.d/rc.firewall−iptables file just after the network is up. If you want a 

more detailed guide and/or a stronger firewall ruleset, I recommend you check out Section 10 of 

TrinityOS found in the links section at the bottom of this HOWTO. 

Notes on how users might want to change the above firewall ruleset: 

You could also have IP Masquerading enabled on a PER MACHINE basis instead of the above method, 

which is enabling an ENTIRE TCP/IP network. For example, say if I wanted only the 192.168.0.2 and 

192.168.0.8 hosts to have access to the Internet and NOT any of the other internal machines. I would change 

the in the "Enable simple IP forwarding and Masquerading" section (shown above) of the 

/etc/rc.d/rc.firewall−iptables ruleset. 

#!/bin/sh 

# 

# Partial IPTABLES config to enable simple IP forwarding and Masquerading 


# v0.61 

# 

# NOTE: The following is an example to allow only IP Masquerading for the 

# 192.168.0.2 and 192.168.0.8 machines with a 255.255.255.0 or a 

# "/24" subnet mask connecting to the Internet on interface eth0. 

# 

# ** Please change the network number, subnet mask, and the Internet 

# ** connection interface name to match your internal LAN setup 

# 

echo " − Setting the default FORWARD policy to DROP" 


echo " − Enabling SNAT (IPMASQ) functionality on $EXTIF" 

$IPTABLES −t nat −A POSTROUTING −o $EXTIF −s 192.168.0.2/32 −j MASQUERADE 

$IPTABLES −t nat −A POSTROUTING −o $EXTIF −s 192.168.0.8/32 −j MASQUERADE 

echo " − Setting the FORWARD policy to 'DROP' all incoming / unrelated traffic" 

$IPTABLES −A INPUT −i $EXTIF −m state −−state NEW,INVALID −j DROP 

$IPTABLES −A FORWARD −i $EXTIF −m state −−state NEW,INVALID −j DROP 

Common mistakes: 

It appears that a common mistake with new IP Masq users is to make the first command simply the following: 

IPTABLES: 

−−−−−−−−− 

iptables −t nat −A POSTROUTING −j MASQUERADE 

Do NOT make your default policy MASQUERADING. Otherwise, someone can manipulate their routing 

tables to tunnel straight back through your gateway, using it to masquerade their OWN identity! 

Again, you can add these lines to the /etc/rc.d/rc.firewall−iptables file, one of the other rc 

files you prefer, or do it manually every time you need IP Masquerade. 

Please see Section 6.4.1 for a detailed guide on a strong IPTABLES ruleset example. For additional details on 

IPTABLES usage, please refer to http://www.netfilter.org/ for the primary IPTABLES site. 

3.4.2. Configuring IP Masquerade on Linux 2.2.x Kernels 

Please note that IPFWADM is no longer the firewall tool for manipulating IP Masquerading rules for both 

the 2.1.x and 2.2.x kernels. These new kernels now use the IPCHAINS toolkit. For a more detailed reason for 

this change, please see Chapter 7. 

Create the file /etc/rc.d/rc.firewall−ipchains with the following initial SIMPLE ruleset: 

 


#!/bin/sh 

# 

# rc.firewall−ipchains 

# 

# − Initial SIMPLE IP Masquerade test for 2.1.x and 2.2.x kernels 

# using IPCHAINS. 

# 







FWVER="1.23" 

# 


# 1.22 − ruleset now uses modprobe instead of insmod 



# 1.20 − Updated the script to use environment vars 

# 1.01 − Original version 

echo −e "\n\nLoading simple rc.firewall−ipchains : version $FWVER..\n" 

# The location of the ipchains and kernel module programs 

# 

# If your Linux distribution came with a copy of ipchains, 

# most likely all the programs will be located in /sbin. If 

# you manually compiled ipchains, the default location will 

# be in /usr/local/sbin 

# 

# ** Please use the "whereis ipchains" command to figure out 



# 

IPCHAINS=/sbin/ipchains 

#IPTABLES=/usr/local/sbin/ipchains 




# 



# is where the NATing will occur and the internal network 

# should preferably be addressed with a RFC1918 private addressing 

# scheme. 

# 


# 



# 


# 


# 

# ** Please change this to reflect your specific configuration ** 

# 

EXTIF="eth0" 

INTIF="eth1" 



# Network Address of the Internal Network 

# 

# This example rc.firewall−ipchains file uses the 192.168.0.0 network 

# with a /24 or 255.255.255.0 netmask. 

# 

# ** Change this variable to reflect your specific setup ** 


# 

INTLAN="192.168.0.0/24" 

echo −e " Internal Interface: $INTLAN\n" 

# Load all required IP MASQ modules 

# 

# NOTE: Only load the IP MASQ modules you need. All current IP MASQ modules 

# are shown below but are commented out from loading. 

echo " loading required IPMASQ kernel modules.." 

# Needed to initially load modules 

# 

$DEPMOD −a 

echo −en " Loading modules: " 

# Supports the proper masquerading of FTP file transfers using the PORT method 

# 

echo −en "FTP, " 

$MODPROBE ip_masq_ftp 

# Supports the masquerading of RealAudio over UDP. Without this module, 

# RealAudio WILL function but in TCP mode. This can cause a reduction 

# in sound quality 

# 

#echo −en "RealAudio, " 

$MODPROBE ip_masq_raudio 

# Supports the masquerading of IRC DCC file transfers 

# 

#echo −en "Irc, " 

#$MODPROBE ip_masq_irc 

# Supports the masquerading of Quake and QuakeWorld by default. This modules is 

# for for multiple users behind the Linux MASQ server. If you are going to 

# play Quake I, II, and III, use the second example. 

# 

# NOTE: If you get ERRORs loading the QUAKE module, you are running an old 

# −−−−− kernel that has bugs in it. Please upgrade to the newest kernel. 

# 

#echo −en "Quake, " 

#Quake I / QuakeWorld (ports 26000 and 27000) 

#$MODPROBE ip_masq_quake 

# 

#Quake I/II/III / QuakeWorld (ports 26000, 27000, 27910, 27960) 

#$MODPROBE ip_masq_quake 26000,27000,27910,27960 

# Supports the masquerading of the CuSeeme video conferencing software 

# 

#echo −en "CuSeeme, " 

#$MODPROBE ip_masq_cuseeme 

#Supports the masquerading of the VDO−live video conferencing software 

# 

#echo −en "VdoLive " 

#$MODPROBE ip_masq_vdolive 

echo ". Done loading modules." 





# 



# 


# to 


# 

echo " enabling forwarding.." 


#CRITICAL: Enable automatic IP defragmenting since it is disabled by default 

# in 2.2.x kernels. This used to be a compile−time option but the 

# behavior was changed in 2.2.12 

# 

echo " enabling AlwaysDefrag.." 

echo "1" > /proc/sys/net/ipv4/ip_always_defrag 


# 

# If you get your IP address dynamically from SLIP, PPP, or DHCP, enable this 

# following option. This enables dynamic−ip address hacking in IP MASQ, 

# making the life with Diald and similar programs much easier. 

# 

#echo " enabling DynamicAddr.." 

#echo "1" > /proc/sys/net/ipv4/ip_dynaddr 

# Enable the LooseUDP patch which some Internet−based games require 

# 

# If you are trying to get an Internet game to work through your IP MASQ box, 

# and you have set it up to the best of your ability without it working, try 

# enabling this option (delete the "#" character). This option is disabled 

# by default due to possible internal machine UDP port scanning 

# vulnerabilities. 

# 

#echo " enabling LooseUDP.." 

#echo "1" > /proc/sys/net/ipv4/ip_masq_udp_dloose 


# 


# The default for FORWARD is REJECT 

# 






# 

echo " clearing any existing rules and setting default policy.." 

$IPCHAINS −P input ACCEPT 

$IPCHAINS −P output ACCEPT 

$IPCHAINS −P forward REJECT 

$IPCHAINS −F input 

$IPCHAINS −F output 


$IPCHAINS −F forward 

# MASQ timeouts 

# 

# 2 hrs timeout for TCP session timeouts 

# 10 sec timeout for traffic after the TCP/IP "FIN" packet is received 

# 160 sec timeout for UDP traffic (Important for MASQ'ed ICQ users) 

# 

echo " setting default timers.." 

$IPCHAINS −M −S 7200 10 160 

# DHCP: For people who receive their external IP address from either DHCP or 

# BOOTP for connctions such as DSL or Cablemodem users, it is necessary 

# to use the following before the deny command. 

# 

# This example is currently commented out. 

# 

# 

#$IPCHAINS −A input −j ACCEPT −i $EXTIF −s 0/0 67 −d 0/0 68 −p udp 


# 

# NOTE: The following is an example for an internal LAN address in the 


# connecting to the Internet on interface eth0. 

# 

# ** Please change this network number, subnet mask, and your Internet 


# 

echo " enabling IPMASQ functionality on $EXTIF" 

$IPCHAINS −P forward DENY 

$IPCHAINS −A forward −i $EXTIF −s $INTLAN −j MASQ 

echo −e "\nrc.firewall−ipchains v$FWVER done.\n" 

 

Once you are finished with editing the /etc/rc.d/rc.firewall−ipchains ruleset, make it executable by typing in 

chmod 700 /etc/rc.d/rc.firewall−ipchains 

Now that the firewall ruleset is ready, you need to let it run after every reboot. You could either do this by 

running it by hand everytime (such a pain) or add it to the boot scripts. We have covered two methods below: 

Redhat (SyS−V style) and Slackware (BSD style) 

1. Redhat and Redhat−derived distros: 

• 



/etc/rc.d/init.d/. The first method is the easiest but isn't doing things the Sys−V way. All you have to 

do is add the line: 

echo "Loading the rc.firewall ruleset.." 

/etc/rc.d/rc.firewall−ipchains 

to the end of the /etc/rc.d/rc.local file and thats it (as described earlier in the HOWTO). 

The problem with this approach is that the firewall isn't executed until the last stages of booting. The 

preferred approach is to have the firewall loaded just after the networking subsystem is loaded. To do 


this, copy the following file into the /etc/rc.d/init.d directory: 

 

#!/bin/sh 

# 

# chkconfig: 2345 11 89 

# 

# description: Loads the rc.firewall−ipchains ruleset. 

# 

# processname: firewall−ipchains 


# config: /etc/rc.d/rc.firewall−ipchains 

# probe: true 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

# v08/29/02 

# 



# 



# 

# Updates 

# −−−−−−− 

# 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 




# This line no longer work with bash2 

#[ ${NETWORKING} = "no" ] && exit 0 

# This should be OK. 




# 




# 




# 



case "$1" in 

start) 


;; 



2. Slackware: 

stop) 

echo −e "\nFlushing firewall and setting default policies to REJECT\n" 

$IPCHAINS −P input REJECT 

$IPCHAINS −P output REJECT 

$IPCHAINS −P forward REJECT 




;; 

restart) 

$0 stop 

$0 start 

;; 

status) 

$IPCHAINS −L 

;; 

mlist) 

$IPCHAINS −M −L 

;; 

*) 

echo "Usage: firewall−ipchains {start|stop|status|mlist}" 

exit 1 

esac 

exit 0 

 





# 

chmod 700 /etc/rc.d/init.d/firewall−ipchains 

Now, make the ruleset load upon reboot: 

#Redhat style 

# 

chkconfig −−level=345 firewall−ipchains on 

That's it! Now upon boot, the firewall will be loaded automatically. Just to make sure, run the 

command to see that the firewall should start upon reboot by running the command: 

#Redhat style 

# 

chkconfig −−list firewall−ipchains 


# 

firewall−ipchains 0:off 1:off 2:off 3:on 4:on 5:on 6:off 

• There are two ways to load things in Slackware: /etc/rc.d/rc.local or editing the /etc/rc.d/rc.inet2 file. 

The first method is the easiest but isn't the most secure (see below). All you have to do is append the 

following lines to the /etc/rc.d/rc.local file: 



echo "Loading the rc.firewall−ipchains ruleset.." 






startup script to load the /etc/rc.d/rc.firewall−ipchains file just after the network is up. If you want a 




You could also have IP Masquerading enabled on a PER MACHINE basis instead of the above method, 

which is enabling an ENTIRE TCP/IP network. For example, say if I wanted only the 192.168.0.2 and 

192.168.0.8 hosts to have access to the Internet and NOT any of the other internal machines. I would change 

the in the "Enable simple IP forwarding and Masquerading" section (shown above) of the 

/etc/rc.d/rc.firewall−ipchains ruleset. 

#!/bin/sh 

# 


# v1.01 

# 

# NOTE: The following is an example used in addition to the simple 

# IPCHAINS ruleset anove to allow only IP Masquerading for the 

# 192.168.0.2 and 192.168.0.8 machines with a 255.255.255.0 or a 

# "24" bit subnet mask connecting to the Internet on interface $EXTIF. 

# 

# ** Please change the network number, subnet mask, and the Internet 


# 


$IPCHAINS −A forward −i $EXTIF −s 192.168.0.2/32 −j MASQ 

$IPCHAINS −A forward −i $EXTIF −s 192.168.0.8/32 −j MASQ 


What appears to be a common mistake with new IP MASQ users is to make the first command: 

$IPCHAINS −P forward masquerade 

Do NOT make your default policy MASQUERADING. Otherwise, someone can manipulate their routing 

tables to tunnel straight back through your gateway, using it to masquerade their OWN identity! 

Again, you can add these lines to the /etc/rc.d/rc.firewall−ipchains file, one of the other rc 

files you prefer, or do it manually every time you need IP Masquerade. 

Please see Section 6.4.2 for a detailed guide on IPCHAINS and a strong IPCHAINS ruleset example. For 

additional details on IPCHAINS usage, please refer to http://www.netfilter.org/ipchains/ for the primary 

IPCHAINS site or the Linux IP CHAINS HOWTO Backup site 



3.4.3. Configuring IP Masquerade on Linux 2.0.x Kernels 

Create the file /etc/rc.d/rc.firewall−ipfwadm with the following initial SIMPLE ruleset: 

#!/bin/sh 

# 

# rc.firewall−ipfwadm 

# 

# A Initial SIMPLE IP Masquerade setup for 2.0.x kernels using IPFWADM 

# 

FWVER="2.03" 

# 




# 

# 





# 

echo −e "\n\nLoading simple rc.firewall−ipfwadm version $FWVER..\n" 


# 



# is where the NATing will occur and the internal network 

# should preferably be addressed with a RFC1918 private addressing 

# scheme. 

# 


# 



# 


# 


# 

# ** Please change this to reflect your specific configuration ** 

# 

EXTIF="eth0" 

INTIF="eth1" 



# Network Address of the Internal Network 

# 

# This example rc.firewall−ipfwadm file uses the 192.168.0.0 network 

# with a /24 or 255.255.255.0 netmask. 

# 

# ** Change this variable to reflect your specific setup ** 

# 

INTLAN="192.168.0.0/24" 

echo −e " Internal Interface: $INTLAN\n" 



# 

# NOTE: Only load the IP MASQ modules you need. All current available IP 

# MASQ modules are shown below but are commented out from loading. 

echo −en "Loading modules: " 


# 

/sbin/depmod −a 


# 

echo −en "FTP, " 

/sbin/modprobe ip_masq_ftp 




# 

#echo −en "RealAudio, " 

#/sbin/modprobe ip_masq_raudio 


# 

#echo −en "Irc, " 

#/sbin/modprobe ip_masq_irc 

# Supports the masquerading of Quake and QuakeWorld by default. These modules 

# are for multiple users behind the Linux MASQ server. If you are going to 


# 



# 

#echo −en "Quake, " 


#/sbin/modprobe ip_masq_quake 

# 


#/sbin/modprobe ip_masq_quake 26000,27000,27910,27960 


# 

#echo −en "CuSeeme, " 

#/sbin/modprobe ip_masq_cuseeme 


# 

#echo −en "VdoLive, " 

#/sbin/modprobe ip_masq_vdolive 

echo ". Done loading modules." 


#CRITICAL: Enable IP forwarding since it is disabled by default 

# 



# 



# to 


# 

echo " enabling forwarding.." 




# 

# This used to be a compile−time option but the behavior was changed 

# in 2.2.12. This option is required for both 2.0 and 2.2 kernels. 

# 

echo " enabling AlwaysDefrag.." 



# 

# If you get your Internet IP address dynamically from SLIP, PPP, or DHCP, 

# enable the following option. This enables dynamic−ip address hacking in 

# IP MASQ, making the life with DialD, PPPd, and similar programs much easier. 

# 

#echo " enabling DynamicAddr.." 



# 


# The default for FORWARD is REJECT 

# 






# 

echo " clearing any existing rules and setting default policy.." 

/sbin/ipfwadm −I −p accept 

/sbin/ipfwadm −O −p accept 

/sbin/ipfwadm −F −p reject 

/sbin/ipfwadm −I −f 

/sbin/ipfwadm −O −f 

/sbin/ipfwadm −F −f 


# 



# 160 sec timeout for UDP traffic (Important for MASQ'ed ICQ users) 

# 

echo " setting default timers.." 

/sbin/ipfwadm −M −s 7200 10 160 

# DHCP: For people who receive their external IP address from either DHCP or 

# BOOTP such as DSL or Cablemodem users, it is necessary to use the 

# following before the deny command. 

# 

# This example is currently commented out. 

# 


# 

#/sbin/ipfwadm −I −a accept −S 0/0 67 −D 0/0 68 −W $EXTIF −P udp 


# 

# NOTE: The following is an example for an internal LAN address in the 


# connecting to the Internet on interface eth0. 

# 


# ** connection interface name to match your internal LAN setup. 

# 

echo " enabling IPMASQ functionality on $EXTIF" 

/sbin/ipfwadm −F −p deny 

/sbin/ipfwadm −F −a m −W $EXTIF −S $INTLAN −D 0.0.0.0/0 

echo −e "\nrc.firewall−ipfwadm v$FWVER done.\n" 

 

Once you are finished with editing the /etc/rc.d/rc.firewall−ipfwadm ruleset, make it executable by typing in 

"chmod 700 /etc/rc.d/rc.firewall−ipfwadm" 

Now that the firewall ruleset is ready to go, you need to let it run after every reboot. You could either do this 

by running it by hand everytime (such a pain) or add it to the boot scripts. We have covered two methods 

below: Redhat (SyS−V style) and Slackware (BSD style) 

Redhat and Redhat−derived distros: 

• 


/etc/rc.d/init.d/. The first method is the easiest but isn't doing it the Sys−V way. All you have to do is 

add the line: 

echo "Loading the rc.firewall−ipfwadm ruleset.." 

/etc/rc.d/rc.firewall−ipfwadm 

The problem with this approach is that the firewall isn't executed until the last stages of booting. The 

preferred approach is to have the firewall loaded just after the networking subsystem is loaded. To do 

this, copy the following file into the /etc/rc.d/init.d directory: 

 


#!/bin/sh 

# 

# chkconfig: 2345 11 89 

# 

# description: Loads the rc.firewall−ipfwadm ruleset. 

# 

# processname: firewall−ipfwadm 


# config: /etc/rc.d/rc.firewall−ipfwadm 

# probe: true 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

# v02/09/02 

# 




# 



# 

# Updates 

# −−−−−−− 

# 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 




# This line no longer work with bash2 

#[ ${NETWORKING} = "no" ] && exit 0 

# This should be OK. 




# 




# 




# 

IPFWADM=/sbin/ipfwadm 


case "$1" in 

start) 


;; 

stop) 

echo −e "\nFlushing firewall and setting default policies to REJECT\n" 

$IPFWADM −I −p REJECT 

$IPFWADM −O −p REJECT 

$IPFWADM −F −p REJECT 

$IPFWADM −I −f 

$IPFWADM −O −f 

$IPFWADM −F −f 

;; 

restart) 

$0 stop 

$0 start 

;; 

status) 

$IPFWADM −l 

;; 



Slackware: 

• 

mlist) 

$IPFWADM −M −l 

;; 

*) 

echo "Usage: firewall−ipfwadm {start|stop|status|mlist}" 

exit 1 

esac 

exit 0 

 





# 

chmod 700 /etc/rc.d/init.d/firewall−ipfwadm 

Now, make the ruleset load upon reboot: 

#Redhat style 

# 

chkconfig −−level=345 firewall−ipfwadm on 

That's it! Now upon boot, the firewall will be loaded automatically. Just to make sure, run the 

command to see that the firewall should start upon reboot by running the command: 

#Redhat style 

# 

chkconfig −−list firewall−ipfwadm 


# 

firewall−ipfwadm 0:off 1:off 2:off 3:on 4:on 5:on 6:off 

There are two ways to automatically load things in Slackware: /etc/rc.d/rc.local or editing the 

/etc/rc.d/rc.inet2 file. The first method is the easiest but isn't the most secure (see below). All you 

have to do is append the following lines to the /etc/rc.d/rc.local file: 

echo "Loading the rc.firewall−ipfwadm ruleset.." 






startup script to load the /etc/rc.d/rc.firewall−ipfwadm file just after the network is up. If you want a 




You could have also enabled IP Masquerading on a PER MACHINE basis instead of the above method 

enabling an ENTIRE TCP/IP network. For example, say if I wanted only the 192.168.0.2 and 192.168.0.8 

hosts to have access to the Internet and NOT any of the other internal machines. I would change the in the 



"Enable simple IP forwarding and Masquerading" section (shown above) of the /etc/rc.d/rc.firewall−ipfwadm 

ruleset. 


# v2.01 

# 

# NOTE: The following is an example to only allow IP Masquerading for the 

# 192.168.0.2 and 192.168.0.8 machines with a 255.255.255.0 or a "24" 

# bit subnet mask connected to the Internet on interface eth0. 

# 



# 

# Please use the following in ADDITION to the simple rulesets above for 

# specific MASQ networks. 

# 

/sbin/ipfwadm −F −p deny 

/sbin/ipfwadm −F −a m −W $EXTIF −S 192.168.0.2/32 −D 0.0.0.0/0 

/sbin/ipfwadm −F −a m −W $EXTIF −S 192.168.0.8/32 −D 0.0.0.0/0 


What appears to be a common mistake with new IP Masq users is to make the first command: 

ipfwadm −F −p masquerade 

Do NOT make your default policy MASQUERADING. Otherwise, someone who has the ability to 

manipulate their routing tables will be able to tunnel straight back through your gateway, using it to 

masquerade their OWN identity! 

Again, you can add these lines to the /etc/rc.d/rc.firewall−ipfwadm file, one of the other rc files 

(if you prefer), or manually add those lines every time you need IP Masquerade. 

Please see Section 6.4.3 and Section 6.4.3for a detailed guide and stronger examples of IPCHAINS and 

IPFWADM ruleset examples. 


Chapter 4. Configuring the other internal to−be 

MASQed machines 

Besides setting the appropriate IP address for each internal MASQed machine (either statically or though 

DHCP), you should also set each internal machine with the appropriate gateway IP address of the Linux 

MASQ server and required DNS servers. In general, this is rather straight forward. You simply enter the 

address of your Linux host (192.168.0.1 is used throughout this HOWTO) as the machine's gateway address. 

For the Domain Name Service (DNS), you add in any DNS servers that are available to you to use. The most 

apparent one(s) should be the DNS servers that your Linux server uses. You can optionally add any "domain 

search" suffix as well for quicker connections, etc. 

After you have properly reconfigured the internal MASQed machines, remember to restart their appropriate 

network services or reboot them if need be. 

The following configuration instructions assume that you are using a Class C network with 192.168.0.1 as 

your Linux MASQ server's address. Please note that 192.168.0.0 and 192.168.0.255 are reserved TCP/IP 

address per RFC1918 for uses just like enabling IP Masquerade services. 

As it stands, the following Platforms have been tested as internal MASQed machines. This is only an 

EXAMPLE of all compatible OSes out there: 

• Apple Macintosh OS and OS−X (with MacTCP or Open Transport or the BSD TCP/IP stack) 

• AT&T Unix (Caldera) 

• *BSD systems including Free/Net/Open/BSDi/386/etc. 

• Commodore Amiga (with AmiTCP or AS225−stack) 

• Digital VAX Stations 3520 and 3100 with UCX (TCP/IP stack for VMS) 

• Digital Ultrix, Digital Unix (Compaq Tru/64) 

• HP HP/UX 

• IBM AIX running on RS/6000, PowerPC, etc. 

• IBM OS/2 (including Warp v3) 

• IBM OS400 running on a AS/400 

• Linux distributions from vendors like Caldera, Corel, Debian, Mandrake, Redhat, Slackware, SuSe, 

etc. running various kernels like 1.2.x, 1.3.x, 2.0.x, 2.1.x, 2.2.x, 2.3.x, 2.4.x, etc. 

• Microsoft DOS (with NCSA Telnet package, DOS Trumpet works partially) 

• Microsoft Windows 3.1 (with the Netmanage or FTP packages) 

• Microsoft Windows For Workgroup 3.11 (with a TCP/IP package) 

• Microsoft Windows 95, OSR2, 98, 98se, Me 

• Microsoft Windows NT 3.51, 4.0, 2000, XP − (both workstation (professional) and server versions) 

• Novell Netware 4.01, 5.x, etc. with the TCP/IP service 

• SCO Openserver (v3.2.4.2 and 5) and UnixWare (AT&T Unix) 

• Sun Solaris 2.51, 2.6, 7, 8 

• heheh.. what else am I missing? 

4.1. Configuring Microsoft Windows 95 and OSR2 

1. ** Please note that some prompts might be different based upon the build version of Windows95 you 

are running ** 

Chapter 4. Configuring the other internal to−be MASQed machines 60

If you haven't installed your network card and adapter driver, do so now. Descriptions to perform this 

step is beyond the scope of this document and though it is fairly simple, if you haven't done this 

before, please seek assitance. 

2. Go to the 'Control Panel' −−> 'Network'. 

3. Click on Add −−> Protocol −−> Manufacture: Microsoft −−> Protocol: 'TCP/IP protocol' if you 

don't already have it installed. 

4. Highlight the TCP/IP item bound to your correct Windows95 network card e.g. (TCP/IP −−> Intel 

EtherExpress Pro/100+) and select 'Properties'. Here, you have two options: configure a static 

address or use DHCP. Static addresses are simple but require that you NEVER configure duplication 

IPs on different machines. The alternative is DHCP which automatically configures all 

DHCP−enabled workstations things like IP addresses, DNS servers, etc. from a central server 

(typically the Linux MASQ server). 

DHCP enabled: 

To use DHCP, simply click on the "Use DHCP to assign addresses" button. Please note that 

configuring a DHCP server is beyond the scope of this HOWTO but it is fully covered in TrinityOS 

and other Linux HOWTOs. 

Static Addresses: 


Now goto the 'IP Address' tab and set IP Address to 192.168.0.x, (1 < x < 255), and set the Subnet 

Mask to 255.255.255.0 

5. Now select the "Gateway" tab and add 192.168.0.1 as your gateway under 'Gateway' and hit "Add". 

6. Under the 'DNS Configuration' tab, make sure to put enter in a name for this machine and specify 

your official domain name. If you don't have your own domain, enter in the domain of your ISP. 

Next, you need to specify the DNS servers you plan on using. 

DHCP: No entries are required as this is configured dynamically via DHCP. 

STATIC: Add all of the DNS servers that your Linux MASQ server uses (usually found in 

/etc/resolv.conf). Usually these DNS servers are located at your ISP though you could be 

running either your own Caching or Authoritative DNS server on your Linux MASQ server as well. 

Again, setting up DNS services is beyond the scope of this HOWTO but it is covered by TrinityOS as 

well as the LDP's DNS HOWTO. 

Optionally, you can add any appropriate domain search suffixes as well. This allows users to simply 

type in the hostname of the destination computer instead of the fully qualified domain name (FQDN). 

This is similar to the PATH function for finding common Unix commands. 

7. Leave all of the other settings alone as they are unless (even dangerous) if you don't know what you're 

doing. 

8. Click 'OK' in all dialog boxes and restart your system. 

9. As an initial test, Ping the Linux MASQ server to test the network connection: 'Start/Run', type: 

ping 192.168.0.1(This is only an INTERNAL LAN connection test, you might not be able to 

ping the outside world yet.) If you don't see "replies" to your PINGs, please verify your network 

configuration. 

10. You can optionally create a HOSTS file in the C:\Windows directory so that you can ping the 

"hostname" of the machines on your LAN without the need for a DNS server. There is an example 

called HOSTS.SAM in the C:\windows directory for an example. 


4.2. Configuring Windows NT 


1. If you haven't installed your network card and adapter driver, do so now. Descriptions to perform this 

task is beyond the scope of this document. 

2. Go to 'Control Panel' −−> 'Network' −−> Protocols 

3. Add the TCP/IP Protocol and related Components from the 'Add Software' menu if you don't have 

TCP/IP service installed already. 

4. Under 'Network Software and Adapter Cards' section, highlight the 'TCP/IP Protocol' in the 

'Installed Network Software' selection box. 

5. In 'TCP/IP Configuration', select the appropriate adapter, e.g. [1]Intel EtherExpress 

Pro/100+. Then set the IP Address to 192.168.0.x (1 < x < 255), then set the Subnet Mask to 

255.255.255.0 and Default Gateway to 192.168.0.1. 

6. Do not enable any of the following options (unless you know what you are doing): 

♦ 'Automatic DHCP Configuration' : Unless you have a DHCP server running on your 

network. 

♦ Put anything in the 'WINS Server' input areas : Unless you have setup one or more WINS 

servers. 

♦ Enable IP Forwardings : Unless you are routing on your NT machine and really 

−REALLY− know EXACTLY what you're doing. 

7. Click 'DNS', fill in the appropriate information that your Linux host uses (usually found in 

/etc/resolv.conf) and then click 'OK' when you're done. 

8. Click 'Advanced', be sure to DISABLE 'DNS for Windows Name Resolution' and 'Enable 

LMHOSTS lookup' unless you known what these options do. If you want to use a LMHOSTS file, it 

is stored in C:\winnt\system32\drivers\etc. 

9. Click 'OK' on all dialog boxes and restart the system. 

10. As an initial test, ping the Linux MASQ server to test the network connection: 

'File/Run', type: ping 192.168.0.1(This is only an INTERNAL LAN connection test, you you 

might not be able to ping the outside world yet.) If you don't see any "replies" to your PINGs, please 

verify your network configuration. 

4.3. Configuring Windows for Workgroup 3.11 

1. If you haven't installed your network card and adapter driver, do so now. Descriptions to perform this 

task is beyond the scope of this document. 

2. Install the TCP/IP 32b package if you haven't already. 

3. In 'Main'/'Windows Setup'/'Network Setup', click on 'Drivers'. 

4. Highlight 'Microsoft TCP/IP−32 3.11b' in the 'Network Drivers' section, click 'Setup'. 

5. Set the IP Address to 192.168.0.x (1 < x < 255), then set the Subnet Mask to 255.255.255.0 and 

Default Gateway to 192.168.0.1 

6. Do not enable any of the following options (unless you know what you are doing): 

♦ 'Automatic DHCP Configuration' : Unless you have a DHCP server running on your 

network. 

♦ Put anything in the 'WINS Server' input areas : Unless you have setup one or more WINS 

servers. 

7. Click 'DNS', fill in the appropriate information your Linux host uses (usually found in 

/etc/resolv.conf). Then click 'OK' when you're done with it. 

8. Click 'Advanced', check 'Enable DNS for Windows Name Resolution' and 'Enable LMHOSTS 


lookup' found in c:\windows. 

9. Click 'OK' in all dialog boxes and restart the system. 

10. As an initial test, ping the linux box to test the network connection: 'File/Run', type: ping 

192.168.0.1 (This is only an INTERNAL LAN connection test so you might not be able to ping 

the outside world yet.) If you don't see "replies" to your PINGs, please verify your network 


4.4. Configuring UNIX Based Systems 

1. If you haven't installed your network card and either re−configured the network subsystem or 

recompiled your kernel with the appropriate adapter driver, do so now. Descriptions to perform this 

task is beyond the scope of this document but are covered in the Networking HOWTO. 

2. Install TCP/IP networking, such as the net−tools package, if you don't have it already. 

3. Set IPADDR to 192.168.0.x (1 < x < 255), then set NETMASK to 255.255.255.0, GATEWAY to 

192.168.0.1, and BROADCAST to 192.168.0.255. 

♦ Redhat (Mandrake / TurboLinux / etc): You can edit the 

/etc/sysconfig/network−scripts/ifcfg−eth0 file, or simply do so through the 

Control Panel (Linuxconf). 

♦ Slackware: You need to edit the /etc/rc.d/rc.inet1 file to configure the network subsystem. 

♦ To Add: Debian, Suse, Caldera, etc. Please email dranch@trinnet.net if you can tell me what 

distro uses what files to configure the networking subsystem. 

Beyond this, most Linux distributions use significantly different network configuration mechanisms 

let alone other UNIXes such as SunOS, BSDi, Solaris, AIX, TruUnix, FreeBSD, etc.). Please refer to 

your specific UNIX documentation for more details. 

4. Add your domain name service (DNS) and domain search suffix in /etc/resolv.conf and for 

the appropreiate UNIX versions, edit the /etc/nsswitch.conf file to enable DNS services. 

5. You may also want to update your /etc/networks file depending on your version of UNIX and 

the system's settings. 

6. Restart the appropriate services, or simply restart your system. 

7. As an initial test, run the ping command: ping 192.168.0.1 to test the connection to your 

gateway machine. (This is only an INTERNAL LAN connection test, so you might not be able to 

ping the outside world yet.) If you don't see "replies" to your PINGs, please verify your network 


4.5. Configuring DOS using NCSA Telnet package 

1. If you haven't installed your network card, do so now. Descriptions to perform this task is beyond the 

scope of this document. 

2. Load the appropriate packet driver. For example: an NE2000 Ethernet card set for I/O port 300 and 

IRQ 10, would need to be issued nwpd 0x60 10 0x300 

3. Make a new directory, and then unpack the NCSA Telnet package: pkunzip tel2308b.zip 

4. Use a text editor to open the config.tel file 

5. Set myip=192.168.0.x (1 < x < 255), and netmask=255.255.255.0 

6. In this example, you should set hardware=packet, interrupt=10, ioaddr=60 

7. You should have at least one individual machine specified as the gateway, i.e. the Linux host: 

name=default 

host=yourlinuxhostname 

hostip=192.168.0.1 



8. 


gateway=1 

Have another specified as a domain name service: 

name=dns.domain.com ; hostip=123.123.123.123; nameserver=1 

Note: substitute the appropriate information about the DNS according what your Linux host uses 

9. Save your config.tel file 

10. As an initial test, ping the Linux MASQ server to test the network connection: ping 

192.168.0.1 If you don't receive any replies, please verify your network configuration. 

4.6. Configuring MacOS Based System Running MacTCP 

1. If you haven't installed the appropriate driver software for your Ethernet adapter, do so now. 

Descriptions to perform this task is beyond the scope of this document. 

2. Open the MacTCP control panel. Select the appropriate network driver (Ethernet, NOT EtherTalk) 

and click on the 'More...' button. 

3. Under 'Obtain Address:', click 'Manually'. 

4. Under 'IP Address:', select class C from the popup menu. Ignore the rest of the dialog box sections. 

5. Fill in the appropriate information under 'Domain Name Server Information:'. 

6. Under 'Gateway Address:', enter 192.168.0.1 

7. Click 'OK' to save the settings. In the main window of the MacTCP control panel, enter the IP 

address of your Mac (192.168.0.x, 1 < x < 255) in the 'IP Address:' box. 

8. Close the MacTCP control panel. If a dialog box pops up, notifying you to do so, then restart the 

system. 

9. You may optionally ping the Linux box to test the network connection. If you have the freeware 

program MacTCP Watcher , click on the 'Ping' button, and enter the address of your Linux box 

(192.168.0.1) in the dialog window that pops up. (This is only an INTERNAL LAN connection test, 

you can't ping the outside world yet.) If you don't see "replies" to your PINGs, please verify your 

network configuration. 

10. You can optionally create a Hosts file in your System Folder so that you can use the hostnames of 

the machines on your LAN. The file should already exist in your System Folder, and should contain 

some (commented−out) sample entries which you can modify according to your needs. 

4.7. Configuring MacOS Based System Running Open 

Transport 



2. Open the TCP/IP Control Panel and choose 'User Mode ...' from the Edit menu. Make sure the user 

mode is set to at least 'Advanced' and click the 'OK' button. 

3. Choose 'Configurations...' from the File menu. Select your 'Default' configuration and click the 

'Duplicate...' button. Enter 'IP Masq' (or something to let you know that this is a special 

configuration) in the 'Duplicate Configuration' dialog, it will probably say something like 'Default 

copy'. Then click the 'OK' button, and the 'Make Active' button 

4. Select 'Ethernet' from the 'Connect via:' pop−up. 

5. Select the appropriate item from the 'Configure:' pop−up. If you don't know which option to choose, 

you probably should re−select your 'Default' configuration and quit. I use 'Manually'. 

6. Enter the IP address of your Mac (192.168.0.x, 1 < x < 255) in the 'IP Address:' box. 

7. Enter 255.255.255.0 in the 'Subnet mask:' box. 

8. Enter 192.168.0.1 in the 'Router address:' box. 


9. Enter the IP addresses of your domain name servers in the 'Name server addr.:' box. 

10. Enter the name of your Internet domain (e.g. 'microsoft.com') in the 'Starting domain name' box 

under 'Implicit Search Path:'. 

11. The following procedures are optional. Incorrect values may cause erratic behavior. If you're not sure, 

it's probably better to leave them blank, unchecked and/or un−selected. Remove any information from 

those fields, if necessary. As far as I know, there is no way to use the TCP/IP dialogs to tell the 

system not to use a previously selected alternate "Hosts" file. If you know, I would be interested. 

Check the '802.3' if your network requires 802.3 frame types. 

12. Click the 'Options...' button to make sure that the TCP/IP is active. I use the 'Load only when 

needed' option. If you continuously run and quit TCP/IP applications without rebooting your 

machine, you may find that unchecking the 'Load only when needed' option will prevent/reduce the 

effects on your machine's memory management. With the item unchecked, the TCP/IP protocol stacks 

are always loaded and available for use. If checked, the TCP/IP stacks are automatically loaded when 

needed and un−loaded when not. It's the loading and unloading process that can cause your machine's 

memory to become fragmented. 

13. You may ping the Linux box to test the network connection. If you have the freeware program 

MacTCP Watcher, click on the 'Ping' button, and enter the address of your Linux box (192.168.0.1) 

in the dialog box that pops up. (This is only an INTERNAL LAN connection test, you can't ping the 

outside world yet.) If you don't see "replies" to your PINGs, please verify your network configuration. 

14. You can optionally create a Hosts file in your System Folder so that you can use the hostnames of 

the machines on your LAN. The file may or may not already exist in your System Folder. If so, it 

should contain some (commented−out) sample entries which you can modify according to your needs. 

If not, you can get a copy of the file from a system running MacTCP, or just create your own (it 

follows a subset of the Unix /etc/hosts file format, described on RFC1035). Once you've created 

the file, open the TCP/IP control panel, click on the 'Select Hosts File...' button, and open the 

Hosts file. 

15. Click the close box or choose 'Close' or 'Quit' from the File menu, and then click the 'Save' button to 

save the changes you have made. 

16. The changes take effect immediately, but rebooting the system won't hurt. 

4.8. Configuring Novell network using DNS 



2. Downloaded tcpip16.exe from The Novell LanWorkPlace page 

3. edit c:\nwclient\startnet.bat: (here is a copy of mine) 

SET NWLANGUAGE=ENGLISH 

LH LSL.COM 

LH KTC2000.COM 

LH IPXODI.COM 

LH tcpip 

LH VLM.EXE 

F: 

4. edit c:\nwclient\net.cfg: (change link driver to yours i.e. NE2000) 

Link Driver KTC2000 

Protocol IPX 0 ETHERNET_802.3 

Frame ETHERNET_802.3 

Frame Ethernet_II 

FRAME Ethernet_802.2 

NetWare DOS Requester 

FIRST NETWORK DRIVE = F 



USE DEFAULTS = OFF 

VLM = CONN.VLM 

VLM = IPXNCP.VLM 

VLM = TRAN.VLM 

VLM = SECURITY.VLM 

VLM = NDS.VLM 

VLM = BIND.VLM 

VLM = NWP.VLM 

VLM = FIO.VLM 

VLM = GENERAL.VLM 

VLM = REDIR.VLM 

VLM = PRINT.VLM 

VLM = NETX.VLM 

Link Support 

Buffers 8 1500 

MemPool 4096 

Protocol TCPIP 

PATH SCRIPT C:\NET\SCRIPT 

PATH PROFILE C:\NET\PROFILE 

PATH LWP_CFG C:\NET\HSTACC 

PATH TCP_CFG C:\NET\TCP 

ip_address 192.168.0.xxx 

ip_router 192.168.0.1 

Change the IP address in the above "ip_address" field (192.168.0.x, 1 < x 

< 255) and finally create c:\bin\resolv.cfg: 

SEARCH DNS HOSTS SEQUENTIAL 

NAMESERVER xxx.xxx.xxx.xxx 

NAMESERVER yyy.yyy.yyy.yyy 

5. Now edit the above "NAMESERVER" entries and replace them with the correct IP addresses for your 

local DNS server. 

6. Issue a ping command: ping 192.168.0.1 to test the connection to your gateway machine. 

(This is only an INTERNAL LAN connection test, you can't ping the outside world yet.) If you don't 

see "replies" to your PINGs, please verify your network configuration. 

4.9. Configuring OS/2 Warp 




2. Install the TCP/IP protocol if you don't have it already. 

3. Go to Programs/TCP/IP (LAN) / TCP/IP Settings 

4. In 'Network', add your TCP/IP Address (192.168.0.x) and set your netmask (255.255.255.0) 

5. Under 'Routing', press 'Add'. Set the Type to 'default' and type the IP Address of your Linux Box in 

the Field 'Router Address'. (192.168.0.1). 

6. Set the same DNS (Nameserver) Address that your Linux host uses in 'Hosts'. 

7. Close the TCP/IP control panel. Say yes to the following question(s). 

8. Reboot your system 

9. You may ping the Linux box to test the network configuration. Type 'ping 192.168.0.1' in a 

'OS/2 Command prompt Window'. When ping packets are received all is ok. 


4.10. Configuring OS/400 on a IBM AS/400 

The descriptions to configure TCP/IP on OS/400 version V4R1M0 running on a AS/400 is beyond the scope 

of this document. 

1) To perform any communications configuration tasks on your AS/400, you must have the special authority 

of *IOSYSCFG (I/O System Configuration) defined in your user profile. You can check the characteristics of 

your user profile with the DSPUSRPRF command. 

2) Type GO CFGTCP command th reach the Configure TCP/IP menu. 

3) Select Option 2 − Work with TCP/IP Routes. 


4) Enter a 1 on the Opt field to add a route. * In Route Destination type *DFTROUTE * In Subnet Mask type 

*NONE * In Type of Service type *NORMAL * In Nex Hop type the address of your gataway (the Linux 

box) 

4.11. Configuring Other Systems 

The same logic should apply to setting up other platforms. Consult the sections above. If you're interested in 

writing about any of systems that have not been covered yet, please send a detail setup instruction to 

ambrose@writeme.com and dranch@trinnet.net. 


Chapter 5. Testing IP Masquerade 

Finally, it's time to give IP Masquerading an official try after all this hard work. If you haven't already 

rebooted your Linux box, do so to make sure the machines boots ok, executes the /etc/rc.d/rc.firewall−* 

ruleset, etc. Next, make sure that both the internal LAN connection and connection of your Linux hosts to the 

Internet is okay. 

Follow these −11− tests to make sure all aspects of your MASQ setup is running properly: 

5.1. Loading up the rc.firewall ruleset 

Step One: run the correct firewall for your machine via command "/etc/rc.d/rc.firewall−[iptables / ipchains 

/ipfwadm]". For example, Linux 2.6 users would run "/etc/rc.d/rc.firewall−iptables" 

Does it load with some strange errors? Here are some exmaples and help to fix them: 

• Problem #1: 

ip_tables, Using /lib/modules/2.4.2−2/kernel/net/ipv4/netfilter/ip_tables.o 

/lib/modules/2.4.2−2/kernel/net/ipv4/netfilter/ip_tables.o: init_module: Device 

or resource busy 

Hint: insmod errors can be caused by incorrect module parameters, including 

invalid IO or IRQ parameters 

Run the command "/sbin/lsmod" and make sure the module "ipchains.o" is NOT installed. If it is 

installed, your machine (most likely Redhat−7.x based) is probably trying to load an IPCHAINS 

ruleset which is incompatible with IPTABLES. 

To disable this from happening in the future, run the command: 

chkconfig −−level=2345 ipchains off 

To remove the "ipchains" module without rebooting, run the command: 

/sbin/rmmod ipchains 

and the re−try to load the rc.firewall−* ruleset. 


. 

. 

Creating a DROP chain.. 

iptables v1.2.3: log−level ìnfo' ambiguous 

. 

. 

Your version of IPTABLES it too old. You need to upgrade it with a newer version via an updated 

RPM, DEB, or via compiling up the source. You can get an updated version from your Linux 

distribution manufacturer or from the NetFilter WWW site. This is all covered in the Section 2.6 

section. 


Chapter 5. Testing IP Masquerade 68

No such file: 


Did you copy this rc.firewall−* file from a DOS machine? Load the rc.firewall−* file in a binary 

editor such as ViM (vim −b /etc/rc.d/rc.firewall−*) and make sure that every line is NOT finished 

with a ^M. 

5.2. Testing internal MASQ client PC connectivity 

• Step Two: Testing internal MASQ client PC connectivity 

From an internal MASQed computer, try pinging its local IP address (i.e. ping 192.168.0.10 ). This 

will verify that TCP/IP is correctly working on the local machine. Almost ALL modern operating 

systems have built−in support for the "ping" command. If this ping doesn't work, make sure that 

TCP/IP is correctly configured on the MASQed PC as described earlier in Chapter 4 of this HOWTO. 

The output should look something like the following (hit Control−C to abort the ping): 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

masq−client# ping 192.168.0.10 

PING 192.168.0.10 (192.168.0.10): 56 data bytes 

64 bytes from 192.168.0.10: icmp_seq=0 ttl=255 time=0.8 ms 




^C 

−−− 192.168.0.10 ping statistics −−− 

4 packets transmitted, 4 packets received, 0% packet loss 

round−trip min/avg/max = 0.4/0.5/0.8 ms 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

5.3. Testing internal MASQ client to MASQ server 

connectivity 

• Step Three: Testing internal MASQ client to MASQ server connectivity 

Next, from the same internal MASQed computer, try pinging the the IP address of the Linux MASQ 

server's INTERNAL interface (i.e. ping 192.168.0.1 ). This will verify that TCP/IP is correctly 

working on both the local and Linux MASQ machine. Almost ALL modern operating systems have 

built−in support for the "ping" command. If this ping doesn't work, make sure that TCP/IP is correctly 

configured on the MASQed Server as described by the various Network HOWTOs (URLs can be 

found in the requirements section for your 2.4.x kernel in Section 2.6, 2.2.x kernel in Section 2.7, or 

2.0.x kernel in Section 2.8). The output should look something like the following (hit Control−C to 

abort the ping): 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 







^C 






−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

If the ping failed, check the network connection between the MASQ server and the PC. If it's a 

DIRECT Ethernet connection (no hub or switch), you MUST have a "Ethernet cross−over cable". 

These cables are common and can be found at any computer store. Without this cable, the NICs 

(network cards) will not give you a "LINK" light. If you are using a hub or switch, make sure the 

ports connected to the MASQ server and MASQed client machine have a LINK light. If they do and 

the pings STILL don't work or there is a lot of packet loss, try different ports on the hub/switch (it not 

all that uncommon to have hub/switch ports die). Finally, if things still don't work perfectly, try 

replacing each of the NICs in the machines. You would be surprised how many people I've helped 

have found that their NIC cards were going bad and caused them all kinds of grief. 

5.4. Testing internal MASQ server connectivity 

• Step Four: Testing internal MASQ server connectivity 

On the MASQ server, ping the internal IP address of the MASQ server's network interface card (i.e. 

ping 192.168.0.1). If this ping doesn't work, make sure that TCP/IP is correctly configured on the 

MASQed Server as described by the various Network HOWTOs (URLs can be found in the 

requirements section for your 2.4.x kernel in Section 2.6, 2.2.x kernel in Section 2.7, or 2.0.x kernel in 

Section 2.8). The output should look something like the following (hit Control−C to abort the ping): 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

masq−server# ping 192.168.0.1 






^C 




−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

5.5. Testing internal MASQ server to MASQ client 

connectivity 

• Step Five: Testing internal MASQ server to MASQ client connectivity 

Next from MASQed server, try pinging the IP address of one of the internal MASQ client computers 

(i.e. ping 192.168.0.10 ). This will verify that TCP/IP is correctly working on both the local server 

machine and on the MASQ client machine. If this ping doesn't work, make sure that TCP/IP is 

correctly configured on the MASQed PC as described earlier in Chapter 4 of this HOWTO. Also be 

sure that the cabling is correct (Ethernet: the NICs connecting the internal MASQ PC and the MASQ 

server have the "link" light lit up). If the ping does work, the output should look something like the 

following (hit Control−C to abort the ping): 


−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 







^C 




−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

5.6. Testing External MASQ server Internet connectivity 

• Step Six: Testing external MASQ server to Internet connectivity 

From the MASQ server, ping the external IP address of the MASQ server's EXTERNAL network 

interface that is connected to the Internet. This address might be a Ethernet interface, a PPP interface, 

etc. connection to your ISP. If you don't know what this external IP address is, run the Linux 

command "/sbin/ifconfig" on the MASQ server itself to get the Internet address. The output should 

look something like the following (we are looking for the IP address of eth0): 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

eth0 Link encap:Ethernet HWaddr 00:08:C7:A4:CC:5B 

inet addr:12.13.14.15 Bcast:12.13.14.255 Mask:255.255.255.0 

UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 

RX packets:6108459 errors:0 dropped:0 overruns:0 frame:0 

TX packets:5422798 errors:8 dropped:0 overruns:0 carrier:8 

collisions:4675 txqueuelen:100 

Interrupt:11 Base address:0xfcf0 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

As you can see from the above, the external IP address is "12.13.14.15" for this example. So, now that 

you have your IP address after running the "ipconfig" command, ping your external IP address. This 

will confirm that the MASQ server has full network connectivity. The output should look something 

like the following (hit Control−C to abort the ping): 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 







^C 




−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

If either of these tests doesn't work, you need to go back and double check your network cabling and 

verify that the two network interfaces on the MASQ server are seen in "dmesg". An example of this 

output would be the following towards the END of the "dmesg" command: 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 




. 

. 

PPP: version 2.3.7 (demand dialling) 

TCP compression code copyright 1989 Regents of the University of California 

PPP line discipline registered. 

3c59x.c:v0.99H 11/17/98 Donald Becker 

http://cesdis.gsfc.nasa.gov/linux/drivers/ 

vortex.html 

eth0: 3Com 3c905 Boomerang 100baseTx at 0xfe80, 00:60:08:a7:4e:0e, IRQ 9 

8K word−wide RAM 3:5 Rx:Tx split, autoselect/MII interface. 

MII transceiver found at address 24, status 786f. 

Enabling bus−master transmits and whole−frame receives. 

eth1: 3Com 3c905 Boomerang 100baseTx at 0xfd80, 00:60:97:92:69:f8, IRQ 9 

8K word−wide RAM 3:5 Rx:Tx split, autoselect/MII interface. 

MII transceiver found at address 24, status 7849. 

Enabling bus−master transmits and whole−frame receives. 

Partition check: 

sda: sda1 sda2 < sda5 sda6 sda7 sda8 > 

sdb: 

. 

. 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

Also be sure that the cabling is correct (Ethernet: the NICs connecting the external MASQ server to 

your ISP has the "link" light lit up). Finally, make sure that TCP/IP is correctly configured on the 

MASQed Server as described by the various Network HOWTOs (URLs can be found in the 

requirements section for your 2.4.x kernel in Section 2.6, 2.2.x kernel in Section 2.7, or 2.0.x kernel in 

Section 2.8). 

5.7. Testing internal MASQ client to external MASQ server 

connectivity 

• Step Seven: Testing internal MASQ client to external MASQ server connectivity 

From an internal MASQed computer, ping the IP address of the MASQ server's EXTERNAL TCP/IP 

address obtained in Step FIVE above. This address could be from your Ethernet, PPP, etc. interface 

which is ultimately the address connected to your ISP. This ping test will prove that Linux 

masquerading (ICMP Masquerading specifically) and IP forwarding is working. 

If everthing thing is working correctly, the output should look something like the following (hit 

Control−C to abort the ping): 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 







^C 




−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

If this test doesn't work, first make sure that the "Default Gateway" on the MASQed PC is pointing to 



the IP address on the MASQ −SERVERs− INTERNAL NIC. Also double check that the 

/etc/rc.d/rc.firewall−* script was run without any errors. Just as a test, try re−running the 

/etc/rc.d/rc.firewall−* script now to see if it runs OK. Also, though most kernels support it by default, 

make sure that you enabled "ICMP Masquerading" in the kernel comfiguration and "IP Forwarding" 

in your /etc/rc.d/rc.firewall−* script. 

If you still can't get things to work, take a look at the output from the following commands run on the 

Linux MASQ SERVER: 

♦ "ifconfig" : Make sure the interface for your Internet connection (be it ppp0, eth0, etc.) is UP 

and you have the correct IP address for the Internet connection. An example of this output is 

shown in STEP FIVE above. 

♦ "netstat −rn" : Make sure your default gateway (the column with an IP address in the 

Gateway column) is set. An example of this output might look like: 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

masq−server# netstat −rn 

Kernel IP routing table 

Destination Gateway Genmask Flags MSS Window irtt Iface 

192.168.0.1 0.0.0.0 255.255.255.255 UH 0 16384 0 eth1 

12.13.14.15 0.0.0.0 255.255.255.255 UH 0 16384 0 eth0 

12.13.14.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 

192.168.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1 

127.0.0.0 0.0.0.0 255.0.0.0 U 0 16384 0 lo 

0.0.0.0 12.13.14.1 0.0.0.0 UG 0 16384 0 eth0 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

Notice the very LAST line that starts with 0.0.0.0? Notice that it also has an IP address in the 

"Gateway" field? You should specify an IP address for your specific setup in that field (this is 

typically done automatically when your Internet connection is enabled). 

♦ "cat /proc/sys/net/ipv4/ip_forward" : Make sure it says "1" so that Linux forwarding is 

enabled 

♦ Run the command "/sbin/ipchains −n −L" for 2.2.x users or "/sbin/ipfwadm −F −l" for 2.0.x 

users. Specifically, look for the FORWARDing section to make sure you have MASQ 

enabled. An example of an IPCHAINS output might look like for users using the SIMPLE 

rc.firewall−* ruleset: 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

. 

. 

Chain forward (policy REJECT): 

target prot opt source destination ports 

MASQ all −−−−−− 192.168.0.0/24 0.0.0.0/0 n/a 

ACCEPT all −−−−l− 0.0.0.0/0 0.0.0.0/0 n/a 

. 

. 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

5.8. Testing external MASQ ICMP forwarding 

• Step Eight: Testing external MASQ ICMP forwarding 

From an internal MASQed computer, ping a static TCP/IP address (NOT a machine by DNS name) 

out on the Internet (i.e. ping 152.2.210.80 (this technically the DNS name "metalab.unc.edu" which is 

home of MetaLabs' Linux Archive). If this works, it should look something like the result below and 

this ultimately shows that ICMP Masquerading is working properly. (hit Control−C to abort the ping): 


−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 







^C 




−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

If this didn't work, again check your Internet connection. Make sure that the MASQ server itself can 

ping this address. If this works from the MASQ server, make sure you are using the simple 

rc.firewall−* ruleset and that you have ICMP Masqurading compiled into the Linux kernel. 

Finally, make sure that the ruleset which enables IP MASQ is pointing to the correct EXTERNAL 

interface. PPPoE users should use the MASQ servers's logical PPP interface such as "ppp0" and 

/NOT/ the physical external interface like "eth0". 

5.9. Testing MASQ functionality without DNS 

• Step Nine: Testing MASQ functionality without DNS 

Now try TELNETing to a remote IP address (i.e. telnet 152.2.210.81 ( this is technically the DNS 

name "metalab.unc.edu"). It might take a few seconds to get a login prompt since this is a VERY busy 

server. Did you get a login prompt like shown below? If so, it means that TCP Masquerading is 

running OK. If not, try TELNETing to some other hosts you think will support TELNET like 

198.182.196.55 (www.linux.org). If this still doesn't work, make sure you are using the simple 

rc.firewall−* ruleset for this test. An example of this output might look like (hit Control−D to exit out 

of the TELNET): 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

masq−client# telnet 152.2.210.80 

Trying 152.2.210.80... 

Connected to 152.2.210.80. 

Escape character is '^]'. 

SunOS 5.7 


******************** Welcome to MetaLab.unc.edu ******************* 

To login to MetaLab as a user, connect to login.metalab.unc.edu. 

This machine does not allow public telnet logins. 

login: Connection closed by foreign host. 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 


5.10. Testing MASQ functionality with DNS resolution 

• Step Ten: Testing MASQ functionality with DNS resolution 

Now try TELNETing to a remote machine by DNS name (i.e. "telnet metalab.unc.edu" (IP address 

152.2.210.81). If this works, the output should look like something below. With this test, this shows 

that UDP−based DNS is working fine. 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

masq−client# telnet MetaLab.unc.edu 

Trying 152.2.210.80... 

Connected to 152.2.210.80. 

Escape character is '^]'. 

SunOS 5.7 


******************** Welcome to MetaLab.unc.edu ******************* 

To login to MetaLab as a user, connect to login.metalab.unc.edu. 

This machine does not allow public telnet logins. 

login: Connection closed by foreign host. 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

If this did not work, but Step EIGHT did work, make sure that you have one or more valid DNS 

servers configured on each of the MASQ CLIENT computer(s) as shown in Chapter 4. Please note 

that these DNS servers will typically be your ISP's DNS servers and NOT your local MASQ server. 

Some people might later choose to setup their OWN DNS servers but this is beyond the scope of this 

HOWTO. 

5.11. Testing more MASQ functionality with DNS 

• Step Eleven: Testing more MASQ functionality with DNS 

As a last test, try browsing some 'INTERNET' WWW sites on one of your MASQ Client machines, 

and see if you can reach them. For example, access the Linux Documentation Project site at 

http://www.tldp.org. If you are successful in bringing up that page, you can be fairly certain that 

everything is working FINE! If some WWW or FTP sites have problems, where other sites seem to 

work just fine, see the next step for more ideas. 

If you see The Linux Documentation Project homepage, then CONGRATULATIONS! It's working! 

If the WWW site comes up correctly, then all other standard network tools such as PING, TELNET, 

SSH, and with their related IP MASQ modules loaded: FTP, Real Audio, IRC DCCs, Quake I/II/III, 

CuSeeme, VDOLive, etc. should work fine too! If FTP, IRC, RealAudio, Quake I/II/III, etc. aren't 

working or are performing poorly, verify that their associated Masquerading modules are loaded by 

running "lsmod" and also be sure you are loading the module with any non−default server ports. If 

you don't see your needed module, make sure your /etc/rc.d/rc.firewall−* script is loading them (i.e. 

remove the # character for a given IP MASQ module). 



5.12. Any remaining functional, performance, etc. issues... 

• Step Twelve: Any remaining functional, performance, etc. issues... 

If your system passes all of the above tests, but functionality tests like WWW browsing, FTP, and 

other types of traffic aren't reliable or are slow, I recommend that you read the FAQ section of this 

HOWTO.. specifically the Section 7.15 FAQ entry. There might be other items in the FAQ section 

that will also help you as they have helped many other users in the past. 


Chapter 6. Other IP Masquerade Issues and 

Software Support 

6.1. Problems with IP Masquerade 

Some TCP/IP application protocols will not currently work with Linux IP Masquerading because they either 

assume things about port numbers or encode TCP/IP addresses and/or port numbers in their data stream. 

These latter protocols need specific proxies or IP MASQ modules built into the masquerading code to make 

them work. 

6.2. Incoming services 

By default, Linux IP Masquerading cannot handle incoming services at all but there are a few ways that would 

allow this. 

If you do not require high levels of security, then you can simply forward or redirect IP ports. There are 

various ways to perform this, though the most stable method is to use IPPORTFW. For more information, 

please see Section 6.7. 

If you wish to have some level of authorization on incoming connections, then you will need to either 

configure TCP−wrappers or Xinetd to allow only specific IP addresses to pass. The TIS Firewall Toolkit is a 

good place to look for tools and information. 

More details on incoming security can be found in the TrinityOS document and at IP Masquerade Resource. 

6.3. Supported Client Software and Other Setup Notes 

"** The Linux Masquerade Application list has a lot of good information regarding applications that work 

through Linux IP masquerading. This site was recently taken over by Steve Grevemeyer, who implemented 

it with a full database backend. It's a great resource! " 

Generally, any application that uses standard TCP and UDP should work. If you have any suggestion, hints, 

etc., please see the IP Masquerade Resource for more details. 

6.3.1. Network Clients that −Work− with IP Masquerade 

General Clients: 

Archie 

all supported platforms, file searching client (not all archie clients are supported) 

FTP 

all supported platforms, with the ip_masq_ftp.o kernel module for active FTP connections. 

Gopher client 

all supported platforms 

HTTP 

all supported platforms, WWW surfing 

IRC 

Chapter 6. Other IP Masquerade Issues and Software Support 77

all IRC clients on various supported platforms, DCC is supported via the ip_masq_irc.o module 

NNTP (USENET) 

all supported platforms, USENET news client 

PING 

all platforms, with ICMP Masquerading kernel option 

POP3 

all supported platforms, email clients 

SSH 

all supported platforms, Secure TELNET/FTP clients 

SMTP 

all supported platforms, email servers like Sendmail, Qmail, PostFix, etc. 

TELNET 

all supported platforms, remote session 

TRACEROUTE 

UNIX and Windows based platforms, some variations may not work 

VRML 

Windows(possibly all supported platforms), virtual reality surfing 

WAIS client 

all supported platforms 

Multimedia and Communication Clients: 


All H.323 programs 

− MS Netmeeting, Intel Internet Phone Beta , and other H.323 applications − There are now two 

solutions to accomplish this through IPMASQed connections: 

There is a stable BETA 2.2.x kernel module available on the MASQ WWW site or at 

http://www.coritel.it/projects/nat/implementation.htm to work with Microsoft Netmeeting v3.x code 

on 2.2.x kernels. There is also another module version on the MASQ WWW site specifically for 

Netmeeting 2.x with 2.0.x kernels, but this does not support Netmeeting v3.x. 

Another commercial solution is the Equivalence's PhonePatch H.323 gateway. 

Alpha Worlds 

Windows, Client−Server 3D chat program 

CU−SeeMe 

all supported platforms, with the ip_masq_cuseeme module loaded, please see Section 6.8 for more 

details. 

ICQ 

all supported clients. Requires the Linux kernel to be either compiled with PORTFW support, have 

the ip_masq_icq module (2.2.x and 2.0.x only), or have a SOCKS proxy running. A full description of 

this configuration is in Section 6.9. 

Internet Phone 3.2 

Windows, Peer−to−peer audio communications, users can reach you only if you initiate the call, but 

those users cannot call you without a specific port forwarding setup. See Section 6.7for more details. 

Internet Wave Player 

Windows, network streaming audio 

Powwow 

Windows, Peer−to−peer Text audio whiteboard communications, users can reach you only if you 

initiate the call, but those users cannot call you without a specific port forwarding setup. See Section 

6.7for more details. 


Real Audio Player 

Windows, network streaming audio, higher quality available with the ip_masq_raudio UDP module 

True Speech Player 1.1b 

Windows, network streaming audio 

VDOLive 

Windows, with the ip_masq_vdolive patch 

Worlds Chat 0.9a 

Windows, Client−Server 3D chat program 

Games − See Section 6.10for more details on the LooseUDP patch 

Battle.net 

Works but requires TCP ports 116, 118 and UDP port 6112 IPPORTFWed to the client game 

machine. See Section 6.7for more details. Please note that FSGS and Bnetd servers still require 

IPPORTFW because they have not been re−written to be NAT−friendly. 

BattleZone 1.4 

Works with LooseUDP patch and new NAT−friendly −− email David Ranch for the .DLLs from 

Activision 

Dark Reign 1.4 

Works with LooseUDP patch or requires TCP ports 116 and 118 and UDP port 6112 IPPORTFWed 

to the game machine. See Section 6.7for more details. 

Diablo 


to the game machine. Newer versions of Diablo use only TCP port 6112 and UDP port 6112. See 

Section 6.7for more details. 

Heavy Gear 2 


to the game machine. See Section 6.7for more details. 

Quake I/II/III 

Works right out of the box but requires the ip_masq_quake module if there are more than one Quake 

I/II/III player behind a MASQ box. Also, this module only supports Quake I and QuakeWorld by 

default. If you need to support Quake II or non−default server ports, please see the module install 

section of Section 3.4.3 and Section 3.4.2 rulesets. 

StarCraft 

Works with the LooseUDP patch, IPPORTFWing TCP, and UDP ports 6112 to the internal MASQed 

game machine. See Section 6.7for more details. 

WorldCraft 

Works with LooseUDP patch 

Other Clients: 


Linux net−acct package 

Linux, network administration−account package 

NCSA Telnet 2.3.08 

DOS, a suite containing telnet, ftp, ping, etc. 

PC−anywhere for Windows 

MS−Windows remotely controls a PC over TCP/IP, but only works if it is a client, but not a host 

without a specific port forwarding setup. See Section 6.7for more details. 

Socket Watch 

uses NTP − network time protocol 


6.3.2. Clients that do not have full support in IP MASQ: 

Intel Streaming Media Viewer Beta 1 

Cannot connect to server 

Netscape CoolTalk 

Cannot connect to opposite side 

WebPhone 

Cannot work at present (it makes invalid assumptions about addresses). 

6.4. Stronger firewall rulesets to run after initial testing 

6.4.1. Stronger IP Firewall (IPTABLES) rulesets 

 

#!/bin/sh 

# 

# rc.firewall−iptables−stronger 

# 

FWVER=0.88s 


# An example of a stronger IPTABLES firewall with IP Masquerade 

# support for 2.4.x kernels. 

# 

# Log: 

# 

# 0.88s − Updated the commands for dynamically addresses machines and 

# to point to an expanded FAQ section for more information 

# 

# 0.87s − Removed the unused drop−and−logit chain as it was only later 

# being deleted anyway 

# 0.86s − Fixed a typo that had a preceeding ; instead of a # 

# 0.85s − renamed from rc.firewall−2.4−stronger to rc.firewall−iptables− 

# stronger to reflect this script works for all IPTABLES enabled 

# platforms including 2.6.x kernels 

# − fixed an incorrect /24 netmask for the INTIP variable 

# − removed the unneeded SED variable 

# 0.84s − Changed the defaults from 192.168.1.0 to 192.168.0.x to align 

# with the rest of the IPMASQ howto 

# 0.83s − Added additional comments to make PORTFW configs more obvious 

# 0.82s − Added a special ICMP filter to work around a Netfilter security 

# issue 

# − renamed the drop−and−log−it rule to reject−and−log−it 

# 0.81s − Added an additional comment in the INPUT section for NOT 

# allowing all traffic in, but only select traffic 

# 0.80s − Added a DISABLED ip_nat_irc kernel module section, changed the 

# default of the ip_conntrack_irc to NOT load by default, and 

# added additional kernel module comments 

# 0.79s − ruleset now uses modprobe instead of insmod 

# 0.78s − REJECT is not a legal policy yet; back to DROP 

# 0.77s − Changed the default block behavior to REJECT not DROP 

# 0.76s − Added a comment about the OPTIONAL WWW ruleset and a comment 

# where to put optional PORTFW commands 

# 0.75s − Added clarification that PPPoE users need to use 


# 0.74s − Changed the EXTIP command to work on NON−English distros 

# 0.73s − Added comments in the output section that DHCPd is optional 



# and changed the default settings to disabled 

# 0.72s − Changed the filter from the INTNET to the INTIP to be 

# stateful; moved the command VARs to the top and made the 

# rest of the script to use them 

# 0.70s − Added a disabled examples for allowing internal DHCP 

# and external WWW access to the server 

# 0.63s − Added support for the IRC module 

# 0.62s − Initial version based upon the basic 2.4.x rc.firewall 

echo −e "\nLoading rc.firewall−iptables−STRONGER − version $FWVER..\n" 


# 




# 




# 

#IPTABLES=/sbin/iptables 


# 

LSMOD=/sbin/lsmod 



GREP=/bin/grep 

AWK=/bin/awk 

IFCONFIG=/sbin/ifconfig 


# 



# is where the natting will occur and the internal network 

# should preferably be addressed with a RFC1918 private address 

# scheme. 

# 


# 



# 


# 


# 

EXTIF="eth0" 

INTIF="eth1" 



echo " −−−" 

# Specify your Static IP address here or let the script take care of it 

# for you. 

# 

# If you prefer to use STATIC addresses in your firewalls, un−# out the 

# static example below and # out the dynamic line. If you don't care, 


# just leave this section alone. 

# 

# If you have a DYNAMIC IP address, the ruleset already takes care of 

# this for you. Please note that the different single and double quote 

# characters and the script MATTER. 

# 

# 

# PPP and DHCP (Cablemodem and DSL ) users: 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

# PPP: If you get your TCP/IP address via DHCP, **you will need ** to 

# enable the # #ed out command below underneath the PPP section AND 

# replace the word "eth0" with the name of your EXTERNAL Internet 

# connection (ppp0, ippp0, etc) on the lines for "ppp−ip" and "extip". 

# 

# DHCP and PPP users: The remote DHCP or PPP server can and will change 

# IP addresses on you over time. To deal with this, users should configure 

# their DHCP or PPP client to re−run the rc.firewall−* ruleset everytime 

# the IP address is changed. Please see the "masq−and−dyn−addr" FAQ entry 

# in the IPMASQ howto for full details on how to do this. 

# 

# 

# Determine the external IP automatically: 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

# 

# The following line will determine your external IP address. This 

# line is somewhat complex and confusing but it will also work for 

# all NON−English Linux distributions: 

# 

EXTIP="`$IFCONFIG $EXTIF | $AWK \ 

/$EXTIF/'{next}//{split($0,a,":");split(a[2],a," ");print a[1];exit}'`" 

# For users who wish to use STATIC IP addresses: 

# 

# # out the EXTIP line above and un−# out the EXTIP line below 

# 

#EXTIP="your.static.PPP.address" 

echo " External IP: $EXTIP" 

echo " −−−" 

# Assign the internal TCP/IP network and IP address 

INTNET="192.168.0.0/24" 

INTIP="192.168.0.1/32" 

echo " Internal Network: $INTNET" 

echo " Internal IP: $INTIP" 

echo " −−−" 

# Setting a few other local variables 

# 

UNIVERSE="0.0.0.0/0" 


#====================================================================== 

#== No editing beyond this line is required for initial MASQ testing == 

# Need to verify that all modules have all required dependencies 

# 

echo " − Verifying that all kernel modules are ok" 

$DEPMOD −a 


echo −en " Loading kernel modules: " 


# With the new IPTABLES code, the core MASQ functionality is now either 

# modular or compiled into the kernel. This HOWTO shows ALL IPTABLES 

# options as MODULES. If your kernel is compiled correctly, there is 

# NO need to load the kernel modules manually. 

# 

# NOTE: The following items are listed ONLY for informational reasons. 

# There is no reason to manual load these modules unless your 

# kernel is either mis−configured or you intentionally disabled 

# the kernel module autoloader. 

# 

# Upon the commands of starting up IP Masq on the server, the 

# following kernel modules will be automatically loaded: 

# 

# NOTE: Only load the IP MASQ modules you need. All current IP MASQ 

# modules are shown below but are commented out from loading. 

# =============================================================== 

#Load the main body of the IPTABLES module − "ip_tables" 

# − Loaded automatically when the "iptables" command is invoked 

# 


# 

echo −en "ip_tables, " 

# 

#Verify the module isn't loaded. If it is, skip it 

# 

if [ −z "` $LSMOD | $GREP ip_tables | $AWK {'print $1'} `" ]; then 

$MODPROBE ip_tables 

fi 

#Load the IPTABLES filtering module − "iptable_filter" 

# 

# − Loaded automatically when filter policies are activated 

#Load the stateful connection tracking framework − "ip_conntrack" 

# 

# The conntrack module in itself does nothing without other specific 

# conntrack modules being loaded afterwards such as the "ip_conntrack_ftp" 

# module 

# 

# − This module is loaded automatically when MASQ functionality is 

# enabled 

# 


# 

echo −en "ip_conntrack, " 

# 


# 

if [ −z "` $LSMOD | $GREP ip_conntrack | $AWK {'print $1'} `" ]; then 

$MODPROBE ip_conntrack 

fi 

#Load the FTP tracking mechanism for full FTP tracking 

# 




# 

echo −e "ip_conntrack_ftp, " 

# 


# 

if [ −z "` $LSMOD | $GREP ip_conntrack_ftp | $AWK {'print $1'} `" ]; then 

$MODPROBE ip_conntrack_ftp 

fi 

#Load the IRC tracking mechanism for full IRC tracking 

# 

# Disabled by default −− insert a "#" on the next few lines to activate 

# 

# echo −en " ip_conntrack_irc, " 

# 


# 

# if [ −z "` $LSMOD | $GREP ip_conntrack_irc | $AWK {'print $1'} `" ]; then 

# $MODPROBE ip_conntrack_irc 

# fi 

#Load the general IPTABLES NAT code − "iptable_nat" 

# − Loaded automatically when MASQ functionality is turned on 

# 


# 

echo −en "iptable_nat, " 

# 


# 

if [ −z "` $LSMOD | $GREP iptable_nat | $AWK {'print $1'} `" ]; then 

$MODPROBE iptable_nat 

fi 

#Loads the FTP NAT functionality into the core IPTABLES code 

# Required to support non−PASV FTP. 

# 


# 

echo −e "ip_nat_ftp" 

# 


# 

if [ −z "` $LSMOD | $GREP ip_nat_ftp | $AWK {'print $1'} `" ]; then 

$MODPROBE ip_nat_ftp 

fi 

#Loads the IRC NAT functionality (for DCC) into the core IPTABLES code 

# 

# DISABLED by default −− delete the "#" on the next few lines to activate 

# 

# echo −e "ip_nat_irc" 

# 


# 

# if [ −z "` $LSMOD | $GREP ip_nat_irc | $AWK {'print $1'} `" ]; then 

# $MODPROBE ip_nat_irc 


# fi 

echo " −−−" 


# Just to be complete, here is a partial list of some of the other 

# IPTABLES kernel modules and their function. Please note that most 

# of these modules (the ipt ones) are automatically loaded by the 

# master kernel module for proper operation and don't need to be 

# manually loaded. 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

# 

# ip_nat_snmp_basic − this module allows for proper NATing of some 

# SNMP traffic 

# 

# iptable_mangle − this target allows for packets to be 

# manipulated for things like the TCPMSS 

# option, etc. 

# 

# −− 

# 

# ipt_mark − this target marks a given packet for future action. 

# This automatically loads the ipt_MARK module 

# 

# ipt_tcpmss − this target allows to manipulate the TCP MSS 

# option for braindead remote firewalls. 

# This automatically loads the ipt_TCPMSS module 

# 

# ipt_limit − this target allows for packets to be limited to 

# to many hits per sec/min/hr 

# 

# ipt_multiport − this match allows for targets within a range 

# of port numbers vs. listing each port individually 

# 

# ipt_state − this match allows to catch packets with various 

# IP and TCP flags set/unset 

# 

# ipt_unclean − this match allows to catch packets that have invalid 

# IP/TCP flags set 

# 

# iptable_filter − this module allows for packets to be DROPped, 

# REJECTed, or LOGged. This module automatically 

# loads the following modules: 

# 

# ipt_LOG − this target allows for packets to be 

# logged 

# 

# ipt_REJECT − this target DROPs the packet and returns 

# a configurable ICMP packet back to the 

# sender. 


# 



# 


# to 


# 

echo " Enabling forwarding.." 




# 

# If you get your IP address dynamically from SLIP, PPP, or DHCP, 

# enable the following option. This enables dynamic−address hacking 

# which makes the life with Diald and similar programs much easier. 

# 

echo " Enabling DynamicAddr.." 


echo " −−−" 

############################################################################# 

# 

# Enable Stronger IP forwarding and Masquerading 

# 

# NOTE: In IPTABLES speak, IP Masquerading is a form of SourceNAT or SNAT. 

# 

# NOTE #2: The following is an example for an internal LAN address in the 

# 192.168.0.x network with a 255.255.255.0 or a "24" bit subnet 

# mask connecting to the Internet on external interface "eth0". 

# This example will MASQ internal traffic out to the Internet 

# but not allow non−initiated traffic into your internal network. 

# 

# 

# ** Please change the above network numbers, subnet mask, and your 

# 


# 

# Unless specified, the defaults for INPUT, OUTPUT, and FORWARD to DROP 

# 

# You CANNOT change this to REJECT as it isn't a vaild policy setting. 

# If you want REJECT, you must explictly REJECT at the end of a giving 

# INPUT, OUTPUT, or FORWARD chain 

# 

echo " Clearing any existing rules and setting default policy to DROP.." 

$IPTABLES −P INPUT DROP 


$IPTABLES −P OUTPUT DROP 




$IPTABLES −F −t nat 

#Not needed and it will only load the unneeded kernel module 

# 

#$IPTABLES −F −t mangle 

# Delete all User−specified chains 

$IPTABLES −X 

# Reset all IPTABLES counters 

$IPTABLES −Z 


#Configuring specific CHAINS for later use in the ruleset 

# 


# NOTE: Some users prefer to have their firewall silently 

# "DROP" packets while others prefer to use "REJECT" 

# to send ICMP error messages back to the remote 

# machine. The default is "REJECT" but feel free to 

# change this below. 

# 

# NOTE: Without the −−log−level set to "info", every single 

# firewall hit will goto ALL vtys. This is a very big 

# pain. 

# 

echo " Creating a DROP chain.." 

$IPTABLES −N reject−and−log−it 

$IPTABLES −A reject−and−log−it −j LOG −−log−level info 

$IPTABLES −A reject−and−log−it −j REJECT 

echo −e "\n − Loading INPUT rulesets" 


####################################################################### 

# INPUT: Incoming traffic from various interfaces. All rulesets are 

# already flushed and set to a default policy of DROP. 

# 

# loopback interfaces are valid. 

# 

$IPTABLES −A INPUT −i lo −s $UNIVERSE −d $UNIVERSE −j ACCEPT 

# local interface, local machines, going anywhere is valid 

# 

$IPTABLES −A INPUT −i $INTIF −s $INTNET −d $UNIVERSE −j ACCEPT 

# remote interface, claiming to be local machines, IP spoofing, get lost 

# 

$IPTABLES −A INPUT −i $EXTIF −s $INTNET −d $UNIVERSE −j reject−and−log−it 

# external interface, from any source, for ICMP traffic is valid 

# 

# If you would like your machine to "ping" from the Internet, 

# enable this next line 

# 

#$IPTABLES −A INPUT −i $EXTIF −p ICMP −s $UNIVERSE −d $EXTIP −j ACCEPT 

# remote interface, any source, going to the MASQ servers IP address is valid 

# 

# ENABLE this line if you want ALL Internet traffic to connect to your 

# the various servers running on the MASQ server. This includes 

# web servers, ssh servers, dns servers, etc. 

# 

# I DON'T recommend you enable this rule. Instead, only enable specific 

# access to select server ports under the "OPTIONAL INPUT Section". 

# An example of enabling HTTP (WWW) has been given below: 

# 

# 

#$IPTABLES −A INPUT −i $EXTIF −s $UNIVERSE −d $EXTIP −j ACCEPT 

# Allow any related traffic coming back to the MASQ server in. 

# 


# STATEFULLY TRACKED 

# 

$IPTABLES −A INPUT −i $EXTIF −s $UNIVERSE −d $EXTIP −m state −−state \ 

ESTABLISHED,RELATED −j ACCEPT 

# −−−−− Begin OPTIONAL INPUT Section −−−−− 

# 

# DHCPd − Enable the following lines if you run an INTERNAL DHCPd server 

# 

#$IPTABLES −A INPUT −i $INTIF −p tcp −−sport 68 −−dport 67 −j ACCEPT 

#$IPTABLES −A INPUT −i $INTIF −p udp −−sport 68 −−dport 67 −j ACCEPT 

# HTTPd − Enable the following lines if you run an EXTERNAL WWW server 

# 

# NOTE: This is NOT needed for simply enabling PORTFW. This is ONLY 

# for users that plan on running Apache on the MASQ server itself 

# 

#echo −e " − Allowing EXTERNAL access to the WWW server" 

#$IPTABLES −A INPUT −i $EXTIF −m state −−state NEW,ESTABLISHED,RELATED \ 

# −p tcp −s $UNIVERSE −d $EXTIP −−dport 80 −j ACCEPT 

# 

# −−−−− End OPTIONAL INPUT Section −−−−− 

# Catch all rule, all other incoming is denied and logged. 

# 

$IPTABLES −A INPUT −s $UNIVERSE −d $UNIVERSE −j reject−and−log−it 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

echo −e " − Loading OUTPUT rulesets" 


####################################################################### 

# OUTPUT: Outgoing traffic from various interfaces. All rulesets are 

# already flushed and set to a default policy of DROP. 

# 

# Workaround bug in netfilter 

# See http://www.netfilter.org/security/2002−04−02−icmp−dnat.html 

# 

$IPTABLES −A OUTPUT −m state −p icmp −−state INVALID −j DROP 

# loopback interface is valid. 

# 

$IPTABLES −A OUTPUT −o lo −s $UNIVERSE −d $UNIVERSE −j ACCEPT 

# local interfaces, any source going to local net is valid 

# 

$IPTABLES −A OUTPUT −o $INTIF −s $EXTIP −d $INTNET −j ACCEPT 

# local interface, MASQ server source going to the local net is valid 

# 

$IPTABLES −A OUTPUT −o $INTIF −s $INTIP −d $INTNET −j ACCEPT 

# outgoing to local net on remote interface, stuffed routing, deny 


# 

$IPTABLES −A OUTPUT −o $EXTIF −s $UNIVERSE −d $INTNET −j reject−and−log−it 

# anything else outgoing on remote interface is valid 

# 

$IPTABLES −A OUTPUT −o $EXTIF −s $EXTIP −d $UNIVERSE −j ACCEPT 

# −−−−− Begin OPTIONAL OUTPUT Section −−−−− 

# 

# DHCPd − Enable the following lines if you run an INTERNAL DHCPd server 

# − Remove BOTH #s all the #s if you need this functionality. 

# 

#$IPTABLES −A OUTPUT −o $INTIF −p tcp −s $INTIP −−sport 67 \ 

# −d 255.255.255.255 −−dport 68 −j ACCEPT 

#$IPTABLES −A OUTPUT −o $INTIF −p udp −s $INTIP −−sport 67 \ 

# −d 255.255.255.255 −−dport 68 −j ACCEPT 

# 

# −−−−− End OPTIONAL OUTPUT Section −−−−− 

# Catch all rule, all other outgoing is denied and logged. 

# 

$IPTABLES −A OUTPUT −s $UNIVERSE −d $UNIVERSE −j reject−and−log−it 

echo −e " − Loading FORWARD rulesets" 

####################################################################### 

# FORWARD: Enable Forwarding and thus IPMASQ 

# 

# −−−−− Begin OPTIONAL FORWARD Section −−−−− 

# 

# Put PORTFW commands here 

# 

# −−−−− End OPTIONAL FORWARD Section −−−−− 


echo " − FWD: Allow all connections OUT and only existing/related IN" 

$IPTABLES −A FORWARD −i $EXTIF −o $INTIF −m state −−state ESTABLISHED,RELATED \ 

−j ACCEPT 

$IPTABLES −A FORWARD −i $INTIF −o $EXTIF −j ACCEPT 

# Catch all rule, all other forwarding is denied and logged. 

# 

$IPTABLES −A FORWARD −j reject−and−log−it 

echo " − NAT: Enabling SNAT (MASQUERADE) functionality on $EXTIF" 

# 

#More liberal form 

#$IPTABLES −t nat −A POSTROUTING −o $EXTIF −j MASQUERADE 

# 

#Stricter form 

$IPTABLES −t nat −A POSTROUTING −o $EXTIF −j SNAT −−to $EXTIP 

####################################################################### 


echo −e "\nrc.firewall−iptables−stronger $FWVER done.\n" 

 

To automatically start this stronger firewall ruleset at the proper time, please see the end of the Section 3.4.2 

section for full details. Please make sure you make the correct "rc.firewall−iptables" to 

"rc.firewall−iptables−stronger" substitutions!! 

6.4.2. Stronger IP Firewall (IPCHAINS) rulesets 

This section provides a more in−depth guide to using the 2.2.x firewall tool, IPCHAINS. See above sections 

for IPFWADM rulesets. 

This example is for a firewall/masquerade system behind a PPP link with a static PPP address (dynamic PPP 

instructions are included but disabled). The trusted interface is 192.168.0.1 and the PPP interface IP address 

has been changed to protect the guilty :−). I have listed each incoming and outgoing interface individually to 

catch IP spoofing as well as stuffed routing and/or masquerading. A nything not explicitly allowed is 

FORBIDDEN (well.. rejected actually). If your IP MASQ box breaks after implementing this 

rc.firewall−ipchains−stronger script, be sure that you edit it for your configuration and check your 

/var/log/messages or /var/adm/messages SYSLOG file for any firewall errors. 

For more comprehensive examples of a strong IP Masqueraded IPFWADM rulesets for PPP, Cablemodem 

users, etc., please see TrinityOS − Section 10 and GreatCircle's Firewall WWW page 

NOTE #1: −−− UPDATE YOUR KERNEL −−− Linux 2.2.x kernels less than version 2.2.20 contain several 

different security vulnerabilities (some were MASQ specific). Kernels less than 2.2.20 have a few local 

vulnerabilities. Kernel versions less than 2.2.16 have a TCP root exploit vulnerability and versions less than 

2.2.11 have a IPCHAINS fragmentation bug. Because of these issues, users running a firewall with strong 

IPCHAINS rulesets are open to possible instrusion. Please upgrade your kernel to a fixed version. 

NOTE #2: If you get a dynamically assigned TCP/IP address from your ISP (PPP, DSL, Cablemodems, etc.), 

you CANNOT load this strong ruleset upon booting. You will either need to reload this firewall ruleset 

EVERY TIME you get a new IP address or make your /etc/rc.d/rc.firewall−ipchains−stronger ruleset more 

intelligent. To do this for various types of connections such as PPP or DHCP users, please see the Section 7.8 

FAQ entry for all the details. 

Please also be aware that there are several GUI Firewall creation tools available as well. Please see 

Chapter 7for full details. 

Lastly, if you are using a STATIC PPP IP address, change the "EXTIF="your.static.PPP.address"" line to 

reflect your address. 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

 


#!/bin/sh 

# 

# /etc/rc.d/rc.firewall−ipchains−stronger: An example of a Stronger IPCHAINS 

# firewall ruleset for 2.2 kernels 

# 

FWVER=0.75s 


# 

# Log: 



# 

# 0.74s − renamed from rc.firewall−2.2−stronger to 

# rc.firewall−ipchains−stronger to better reflect that this ruleset can 

# can run on different major kernel versions 

# − removed unused SED variable 

# 0.73s − Added additional comments to make PORTFW configs more obvious 

# 0.72s − #ed out the rule that would allow all traffic destined for the 

# MASQ server itself to be accepted. Use the OPTIONAL INPUT 

# section to only allow explicit services. 

# − Fixed an INTLAN rule that was allowing traffic from ANY IP address 

# instead of the proper INTIP IP address only. This aligns the 

# IPCHAINS ruleset with the IPTABLES and IPFWADM ruleset examples 

# 0.71s − ruleset now uses modprobe instead of insmod 

# 0.70s − Added missing execution variables 

# − fixed a missing −p tcp for the commented HTTPd section 

# 0.65s − Added comments HTTPd rules to the INPUT and OUTPUT section 

# − Added a comment where to insert IPPORTFW commands 

# 0.60s − Changed the EXTIP command to work on NON−English distros 

# − Updated the CASE of some of the script variables 

# 

echo −e "\nLoading rc.firewall−ipchains−stronger : version $FWVER..\n" 


# 




# 




# 


LSMOD=/sbin/lsmod 



GREP=/bin/grep 

AWK=/bin/awk 

IFCONFIG=/sbin/ifconfig 

PATH=/sbin:/bin:/usr/sbin:/usr/bin 

# Global variables 

# −−−−−−−−−−−−−−−− 

# ALL PPP and DHCP users must set this for the correct EXTERNAL and 

# INTERNAL interfaces names. Examples: eth0, ppp0, ippp0, etc. 

# See more info about this below. 

# 

EXTIF="ppp0" 

INTIF="eth0" 

# The INTERNAL IP address 

# 

INTIP="192.168.0.1/32" 



INTNET="192.168.0.0/24" 

echo " Internal IP: $INTIP" 

echo " Internal Network: $INTNET" 


# 


# are shown below but are commented from loading. 


# 

$DEPMOD −a 



# 

$MODPROBE ip_masq_ftp 




# 

$MODPROBE ip_masq_raudio 


# 

#$MODPROBE ip_masq_irc 

# Supports the masquerading of Quake and QuakeWorld by default. These modules are 

# for multiple users behind the Linux MASQ server. If you are going to 


# 



# 


#$MODPROBE ip_masq_quake 

# 


#$MODPROBE ip_masq_quake 26000,27000,27910,27960 


# 

#$MODPROBE ip_masq_cuseeme 


# 

#$MODPROBE ip_masq_vdolive 

#CRITICAL: Enable IP forwarding since it is disabled by default 

# 



# 


# to 


# 




#CRITICAL: Enable automatic IP defragmentation since it is disabled by default 

# in 2.2.x kernels 

# 

# This used as a compile−time option but the behavior was changed 

# in 2.2.12. It should also be noted that some distributions have 

# removed this option from the /proc table. If this entry isn't 

# present in your /proc, don't worry about it. 

# 



# 


# following option. This enables dynamic−ip address hacking in IP MASQ, 

# making life with Diald and similar programs much easier. 

# 


# Enable the LooseUDP patch which some Internet−based games require 

# 

# If you are trying to get an Internet game to work through your IP MASQ box, 

# and you configured it to the best of your ability without it working, try 

# enabling this option (delete the "#" character). This option is disabled 

# by default due to possible internal machine UDP port scanning 

# vulnerabilities. 

# 

#echo "1" > /proc/sys/net/ipv4/ip_masq_udp_dloose 

# Specify your Static IP address here. 

# 

# If you have a DYNAMIC IP address, you need to make this ruleset recognize 

# your IP address everytime you get a new IP. To do this, enable the 

# following one−line script. (Please note that the different single and 

# double quote characters MATTER). 

# 

# 

# DHCP users (Cablemodem and DSL ) users: 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

# If you get your TCP/IP address via DHCP, **you will need ** to enable the 

# #ed out command below underneath the PPP section AND replace the word 

# "ppp0" with the name of your EXTERNAL Internet connection (eth0, eth1, etc) 

# on the lines for "ppp−ip" and "EXTIP". 

# 






# 

# 


# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

# 



# all NON−English Linux distributions. 


# 

# Make sure the EXTIF variable above is set to reflect the name 

# of your Internet connection 

# 

EXTIP="`$IFCONFIG $EXTIF | $AWK \ 

/$EXTIF/'{next}//{split($0,a,":");split(a[2],a," ");print a[1];exit}'`" 


# 



# 60 sec timeout for UDP traffic (MASQ'ed ICQ users must enable a 30sec 

# firewall timeout in ICQ itself) 

# 

$IPCHAINS −M −S 7200 10 60 

############################################################################# 

# Incoming, flush and set default policy of reject. Actually the default policy 

# is irrelevant because there is a catch all rule with deny and log. 

# 


$IPCHAINS −P input REJECT 


# 

$IPCHAINS −A input −i $INTIF −s $INTNET −d 0.0.0.0/0 −j ACCEPT 


# 

$IPCHAINS −A input −i $EXTIF −s $INTNET −d 0.0.0.0/0 −l −j REJECT 


# 




# 




# 

# 

#$IPCHAINS −A input −i $EXTIF −s 0.0.0.0/0 −d $EXTIP/32 −j ACCEPT 


# 

$IPCHAINS −A input −i lo −s 0.0.0.0/0 −d 0.0.0.0/0 −j ACCEPT 

# −−−−− Begin OPTIONAL INPUT Section −−−−− 

# 


# HTTPd − Enable the following lines if you either run a WWW server on 

# the IPMASQ server −OR− plan on PORTFW'ing HTTP traffic to 

# an internal WWW server 

# 

#$IPCHAINS −A input −i $EXTIF −p tcp −s 0.0.0.0/0 −d $EXTIP 80 −j ACCEPT 


# 

# −−−−− End OPTIONAL INPUT Section −−−−− 

# catch all rule, all other incoming is denied and logged. pity there is no 

# log option on the policy but this does the job instead. 

# 

$IPCHAINS −A input −s 0.0.0.0/0 −d 0.0.0.0/0 −l −j REJECT 

############################################################################# 

# Outgoing, flush and set default policy of reject. Actually the default policy 


# 


$IPCHAINS −P output REJECT 


# 

$IPCHAINS −A output −i $INTIF −s $INTIP −d $INTNET −j ACCEPT 


# 

$IPCHAINS −A output −i $EXTIF −s 0.0.0.0/0 −d $INTNET −l −j REJECT 

# outgoing from local net on remote interface, stuffed masquerading, deny 

# 

$IPCHAINS −A output −i $EXTIF −s $INTNET −d 0.0.0.0/0 −l −j REJECT 


# 

$IPCHAINS −A output −i $EXTIF −s $EXTIP/32 −d 0.0.0.0/0 −j ACCEPT 


# 

$IPCHAINS −A output −i lo −s 0.0.0.0/0 −d 0.0.0.0/0 −j ACCEPT 

# −−−−− Begin OPTIONAL OUTPUT Section −−−−− 

# 

# HTTPd − Enable the following lines if you either run a WWW server on 

# the IPMASQ server −OR− plan on PORTFW'ing HTTP traffic to 

# an internal WWW server 

# 

#$IPCHAINS −A output −i $EXTIF −p tcp −s $EXTIP 80 −d 0.0.0.0/0 −j ACCEPT 

# 

# −−−−− End OPTIONAL OUTPUT Section −−−−− 


# catch all rule, all other outgoing is denied and logged. pity there is no 


# 

$IPCHAINS −A output −s 0.0.0.0/0 −d 0.0.0.0/0 −l −j REJECT 

############################################################################# 

# Forwarding, flush and set default policy of deny. Actually the default policy 


# 




# −−−−− Begin OPTIONAL FORWARD Section −−−−− 

# 

# Put PORTFW commands here 

# 

# −−−−− End OPTIONAL FORWARD Section −−−−− 

# Masquerade from local net on local interface to anywhere. 

# 

$IPCHAINS −A forward −i $EXTIF −s $INTNET −d 0.0.0.0/0 −j MASQ 

# 

# catch all rule, all other forwarding is denied and logged. pity there is no 


# 

$IPCHAINS −A forward −s 0.0.0.0/0 −d 0.0.0.0/0 −l −j REJECT 

#End of file. 

 


section for full details. Please make sure you make the correct "rc.firewall−ipchains" to 

"rc.firewall−ipchains−stronger" substitutions!! 

With IPCHAINS, you can block traffic to a particular site using the "input", "output", and/or "forward" rules. 

Remember that the set of rules are scanned from top to bottom and "−A" tells IPCHIANS to "append" this 

new rule to the existing set of rules. So with this in mind, any specific restrictions need to come before any 

global rules. For example: 

Using "input" rules: 

Probably the fastest and most efficient method to block traffic, but this method only stops the MASQed 

machines and NOT the firewall machine itself. Of course, you might want to allow that combination. 

Anyway, to block 204.50.10.13: 

In the /etc/rc.d/rc.firewall−ipchains−stronger ruleset: 

... start of "input" rules ... 

# reject and log local interface, local machines going to 204.50.10.13 

# 

ipchains −A input −s 192.168.0.0/24 −d 204.50.10.13/32 −l −j REJECT 


# 

ipchains −A input −s 192.168.0.0/24 −d 0.0.0.0/0 −l −j ACCEPT 

... end of "input" rules ... 

Using "output" rules: 


This is the slower method to block traffic because the packets must go through masquerading before they are 

dropped. Yet, this rule even stops the firewall machine from accessing the forbidden site. 

... start of "output" rules ... # reject and log outgoing to 204.50.10.13 # ipchains −A output −s $ppp_ip/32 −d 

204.50.10.13/32 −l −j REJECT # anything else outgoing on remote interface is valid # ipchains −A output −s 


$ppp_ip/32 −d 0.0.0.0/0 −l −j ACCEPT ... end of "output" rules ... 

Using "forward" rules: 

Probably slower than "input" rules for blocking traffic, this only stops masqueraded machines (e.g. internal 

machines). The firewall machine can still reach forbidden site(s). 

... start of "forward" rules ... # Reject and log from local net on PPP interface to 204.50.10.13. # ipchains −A 

forward −i ppp0 −s 192.168.0.0/24 −d 204.50.10.13/32 −l −j REJECT # Masquerade from local net on local 

interface to anywhere. # ipchains −A forward −i ppp0 −s 192.168.0.0/24 −d 0.0.0.0/0 −j MASQ ... end of 

"forward" rules ... 

No need for a special rule to allow machines on the 192.168.0.0/24 network to go to 204.50.11.0. Why? It is 

already covered by the global MASQ rule. 

NOTE: Unlike IPFWADM, IPCHIANS has only one way of coding the interfaces name. IPCHAINS uses the 

"−i eth0" option where as IPFWADM had both "−W" for the interface name and "−V" for the interface's IP 

address. 

6.4.3. Stronger IP Firewall (IPFWADM) Rulesets 

This section provides a more in−depth guide on using the 2.0.x firewall tool, IPFWADM. See below for 

IPCHAINS rulesets 

This example is for a firewall/masquerade system behind a PPP link with a static PPP address (dynamic PPP 

instructions are included but disabled). The trusted interface is 192.168.0.1 and the PPP interface IP address 

has been changed to protect the guilty :). I have listed each incoming and outgoing interface individually to 

catch IP spoofing as well as stuffed routing and/or masquerading. Anything not explicitly allowed is 

FORBIDDEN (well.. rejected, actually). If your IP MASQ box breaks after implementing this 

rc.firewall−ipfwadm−stronger script, be sure that you edit it for your configuration and check your 

/var/log/messages or /var/adm/messages SYSLOG file for any firewall errors. 

For more comprehensive examples of a strong IP Masqueraded IPFWADM rulesets for PPP, Cablemodem 

users, etc., please see TrinityOS − Section 10 and GreatCircle's Firewall WWW page 






Please also be aware that there are several GUI Firewall creation tools available as well. Please see 

Chapter 7for full details. 

Lastly, if you are using a STATIC PPP IP address, change the "ppp_ip="your.static.PPP.address"" line to 

reflect your address. 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

 



#!/bin/sh 

# 

# /etc/rc.d/rc.firewall−ipfwadm−stronger: An example of a semi−STRONG 

# IPFWADM firewall ruleset for 2.0 kernels 

# 

FWVER=0.74s 

# 

# Log: 



# 

# 0.73s − renamed from rc.firewall−2.0−stronger to 

# rc.firewall−ipfwadm−stronger 

# 

# 0.72s − #ed out the rule that would allow all traffic destined for the 

# MASQ server itself to be accepted. Use the OPTIONAL INPUT 

# section to only allow explicit services. 

PATH=/sbin:/bin:/usr/sbin:/usr/bin 

# testing, wait a bit then clear all firewall rules. 

# uncomment the following lines if you want the firewall to automatically 

# disable after 10 minutes. 

# 

# Disabled by default 

# 

# (sleep 600; \ 

# ipfwadm −I −f; \ 

# ipfwadm −I −p accept; \ 

# ipfwadm −O −f; \ 

# ipfwadm −O −p accept; \ 

# ipfwadm −F −f; \ 

# ipfwadm −F −p accept; \ 

# ) & 


# 


# are shown below but are commented from loading. 


# 

/sbin/depmod −a 



# 

/sbin/modprobe ip_masq_ftp 




# 

#/sbin/modprobe ip_masq_raudio 


# 

#/sbin/modprobe ip_masq_irc 

# Supports the masquerading of Quake and QuakeWorld by default. This modules is 



# for multiple users behind the Linux MASQ server. If you are going to 


# 



# 


#/sbin/modprobe ip_masq_quake 

# 


#/sbin/modprobe ip_masq_quake 26000,27000,27910,27960 


# 

#/sbin/modprobe ip_masq_cuseeme 


# 

#/sbin/modprobe ip_masq_vdolive 

#CRITICAL: Enable IP forwarding, since it is disabled by default 

# 

# Redhat Users: you may try changing the options in /etc/sysconfig/network 

# from: 

# 


# to 


# 



# in 2.2.x kernels 

# 

# This used to be a compile−time option but the behavior was changed 

# in 2.2.12 

# 



# 


# following option. This allows dynamic−ip address hacking in IP MASQ, 

# making the life with Diald and similar programs much easier. 

# 


# Specify your Static IP address here. 

# 

# If you have a DYNAMIC IP address, you need to make this ruleset understand 

# your IP address everytime you get a new IP. To do this, enable the 

# following one−line script. (Please note that the different single and 

# double quote characters MATTER). 

# 

# 

# DHCP (Cablemodem and DSL) and PPP users: 

# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 


# If you get your TCP/IP address a dynamic IP address **you will need ** to 

# enable the #ed out command below underneath the PPP section AND replace the word 

# "ppp0" with the name of your EXTERNAL Internet connection (eth0, eth1, 

# etc). 

# 






# 

# 

# PPP and DHCP Users: 

# −−−−−−−−−−−−−−−−−−− 

# Remove the # on the line below and place a # in front of the line after that. 

# 

#ppp_ip="`/sbin/ifconfig ppp0 | grep 'inet addr' | awk '{print $2}' | sed −e 's/.*://'`" 

# 

ppp_ip="your.static.PPP.address" 


# 





# 


############################################################################# 

# Incoming, flush and set default policy of reject. Actually the default policy 


# 

/sbin/ipfwadm −I −f 

/sbin/ipfwadm −I −p reject 


# 

/sbin/ipfwadm −I −a accept −V 192.168.0.1 −S 192.168.0.0/24 −D 0.0.0.0/0 


# 

/sbin/ipfwadm −I −a reject −V $ppp_ip −S 192.168.0.0/24 −D 0.0.0.0/0 −o 


# 




# 




# 

# 

#/sbin/ipfwadm −I −a accept −V $ppp_ip −S 0.0.0.0/0 −D $ppp_ip/32 





# 

/sbin/ipfwadm −I −a accept −V 127.0.0.1 −S 0.0.0.0/0 −D 0.0.0.0/0 

# catch all rule, all other incoming is denied and logged. pity there is no 


# 

/sbin/ipfwadm −I −a reject −S 0.0.0.0/0 −D 0.0.0.0/0 −o 

############################################################################# 

# Outgoing, flush and set default policy of reject. Actually the default policy 


# 

/sbin/ipfwadm −O −f 

/sbin/ipfwadm −O −p reject 


# 

/sbin/ipfwadm −O −a accept −V 192.168.0.1 −S 0.0.0.0/0 −D 192.168.0.0/24 


# 

/sbin/ipfwadm −O −a reject −V $ppp_ip −S 0.0.0.0/0 −D 192.168.0.0/24 −o 


# 



# 



# 

/sbin/ipfwadm −O −a accept −V $ppp_ip −S $ppp_ip/32 −D 0.0.0.0/0 


# 

/sbin/ipfwadm −O −a accept −V 127.0.0.1 −S 0.0.0.0/0 −D 0.0.0.0/0 

# catch all rule, all other outgoing is denied and logged. pity there is no 


# 

/sbin/ipfwadm −O −a reject −S 0.0.0.0/0 −D 0.0.0.0/0 −o 

############################################################################# 

# Forwarding, flush and set default policy of deny. Actually the default policy 


# 

/sbin/ipfwadm −F −f 

/sbin/ipfwadm −F −p reject 

# Masquerade from local net on local interface to anywhere. 

# 

/sbin/ipfwadm −F −a masquerade −W ppp0 −S 192.168.0.0/24 −D 0.0.0.0/0 

# 

# catch all rule, all other forwarding is denied and logged. Pity there is no 


# 

/sbin/ipfwadm −F −a reject −S 0.0.0.0/0 −D 0.0.0.0/0 −o 


#End of file. 

 


section for full details. Please make sure you make the correct "rc.firewall−ipfwadm" to 

"rc.firewall−ipfwadm−stronger" substitutions!! 

With IPFWADM, you can block traffic to a particular site using the −I, −O or −F rules. Remember that the set 

of rules are scanned top to bottom and "−a" tells IPFWADM to "append" this new rule to the existing set of 

rules. So with this in mind, any specific restrictions need to come before global rules. For example: 

Using −I (input ) rules: 

Probably the fastest and most efficient method to block traffic but it only stops the MASQed machines, and 

NOT the the firewall machine itself. Of course, you might want to allow that combination. 

Anyway, to block 204.50.10.13: 

In the /etc/rc.d/rc.firewall−ipfwadm−stronger ruleset: ... start of −I rules ... # reject and log local interface, 

local machines going to 204.50.10.13 # /sbin/ipfwadm −I −a reject −V 192.168.0.1 −S 192.168.0.0/24 −D 

204.50.10.13/32 −o # local interface, local machines, going anywhere is valid # /sbin/ipfwadm −I −a accept 

−V 192.168.0.1 −S 192.168.0.0/24 −D 0.0.0.0/0 ... end of −I rules ... 

Using −O (output) rules: 

This is the slower method to block traffic because the packets go through masquerading first before they are 

dropped. Yet, this rule even stops the firewall machine from accessing the forbidden site. 

... start of −O rules ... # reject and log outgoing to 204.50.10.13 # /sbin/ipfwadm −O −a reject −V $ppp_ip −S 

$ppp_ip/32 −D 204.50.10.13/32 −o # anything else outgoing on remote interface is valid # /sbin/ipfwadm −O 

−a accept −V $ppp_ip −S $ppp_ip/32 −D 0.0.0.0/0 ... end of −O rules ... 

Using −F (forward) rules: 


Probably slower than −I (input) rules for blocking traffic, this still only stops masqueraded machines (e.g. 

internal machines). The firewall machine can still reach forbidden site(s). 

... start of −F rules ... # Reject and log from local net on PPP interface to 204.50.10.13. # /sbin/ipfwadm −F −a 

reject −W ppp0 −S 192.168.0.0/24 −D 204.50.10.13/32 −o # Masquerade from local net on local interface to 

anywhere. # /sbin/ipfwadm −F −a masquerade −W ppp0 −S 192.168.0.0/24 −D 0.0.0.0/0 ... end of −F rules ... 

There is no need for a special rule to allow machines on the 192.168.0.0/24 network to go to 204.50.11.0. 

Why? It is already covered by the global MASQ rule. 

NOTE: There is more than one way of coding the interfaces in the above rules. For example instead of "−V 

192.168.255.1" you can code "−W eth0", instead of "−V $ppp_ip" , you can use "−W ppp0". The "−V" 

method was phased out with the imgration to IPCHAINS, but for IPFWADM users, its more of a personal 

choice and documentation. 


6.5. IP Masquerading multiple internal networks 

Masquerading more than one internal network is fairly simple. You need to first make sure that all of your 

networks are running correctly (both internal and external). You then need to enable traffic to pass to both the 

other internal interfaces and to be MASQed to the Internet. 

Next, you need to enable Masquerading on the INTERNAL interfaces. This example uses a total of THREE 

interfaces: EXTIF stands for the eth0 interface which is the EXTERNAL connection to the Internet. INTIF 

stands for the eth1 interface and is the 192.168.0.0 network. Finally, INTIF2 stands for the eth2 interface and 

is the 192.168.1.0 network. Both INTIF and INTIF2 will be MASQed out of interface eth0 or EXTIF. In your 

rc.firewall−* ruleset next to the existing MASQ at the very end of the ruleset, add the following: 

6.5.1. iptables support for multiple internal lans 

• 

# 2.6.x and 2.4.x kernels with IPTABLES 

# 

# The following rules build upon the rc.firewall−iptables−stronger ruleset. 

# Please see that ruleset in Section 6 for how all variables get set, etc. 

#Enable internal interfaces to communication between each other 

# 

$IPTABLES −A FORWARD −i $EXTIF −o $INTIF2 −m state −−state ESTABLISHED,RELATED \ 

−j ACCEPT 

$IPTABLES −A FORWARD −i $INTIF −o $INTIF2 −m state −−state ESTABLISHED,RELATED \ 

−j ACCEPT 

$IPTABLES −A FORWARD −i $INTIF2 −o $INTIF −m state −−state ESTABLISHED,RELATED \ 

−j ACCEPT 

$IPTABLES −t nat −A POSTROUTING −o $EXTIF −j SNAT −−to $EXTIP 

6.5.2. ipchains support for multiple internal lans 

• 


# 2.2.x kernels with IPCHAINS 

# 

# The following rules build upon the rc.firewall−ipchains−stronger ruleset. 



$IPCHAINS −A forward −i eth1 −d 192.168.0.0/24 −j ACCEPT 

$IPCHAINS −A forward −i eth2 −d 192.168.1.0/24 −j ACCEPT 

#Enable internal interfaces to MASQ out to the Internet 

$IPCHAINS −A forward −j MASQ −i eth0 −s 192.168.0.0/24 −d 0.0.0.0/0 

$IPCHAINS −A forward −j MASQ −i eth0 −s 192.168.1.0/24 −d 0.0.0.0/0 

6.5.3. ipfwadm support for multiple internal lans 

• # 2.0.x kernels with IPFWADM 

# 

# The following rules build upon the rc.firewall−ipfwadm−stronger ruleset. 





/sbin/ipfwadm −F −a accept −V 192.168.0.1 −D 192.168.1.0/24 

/sbin/ipfwadm −F −a accept −V 192.168.1.1 −D 192.168.0.0/24 

#Enable internal interfaces to MASQ out to the Internet 

/sbin/ipfwadm −F −a masq −W eth0 −S 192.168.0.0/24 −D 0.0.0.0/0 

/sbin/ipfwadm −F −a masq −W eth0 −S 192.168.1.0/24 −D 0.0.0.0/0 

Please note that it is CORRECT to have "eth0" specified multiple times for the exmples shown above. The 

reason for this is the Linux kernel needs to know which interface is used for OUTGOING traffic. Since eth0 

in the above examples is the Internet connection, it is listed for each internal interface. 

6.6. IP Masquerade and Dial−on−Demand Connections 

1. If you would like to setup your network to automatically dial up the Internet, either the Diald demand 

dial−up or new versions of the PPPd packages will be of great utility. Both Diald and PPPd are very 

powerful in their configuration flexibility. 

2. To setup PPPd for Dial−on−Demand, pease check out the PPPd Homepage 

3. To setup Diald, please check out the Diald Homepage or TrinityOS − Section 23 

4. Once Dial on Demand and IP Masq have been setup properly, any MASQed client machines that 

initiate a web, telnet or ftp session will make the Linux box dynamically bring up its Internet link. 

5. There is a timeout that will occur with the first connection. This is inevitable if you are using analog 

modems. The time taken to establish the modem link and the PPP connections may cause your client 

program (WWW browser, etc.) to stop. This isn't common though. If this does happen, just retry that 

Internet traffic request (say a WWW page) again and it should come up fine. You can also try setting 

echo "1" > /proc/sys/net/ipv4/ip_dynaddr kernel option to help with this initial setup. 

6.7. Port Forwarding with IPTABLES or external tools like 

IPPORTFW, IPMASQADM, IPAUTOFW, REDIR, UDPRED, 

and other Port Forwarding tools 

IPTABLES as well as IPPORTFW, IPAUTOFW, REDIR, UDPRED, and other programs offer generic TCP 

and/or UDP port forwarding for Linux IP Masquerade. These tools are typically used with or as a replacement 

for specific IP MASQ modules to get a specific network traffic through the MASQ server. 

With port forwarders, you can redirect data connections from the Internet to an internal, privately addressed 

machine behind your IP MASQ server. This forwarding ability includes network protocols such as TELNET, 

WWW, and SMTP. Protocols such as FTP, legacy ICQ, and others require special handling via kernel 

modules (see below). 

NOTE: If you are just looking to do simple port forwarding but you don't need Masquerading support, you 

don't have any choice. You will STILL NEED to enable IP Masquerading support in the kernel AND either 

run a IPTABLES, IPCHAINS, or IPFWADM ruleset to be able to use Linux's port forwarding tools. 

So why all the different choices? IPTABLES, IPPORTFW, MARK (MFW), IPMASQADM (PORTFW or 

AUTOFW), IPAUTOFW, REDIR, and UDPRED (all URLs are in Section 3.2.3) are the various tools 

available to IP MASQ users to allow this type of functionality depending on their kernel version. Later, as the 

Linux IP Masquerade feature matured, many of these tools were eventually replaced by the IPTABLES or 



PORTFW and MARK systems which are far more intelligent solutions. 

For the later 2.2.x kernels, the IPMASQADM tool combined the legacy IPAUTOFW and IPPORTFW 2.0.x 

kernel tools into one binary. Both the IPMASQADM tool for 2.2.x kernels as well as IPTABLES for the 2.6.x 

and 2.4.x kernels also supports a new mechanism called "MARK" or MFW for PORTFW functionality. The 

MARK system works where a specific IPTABLES or IPCHAINS ruleset would match a given packet 

sequence. Once matched, the tool would "mark" these packets. Later, the IPMASQADM tool or a specific 

IPTABLE "table" could be instructed to change these packets as needed and forward them off to their desired 

internal destination. Currently, this HOWTO doesn't cover the MARK solution but it will in the future. 

Anyway, because of the availablity of the newer tools, it is *HIGHLY DISCOURAGED* to use the old tools 

such as IPAUTOFW (even AUTOFW in IPMASQADM) and REDIR because they don't properly notify the 

Linux kernel of their presence and can ultimately CRASH your Linux server with extreme use. 

NOTE #2: With enabling PORTFW functionality in ANY 2.2/2.0 Linux kernel (2.6.x. and 2.4.x users won't 

use these specific tools anyway), internal machines typically CANNOT use the same "external" 

PORTFWed IP address to access a given internal" machine. To put it another way, PORTFW was only 

intended to be used with "external" computers on the Internet. If this is an issue for you, you can also use the 

REDIR tool for older 2.2.x and 2.0.x kernels to let internal machines get redirected to the internal servers too. 

2.6.x and 2.4.x kernels users running IPTABLES solves this issue once and for all and is fully covered in a 

FAQ entry in Section 7.19 below. If you would like a technical explination on why this internal/external 

forwarding doesn't work, please page down towards the bottom of the 2.2.x PORTFW section for a note from 

Juan Jose Ciarlante. 

NOTE #3: The forwarding of non−NAT friendly traffic such as FTP server traffic to an internal MASQed 

FTP server, known as PORTFW FTP, is now fully supported in the 2.6.x and 2.4.x kernels as well as in the 

2.2.x kernels via a BETA version FTP kernel module (does NOT come with the stock Linus kernels). It 

should also be noted that you can also PORTFW FTP traffic using an external FTP proxy program (not 

covered in this HOWTO). It should be noted that the Beta 2.2.x FTP kernel module code is still experimental 

and some people get better results simply using ACTIVE FTP sessions compared to PASSIVE connections. 

Interestingly enough, other people have seen the exact opposite behavior. Please let us know what your results 

are like. More about this is covered below in both the 2.2.x and 2.0.x sections as the solutions require the use 

of different patches. 

WARNING! Before jumping right into installing ANY of these tools, it needs to be mentioned that network 

security can be an issue with ANY PORT FORWARD tool. The reason for this is because these tools basically 

create a hole in strong packet firewalls for the required TCP/UDP ports. Though this doesn't pose any threat to 

your Linux machine, it might be an issue to the PORTFW'ed internal machine(s). No worries though, this is 

what Steven Clarke (the author of IPPORTFW) had to say about that: 

"Port Forwarding is only called within masquerading functions so it 

fits inside the same IPFWADM/IPCHAINS rules. Masquerading is an extension to 

IP forwarding. Therefore, ipportfw only sees a packet if it fits 

both the input and masquerading ipfwadm rule sets." 

What that means in English is that if you have a strong packet firewall running, PORTFW doesn't directly 

bypass any of that security. You will still be able to allow or deny specific IPs and/or domains to this new 

PORTFW'ed resource if you so wish. 

With this said, it's important to have a strong firewall ruleset. Please see Section 6.4.1, Section 6.4.2, and 

Section 6.4.3 for more details on getting strong rulesets. 


• 2.6.x and 2.4.x kernels have PORTFW functional already built in using the IPTABLES tool. 2.2.x and 

2.0.x kernel kernel users will need to re−compile the Linux kernel to support PORTFW. It should be 

noted that some Linux distribution kernels might have this already done for you. 

• The latest 2.2.x kernel users will already have the PORTFW kernel option available to them though 

you might still need to recompile the kernel via the normal kernel "make" procedures. 

• 2.0.x users will need to apply a simple kernel option patch to have access to then enable this via the 

normal kernel "make" procedures. 

6.7.1. IPTABLES−based PORTFWD'ing: Using IPTABLES's PREROUTING 

option for 2.6.x and 2.4.x kernels 

As mentioned before, a port forward or "PORTFW" allows a single or range of TCP/IP ports from the 

external side of the network to be forwarded into the inside network. 

Unlike ALL previous Linux kernels, the 2.6.x and 2.4.x series kernels now allows for full PORTFW, 

PORTFW FTP, and PORTFW REDIR functionality within the "iptables" tool itself. 

NOTE: Once you enable a port forwarder on say port 80 (forward WWW traffic through the MASQ server to 

an internal WWW server), that port will no longer be used by the Linux IP Masquerade server itself. To be 

more specific, if you have a WWW server already running on the MASQ server, enabling PORTFW will now 

give all Internet users acces to the WWW pages from the −INTERNAL− WWW server and not the pages on 

your IP MASQ server. 

To enable port forwarding on an IPTABLES (2.6.x or 2.4.x kernel): 

• Edit the /etc/rc.d/rc.firewall−iptables ruleset and place the lines shown below just ABOVE the "FWD: 

Allow all connections OUT and only existing and related ones IN" line 

(in the "Optional FORWARD section"). Please be sure to replace the word "$EXTIP" with your 

specific Internet IP address. 

• NOTE: Unlike the 2.2.x and 2.0.x kernels, PORTFWed traffic does *not* traverse the INPUT or 

OUTPUT rules. It only traverses the FORWARD rule. 

• NOTE: If you get a dynamically assigned TCP/IP address from your ISP (PPP, DSL, Cablemodems, 

etc.), you CANNOT load this strong ruleset upon booting. You will either need to reload this firewall 

ruleset EVERY TIME you get a new IP address or make your /etc/rc.d/rc.firewall−ipchains−stronger 

ruleset more intelligent. To do this for various types of connections such as PPP or DHCP users, 

please see the Section 7.8 FAQ entry for all the details. 

/etc/rc.d/rc.firewall−* 


#echo "Enabling PORTFW Redirection on the external LAN.." 

# 

# This will forward ALL port 80 traffic from the external IP address 

# to port 80 on the 192.168.0.10 machine 

# 

# Be SURE that when you add these new rules to your rc.firewall−*, you 

# add them before a direct or implict DROP or REJECT. 

# 

PORTFWIP="192.168.0.10" 

# NOTE: If you are using the basic rc.firewall−iptables ruleset, you 



# will need to enable the following EXTIP option. Users of the 

# rc.firewall−iptables−stronger ruleset already have this defined. 

# 

# *PLEASE* look over the rc.firewall−iptables−stronger ruleset for more 

# specific issues regarding dynamic vs. static IP addresses 

# 

# 


# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

# 



# all NON−English Linux distributions: 

# 

# DISABLED by default −− to enable, REMOVE both the "#" characters below 

# 

#EXTIP="`$IFCONFIG $EXTIF | $AWK \ 

#/$EXTIF/'{next}//{split($0,a,":");split(a[2],a," ");print a[1];exit}'`" 

# Allow forwarding of new and existing port 80 connections from the external 

# interface. This rule is required as our default FORWARD policy is DENY. 

# 

$IPTABLES −A FORWARD −i $EXTIF −o $INTIF −p tcp −−dport 80 −m state \ 

−−state NEW,ESTABLISHED,RELATED −j ACCEPT 

#Enable PORTFW of this port 80 traffic from the external interface 

# 

$IPTABLES −A PREROUTING −t nat −p tcp −d $EXTIP −−dport 80 −m state \ 

−−state NEW,ESTABLISHED,RELATED −j DNAT −−to $PORTFWIP:80 

That's it! Just re−run your /etc/rc.d/rc.firewall−iptables ruleset and test it out! If you would like to learn more 

about this, please see the Section 7.19 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

Running the rc.firewall−iptables−stronger ruleset? Good for you! To get PORTFW running with this ruleset, 

it's very easy. The following example is for HTTP (WWW) traffic to be PORTFWed to the IP address 

indicated by the $PORTFWIP variable: 

• Take the ruleset lines shown above (for the generic rc.firewall−* ruleset) and place them just *after* 

the "#FORWARD Enable Forwarding and thus IPMASQ" comment lines. It should also be 

noted that you DO NOT have to enable the "HTTPD" rules under the "Optional 'INPUT' section" for 

2.4−based kernels. With 2.4 kernels, those lines are ONLY used if you want to allow HTTP traffic to 

terminate on the MASQ server's own local httpd process (usually Apache). 

PORTFW FTP: If you have the "ip_conntrack_ftp" and "ip_nat_ftp" kernel modules loaded into kernel space 

(as already done in the rc.firewall−iptables script), the simple PREROUTING command like the one shown 

above changed for for port "21" should do the trick. This is much easier than the configuration for the older 

IPCHAINS / IPFWADM tools for the 2.2.x / 2.0.x kernels! 

Please note, if you setup PORTFW to redirect traffic to an internal FTP server that is running on a 

NON−standard FTP port, say port 8021 instead of the usual port 21, you MUST tell the "ip_conntrack_ftp" 

module to be aware of the new FTP port. The reason for this configuration change is that FTP is not a 

NAT−friendly protocol. By telling the FTP NAT module about this non−standard FTP port, the NAT module 

and do it's job again. To enable this, edit your rc.firewall−* file and change the loading of the FTP module to 


look something like this: 

/sbin/insmod ip_conntrack_ftp ports=21,8021 

PORTFW Redirection of Internal requests: 

In the past, if users PORTFWed port 80 on their EXTERNAL IP address to some internal machine, only the 

machines out on the Internet would be able to properly reach this internal WWW server. If you tried to contact 

this internal WWW server via the MASQ server's EXTERNAL address, it would fail. Fortunately, there are 

workarounds for all Linux kernels. IPTABLES for the 2.6.x and 2.4.x kernels supports this functionality 

natively. IPCHAINS support for the 2.2.x kernels and IPFWADM support for the 2.0.x kernels need to use the 

external REDIR tool (not currently covered in the HOWTO). For those of you who understand IPTABLES 

fairly well, the rule of thumb is a PREROUTING/DNAT rule is for enabling PORTFW from *external* 

machines and a POSTROUTING/SNAT rule is for enabling PORTFW from *internal* machines. 

To support redirection like this from an internal host, add a rule like the one shown below ABOVE the "Catch 

all" FORWARDing rule in the rc.firewall−* file. The following example will REDIRECT all external as well 

as internal WWW traffic to the internal machine noted as PORTFWIP (192.168.0.10). This traffic will have 

the source IP address of the IP Masq server's internal IP address. Please change the IP address of the 

PORTFWIP variable to reflect your specific configuration: 

#The following rule should be configured in *addition* to the above rule 

# for enabling external to internal PORTFWing 

$IPTABLES −t nat −A POSTROUTING −d $PORTFWIP −s $INTLAN −p tcp \ 

−−dport 80 −m state −−state NEW,ESTABLISHED,RELATED −j SNAT \ 

−−to $INTIP 

6.7.2. IPMASQADM−based PORTFWD'ing: Using IPMASQADM with 2.2.x 

kernels 

First, make sure you have the newest 2.2.x kernel uncompressed into /usr/src/kernel/linux. If you haven't 

already done this, please see Section 3.2.2 section for full details. Next, download the "ipmasqadm.c" program 

from Section 2.7 into the /usr/src/kernel directory. 

Next, you'll need to compile the 2.2.x kernel as shown in Section 3.2.2 section. Be sure to say "YES" to the 

IPPORTFW option when you configure the kernel. Once the kernel compile is complete and you have 

rebooted, return to this section. 

Now, compile and install the IPMASQADM tool: 


cd /usr/src 

tar xzvf ipmasqadm−x.tgz 

cd ipmasqadm−x 

make 

make install 

Now, for this example, we are going to allow ALL WWW Internet traffic (port 80) hitting your Internet 

TCP/IP address to be forwarded to the internal Masqueraded machine at IP address 192.168.0.10. 

PORTFW FTP: As mentioned above, there are two solutions for forwarding FTP server traffic to an internal 

MASQed PC. The first solution *IS* a BETA level IP_MASQ_FTP module for 2.2.x kernels to PORT 

Forward FTP connections to an internal MASQed FTP server. The other method is using a FTP proxy 

program (the URL is in Section 2.7. It should also be noted that the FTP kernel module also supports the 


adding of additional PORTFW FTP ports on the fly without the requirement of unloading and reloading the 

IP_MASQ_FTP module and thus breaking any existing FTP transfers. You can find more about this new code 

at the IPMASQ WWW site at http://ipmasq.webhop.net;. There are also examples and some additional 

information about PORTFWed FTP connection below in the 2.0.x. kernel section. 

NOTE: Once you enable a port forwarder on port 80, that port can no longer be used by the Linux IP 

Masquerade server. To be more specific, if you have a WWW server already running on the MASQ server, a 

port forward will now give all Internet users the WWW pages from the −INTERNAL− WWW server and not 

the pages on your IP MASQ server. 

Anyway, to enable port forwarding for HTTPd: 

• Edit the /etc/rc.d/rc.firewall−* ruleset and ENABLE the "Optional" "HTTP" sections in both the 

INPUT and OUTPUT subsections. 

• Add the following lines shown below JUST BELOW the "ipchains −P forward DENY" rule 

(in the "Optional FORWARD section"). Be sure to replace the "$EXTIP" variable's contents with 

your EXTERNAL Internet IP address on the IPMASQ server. 







#echo "Enabling IPPORTFW Redirection on the external LAN.." 

# 



# 

PORTFWIP="192.168.0.10" 

/usr/sbin/ipmasqadm portfw −f 

/usr/sbin/ipmasqadm portfw −a −P tcp −L $EXTIP 80 −R $PORTFWIP 80 

That's it! Just re−run your /etc/rc.d/rc.firewall−* ruleset and test it out! 

If you get the error message "ipchains: setsockopt failed: Protocol not available", you AREN'T running your 

new IPPORTFW enabled kernel. Make sure that you moved the new kernel over, re−run LILO, and then 

reboot again. If you are sure you are running your new kernel, run the command "ls /proc/net/ip_masq" and 

make sure the "portfw" file exists. If it doesn't, you must have made an error when configuring your kernel. 

Try again. 


It should be mention that this IPMASQ HOWTO currently does *NOT* provide any explination or examples 

on how to use the REDIR tool. If you need help setting it up for 2.2.x kernels, send me an email. For those 

who want to understand why PORTFW cannot redirect traffic for both external and internal interfaces (what 

the REDIR tool fixes), here is an email from Juanjo that better explains it. 

From Juanjo Ciarlante 

−− 



If I use: 

> 

> ipmasqadm portfw −a −P tcp −L 1.2.3.4 80 −R 192.168.2.3 80 

> 

>Everything works great from the outside but internal requests for the same 

>1.2.3.4 address fail. Are there chains that will allow a machine on localnet 

>192.168.2.0 to accesss www.periapt.com without using a proxy? 

Actually not. 

I usually setup a ipmasqadm rule for outside, *AND* a port 

redirector for inside. This works because ipmasqadm hooks before 

redir will get the eventual outside connection, _but_ leaves things 

ok if not (stated by APPROPIATE rules). 

The actual "conceptual" problem comes from the TRUE client (peer) IP 

goal (thanks to masq) being in the same net as target server. 

The failing scenario for "local masq" is : 

client: 192.168.2.100 

masq: 192.168.2.1 

serv: 192.168.2.10 

1)client−>server packet 

a) client: 192.168.2.100:1025 −> 192.168.2.1:80 [SYN] 

b) (masq): 192.168.2.100:1025 −> 192.168.2.10:80 [SYN] 

(and keep 192.168.2.1:61000 192.168.2.100:1025 related) 

c) serv: gets masqed packet (1b) 

2)server−>client packet 

a) serv: 192.168.2.10:80 −> 192.168.2.100:1025 [SYN,ACK] 

b) client: 192.168.2.100:1025 −> 192.168.2.10:80 [RST] 

Now take a moment to compare (1a) with (2a). 

You see, the server replied DIRECTLY to client bypassing masq (not 

letting masq to UNDO the packet hacking) because it is in SAME net, so 

the client resets the connection. 

hope I helped. 

Warm regards 

Juanjo 


6.7.3. IPPORTFW−based PORTFWD'ing: Using IPPORTFW on 2.0.x 

kernels 

First, make sure you have the newest 2.0.x kernel uncompressed into /usr/src/kernel. If you haven't already 

done this, please see Section 3.2.3 for full details. Next, download the "ipportfw.c" program and the 

"subs−patch−x.gz" kernel patch from Section 3.2.3 into the /usr/src/ directory. 

NOTE: Please replace the "x" in the "subs−patch−x.gz" file name with the most current version available on 

the site. 

Next, if you plan on port forwarding FTP traffic to an internal server, you will have to apply an additional 

NEW IP_MASQ_FTP module patch found in Section 3.2.3. More details regarding this are later in this 

section. Please note that this is NOT the same patch as for the 2.2.x kernels so some functionality such as the 

dynamic FTP PORT functionality is not present. 


Now, copy the IPPORTFW patch (subs−patch−x.gz) into the Linux directory 

cp /usr/src/subs−patch−1.37.gz /usr/src/linux 

Next, apply the kernel patch to create the IPPORTFW kernel option: 

cd /usr/src/linux 

zcat subs−patch−1.3x.gz | patch −p1 

Ok, time to compile the kernel as shown in Section 3.2.3. Be sure to say YES to the IPPORTFW option now 

available when you configure the kernel. Once the compile is complete and you have rebooted, return to this 

section. 

Now with a newly compiled kernel, please compile and install the actual "IPPORTFW" program 

cd /usr/src 

gcc ipportfw.c −o ipportfw 

mv ipportfw /usr/local/sbin 

Now, for this example, we are going to allow ALL WWW Internet traffic (port 80) hitting your Internet 

TCP/IP address to then be forwarded to the internal Masqueraded machine at IP address 192.168.0.10. 


Masquerade server. To be more specific, if you have a WWW server already running on the MASQ server 

and then you port forward port 80 to an internal MASQed computer, ALL internet users will see the WWW 

pages pages from the −INTERNAL− WWW server and not the pages on your IP MASQ server. This only 

performs a port forward to some other port, say 8080, to your internal MASQ machine. Though this will 

work, all Internet users will have to append :8080 to the URL to then contact the internal MASQed WWW 

server. 

Anyway, to enable port forwarding, edit the /etc/rc.d/rc.firewall−* ruleset. Add the follow lines but be sure to 

replace the word "$extip" with your Internet IP address. 








# 



# 

/usr/local/sbin/ipportfw −C 

/usr/local/sbin/ipportfw −A −t$extip/80 −R 192.168.0.10/80 


If you get the error message "ipfwadm: setsockopt failed: Protocol not available", you AREN'T running your 

new kernel. Make sure that you moved the new kernel over, re−run LILO, and then reboot again. 

Port Forwarding FTP servers: 



If you plan on port forwarding FTP to an internal machine, things get more complicated. The reason for this is 

because the standard IP_MASQ_FTP kernel module wasn't written for this though some users report that it 

works without any problems. Personally, without the patch, I've heard that extended file transfers in excess of 

30 minutes will fail without the patch while other users swear that it works flawlessly. Anyway, I recommend 

that you try the following PORTFW instruction with the STOCK ip_masq_ftp module and see if it works for 

you. If it doesn't, try using the modified ip_masq_ftp module. 

For those who need the module, Fred Viles wrote a modified IP_MASQ_FTP module to make things work. If 

you are curious what EXACTLY are the issues, download the following archive since Fred documents it quite 

well. Also understand that this patch is somewhat experimental and should be treated as such. It should be 

noted that this patch is ONLY available for the 2.0.x kernels though there is a different patch available for 

2.2.x kernels. 

So, to get the 2.0.x patch working, you need to: 

• FIRST, apply the IPPORTFW kernel patch as shown earlier in this section. 

• Download the "msqsrv−patch−36" from Fred Viles's FTP server in Section 3.2.3and put it into 

/usr/src/linux. 

• Patch the kernel with this new code by running "cat msqsrv−patch−36 | patch −p1" 

• Next, replace the original "ip_masq_ftp.c" kernel module with the new one 

mv /usr/src/linux/net/ipv4/ip_masq_ftp.c /usr/src/linux/net/ipv4/ip_masq_ftp.c.orig 

mv /usr/src/linux/ip_masq_ftp.c /usr/src/linux/net/ipv4/ip_masq_ftp.c 

• Lastly, build and install the kernel with this new code in place. 

Once this is complete, edit the /etc/rc.d/rc.firewall−* ruleset and add the following lines, but be sure to replace 

the word "$extip" with your Internet IP address. 

This example, like the one above, will allow ALL FTP Internet traffic (port 21) hitting your Internet TCP/IP 

address to then be forwarded to the internal Masqueraded machine at IP address 192.168.0.10. 


Masquerade server. To be more specific, if you have an FTP server already running on the MASQ server, a 

port forward will now give all Internet users the FTP files from the −INTERNAL− FTP server and not the 

files on your IP MASQ server. 




# 

/usr/local/sbin/ipportfw −C 

/usr/local/sbin/ipportfw −A −t$extip/21 −R 192.168.0.10/21 

#NOTE: If you are using multiple local port numbers to PORTFW 

# to multuple internal FTP servers (say, 21, 2121, 2112, 

# etc, you need to configure the ip_masq_ftp nodule to 

# listen to these ports. To do this, edit the 

# /etc/rc.d/rc.firewall−* script as shown in this HOWTO 

# to look like: 

# 

# /sbin/modprobe ip_masq_ftp ports=21,2121,2112 

# 


# Re−run the /etc/rc.d/rc.firewall−* script for these changes to 

# take effect. 

#Please note that PORTFWing port 20 is probably NOT required 

# for ACTIVE connections as the internal FTP server will 

# initiate this port 20 connection and it will be properly 

# handled by the classic MASQ mechanisms. 



It's not clear if the REDIR tool will compile or work against legacy 2.0.x kernels. It should also be mentioned 

that this IPMASQ HOWTO currently does *NOT* provide any explination or examples on how to use the 

REDIR tool. If you need help setting it up, send me an email. If you would like to learn more about REDIR, 

please see the above section for the 2.2.x kernels. 

6.8. CU−SeeMe and Linux IP−Masquerade 

Linux IP Masquerade supports CuSeeme via the "ip_masq_cuseeme" kernel module. This kernel modules 

should be loaded in the /etc/rc.d/rc.firewall−* script. Once the "ip_masq_cuseeme" module is installed, you 

should be able to both initiate and receive CuSeeme connections to remote reflectors and/or users. 

NOTE: It is recommended to use the IPPORTFW tool instead of the old IPAUTOFW tool for running 

CuSeeme. 

If you need more explicit information on configuring CuSeeme, see Michael Owings's CuSeeMe page for a 

Mini−HOWTO or The IP Masquerade Resources for a mirror of the Mini−HOWTO. 

6.9. Mirabilis ICQ 


ICQ, the instant messaging client now owned by AOL, has changed over the years. All modern ICQ clients 

are NAT friendly and thus DON'T require any special NAT modules, PORTFW tricks, etc. 

IF, for some reason, you want to run an OLD ICQ client, you can read this section. If not, just IGNORE all 

this info. I am leaving this in the HOWTO demonstrate large a LARGE PORTFW example. 

There are three methods of getting ICQ to work behind a Linux MASQ server. These solutions include the use 

the ICQ Masq module (for 2.2.x and 2.0.x kernels), using IPPORTFW for basic ICQ functionality, or setting 

up a SOCKS proxy server. 

MODULE: The ICQ module was written for the older generation of ICQ clients for both the 2.2.x and 2.0.x 

kernels. This module allows for the simple setup of multiple ICQ users behind a MASQ server. It also doesn't 

require any special changes to the ICQ client(s). Recently, AOL changed the protocol and ports used for ICQ. 

Because of this, many users might find that the ip_masq_icq module will no longer help them. For users of the 

older ICQ clients, the 2.2.x version of the module supports file transfer and read−time chat. The 2.0.x kernel 

module doesn't support file transfers and there is no module available for the 2.4.x kernels. 

PORTFW: Your next option is to use port forwarding. With port forwarding, basic ICQ chat will work but file 

transfers might not be very reliable. Please see below for an example of how to configure ICQ PORTFW. 


SOCKS: Finally, your last and possibly best option is to setup a SOCKS proxy server on your Linux machine. 

This service can happily co−exist with the MASQ service and ICQ should be fully functional regardless of 

what Linux kernel version you are running. The use of a SOCKS server will require ALL ICQ clients to be 

reconfigured to use it and the installation and configuration of a SOCKS server has nothing to do with IP 

Masquerade. Because of this, SOCKS is not covered in this HOWTO. 

If you are interested in Andrew Deryabin's djsf@usa.net ICQ IP Masq module for the 2.2.x and 2.0.x kernels, 

please see Section 2.7 for details. 

To use port forwarding (PORFW)for ICQ, you will have to make some changes on both Linux and ICQ 

clients but all ICQ messaging, URLs, chat, and some file transfers should work. 

• First, you need to be running a Linux kernel with IPPPORTFW enabled. Please see Section 6.7for 

more details. 

• Next, you need to add the following lines to your /etc/rc.d/rc.firewall−* file. This example assumes 

that 10.1.2.3 is your external Internet IP address and your internal MASQed ICQ machine is 

192.168.0.10: 

• The following example is for a 2.2.x kernel with IPCHAINS: 

I have included two examples here for the user: Either one would work fine: 

Example #1 


/usr/local/sbin/ipmasqadm portfw −a −P tcp −L 10.1.2.3 2000 −R 192.168.0.10 2000 





















Example #2 

port=2000 

while [ $port −le 2020 ] 

do 

/usr/local/sbin/ipmasqadm portfw −a −P tcp −L 10.1.2.3 $port −R 192.168.0.10 $port 

port=$((port+1)) 

done 

• The following example is for a 2.0.x kernel with IPFWADM: 

I have included two examples here for the user: Either one would work fine: 


Example #1 

/usr/local/sbin/ipportfw −A −t10.1.2.3/2000 −R 192.168.0.10/2000 





















Example #2 


port=2000 

while [ $port −le 2020 ] 

do 

/usr/local/sbin/ipportfw −A t10.1.2.3/$port −R 192.168.0.10/$port 

port=$((port+1)) 

done 

• Once your new rc.firewall−* is ready, reload the ruleset to make sure things are OK by simply typing 

in "/etc/rc.d/rc.firewall−*". If you get any errors, you either don't have IPPORTFW support in the 

kernel or you made a typo in the rc.firewall file. 

• Now, in ICQ's Preferences−−>Connection, configure it to be "Behind a LAN" and "Behind a firewall 

or Proxy". Now, click on "Firewall Settings" and configure it to be "I don't use a SOCK5 proxy". Also 

note that it was previously recommended to change ICQ's "Firewall session timeouts" to "30" seconds 

BUT many users have found that ICQ becomes unreliable. It has been found that ICQ is more reliable 

with its stock timeout setting (don't enable that ICQ option) and simply change MASQ's timeout to 

160 seconds. You can see how to change this timeout in Section 3.4.3 and Section 3.4.2 rulesets. 

Finally, click on Next and configure ICQ to "Use the following TCP listen ports.." from "2000" to 

"2020". Now click done. 

Now ICQ will tell you that you will have to restart ICQ for the changes to take effect. To be honest, I 

had to REBOOT the Windows9x machine in order for things to work right but some users might say 

otherwise. So.. try it both ways. 

• A user once told me that by simply portforwarding port 4000 to his ICQ machine, it worked perfectly. 

He reported that EVERYTHING worked fine (even chat, file transfers, etc) WITHOUT 

re−configuring ICQ from its default settings. Your mileage might vary on this topic but I thought you 

might like to hear about this alternative configuration. 



6.10. Gamers: The LooseUDP patch 

The LooseUDP patch allows semi−NAT−friendly games that usually use UDP connections to both WORK 

behind a Linux IP Masquerade server. 

What the LooseUDP patch does is allow ALL UDP packets to be NATed through the MASQ box without any 

checks or expiration. This liberal forwarding method is considered insecure by many and is disabled in 

modern 2.2.x kernels. The 2.4.x kernels with it's IPTABLES stateful UDP inspection only allows incoming 

UDP packets into the machine (and thus MASQ) if there was already an outbound UDP packet to that same 

host in it's stateful table. If the MASQ host hasn't sent a UDP packet to the remote host within ~30 seconds, 

the return UDP table entry is deleted. Because of this, IPTABLES removes most of the need for the 

LooseUDP patch as it does it in a more secure fashion. 

Currently, LooseUDP is available as a patch for 2.0.36+ kernels and it is already built into 2.2.3+ kernels 

though it is now DISABLED by DEFAULT in 2.2.16+ (please see the example rc.firewal ruleset comments 

for details). 

To get LooseUDP running on a 2.0.x kernel, follow the following steps: 

• Have the newest 2.0.x kernel sources uncompressed in the /usr/src/linux directory 

• ABSOLUTELY REQUIRED for v2.0.x: Download and install the IPPORTFW patch from Section 

3.2.3and as described in Section 6.7of the HOWTO. 

• Download the LooseUDP patch from Section 3.2.3 

Now, put the LooseUDP patch in the /usr/src/linux directory. Once this is done, type in: 

For a compressed patch file: zcat loose−udp−2.0.36.patch.gz | patch −p1 

For a NON−compressed patch file: cat loose−udp−2.0.36.patch | patch −p1 

Now, depending on the version of your "patch", You will then see the following text: 

patching file `CREDITS' 

patching file `Documentation/Configure.help' 

patching file ìnclude/net/ip_masq.h' 

patching file `net/ipv4/Config.in' 

patching file `net/ipv4/ip_masq.c' 

• If you see the text "Hunk FAILED" only ONCE and ONLY ONCE at the very beginning of the 

patching, don't be alarmed. You probably have an old patch file (this has been fixed) but it still works. 

If it fails completely, make sure you have applied the IPPORTFW kernel patch FIRST. 

Once the patch is installed, re−configure the kernel as shown in Section 3.2.3 and be sure to say "Y" 

to the "IP: loose UDP port managing (EXPERIMENTAL) (CONFIG_IP_MASQ_LOOSE_UDP) 

[Y/n/?]" option. 

To get LooseUDP running on a 2.2.x kernel, follow the following steps: 

• In the /etc/rc.d/rc.firewall−* script, goto the BOTTOM of the file and find the LooseUDP section. 

Change the "0" in the line: echo "0" > /proc/sys/net/ipv4/ip_masq_udp_dloose to 

a "1" and re−run the rc.firewall−* ruleset. An example of this is given in both Section 3.4.2 example 

and Section 6.4.3 example. 



NOTE: The LooseUDP code is /not/ available (?required?) for the 2.4.x kernels 

• See the begining of this section for all the details. Basically, the old 2.0.x / 2.2.x LooseUDP code was 

considered a security issue. Because of this, it was removed from the kernel. Fortunately, some games 

that used to require the LooseUDP code on the 2.2.x IPCHAINS system might work just final under 

the 2.4.x IPTABLES kernels. If the games don't work, I'm not aware of a solution for you. Sorry. 

Once you are running the new LooseUDP enabled kernel, you should be good to go for most NAT−friendly 

games. Some URLs have been given for patches to make games like BattleZone and others NAT friendly. 

Please see Section 6.3.1 for more details. 



If you can think of any useful FAQ suggestions, please send it to dranch@trinnet.net. Please clearly state the 

question and an appropriate answer (if you have it). Thank you! 

7.1. ( Distro ) − What Linux Distributions support IP 

Masquerading? 

If your Linux distribution doesn't support IP MASQ out of the box, don't worry. All you have to do is to 

re−compile the kernel as shown above in this HOWTO. 

NOTE: If you can help us fill out this table, please email dranch@trinnet.net. 

• Caldera < v1.2 : NO − ? 

• Caldera v1.3 : YES − 2.0.35 based 

• Caldera v2.2 : YES − 2.2.5 based 

• Caldera eServer v2.3 : YES − ? based 

• Debian v1.3 : NO − ? 

• Debian v2.0 : NO − ? 

• Debian v2.1 : YES − 2.2.1 based 



• DLX Linux v? : ? − ? 

• DOS Linux v? : ? − ? 

• FloppyFW v1.0.2 : ? − ? 

• Gentoo v1.4 : YES − ? Based 

• Hal91 Linux v? : ? − ? 

• Linux PPC vR4 : NO − ? 

• Linux Pro v? : ? − ? 

• LinuxWare v? : ? − ? 

• Mandrake v5.3 : YES − ? 

• Mandrake v6.0 : YES − 2.2.5 


• Mandrake v7.0 : YES − 2.2.14 

• Mandrake v7.1 : YES − 2.2.15 

• Mandrake v7.2 : YES − 2.2.17 


• Mandrake v8.1 : YES − 2.4.8 



• Mandrake v9.1 : YES − 2.4.21−pre 

• MkLinux v? : ? − ? 

• MuLinux v3rl : YES − ? 

• Redhat < v4.x : NO − ? 

• Redhat v5.0 : YES − ? 

• Redhat v5.1 : YES − 2.0.34 based 




Chapter 7. Frequently Asked Questions 118




• Redhat v7.3 : YES − 2.4.? based 

• Redhat v8.0 : YES − 2.4.18? based 


• Slackware v3.0 : ? − ? 



• Slackware v3.3 : ? − 2.0.34 based 




• Slackware v3.9 : ? − 2.0.37pre10 based 

• Slackware v4.0 : YES − 2.2.6 based 




• Slackware v8.1 : YES − ? based 

• Stampede Linux v? : ? − ? 

• SuSE v5.2 : YES − 2.0.32 base 

• SuSE v5.3 : YES − ? 

• SuSE v6.0 : YES − 2.0.36 based 



• Tomsrbt Linux v? : ? − ? 

• TurboLinux Lite v4.0 : YES − ? 

• TurboLinux v6.0 : YES − 2.2.12 based 

• TriLinux v? : ? − ? 

• Yggdrasil Linux v? : ? − ? 

7.2. ( Requirements ) − What are the minimum hardware 

requirements and any limitations for IP Masquerade? How 

well does it perform? 

A 486/66 box with 16MB of RAM was more than sufficient to fill a 1.54Mb/s T1 100%! MASQ has also been 

known to run quite well on 386SX−16s with 8MB of RAM. Yet, it should be noted that Linux IP Masquerade 

starts thrashing the system with more than 500 MASQ entries. 

The only application that I know which can temporarily break Linux IP Masquerade, is GameSpy. Why? 

When it refreshes its lists, it creates 10,000s of quick connections in a VERY short period of time. Until these 

sessions timeout, the MASQ tables become "FULL". See Section 7.23 of the FAQ for more details. 

While we are at it: 


There is a hard limit of 4096 concurrent connections each for TCP & UDP. This limit can be changed by 

fiddling the values in /usr/src/linux/net/ipv4/ip_masq.h − a maximum limit of 32000 should by OK. If you 

want to change the limit − you need to change the PORT_MASQ_BEGIN & PORT_MASQ_END values to 

get an appropriately sized range above 32K and below 64K. 



7.3. ( Errors ) − When I run my specific rc.firewall−* ruleset, I 

get "command not found" errors. Why? 

First off, when I say rc.firewall−*, what I really mean is to use one of the various types of firewall rulesets 

depending on what kernel version you're running. Your options from this HOWTO include: 

rc.firewall−iptables or rc.firewall−iptables−stronger or rc.firewall−ipchains or rc.firewall−ipchains−stronger 

or rc.firewall−ipfwadm or rc.firewall−ipfwadm−stronger. 

Next, how did you put the rc.firewall−* onto your machine? Did you cut&paste it into a TELNET window, 

FTP it from a Windows/DOS machine, etc? Try this.. log into your Linux box and run "vim −b 

/etc/rc.d/rc.firewall−*" and see if all your lines end in a ^M. If they do, delete all the ^M and try again. 

7.4. ( Still wont work ) − I've checked all my configurations, I 

still can't get IP Masquerade to work. What should I do? 

• Stay calm. Get yourself a cup of tea, coffee, soda, etc., and have a rest. Once your mind is clear, try 

the suggestions mentioned below. Setting up Linux IP Masquerading is NOT hard but there are 

several concepts that will be new to you. 

• Again, go through all the steps in Chapter 5. 99% of all first−time Masquerade users who have 

problems haven't looked here. 

• Check the IP Masquerade Mailing List Archives, most likely your questions or problems are a 

common one and can be found in a simple Archive search. 

• Check out the TrinityOS document. It covers IP Masquerading for both the 2.0.x and 2.2.x kernels 

and MANY other topics including PPPd, DialD, DHCP, DNS, Sendmail, etc. 

• Make sure that you aren't running ROUTED or GATED. To verify, run "ps aux | grep −e routed −e 

gated" 

• Post your question to the IP Masquerade Mailing List (see next the FAQ section for details). Please 

only use this if you cannot find the answer from the IP Masquerading Archive. Be sure to include all 

the information requested in Chapter 5 in your email!! 

• Post your question to a related Linux NNTP newsgroup. 

• Send email to ambrose@writeme.com and dranch@trinnet.net. You have a better chance of getting a 

reply from the IP Masquerading Email list than either of us. 

• Check your configurations again :−) 

7.5. ( Email list ) − How do I join or view the IP Masquerade 

and/or IP Masqurade Developers mailing lists and archives? 

There are two ways to join the two Linux IP Masquerading mailing lists. The first way is to send an email to 

masq−request@indyramp.com. To join the Linux IP Masquerading Developers mailing list, send an email to 

masq−dev−request@indyramp.com. Please see the bullet below for more details. 

• Subscribe via email: Now put the word "subscribe" in either the subject or body of the e−mail 

message. If you want to only subscribe to the Digest version of either the main MASQ or 

MASQ−DEV list (all e−mails on the given list during the week are sent to you in one big email), put 

the words "subscribe digest" in either the subject or body of the e−mail message. 



Once the server receives your request, it will subscribe you to your requested list and give you a 

PASSWORD. Save this password as you will need it to later unsubscribe from the list or change your 

options. 

The second method is to use a WWW browser and subscribe via a form at 

http://home.indyramp.net/mailman/listinfo/masq for the main MASQ list or 

http://home.indyramp.net/mailman/listinfo/masq−dev for the MASQ−DEV list. 

Once subscribed, you will get emails from your subscribed list. It should be also noted that both subscribed 

and NON−subscribed users can access the two list's archives. To do this, please see the above two WWW 

URLs for more details. 

Lastly, please note that you can only post to the MASQ list from the original account/address you used to 

subscribe. 

If you have any problems regarding the mailing list or the mailing list archive, please contact Robert Novak. 

7.6. ( NAT vs. Proxy ) − How does IP Masquerade differ from 

Proxy or NAT services? 

Proxy: Proxy servers are available for: Win95, NT, Linux, Solaris, etc. 

Pro: + (1) IP address ; cheap 

+ Optional caching for better performance (WWW, etc.) 

Con: − All applications behind the proxy server must both SUPPORT 

proxy services (SOCKS) and be CONFIGURED to use the Proxy 

server 

− Screws up WWW counters and WWW statistics 

A proxy server uses only (1) public IP address, like IP MASQ, and acts 

as a translator to clients on the private LAN (WWW browser, etc.). 

This proxy server receives requests like TELNET, FTP, WWW, 

etc. from the private network on one interface. It would then in turn, 

initiate these requests as if someone on the local box was making the 

requests. Once the remote Internet server sends back the requested 

information, it would re−translate the TCP/IP addresses back to the 

internal MASQ client and send traffic to the internal requesting host. 

This is why it is called a PROXY server. 

Note: ANY applications that you might want to use on the 

internal machines *MUST* have proxy server support 

like Netscape and some of the better TELNET and FTP 

clients. Any clients that don't support proxy servers 

won't work. 

Another nice thing about proxy servers is that some of them 

can also do caching (Squid for WWW). So, imagine that you have 50 

proxied hosts all loading Netscape at once. If they were installed 

with the default homepage URL, you would have 50 copies of the same 

Netscape WWW page coming over the WAN link for each respective computer. 

With a caching proxy server, only one copy would be downloaded by the 

proxy server and then the proxied machines would get the WWW page from 

the cache. Not only does this save bandwidth on the Internet 

connection, it will be MUCH MUCH faster for the internal proxied 

machines. 



MASQ: IP Masq is available on Linux and a few ISDN routers such 

or as the Zytel Prestige128, Cisco 770, NetGear ISDN routers, etc. 

1:Many 

NAT 

Pro: + Only (1) IP address needed (cheap) 

+ Doesn't require special application support 

+ Uses firewall software so your network can become 

more secure 

Con: − Requires a Linux box or special ISDN router 

(though other products might have this.. ) 

− Incoming traffic cannot access your internal LAN 

unless the internal LAN initiates the traffic or 

specific port forwarding software is installed. 

Many NAT servers CANNOT provide this functionality. 

− Special protocols need to be uniquely handled by 

firewall redirectors, etc. Linux has full support 

for this (FTP, IRC, etc.) capabilty but many routers 

do NOT (NetGear DOES). 

Masq or 1:Many NAT is similar to a proxy server in the sense that the 

server will perform IP address translation and fake out the remote server 

(WWW for example) as if the MASQ server made the request instead of an 

internal machine. 

The major difference between a MASQ and PROXY server is that MASQ servers 

don't need any configuration changes to all the client machines. Just 

configure them to use the linux box as their default gateway and everything 

will work fine. You WILL need to install special Linux modules for things 

like RealAudio, FTP, etc. to work)! 

Also, many users operate IP MASQ for TELNET, FTP, etc. *AND* also setup a 

caching proxy on the same Linux box for WWW traffic for the additional 

performance. 

NAT: NAT servers are available on Windows 95/NT, Linux, Solaris, and some 

of the better ISDN routers (not Ascend) 

Pro: + Very configurable 

+ No special application software needed 

Con: − Requires a subnet from your ISP (expensive) 

Network Address Translation is the name for a box that would have a pool of 

valid IP addresses on the Internet interface which it can use. Whenever the 

Internal network wanted to go to the Internet, it associates an available 

VALID IP address from the Internet interface to the original requesting 

PRIVATE IP address. After that, all traffic is re−written from the NAT 

public IP address to the NAT private address. Once the associated PUBLIC 

NAT address becomes idle for some pre−determined amount of time, the 

PUBLIC IP address is returned back into the public NAT pool. 

The major problem with NAT is, once all of the free public IP addresses are 

used, any additional private users requesting Internet service are out of 

luck until a public NAT address becomes free. 

For an excellent and very comprehensive description of the various forms of NAT, please see: 


• http://www.suse.de/~mha/linux−ip−nat/diplom/nat.html/ 

Here is another good site to learn about NAT, although many of the URLs are old but still valid: 

• http://www.linas.org/linux/load.html/ 

7.7. ( GUI ) − Are there any GUI firewall 

creation/management tools? 

Yes! They vary in the type of user interface, complexity, etc. but they are quite good though most are only for 

the IPFWADM tool so far. Here is a short list of available tools in alphabetical order. If you know of any 

others or have any thoughts on which ones are good/bad/ugly, please email David. 

• John Hardin's IPFWADM Dot file generator − a IPCHAINS version is in the works 

• Sonny Parlin's fBuilder: From the author of FWCONFIG, this new solution is fully WWW based and 

offers redundancy options, etc for both IPCHAINS and NetFilter. 

• William Stearns's Mason − A Build−a−ruleset on−the−fly type system 

7.8. ( MASQ and Dynamic IPs ) − Does IP Masquerade work 

with dynamically assigned IP addresses? 

Yes, it works with either dynamic IP addressed assigned by your ISP via either PPP or a DHCP server 

(common for Cablemodem and DSL users). As long as you have a valid Internet IP address, it should work. 

Of course, static IPs work too. If you plan on implementing a strong IPTABLES, IPCHAINS, or IPFWADM 

ruleset and/or plan on using a Port forwarder, your ruleset will have to be re−executed everytime your IP 

address changes. 

So, re−running the rc−firewall−* ruleset really depends on which method you get your IP addresses: 

• 

• 

PPP: The /etc/ppp/ip−up script is always run when a PPP connection comes up. Because of this, we 

can make the rc.firewall−* go get the new PPP IP address and update the firewall ruleset. If the 

/etc/ppp/ip−up file doesn't exist or if does exists, simply edit that file and append a line containing the 

name of your chosen firewall ruleset. For example, to run the SIMPLE IPTABLES ruleset: 



DHCP: If you get your IP address via DHCP, common for Cablemodem and DSL users, it's easy get 

the rc.firewall−* ruleset to run when you get a new IP lease. How this happens depends on what 

DHCP client your distribution uses: 

♦ dhclient : Most modern Linux distributions use dhclient from ISC. To re−run your specific 

rc−firewall−* script when your system gets a new IP address, add the following line to the 

/etc/dhclient−exit−hooks file. Please note that this example is loging the SIMPLE IPTABLES 

ruleset. Please use the correct rc.firewall−* name for your specific setup: 


♦ pump : Many older Redhat distributions use this DHCP client. To re−run the rc−firewall−* 

script when your system gets a new IP address, add the following line to the /etc/pump.conf 

file. Please note that this example is loading the SIMPLE IPTABLES ruleset. Please use the 


♦ 

correct rc.firewall−* name for your specific setup: 

script /etc/rc.d/rc.firewall−iptables 

dhcpcd : Many older distros use this DHCP client. To re−run the rc−firewall−* script when 

your system gets a new IP address depends on which verion of dhcpcd you're using. 

For newer dhcpcd client versions, append the following line to the 

/etc/dhcpcd−[interface].exe file. Please note that you have to replace the [interface] text with 

the name of your Interface connecting to the Internet. For this example, we are going run the 

SIMPLE IPTABLES ruleset against the eth0 interface by editing the 

/etc/dhcpc/dhcpcd−eth0.exe file: 



For old dhcpcd client versions, you need to figure out what script starts up dhcpcd (depends 

on the Linux distribution and it's version). From there, you need to replace the specific 

dhcpcd line with the folling line with the correct Internet−facing interface name. For this 

example, dhcpcd will run the SIMPLE IPTABLES ruleset against the eth0 interface: 

dhcpcd −c /etc/rc.d/rc.firewall eth0 

Please also see the top of TrinityOS − Section 10 for additional help with strong firewall rulesets and 

Dynamic IP addresses. 

7.9. ( MASQ and various networks ) − Can I use a cable 

modem (both bi−directional and with modem returns), DSL, 

satellite link, etc. to connect to the Internet and use IP 

Masquerade? 

Yes, as long as Linux supports that network interface, it should work. If you receive a dynamic IP address, 

please see the URL under the "Does IP Masquerade work with dynamically assigned IP" FAQ item above. 

7.10. ( Dial on Demand ) − Can I use Diald or the 

Dial−on−Demand feature of PPPd with IP MASQ? 

Definitely! IP Masquerading is totally transparent to Diald or PPP. The only thing that might become an issue 

is if you use STRONG firewall rulesets with dynamic IP addresses. See the FAQ item, "Does IP Masquerade 

work with dynamically assigned IP addresses?" above for more details. 

7.11. ( Apps ) − What applications are supported with IP 

Masquerade? 

It is very difficult to keep track of a list of "working applications". However, most of the normal Internet 

applications are supported, such as WWW browsing (Netscape, MSIE, etc.), FTP (such as WS_FTP), 

TELNET, SSH, RealAudio, POP3 (incoming email − Pine, Eudora, Outlook), SMTP (outgoing email), etc. A 


somewhat more complete list of MASQ−compatible clients can be found in Section 6.3 in this HOWTO. 

Applications involving more complicated protocols or special connection methods such as video conferencing 

software need special helper tools. 

For more details, please see the Linux IP masquerading Applications page. 

7.12. ( Distro Setup ) − How can I get IP Masquerade running 

on Redhat, Debian, Slackware, etc.? 

No matter which Linux distribution you have, the procedures for setting up IP Masquerade mentioned in this 

HOWTO should apply. Some distributions may have GUI or special configuration files that make the setup 

easier. We tried our best to write this HOWTO as general as possible. 

7.13. ( Timeouts ) − Connections seem to break if I don't use 

them often. Why is that? 

IP Masq, by default, sets its timers for TCP session, TCP FIN, and UDP traffic to 15 minutes. It is recommend 

to use the following settings (as already shown in this HOWTO's /etc/rc.d/rc.firewall−* ruleset) for most 

users: 

Linux 2.4.x with IPTABLES 


IPMASQ timeouts are NOT adjustable under IPTABLES 

Linux 2.2.x with IPCHAINS: 


# 





# 

/ipchains −M −S 7200 10 60 

Linux 2.0.x with IPFWADM: 


# 





# 


7.14. ( Odd Behavior ) − When my Internet connection first 

comes up, nothing works. If I try again, everything then 

works fine. Why is this? 


The reason is because you have a dynamic IP address and when your Internet connection first comes up, IP 

Masquerade doesn't know its IP address. There is a solution to this. In your /etc/rc.d/rc.firewall−* ruleset, add 

the following: 


# 


# following option. This enables dynamic−ip address hacking in IP MASQ, making 

# the life with Diald and similar programs much easier. 

# 


7.15. ( MTU ) − IP MASQ seems to be working fine but some 

sites don't work. This usually happens with WWW and some 

FTP sites. 

Depending on what Linux kernel version you are running on the MASQ server, some will people disagree on 

the real problem. The two following arguments have valid points, are inter−related, and users from each camp 

continue to debate this to this day. 

• 

With modern 2.4.x Linux systems, most users point their finger at the adminstrators of these remote 

broken sites (typically SSL−encrypted WWW sites, etc.) or your MASQ server's upstream router run 

by your ISP. The main though it that these machines are either filtering or not properly responding to 

SOME or ALL FORMS of ICMP packets (specifically ICMP Code 3 Type 4 − Fragmentation 

Needed) messages due to a fray of misplaced security paranoia. 

What does that all mean? Basically, say your machine is connected to the Internet with a MTU of 

1492 bytes (Maximum Transmission Unit −− the maximum packet size your computer can transmit) 

which is common for PPPoE users. At the same time, the remote WWW/FTP site is connected to the 

Internet at a MTU of 1500 bytes. The way that TCP/IP works is that when a TCP connection is being 

negotiated for your HTTP / FTP connection, the remote side will try to verify that a 1500 byte packet 

can reach your computer via the initial TCP "SYN" packet. 

Since the packet is too big for your connection, your upstream router (run by your ISP) will send a 

ICMP 3:4 (fragmentation needed) packet back to the remote WWW / FTP server. Within this packet 

is a recommended smaller MTU size to retry with. The problem is that either your local upstream 

router, some router between you and the remote server, or the remote HTTP / FTP server is either 

misconfigured or has a firewall in front of it that is BLOCKing these ICMP packets. 

• The final UNCOMMON possibility is a debatable 2.0 / 2.2 kernel bug in the IPMASQ code. Some 

users point their finger to the fact that IPMASQ might have problems with ICMP packets that have 

the DF or "Don't Fragment" bit set. Basically, when a MASQ box connects to the Internet with an 

MTU of anything less than 1500, some packets will have the DF field set. Though changing the MTU 

of the MASQ server's Internet link to 1500 seemingly fixes the problem, the possible bug is still there. 

What is believed to be happening in these older kernels is that the MASQ code is not properly 

re−writing the return IP addresses of the ICMP 3 Sub 4 code packets back to the originating MASQed 

computer. Because of this, these critical packets get dropped. 

No worries though. A there are several perfectly good ways to fix this nasty MTU problem: 

• Enable PMTU clamping in PPPoE 



• 


This solution is mostly for modern 2.4.x and 2.2.x kernel users connected to the Internet via a PPPoE 

DSL or Cablemodem connections. This solution allows for changes to be done ONLY on the MASQ 

server itself and not on all of the internal MASQ clients. 

Enable PMTU clamping via IPTABLES 

This solution is only modern 2.4.x kernel users connected via ANY type of Internet connection. This 

solution allows for changes to be done ONLY on the MASQ server itself and not on all of the internal 

MASQ clients. 

• Change your MASQ server's Internet Link MTU 

This solution will work for any Linux kernel version but is is NOT a solution if you have a PPPoE 

connection for DSL or Cablemodem users. 

It should be noted that some users will balk at this solution because it can hurt some latency specific 

programs like TELNET and Internet games but the impact is only slight. On the other hand, most 

HTTP and FTP traffic will SPEED UP! 

• Change the MTU of all internal MASQed machines 

This solution requires the most work as you have to make minor changes to ALL of the internal 

IPMASQed machines. Basically, you would be changing the MTU on all of the internal machines to 

match the MTU of your MASQ server's Internet connection. Fortunately, this solution is usually 

bulletproof where as some of the other solutions mentioned in this section might rarely not work. 

7.15.1. Enabling PMTU Clamping for PPPoE and some PPP Users: 

For those users who use PPPoE clients for (DSL / Cablemodem) or PPP (Dialup), your Internet connection is 

NOT "eth0" (for example) but usually "ppp0". In addition to this, your Internet link's MTU or Maximum 

Transmission Unit (largest packet you can transmit over the Internet) isn't 1500 bytes but 1492. The 1492 byte 

MTU comes from the link size of Ethernet (1518 bytes) − Ethernet MAC overhead (18) = 1500. Then you 

subtract the PPPoE header (8 bytes) == MTU of 1492. This overhead isn't a big deal but sometimes ISPs or 

remote Internet sites do stupid things to break PPPoE or non−1500 byte MTU connected machines. 

You can find more info about this topic on the web. Specifically, here is good presentaion on the topic: 

mss−talk presentation (PDF). Here is the entire Write up and other good info 

To enable clamping in both the RP or PPPd PPPoE clients, add the following line to your /etc/ppp/pppoe.conf 

file: 

# − If you have a computer acting as a gateway for a LAN, choose "1412". 

# The setting of 1412 is safe for either setup, but uses slightly more 

# CPU power. 

# 

CLAMPMSS=1412 

7.15.2. Clamping the MSS via IPTABLES: 

As mentioned above for PPPoE users, some ISPs and WWW sites filter critical ICMP packets like MTU Path 

Discovery. Because of this, many users might find more Internet sites work but others hang or work poorly. 

Fortunately, recent IPTABLES have added PMTU Clamping support which should help you. If your using 

IPTABLES and think you're hitting this issue, try adding the following line to the end of your 



rc.firewall−iptables ruleset. It should be noted that there is no PMTU clamping support in IPCHAINS. 

iptables −I FORWARD −p tcp −−tcp−flags SYN,RST SYN −j TCPMSS −−clamp−mss−to−pmtu 

If this line causes an error when you re−run the rc.firewall−iptables* firewall rulesets, you might need to 

upgrade your version of IPTABLES which includes the "TCPMSS" IPTABLES module. 

7.15.3. Changing the External MTU of the MASQ server: 

This solution usually only applies to DIALUP users since PPPoE users cannot INCREASE their MTU 

because of PPPoE's header overhead. 

To use this solution, first see what your current MTU for your Internet link is. To do so, run "/bin/ifconfig" on 

the MASQ server. Look at the lines that corresponds to your Internet connection and look for the MTU (for 

example, ppp0). This NEEDs to be set to 1500. Usually, Ethernet links will default to 1500 for Ethernet but 

serial / DIALUP modem PPP links might default to 576. 

• To change the MTU on a standard Ethernet link to your bridged or routed DSL, Cablemodem, etc. 

connection, you need to edit the correct network startup scripts for your Linux distribution. Please see 

the TrinityOS − Section 16 document for network optimizations. 

• To change the MTU issue on a PPP (not PPPoE) Internet link, edit your "/etc/ppp/options" file and 

towards the top, add the following text on two seperate lines, add: 

mtu 1500 

mru 1500 

Save these new changes and then restart PPP. Like shown above, verify that your PPP link has the 

correct MTU and MTU. 

CUA Users: Lastly, though this isn't a common problem, some Linux 2.0.x kernel users have found a 

MTU solution to the following problem. With PPP users, verify what port is your PPPd code 

connecting to. Is it a /dev/cua* port or a /dev/ttyS* port? It NEEDS to be a /dev/ttyS* port as the 

/dev/cua* device ststem has been deprecated and it breaks some things in very odd ways. 

7.15.4. Changing the MTU of various operating systems: 

If you reconfigure ALL of your MASQed PCs to use the SAME MTU as your external Internet link's MTU 

(for example, 1492 for PPPoE users), everything should work fine and this method is sometimes the MOST 

EFFECTIVE way of fixing things. This is including ALL of the solutions mentioned above. But doing things 

this way can be a lot of work if there are lots of internal MASQed machines or be even impossible to do if you 

don't have administrative access to all the internal MASQed machines. 

Follow these simple steps for your respective operating system: 

The follow examples utilize an MTU of 1492 for typical PPPoE connections for some DSL and Cablemodem 

users. It is recommended to use the HIGHEST values possible for all connections that are 128Kb/s and faster. 

It should be noted that some PPPoE ISPs might require an MTU setting of 1460 (not 1492) for proper 

connectivity due to additional overhead in the ISP's internal network. 

The only real reason to use smaller MTUs than 1492 or 1460 is to lower your Internet link's latency but at the 

cost of throughput. Please see http://www.ecst.csuchico.edu/~dranch/PPP/ppp−performance.html#mtu for 


more details on this topic. 

If you know how to make similar changes like these to other OSes like OS/2, MacOS, etc. please email David 

Ranch so it can be included in the HOWTO. 

7.15.4.1. Changing the MTU on Linux: 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

1. The setting of MTU can vary from Linux distribution to distribution. 

For Redhat: You need to edit the various "ifconfig" statements in 

the /sbin/ifup script 

For Slackware: You need to edit the various "ifconfig" statements in 

the /etc/rc.d/rc1.inet 

2. Here is one good, any−distribution−will−work example, edit the 

/etc/rc.d/rc.local file and put the following at the END of the file: 

echo "Changing the MTU of ETH0" 

/sbin/ifconfig eth0 mtu 1492 

Replace "eth0" with the interface name that is the machine's upstream 

connection which is connected to the Internet. 

3. For advanced options like "TCP Receive Windows" and such, detailed examples 

on how to edit the respective networking scripts for your specific Linux 

distro, etc., please see Chapter 16 of 

http://www.ecst.csuchico.edu/~dranch/LINUX/index−linux.html#trinityos 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

7.15.4.2. Changing the MTU on MS Windows 2000 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

1. Making ANY changes to the Registry is inheritantly risky but 

with a backup copy, you should be safe. Proceed at your 

OWN RISK. 

2. Goto Start−−>Run−−>RegEdit 

3. Registry−−>Export Registry File−−>Save a copy of your registry 

to a reliable place 

4. Navigate down to the key: 

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Inter 

faces\ 

Each ID Adapter has default keys for DNS, TCP/IP address, Default Gateway, 

subnet mask, etc. Find the key one that is for your network card. 

5. Create the following Entry: 


type=DWORD 

name="MTU" (Do NOT include the quotes) 

value=1492 (Decimal) (Do NOT include the text "(Decimal)") 

http://support.microsoft.com/support/kb/articles/Q120/6/42.asp?LN=EN−US&SD=gn&FR=0 


*** If you know how to also change the MSS, TCP Window Size, and the 

*** TTL parameters in NT 2000, please email dranch@trinnet.net as I 

*** would love to add it to the HOWTO. 

5. Reboot to let the changes take effect. 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

7.15.4.3. Changing the MTU on MS Windows NT 4.x 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 


with a backup copy, you should be safe. Proceed at your 

OWN RISK. 


3. Registry−−>Export Registry File−−>Save a copy of your registry 

to a reliable place 

4. Create the following keys in the Registry trees, choose two 

possible Registry trees. Multiple entries are for various 

network devices like DialUp Networking (ppp), Ethernet NICs, 

PPTP VPNs, etc. 

http://support.microsoft.com/support/kb/articles/Q102/9/73.asp?LN=EN−US&SD=gn&FR=0 

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Parameters\Tcpip] 

and 

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\\Parameters\Tcpip] 

Replace "" with the respective name of your uplink LAN NIC 

interface 

type=DWORD 

name="MTU" (Do NOT include the quotes) 

value=1492 (Decimal) (Do NOT include the text "(Decimal>") 

(Do NOT include the quotes) 

*** If you know how to also change the MSS, TCP Window Size, and the 

*** TTL parameters in NT 4.x, please email dranch@trinnet.net as I 

*** would love to add it to the HOWTO. 

5. Reboot to make the changes take effect. 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

7.15.4.4. Changing the MTU on MS Windows 98: 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 


with a backup copy, you should be safe. Proceed at your OWN RISK. 



3. You should make a backup copy of your Registry before doing anything. To 

do this, copy the "user.dat" and "system.dat" files from the \WINDOWS 

directory and put them into a safe place. It should be noted that the 


previously mentioned method of using "Regedit: Registry−−>Export Registry 

File−−>Save a copy of your registry" would only perform Registry MERGES 

and NOT do a replacement. 

4. Search though each of the Registry trees that end in "n" (e.g. 0007) 

and have a Registry entry called "IPAddress" which has the IP address 

of your NIC. Under that key, add the following: 

From http://support.microsoft.com/support/kb/articles/q158/4/74.asp 

[Hkey_Local_Machine\System\CurrentControlset\Services\Class\NetTrans\000n] 

type=STRING 

name="MaxMTU" (Do NOT include the quotes) 


5. You can also change the "TCP Receive Window" which sometimes 

increases network performance SUBSTANTIALLY. If you notice your 

throughput has DECREASED, put these items BACK to their original 

settings and reboot. 

[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\MSTCP] 

type=STRING 

name="DefaultRcvWindow" (Do NOT include the quotes) 


type=STRING 

name="DefaultTTL" (Do NOT include the quotes) 



−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

7.15.4.5. Changing the MTU on MS Windows 95: 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 


with a backup copy, you should be safe. Proceed at your OWN RISK. 



3. You should make a backup copy of your Registry before continuing. To 

do this, copy the "user.dat" and "system.dat" files from the \WINDOWS 

directory and put them into a safe place. It should be noted that the 

previously mentioned method of using "Regedit: Registry−−>Export Registry 

File−−>Save a copy of your registry" would only do Registry MERGES and NOT 

do a replacement. 

4. Search through each of the Registry trees that end in "n" (e.g. 0007) 

and have a Registry entry called "IPAddress", which has the IP address 

of your NIC. Under that key, add the following: 

From http://support.microsoft.com/support/kb/articles/q158/4/74.asp 

[Hkey_Local_Machine\System\CurrentControlset\Services\Class\NetTrans\000n] 

type=DWORD 

name="MaxMTU" (Do NOT include the quotes) 



type=DWORD 

name="MaxMSS" (Do NOT include the quotes) 


5. You can also change the "TCP Receive Window" which sometimes 

increases network performance SUBSTANTIALLY. If you notice your 

throughput has DECREASED, put these items BACK to their original 

settings and reboot. 

[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\MSTCP] 

type=DWORD 

name="DefaultRcvWindow" (Do NOT include the quotes) 


type=DWORD 

name="DefaultTTL" (Do NOT include the quotes) 



−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

7.16. ( FTP ) − MASQed FTP clients don't work. 

Check to see that the "ip_masq_ftp" module is loaded. To do this, log into the MASQ server and run the 

command "/sbin/lsmod". If you don't see the "ip_masq_ftp" module loaded, make sure that you followed the 

BASIC /etc/rc.d/rc.firewall−* recommendations found in Section 3.4. If you are implementing your own 

ruleset, make sure you include most of the examples from the HOWTO or you will have many susequent 

problems. 

7.17. ( Performance ) − IP Masquerading seems slow 

There might be a few reasons for this: 

• 


You might be unrealistic about how much available bandwidth is on your modem line. Lets do the 

math for a typical 56k modem connection: 

1. 56k modems = 56,000 bits per second. 

2. You really DON'T have a 56k modem but a 52k modem per US FCC limitations. 

3. You'll almost NEVER get 52k, the best connection I used to get was 48k 

4. 48,000 bits per second is 4,800 BYTES per second (8 bits to a byte + 2 bits for the START 

and STOP RS−232 serial bits) 

5. With an MTU of 1500, you will get (3.2) packets in one second. Since this will involve 

fragmentation, you need to round DOWN to (3) packets per second. 

6. Again with MTU of 1500, thats 3.2 x 40 bytes of TCP/IP overhead (8%) 

7. So the BEST throughput you could hope for is 4.68KB/s w/o compression. Compression, be 

it v.42bis hardware compression, MNP5, or MS/Stac compression can yeild impressive 

numbers on highly compressable stuff like TEXT files, but acutally slow things down when 

transfering pre−compressed files like ZIPs, MP3s, etc. 

Ethernet attached setups (DSL, Cablemodem, LANs, etc) 


• Make sure you don't have both your INTERNAL and EXTERNAL networks running on the same 

network card with the "IP Alias" feature. If you ARE doing this, it can be made to work but it will be 

excessively slow due to high levels of collisions, IRQ usage, etc. It is highly recommended that you 

install another network card for the internal and external networks to have their own interface. 

• Make sure you have the right Ethernet settings for both SPEED and DUPLEX. 

• Some 10Mb/s Ethernet cards and most 100Mb/s cards support FULL Duplex connections. Direct 

connections from an Ethernet card to, say, a DSL modem (without any hubs in between) *CAN* be 

set to FULL DUPLEX but only if the DSL modem supports it. You should also be sure that you have 

Ethernet cables with all eight wires used and that they are in good condition. 

Internal networks that use HUBs −cannot− use Full Duplex. You need either a 10 or 100Mb.s 

Ethernet SWITCH to be able to do this. 

Both auto 10/100Mb/s SPEED negotiation and Full/Half DUPLEX negotiation on Ethernet cards can 

wreck havoc on networks. I recommend to hard code both the NIC speed and duplex into the NIC(s) 

if possible. This is directly possible via Linux NIC kernel modules but isn't directly possible in 

monolithic kernels. You will need to either use MII utililies from Section 8.1 or hardcode the kernel 

source. 

Optimize your MTU and set the TCP Sliding window to at least 8192 

• Though this is COMPLETELY out of the scope of this document, this helps QUITE A BIT with ANY 

network link you have, be it an internal or external PPP, Ethernet, TokenRing, etc. link. For more 

details, this topic is briefly touched in an above section in Section 7.15. For even more details, check 

out the Network Optimization section of TrinityOS − Section 16. 

Serial based modem users with PPP 


• If you have an external modem, make sure you have a good serial cable. Also, many PCs have cheesy 

ribbon cables connecting the serial port from the motherboard or I/O card to the serial port 

connection. If you have one of these, make sure it is in good condition. Personally, I have ferrite coils 

(those grey−black metal like rings) around ALL of my ribbon cables. 

• Make sure your MTU is set to 1500 as described in the FAQ section of this HOWTO above 

• Make sure that your serial port is a 16550A or better UART. Run "dmesg | more" to verify 

• Setup IRQ−Tune for your serial ports. 

On most PC hardware, the use of Craig Estey's IRQTUNE tool and significantly increase serial port 

performance including SLIP and PPP connections. 

• Make sure that your serial port for your PPP connection is running at 115200 (or faster if both your 

modem and serial port can handle it.. a.k.a ISDN terminal adapters) 

♦ 2.0.x kernels: The 2.0.x kernels are kind of an odd ball because you can't directly tell the 

kernel to clock the serial ports at 115200. So, in one of your startup scripts like the 

/etc/rc.d/rc.local or /etc/rc.d/rc.serial file, execute the following commands for a modem on 

COM2: 

♦ setserial /dev/ttyS1 spd_vhi 

♦ In your PPPd script, edit the actual pppd execution line to include the speed "38400" per the 

pppd man page. 

♦ 2.2.x kernels: Unlike the 2.0.x kernels, both the 2.1.x and 2.2.x kernels don't have this 

"spd_vhi" issue. 


All interface types: 

So, in your PPPd script, edit the actual pppd execution line to include the speed "115200" per 

the pppd man page. 

7.18. ( PORTFW ) − IP Masquerading with PORTFWing 

seems to break when my line is idle for long periods 

If you have a DSL or Cablemodem, this behavior is unfortunately quite common. Basically your ISP is 

putting your connection into a very low priority queue to better service non−idle connections. The problem is 

that some enduser's connections will actually be taken OFFLINE until some traffic from the user's 

DSL/Cablemodem connection awakens the ISP's hardware. 

• Some DSL installations can take an idle connection OFFLINE and only be checked for activity once 

every 30 seconds or so. 

• Some Cablemodem setups can set an idle connection into a low priority queue and only be checked 

for activity every minute or so. 

What do I recommend to do? Ping your default gateway every 30 seconds. To do this, edit the 

/etc/rc.d/rc.local file and add the following to the bottom of the file: 

ping −i 30 100.200.212.121 > /dev/null & 

Replace the 100.200.212.121 with your default router (upstream router). 

7.19. ( PORTFW − Locally ) − I can't reach my PORTFWed 

server from the INTERNAL lan 

Basically, say your domain, acme.com, has an external IP address of 1.2.3.4 and you are PORTFWing all 

WWW traffic to an internal machine, say, 192.168.0.20/24. Then an /internal/ user on the 192.169.0.x 

network tries to contact to http://www.acme.com and expects things to work. Well, that isn't going to happen 

with a basic config. Let me explain. Basically, http://www.acme.com is being resolved to the IP of 

http://1.2.3.4 by your chosen DNS server. What really should be happening is the web request should resolve 

that request to http://192.168.0.20. 

See the difference? 

The proper solution to this is to setup a SPLIT DNS server. Internal users would be configured to use an 

/internal/ DNS server which would resolve requests like this with the 192.168.0.20 address when asked for 

www.acme.com. All external users should be serviced by an /external/ DNS server which will will resolve the 

requrest to the 1.2.3.4 IP address. From there, IPTABLES/IPCHAINS/IPFWADM would then PORTFW the 

traffic to the 192.168.0.20 server as normal. 

But you're probably thinking that you don't want to setup a split DNS server and there has to be another way. 

There are a few alternatives! The first alternative is if you only have a few internal machines. Here, you can 

setup a "hosts" file entry on *all* internal machines. That static hosts entry would basically look like: 

www.acme.com 192.168.0.20 



Got it? With that in place, the machine would consult the hosts table before going to the DNS server to 

resolve the host. This works well but if you change the IP address on that internal web server, you're going to 

need to manually update the hosts file on all of those internal machines. If you are interested in doing the 

more scalable split DNS approach, TrinityOS completely covers split and chrooted DNS servers. TrinityOS − 

Section 24 http://www.ecst.csuchico.edu/~dranch/LINUX/index−linux.html#trinityos 

Now, if neither the split DNS nor the hosts file approach interests you, you still have a simple but effective 

alternative to get things working. What you can do is add some specific rules to your rc.firewall−* ruleset. 

Please see the "PORTFW Redirection of Internal requests" section under the Section 6.7 chapter. 

Why didn't I mention this alternative solution first? The main reason is that it's not the ideal solution. The 

primary problem with this approach is that every packet will be going from the internal MASQed client to the 

MASQ server. There, the MASQ server will SNAT each packet to the internal MASQed WWW server's IP 

and then forward it to the internal web server. Once the packet is received and responded to by the web server, 

that response has to go back through all that processing back to the original client machine. This process is 

overly wasteful on both network bandwidth and MASQ server CPU cycles! 

7.20. ( Logs ) − Now that I have IP Masquerading up, I'm 

getting all sorts of weird notices and errors in the SYSLOG 

log files. How do I read the IPTABLES/IPCHAINS/IPFWADM 

firewall errors? 

There is probably a few common things that you are going to see: 

• 

MASQ: Failed TCP Checksum error: You might see this error when a packet coming from the 

Internet gets corrupt in the data section of the packet but the rest of it "seems" ok. When the Linux 

box receives this packet, it will calculate the CRC of the packet and determine that its corrupt. On 

most machines running OSes like Microsoft Windows, they just silently drop the packets but Linux IP 

MASQ reports it. If you get a LOT of them over your PPP link, first follow the FAQ entry above for 

"(Performance) − Masq seems is slow". 

If the (Performance) FAQ tips don't help and you run PPP over dialup or PPPoE, you might try 

adding the line "−vj" (disabled VanJacobson header compression) to your /etc/ppp/options file and 

restart the PPPd connection. 

• Firewall hits: Because you are on the Internet with a decent firewall, you will be surprised with the 

number of users trying to penetrate your Linux box! So what do all these firewall logs mean? 

More so, if they are filling your logs, see the next FAQ entry on thoughts how to reduce all these log 

entries. 

• The following details are from the TrinityOS − Section 10 documentation I also wrote: 

With the use of various firewall rulesets, a given ruleset can either 

DENY (silently drop) or REJECT traffic (sends back a ICMP error). If 

firewall logging is enabled, the errors will show up in the SYSLOG 

"messages" file found at: 

Redhat: /var/log 

Slackware: /var/adm 


If you take a look at one of these firewall logs, you would see something 


like: 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

IPTABLES: 

−−−−−−−−− 

Feb 23 07:37:01 Roadrunner kernel: IPTABLES IN=eth0 OUT= 

MAC=00:50:da:2e:e5:fb:00:03:47:73:c9:d2:08:00 SRC=12.75.147.174 

DST=100.200.0.212 LEN=44 TOS=0x00 PREC=0x00 TTL=64 ID=39034 DF PROTO=TCP 

SPT=4313 DPT=23 WINDOW=32120 RES=0x00 SYN URGP=0 

IPCHAINS: 

−−−−−−−−− 

Feb 23 07:37:01 Roadrunner kernel: input REJECT eth0 PROTO=6 

12.75.147.174:1633 100.200.0.212:23 L=44 S=0x00 I=54054 F=0x0040 T=64 

IPFWADM: 

−−−−−−−− 

Feb 23 07:37:01 Roadrunner kernel: IP fw−in rej eth0 TCP 12.75.147.174:1633 

100.200.0.212:23 L=44 S=0x00 I=54054 F=0x0040 T=64 

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− 

There is a LOT of information in just this one line of SYSLOG. Lets break 

out this example. You should refer back to the original firewall hit as you 

read this. 

−−−−−−−−−−−−−− 


1. =================================================================== 

− This packet firewall "hit" occurred on "Feb 23 07:37:01" 

2. =================================================================== 

− This packet was logged on the "RoadRunner" computer via the kernel 

3. =================================================================== 

− IPTABLES: the SYSLOG prepend string is "iptables" for information purposes 

− IPCHAINS: the packet was stopped on the INPUT chain 

− IPFWADM: the packet was an IP packet 

4. =================================================================== 

− IPTABLES: the packet came IN on interface "eth0" 

− IPCHAINS: the packet was REJECTED (vs. dropped or accepted) 

− IPFWADM: the packet was stopped on INPUT (vs. "fw−out" for OUT or 

"fw−fwd" for FORWARD) 

5. =================================================================== 

− IPTABLES: the packet had NO output interface 

− IPCHAINS: the packet came in on the "eth0" interface 

− IPFWADM: the packet was REJECTED "rej" (vs. "deny" or "accept") 



6. =================================================================== 

− IPTABLES: this display's the MAC address of the source and destination 

Ethetnet MAC address (only relivant for Ethernet networks) 

− IPCHAINS: the packet was IP protocol 6 or TCP 

* If you don't know that protocol 6 is for TCP, look at 

your /etc/protocols file to see what other protocol numbers 

are used for. 

− IPFWADM: the packet on the "eth0" interface 

7. =================================================================== 

− IPTABLES: the packet's source IP address was 12.75.147.174 

− IPCHAINS: the packet's source IP address was 12.75.147.174 

− IPFWADM: the packet was a "TCP" packet 

8. =================================================================== 

− IPTABLES: the packet's destination IP address was 100.200.0.212 

− IPCHAINS: the packet's source PORT was 1633 

− IPFWADM: the packet's source IP address was 12.75.147.174 

9. =================================================================== 

− IPTABLES: the packet's length was 44 bytes 

− IPCHAINS: the packet's destination IP address was 100.200.0.212 

− IPFWADM: the packet's source PORT was 1633 

10. =================================================================== 

− IPTABLES: the packet's TOS markings (type of service which basically 

means class of service) was 0x00 or zero. 

− IPCHAINS: the packet's destination PORT was 23 (telnet) 

* If you don't know that port 23 is for TELNETing, look at 

your /etc/services file to see what other ports are used 

for. 

− IPFWADM: the packet's destination IP address was 100.200.0.212 

11. =================================================================== 

− IPTABLES: the packet's precedense markings (class of service) was 

0x00 or zero. 

− IPCHAINS: the packet's length was 44 bytes 

− IPFWADM: the packet's destination PORT was 23 (telnet) 



for. 

12. ================================================================== 



− IPTABLES: the packet's TTL or Time to Live was 64 or 64 router hops 

* Every router hop over the Internet will subtract (1) from 

this number. Usually, packets will start with a number of 

255 (depends on the operating system) and if that number 

ever reaches (0), it means that realistically, the packet 

was lost in a network loop and should be deleted. 

− IPCHAINS: the packet's TOS markings (type of service which basically 


* divide this by 4 to get the Type of Service (presidence) 

− IPFWADM: the packet was 44 bytes long 

13. ================================================================== 

− IPTABLES: the packet had various TCP flags set such as SYN, SYN+ACK, 

etc. (shown in HEX) 

− IPCHAINS: the packet had various TCP flags set (shown in hex) 

− IPFWADM: the packet's TOS markings (type of service which basically 


14. ================================================================== 

− IPTABLES: the packet's "don't fragment" or DF bit was set from the 

source computer 

− IPCHAINS: the packet had a fragmentation offset of 40 (shown in HEX) 

−−Don't worry if you don't understand this.. 

* A value that started with "0x2..." or "0x3..." means the 

"More Fragments" bit was set so more fragmented packets 

will be coming in to complete this one BIG packet. 

* A value which started with "0x4..." or "0x5..." means that 

the "Don't Fragment" bit was set 

* Any other values are the Fragment offset (divided by 8) to 

be later used to recombine into the original LARGE packet 

− IPFWADM: the packet had various TCP flags set such as SYN, SYN+ACK, 

etc. (shown in HEX) 

15. ================================================================== 

− IPTABLES: the packet was a TCP packet 

− IPCHAINS: the packet's TTL or Time to Live was 64 or 64 router hops 






− IPFWADM: the packet had a fragmentation offset of 40 (shown in HEX) 

−−Don't worry if you don't understand this.. 

* A value that started with "0x2..." or "0x3..." means the 

"More Fragments" bit was set so more fragmented packets 

will be coming in to complete this one BIG packet. 

* A value which started with "0x4..." or "0x5..." means that 

the "Don't Fragment" bit was set 


* Any other values are the Fragment offset (divided by 8) to 

be later used to recombine into the original LARGE packet 

16. ================================================================== 

− IPTABLES: the packet's soure PORT was 4313 

− IPCHAINS: 

− IPFWADM: the packet's TTL or Time to Live was 64 or 64 router hops 






17. ================================================================== 

− IPTABLES: the packet's destination PORT was 23 (telnet) 



for. 


− IPFWADM: 

18. ================================================================== 

− IPTABLES: the packet's TCP window (sliding or selective TCP ack) 

was 32120 bytes 



19. ================================================================== 

− IPTABLES: the packet's TCP reserved bits were 0x00 (HEX) − unused 



20. ================================================================== 

− IPTABLES: the packet's TCP header SYN bit was set 

* IPTABLES displays all the TCP header bits by name and not 

by a HEX dump 



21. ================================================================== 

− IPTABLES: the packet's TCP header URGENT bit was set − rarely used 





7.21. ( Log Reduction ) − My logs are filling up with packet 

hits due to the new "stronger" rulesets. How can I fix this? 

So your realizing that a good firewall is catching a LOT of bad Internet traffic. That's a good thing but it's also 

filling up your logs to the point that you won't read them; that's bad. What to do? 

What you need to figure out is what traffic you DON"T want to log, explicitly match those packets in the 

firewall, and NOT log the packets when you drop them. 

For example, the TrinityOS firewall ruleset in section 10.7 (this would be a "strongest" ruleset in IPMASQ 

speak) gives some ideas: TrinityOS − Section 10.7 

Things I recommend to filter: 


• All RFC1918 address space (TCP/IP address ranges: 10.x.y.z/8, 172.16−31.y.z/12, and 

192.168.y.x/16). You should /never/ receive these packets from an Internet connection. If you do, they 

are most likely spoofed packets 

• Windows File and Print Sharing (Samba or CIFS): ports 137, 138, 139, and 445. Windows machines 

like to talk a lot though most computers don't care what they're saying. 

• Class−D Multicast addresses (if you don't use Multicast): 224.0.0.0/4 

• Class−E and F "future" addresses: 240.0.0.0/5 and 248.0.0.0/5 

To a much lesser extent, you might want to filter other packets. I recommend that you verify that you are 

receiving these specific packet types before you filter them out. 

• RIP (the routing protocol): port 520 

• Some specific forms of ICMP packets − NOT all of them (that will break your machine and IPMASQ 

in general) 

Finally, you'll probably find that some individual TCP/IP address out on the Internet always seem to attack 

your IP. So, in addition to filtering various PORTS like above, you might want to also filter by specific 

SOURCE IP address too. After all, it is *YOUR* firewall. 

7.22. ( MASQ Security ) − Can I configure IP MASQ to allow 

Internet users to directly contact internal MASQed servers? 

Yes! With IPPORTFW, you can allow ALL or only a select few Internet hosts to contact ANY of your 

internal MASQed computers. This topic is completely covered in Section 6.7 in this HOWTO. 

7.23. ( Free Ports ) − I'm getting "kernel: 

ip_masq_new(proto=UDP): no free ports." in my SYSLOG 

files. Whats up? 

One of your internal MASQed machines are creating an abnormally high number of packets destined for the 

Internet. As the IP Masq server builds the MASQ table and forwards these packets out over the Internet, the 


table is quickly filling. Once the table is filled, it will give you this error. 

The only application that I have known which temporarily creates this situation is a gaming program called 

"GameSpy". Why? Gamespy builds a server list and then pings all of the servers in the list (1000s of game 

servers). By creating all these pings, it creates 1,000s of quick connections in a VERY short period of time. 

Until these sessions timeout via the IP MASQ timeouts, the MASQ tables become "FULL". 

So what can you do about it? Realistically, don't use programs that do things like this. If you do get this error 

in your logs, find it and stop using it. If you really like GameSpy, just don't refresh the server too often. 

Regardless, once you stop running this MASQ'ed program, this MASQ error will go away as these 

connections will eventually timeout in the MASQ tables. 

7.24. ( SETSOCKOPT ) − I'm getting "ipfwadm: setsockopt 

failed: Protocol not available" when I try to use IPPORTFW! 

If you get the error message "ipfwadm: setsockopt failed: Protocol not available", you AREN'T running your 

new kernel. Make sure that you moved the new kernel over, re−run LILO, and then reboot again. 

Please see the end of Section 6.7 for full details. 


7.25. ( SAMBA ) − Microsoft File and Print Sharing and 

Microsoft Domain clients don't work through IP Masq! 

To properly support Microsoft's SMB protocol, an IP Masq module would need to be written but there are 

three viable work−arounds. For more details, please see this Microsoft KnowledgeBase article. 

The first way to work around this problem is to configure IPPORTFW from Section 6.7 and portfw TCP ports 

137, 138, and 139 to the internal Windows machine's IP address. Though this solution works, it would only 

work for ONE internal machine. 

The second solution is to install and configure Samba on the Linux MASQ server. With Samba running, you 

can then map your internal Windows File and Print shares onto the Samba server. Then, you can mount these 

newly mounted SMB shares to all of your external clients. Configuring Samba is fully covered in a HOWTO 

found in a Linux Documentation Project and in the TrinityOS document as well. 

The third solution is to configure a VPN (virtual private network) between the two Windows machines or 

between the two networks. This can either be done via the PPTP or IPSEC VPN solutions. There is a Section 

7.35 patch for Linux and also a full IPSEC implementation available for both 2.0.x and 2.2.x kernels. This 

solution would probably be the most reliable and secure method of all three solutions. 

All of these solutions are NOT covered by this HOWTO. I recommend that you look at the TrinityOS 

documentation for IPSEC help and John Hardin's PPTP page for more information. 

Also PLEASE understand that Microsoft's SMB protocol is VERY insecure. Because of this, running 

either Microsoft File and Print sharing or Windows Domain login traffic over the Internet without any 

encryption is a VERY BAD idea. 


7.26. ( IDENT ) − IRC won't work properly for MASQed IRC 

users. Why? 

The main possible reason is because most common Linux distribution's IDENT or "Identity" servers can't deal 

with IP Masqueraded links. No worries though, there are IDENTs out there that will work. 

Installing this software is beyond the scope of this HOWTO but each tool has its own documentation. Here 

are some of the URLs: 

• Oident is a favorite IDENT server for MASQ users. 

• Mident is another popular IDENT server. 

• Sident 

• Other Idents 

Please note that some Internet IRC servers still won't allow multiple connections from the same host even if 

they get Ident info and the users are different though. Complain to the remote sys admin. :−) 

7.27. ( IRC DCC ) − mIRC doesn't work with DCC Sends 

This is a configuration problem on your copy of mIRC. To fix this, first disconnect mIRC from the IRC 

server. Now in mIRC, go to File −−> Setup and click on the "IRC servers tab". Make sure that it is set to port 

6667. If you require other ports, see below. Next, goto File −−> Setup −−> Local Info and clear the fields for 

Local Host and IP Addresses. Now select the checkboxes for "LOCAL HOST" and "IP address" (IP address 

may be checked but disabled). Next under "Lookup Method", configure it for "normal". It will NOT work if 

"server" is selected. That's it. Try to the IRC server again. 

If you require IRC server ports other than 6667, (for example, 6969) you need to edit the 

/etc/rc.d/rc.firewall−* startup file where you load the IRC MASQ modules. Edit this file and the line for 

"modprobe ip_masq_irc" and add to this line "ports=6667,6969". You can add additional ports as long as they 

are separated with commas. 

Finally, close down any IRC clients on any MASQed machines and re−load the IRC MASQ module: 

/sbin/rmmod ip_masq_irc /etc/rc.d/rc.firewall−* 


7.28. ( IP Aliasing ) − Can IP Masquerade work with only 

ONE Ethernet network card? 

Yes and no. With the "IP Alias" kernel feature, users can setup multiple aliased interfaces such as eth0:1, 

eth0:2, etc but its is NOT recommended to use aliased interfaces for IP Masquerading. Why? Providing a 

secure firewall becomes very difficult with a single NIC card. In addition to this, you will experience an 

abnormal amount of errors on this link since incoming packets will almost simultaneously be sent out at the 

same time. Because of all this and NIC cards now costs less than $10, I highly recommend to just get a NIC 

card for each MASQed network segment. 

Users should also understand that IP Masquerading will only work with a physical interface such as eth0, 

eth1, etc. MASQing out an aliased interface such as "eth0:1, eth1:1, etc" will NOT work. In other words, the 

following WILL NOT WORK reliably: 


• It is rumored that you can simply use the destination IP address (the IP address associated with the 

ALIASed interface like eth0:1, etc.) IN PLACE of specifing the interface (eth0:1). This solution is not 

untested −− please email dranch@trinnet.net if you have any positive or negative results 

• /sbin/ipchains −A forward −i eth0:1 −s 192.168.0.0/24 −j MASQ" 

• /sbin/ipfwadm −F −a m −W eth0:1 −S 192.168.0.0/24 −D 0.0.0.0/0 

If you are still interested in using aliased interfaces, you need to enable the "IP Alias" feature in the kernel. 

You will then need to re−compile and reboot. Once running the new kernel, you need to configure Linux to 

use the new interface (i.e. eth0:1, etc.). After that, you can treat it as a normal Ethernet interface with some 

restrictions like the one above. 

7.29. ( Multiple−LANs ) − I have two MASQed LANs but they 

cannot communicate with each other! 

Please see Section 6.5 for full details. 

7.30. ( SHAPING ) − I want to be able to limit the speed of 

specific types of traffic 

I receive lots of emails from people asking something like the following: 

How can I control the internet bandwidth among the LAN PCs behind the IPMASQ 

server? Some times any local pc is downloading and it it will take the majority 

of the bandwidth and thus the other PCs get little bandwidth. 

This topic really doesn't have anything to do with IPMASQ and everthing to do with Linux's built−in traffic 

shaping and rate−limiting abilities. You will find information about this on the LDP such as the ADSL 

Bandwidth HOWTO, the Advanced Rouring HOWTO, and the Bandwidth Limiting HOWTO. Please 

understand this is an advanced topic and any question you may have will be better served from people from 

those forums. 

7.31. ( ACCOUNTING ) − I need to do accounting on who is 

using the network 

Though this doesn't have much to do with IPMASQ, here are a few ideas. If you know of a better solution, 

please email the author of this HOWTO so it can be added to the HOWTO. 

• 

Idea #1: You could run the command: 

IPCHAINS: "ipchains −L −M" 

IPTABLES: "cat /proc/net/ip_conntrack" 


IPFWADM: 

once a second and log all of those entries. You could then write a program to merge this information 

into one large file. Again, this will only provide you with the remote IP address and nothing about the 

content viewed or downloaded. 

• Idea #2: Log every packet: You can match any specific traffic flows but this method will create 

VERY LARGE log files. Unfortunately, these log files aren't very readable and it doesn't tell you 

what was transfered (FTP files, etc.). Fortunately, setting up this form of accounting is easy. 


• Idea #3: Say you want to log all traffic going out onto the internet. You can setup a firewall rule to 

accept port 80 traffic with with the SYN bit set and log it. Now mind you, this will create smaller log 

files than the idea above but you will only know the destination IP address and NOT the WWW pages 

viewed. 

• Idea #4: Transparent Proxy: This method really doesn't use IPMASQ since it requires the installation 

and setup of the Squid HTTP/FTP proxy server. The benefit of this method is that internal users won't 

notice anything different in terms of connectivity but now the SysAdmin gets a LOT more 

information (files downloaded, etc). But, there are pros/cons to setting this up: 

Pro: 

♦ + full logging of all transferred files and issues FTP commands 

♦ + you can enable caching on the proxy server. With caching, you can save bandwidth since 

once a file is downloaded, any identical file requests will be served via the cache and not 

redownloaded via the Internet connection. 

Con: 

♦ − Setting up a transparent proxy is complicated as it requires kernel changes, setting up 

Squid, etc. 

♦ − Could be overkill for a small installation. 

Please see the Advanced Routing HOWTO for more details. 

7.32. ( MULTIPLE IPs − DMZ segments) − I have several 

EXTERNAL IP addresses that I want to PORTFW to several 

internal machines. How do I do this? 

Though technically possible, DON'T do this with IP MASQ. There are far better solutions for this network 

design. 

MASQ is a 1:Many NAT setup which is the incorrect tool to perform what you are looking for. You are 

looking for is either Many:Many NAT solution or a Briding setup. 

NOTE: For users out there who are thinking about enabling multiple IP addresses on one internal NIC using 

"IP Alias" and then just PORTFWeding ALL of those ports (0−65535), and and finally use IPROUTE2 to 

maintain the proper source/destination IP pairs. This has been done SUCCESSFULLY on 2.0.x kernels and 

less successfully on 2.2.x kernels. Regardless of success, that isn't the proper way to do it, it's a total HACK, 

and it is not a supported MASQ configuration. Please, give IPTABLES on the 2.4.x kernels a serious look or 

to a much lesser extent, Section 7.30 IPROUTE2 look for 2.2.x kernels. 

Anyway, for forwarding external IP address to internal hosts, you basically have three possibilites: 

• 1. Route the external IPs 


(This does NOT involve IPMASQ at all but requires special WAN addressing 

and routing setup from your ISP): 

Internet −− Some public WAN −− Linux −− DMZ segment 

IP address Server PUBLIC IPs 

| 

+−−−−−− Internal net 

private IPs 


• 2. 1:1 NAT 

(Most easily done via IPTABLES or with IPCHAINS and IPROUTE2 but still 

some protocols cannot deal with NAT) 

Internet −− Linux −− DMZ segment 

Server Private IPs natted to 1:1 PUBLIC IPs 

| 



• 3. Bridging or ProxyARP: 

The Bridging method is one of the more popular methods that many commercial 

firewalls do and it's very slick. Alternatively, you can use the ProxyARP 

method which works well without some of the complications (or benefits of 

bridging). With both solutions, all public IPs can transparently flow 

through the Linux server to the DMZ but via firewall inspection. 

Internet −− Linux −− DMZ segment 

Server PUBLIC IPs 

| 



Each of these solutions have pros and cons 


Item #1: If you're lucky enough to have an ISP that will set this up for you (pretty rare), all you need to do is 

use basic 'route' commands to get this running. This is the most rebust solution and doesn't require any form of 

IPMASQ or NAT to work. 

Item #2: 1:1 NAT isn't covered in this HOWTO yet but if you need a hand, just email me and I'll give you a 

hand. 

Item #3: ProxyARP is pretty strait forward but only works in specific situations and only works with Ethernet 

networks. Bridging is more powerful but will probably require the re−compiling of the kernel and some 

advanced configuration. Ultimately, neither of these solutions are IPMASQ anymore and thus I can't help you 

set them up. Fortunately, there are other HOWTOs out there that cover this topic: 

• http://www.tldp.org/HOWTO/Proxy−ARP−Subnet/index.html 

• http://www.tldp.org/HOWTO/Adv−Routing−HOWTO/lartc.bridging.html 

• http://www.tldp.org/HOWTO/Ethernet−Bridge−netfilter−HOWTO.html 

• http://bridge.sourceforge.net/ 

NOTE: If you have a bridged DSL or Cablemodem connection (not PPPoE), things are a little more difficult 

because your setup isn't routed. No worries though, check out the Bridge+Firewall Mini HOWTO and the 

Bridge+Firewall+DSL Mini HOWTO. These HOWTOs will teach you how to get your Linux box to support 

multiple IP addresses on a single interface! 

7.33. ( 1:1 NAT ) − I'd like to do 1:1 NAT but I can't figure out 

how to do it 

Please see the previous FAQ entry named Section 7.32 for all the details. 



7.34. ( Netstat ) − I'm trying to use the NETSTAT command 

to show my Masqueraded connections but its not working 

There might be a problem with the "netstat" program in 2.0.x−based Linux distros. After a Linux reboot, 

running "netstat −M" works fine but after a MASQed computer runs some successful ICMP traffic like ping, 

traceroute, etc., you might see something like: 

masq_info.c: Internal Error ìp_masquerade unknown type'. 

The workaround for this is to use the "/sbin/ipfwadm −M −l" command. You will also notice that once the 

listed ICMP masquerade entries timeout, "netstat" works again. 

7.35. ( VPNs ) − I would like to get Microsoft PPTP (GRE 

tunnels) and/or IPSEC (Linux SWAN) tunnels running 

through IP MASQ 

This IS possible for specific modes. Specifically, all of the kernel versions (2.4.x, 2.2.x, and 2.0.x) support 

patches to allow for both ONE or MULTIPLE PPTP users behind a IPMASQ server to connect to the −same− 

PPTP server. The 2.4.x kernels currently have a PPTP module now in the newest versions of the IPTABLES 

program and there is another version available on the IPMASQ WWW site. Please check out John Hardin's 

PPTP Masq page for details. 

7.36. ( Games ) − I want to get the XYZ network game to 

work through IP MASQ but it won't work. Help! 

First, check Steve Grevemeyer's MASQ Applications page. If your solution isn't listed there, try patching your 

Linux kernel with Glenn Lamb's http://ipmasq.webhop.net/files20/loose−udp−2.0.36.patch.gz LooseUDP 

patch which is covered in Section 6.10 above. Also check out Dan Kegel's NAT Page for more information. 

If you are technically inclined, use the program "tcpdump" and sniff your network. Try to find out what 

protocols and port numbers your XYZ game is using. With this information in hand, subscribe to the IP Masq 

email list and email your results for help. 

7.37. ( Stops working ) − IP MASQ works fine for a while but 

then it stops working. A reboot seems to fix this. Why? 

I bet you are using IPAUTOFW and/or you have it compiled into the kernel huh?? This is a known problem 

with IPAUTOFW. It is recommend to NOT even configure IPAUTOFW into the Linux kernel and use 

IPPORTFW option instead. This is covered with more details in Section 6.7. 

7.38. ( SMTP Relay ) − Internal MASQed computers cannot 

send SMTP or POP−3 mail! 

Though this isn't a Masquerading issue but many users do this so it should be mentioned. 


SMTP: The issue is that you are probably using your Linux box as an SMTP relay server and get the 

following error: 

"error from mail server: we do not relay" 

Newer versions of Sendmail and other Mail Transfer Agents (MTAs) disable relaying by default (this is a 

good thing). So do the following to fix this: 

• Sendmail: Enable specific relaying for your internal MASQed machines by editing the 

/etc/sendmail.cw file and add the hostname and domain name of your internal MASQed machine. 

You should also check to see that the /etc/hosts file has the IP address and Fully Qualified Domain 

Name (FQDN) configured in it. Once this is done, you need to restart Sendmail for it to re−read its 

configuration files. This is covered in TrinityOS − Section 25 

POP−3: Some users configure their internal MASQ'ed computer's POP−3 clients to connect to some external 

SMTP server. While this is fine, many SMTP servers out there will try to IDENT your connection on port 

113. Most likely your problem stems around your default Masquerade policy being set to DENY. This is 

BAD. Set it to REJECT and re−run your rc.firewall−* ruleset. 

7.39. ( Source Routing ) − I need different internal MASQed 

networks to exit on different external IP addresses 

Say you have the following setup: You have multiple internal networks and also multiple external IP 

addresses and/or networks. What you want to do is have LAN #1 to only use External IP #1 but you wan LAN 

#2 to use External IP #2. 

Internal LAN −−−−−−−−−−> official IP 


LAN #1 External IP #1 192.168.0.x −−> 123.123.123.11 

LAN #2 External IP #2 192.168.1.x −−> 123.123.123.12 

Basically, what we have described here is routing NOT only on the destination address (typical IP routing) but 

also routing based upon the SOURCE address as well. This is typically called "policy−based routing" or 

"source routing". This functionality is NOT available in 2.0.x kernels, it *IS* available for 2.2.x kernels via 

the IPROUTE2 package, and it is built into the new 2.4.x kernels using IPTABLES. 

First, you have to understand that both IPFWADM and IPCHAINS get involved *AFTER* the routing system 

has decided where to send a given packet. This statement really ought to be stamped in big red letters on all 

IPFWADM/IPCHAINS/IPMASQ documentation. The reason for this is that users MUST first have their 

routing setup correct, then start adding IPFWADM/IPCHAINS and/or Masq features. 

Anyways, for the example case shown above, you will need to persuade the routing system to direct packets 

from 192.168.0.x via 123.123.1233.11 and packets from 192.168.1.x via 123.123.123.12. That is the hardest 

part and adding Masq on top of correct routing is easy. 

To do this fancy routing, you will use IPROUTE2. Because this functionality has NOTHING to do with 

IPMASQ, this HOWTO does not cover this topic in great detail. Please see Section 2.7 for complete URLs 

and documentation for this topic. 


The "iprule" and "iproute" commands are the same as "ip rule" and "ip route" commands (I prefer the former 

since it is easier to search for.) All the commands below are completely untested, if they do not work, please 

let David Ranch know about it but please contact the IPROUTE2 email list for help. This function has 

NOTHING to do with IP Masquerading. 

2.4.x. kernels: 

The following would be integrated into the END of your rc.firewall−iptables ruleset 

EXTIF="eth0" 

INTNET1="192.168.0.0/24" 

INTNET2="192.168.1.0/24" 

EXTIP1="123.123.123.11" 

EXTIP2="123.123.123.12" 

iptables −t nat −A POSTROUTING −o $EXTIF −s $INTNET1 −j SNAT −−to $EXTIP1 

iptables −t nat −A POSTROUTING −o $EXTIF −s $INTNET2 −j SNAT −−to $EXTIP2 

2.2.x. kernels: 


The first few commands only need to be done once at boot, say in /etc/rc.d/rc.local file. 

# Allow internal LANs to route to each other, no masq. 

/sbin/iprule add from 192.168.0.0/16 to 192.168.0.0/16 table main pref 100 

# All other traffic from 192.168.1.x is external, handle by table 101 

/sbin/iprule add from 192.168.1.0/24 to 0/0 table 101 pref 102 

# All other traffic from 192.168.2.x is external, handle by table 102 

/sbin/iprule add from 192.168.2.0/24 to 0/0 table 102 pref 102 

These commands need to be issued when eth0 is configured, perhaps in 

/etc/sysconfig/network−scripts/ifup−post (for Redhat systems). Be sure to 

do them by hand first to make sure they work. 

# Table 101 forces all assigned packets out via 123.123.123.11 

/sbin/iproute add table 101 via 123.123.123.11 

# Table 102 forces all assigned packets out via 123.123.123.12 

/sbin/iproute add table 102 via 123.123.123.12 

At this stage, you should find that packets from 192.168.1.x to the 

outside world are being routed via 123.123.123.11, packets from 

192.168.2.x are routed via 123.123.123.12. 

It is IMPORTANT that these IPROUTE2 rules be run /BEFORE/ the rc.firewall−* 

ruleset is run. 

If everything hangs together, the masq code will see packets being 

routed out on 123.123.123.11 and 123.123.123.12 and will use those addresses 

as the masq source address. 

7.40. ( IPCHAINS rulesets on 2.4.x kernels ) − What the 

ipchains.o module can do on 2.4.x kernels 

Some people would like to continue using their legacy IPCHAINS rulesets on 2.4.x−based kernelw. 

Unfortunately, unless you are only doing packet firewalling and not trying to do any NATing (MASQ), 

PORTFW, or other advanced features, you're in trouble. 


• 


If you ARE only doing IPCHAINS filtering, all you need to do is unload all IPTABLES modules 

shown from the "/sbin/lsmod" command. After that, load the IPCHAINS module by running 

"/sbin/modprobe ipchains". After that, load your IPCHAINS ruleset as normal. 

♦ Please note that if you compiled IPTABLES support statically into the kernel, you CANNOT 

load the "ipchains" module (it shouldn't be even present) as it will conflict with the 

IPTABLES kernel code. Your ONLY option in this case is to recompile your kernel but make 

the IPTABLES and IPCHAINS options as modules. 

So why can't you run IPCHAINS MASQ/PORTFW functionality with a 2.4.x kernel? Once the IPCHAINS 

module is loaded, you CANNOT use any IPTABLES commands or modules since the code conflicts. In 

addition to this, you cannot use any legacy 2.2.x IPCHAINS masq modules on a 2.4.x kernel as the kernels are 

so radically different. Plus, this really shouldn't be an issue as all of this functionality is available via native 

IPTABLES modules now. Finally, you cannot use the IPMASQADM tool with a 2.4.x kernel as the program 

both won't compile and ultimately the PORTFW kernel handlers aren't present anymore (it's now done 

natively by the IPTABLES code). So, considering all of these facts: 

• You cannot run any form of PORTFW on this 2.4.x machine 

• Protocols that require special handling like FTP, IRC, CuSeeme, RealAudio, etc. will no longer work 

Basically, the ipchains kernel module included with the 2.4.x kernels is intended for basic packet firewall 

compatibility and NOT any NAT(MASQ) functionality. 

7.41. ( IPTABLES vs. IPCHAINS vs. IPFWADM ) − Why do the 

2.4.x, 2.2.x, and 2.0.x kernels use different firewall systems? 

IPTABLES supports the following features that IPCHAINS and IPFWADM doesn't: 

• Stateful IPv4 protocol and application tracking 

• Stateful IPv6 protocol tracking 

• True 1:1 and 1:Many NAT 

• Built−in PORTFW functionality 

• See the Section 2.6 section for more details 

IPCHAINS supports the following features that IPFWADM doesn't: 

• "Quality of Service" (QoS support) 

• A TREE style chains system vs. LINEAR system like IPFWADM (Eg. this allows something like "if 

it is ppp0, jump to this chain (which contains its own difference set of rules)" 

• IPCHAINS is more flexible with configuration. For example, it has the "replace" command (in 

addition to "insert" and "add"). You can also negate rules (e.g. "discard any outbound packets that 

don't come from my registered IP" so that you aren't the source of spoofed attacks). 

• IPCHAINS can filter any IP protocol explicitly, not just TCP, UDP, ICMP 

7.42. ( Upgrades ) − I've just upgraded to the x.y.z kernel, 

why isn't IP Masquerade working? 


There are several things you should check assuming your Linux IP Masq box already has proper connections 

to the Internet and your LAN: 

• Make sure you have the necessary features and modules are compiled and loaded. See earlier sections 

for details. 

• Check /usr/src/linux/Documentation/Changes and make sure you have the minimal 

requirement for the network tools installed. 

• Make sure you followed all of the tests in Chapter 5 of the HOWTO. 

• Make sure you are using the proper firewall tool for you kernel be it IPTABLES, IPCHAINS, or 

IPFWADM. 

• If you are doing PORTFW functionality, make sure you use the correct tool for your kernel version. 

IPTABLES has everything built−in, IPCHAINS requires the use of IPMASQADM, and IPFWADM 

reaquires the use of IPPORTFW or IPAUTOFW. This is completely covered in Section 6.7. 

• Go through all setup and configuration again! Usually, it's just a typo or a simple mistake you are 

overlooking. 

7.43. ( EQL ) − I need help with EQL connections and IP 

Masq 

EQL has nothing to do with IP Masq though they are commonly teamed up on Linux boxes. Because of this, I 

recommend checking out the NEW version of Robert Novak's EQL HOWTO for all your EQL needs. 

7.44. ( Wussing out ) − I can't get IP Masquerade to work! 

What options do I have for Windows Platforms? 

Looking to give up a free, reliable, high performance solution that works on minimal hardware and pay a 

fortune for something that needs more hardware, is lower performance and did I say less reliable? (IMHO. 

And yes, I have real life experience with these ;−) 

Okay, it's your call. If you want a Windows NAT and/or proxy solution, here is a decent listing. I don't prefer 

any one of these tools over another, especially since I haven't used them before. 

• Firesock (from the makers of Trumpet Winsock) 

♦ Does Proxy 

♦ http://www.trumpet.com.au 

• Iproute 

♦ DOS program designed to run on 286+ class computers 

♦ requires another box like Linux MASQ 

♦ (UNAVAILABLE) www.mischler.com/iproute/ 

• Microsoft Proxy 

♦ Requires Windows NT Server 

♦ Quite expensive 

♦ http://www.microsoft.com 

• NAT32 



♦ Windows 95/98/NT compatible 

♦ http://www.nat32.com 

♦ Roughly $25 for Win9x and $47 for Win9x and WinNT 

• SyGate 

♦ http://www.sygate.com 

• Wingate 

♦ Does proxy 

♦ Costs roughly $30 for 2−3 IPs 

♦ http://www.wingate.com 

• Winroute 

♦ Does NAT 

♦ WinRoute 


Lastly, do a web search on "MS Proxy Server", "Wingate", "WinProxy", or goto www.download.com. And 

definitely DON'T tell anyone that we sent you. 

7.45. ( Developers ) − I want to help with IP Masquerade 

development. What can I do? 

Join the Linux IP Masquerading DEVELOPERS list and ask the developers there what you can do to help. 

For more details on joining the list, check out Section 7.5 FAQ section. 

Please DON'T ask NON−IP−Masquerade development related questions there!!!! 

7.46. ( More INFO ) − Where can I find more information on 

IP Masquerade? 

You can find more information on IP Masquerade at the Linux IP Masquerade Resource that Ambrose Au 

maintains. 

You can also find more information at Dranch's Linux page where the TrinityOS and other Linux documents 

are kept. 

You may also find more information at The Semi−Original Linux IP Masquerading Web Site maintained by 

Indyramp Consulting, who also provides the IP Masq mailing list. 

Lastly, you can look for specific questions in the IP MASQ and IP MASQ DEV email archives or ask a 

specific question on these lists. Check out Section 7.5 FAQ item for more details. 

7.47. ( Translators ) − I want to translate this HOWTO to 

another language, what should I do? 

Make sure the language you want to translate to is not already covered by someone else. But, most of the 

translated HOWTOs are now OLD and need to be updated. A list of available HOWTO translations are 


available at the Linux IP Masquerade Resource. 


If a copy of a current IP MASQ HOWTO isn't in your proposed language, please download the newest copy 

of the IP−MASQ HOWTO SGML code from the Linux IP Masquerade Resource. From there, begin your 

work while maintaining good SGML coding. For more help on SGML, check out www.sgmltools.org 

7.48. ( Updates ) − This HOWTO seems out of date, are you 

still maintaining it? Can you include more information on 

...? Are there any plans for making this better? 

Yes, this HOWTO is still being maintained. In the past, Ambrose was guilty of being too busy working on 

two jobs and didn't have much time to work on this, my apologies. As of v1.50, David Ranch revamped the 

document and got it current again. 

If you think of a topic that could be included in the HOWTO, please send email dranch@trinnet.net. It will be 

even better if you can provide that information. We will then include the information into the HOWTO once it 

is both found appropriate and tested. Many thanks for your contributions! 

We have a lot of new ideas and plans for improving the HOWTO, such as case studies that will cover 

different network setup involving IP Masquerade, more on security via strong IPFWADM/IPCHAINS 

firewall rulesets, IPCHAINS usage, more FAQ entries, etc. If you think you can help, please do! Thanks. 

7.49. ( Thanks ) − I got IP Masquerade working, it's great! I 

want to thank you guys, what can I do? 

• Can you translate the newer version of the HOWTO to another language? 

• Thank the developers and appreciate the time and effort they spent on this. 

• Join the IP Masquerade email list and support new MASQ users 

• Send an email to us and let us know how happy you are 

• Introduce other users to Linux and help them when they have problems. 


Chapter 8. Miscellaneous 

8.1. Useful Resources 

• IP Masquerade Resource page Will have all the current information for setting up IP Masquerade on 

2.0.x, 2.2.x, and even old 1.2 kernels! 

• Juan Jose Ciarlante's WWW site (mirror) who is one of the current Linux IP Masquerade maintainers. 

• IP Masquerade mailing list Archives contains the recent messages sent to the mailing lists. 

• David Ranch's Linux page including the TrinityOS Linux document and current versions of the 

IP−MASQ−HOWTO.. Topics such as IP MASQ, strong IPFWADM/IPCHAINS rulesets, PPP, Diald, 

Cablemodems, DNS, Sendmail, Samba, NFS, Security, etc. are covered. 

• The IP Masquerading Applications page: A comprehensive list of applications that work or can be 

tuned to work through a Linux IP masquerading server. 

• For users setting up IP Masq on MkLinux, email Taro Fukunaga at tarozax@earthlink.net for a copy 

of his short MkLinux version of this HOWTO. 

• IP masquerade FAQ has some general information 

• Paul Russel's http://www.netfilter.org/ipchains/ doc and its possibly older backup at Linux 

IPCHAINS HOWTO. This HOWTO has lots of information for IPCHAINS usage, as well as source 

and binaries for the ipchains tool. 

• X/OS Ipfwadm page contains sources, binaries, documentation, and other information about the 

ipfwadm package 

• Check out the GreatCircle's Firewall mailing list for a great resource about strong firewall rulesets. 

• The LDP Network Administrator's Guide is a MUST for the beginner Linux administrator trying to 

set up a network. 

• The Linux NET HOWTO is also another comprehensive document on how to setup and configure 

Linux networking. 

• Linux ISP Hookup HOWTO and Linux PPP HOWTO gives you information on how to connect your 

Linux host to the Internet 

• Linux Ethernet−Howto is a good source of information about setting up a LAN running over 

Ethernet. 

• Donald Becker's NIC drivers and Support Utils 

• You may also be interested in Linux Firewalling and Proxy Server HOWTO 

• Linux Kernel HOWTO will guide you through the kernel compilation process 

• Other Linux HOWTOs such as Kernel HOWTO 

• Posting to the USENET newsgroup: comp.os.linux.networking 

8.2. Linux IP Masquerade Resource 

The Linux IP Masquerade Resource is a website dedicated to Linux IP Masquerade information also 

maintained by Ambrose Au. It has the latest information related to IP Masquerade and may have information 

that is not being included in the HOWTO. 

You may find the Linux IP Masquerade Resource at the following locations: 

• http://ipmasq.webhop.net/, Primary Site, redirected to http://ipmasq.webhop.net/ 

• http://ipmasq2.webhop.net/, Secondary Site, redirected to http://www.e−infomax.com/ipmasq 

Chapter 8. Miscellaneous 153

8.3. Thanks to the following supporters.. 

In Alphabetical order: 


• Gabriel Beitler, gabrielb@voicenet.com on providing section 3.3.8 (setting up Novell) 

• Juan Jose Ciarlante, irriga@impsat1.com.ar for contributing his work on the IPMASQADM port 

forward tool, the 2.1.x and 2.2.x kernel code, and the original LooseUDP patch, etc. 

• Steven Clarke, steven@monmouth.demon.co.uk for contributing his IPPORTFW IP port forwarder 

tool 

• Andrew Deryabin, djsf@usa.net for contributing his ICQ MASQ module 

• Ed Doolittle, dolittle@math.toronto.edu for suggestions to −V option in theipfwadm command for 

improved security 

• Matthew Driver, mdriver@cfmeu.asn.au for helping extensively on this HOWTO, and providing 

section 3.3.1 (setting up Windows 95) 

• Ken Eves, ken@eves.com for the FAQ that provides invaluable information for this HOWTO 

• John Hardin, jhardin@impsec.org for his PPTP and IPSEC forwarding tools 

• Glenn Lamb, mumford@netcom.com for the LooseUDP patch 

• Ed. Lott, edlott@neosoft.com for a long list of tested system and software 

• Nigel Metheringham, Nigel.Metheringham@theplanet.net for contributing his version of the IP 

Packet Filtering and IP Masquerading HOWTO, which makes this HOWTO a better and in−depth 

technical document section 4.1, 4.2, and others 

• Keith Owens, kaos@ocs.com.au for providing an excellent guide on ipfwadm section 4.2 on 

correction to ipfwadm −deny option which avoids a security hole, and clarified the status of ping 

over IP Masquerade 

• Michael Owings, mikey@swampgas.com for providing the section about CU−SeeMe and Linux 

IP−Masquerade Teeny How−To 

• Rob Pelkey, rpelkey@abacus.bates.edu for providing section 3.3.6 and 3.3.7 (setting up MacTCP and 

Open Transport) 

• Harish Pillay, h.pillay@ieee.org for providing section 4.5 (dial−on−demand using Diald) 

• Mark Purcell, purcell@rmcs.cranfield.ac.uk for providing section 4.6 (IPautofw) 

• David Ranch, dranch@trinnet.net for maintaining the HOWTO, helping with the Linux IP 

Masquerade Resource Page, the TrinityOS document, ..., too many to list here :−) 

• Paul Russell, rusty@linuxcare.com.au for all his work on IPTABLES, IPCHAINS, the IP Masquerade 

kernel patches in general, etc. The man is a IP NATing fool! 

• Ueli Rutishauser, rutish@ibm.neton providing section 3.3.9 (setting up OS/2 Warp) 

• Steve Grevemeyer, grevemes@tsmservices.com for taking over the IP Masq Applications page from 

Lee Nevo and updating it to a full DB backend. 

• Fred Viles, fv@episupport.com for his patches on proper port forarding of FTP. 

• John B. (Brent) Williams, forerunner@mercury.net on providing section 3.3.7 (setting up Open 

Transport) 

• Enrique Pessoa Xavier, enrique@labma.ufrj.bron the BOOTp setup suggestion 

• All the users on the IP−MASQ email list, masq@indyramp.com for their help and support for all the 

new Linux MASQ users. 

• Other code and documentation developers of IP Masquerade for this great feature 

• Delian Delchev, delian@wfpa.acad.bg 

• David DeSimone (FuzzyFox), fox@dallas.net 

• Jeanette Pauline Middelink, middelin@polyware.iaf.nl 

• Miquel van Smoorenburg, miquels@q.cistron.nl 

• Jos Vos, jos@xos.nl 

• And others who I may have failed to mention here (please let me know) 


• All users sending feedback and suggestions to the mailing list, especially those who reported errors in 

the document and the clients who are supported and not supported 

• We apologize if we have omitted any important names, not included information that some fellow 

users have sent us yet, etc. There were many suggestions and ideas sent but there wasn't enough time 

to verify and integrate these changes. David Ranch is constantly trying his best to incorporate all of 

the information sent to me into the HOWTO. I thank you for the effort, and I hope you understand our 

situation. 

8.4. Reference 

• Original IP masquerade FAQ by Ken Eves 

• IP masquerade mailing list archive by Indyramp Consulting 

• IP Masquerade WWW site by Ambrose Au 

• Ipfwadm page by X/OS 

• Various networking related Linux HOWTOs 

• Some topics covered in TrinityOS by David Ranch 

8.5. ChangeLOG 

TO do − HOWTO: 

• Add the scripted IPMASQADM example to the Forwarders section. Also confirm the syntax. 

• Add a little section on having multiple subnets behind a MASQ server 

• Confirm the IPCHAINS ruleset and make sure it is consistant with the IPFWADM ruleset 

TO DO − WWW page: 

• Update the PPTP patch on the masq site 

• Update the portfw FTP patch 

Changes from 05/22/05 to 11/13/05 

• 11/13/05 − Fix a bug where the PORTFW example rule in section 6.7 was incorrect. Updated the 

IPTABLES PORTFW section to include state tracking for the pre−routing rule, added a 

cross−reference to the PORTFW FAQ entry, and reduced some duplicate PORTFW examples in 

different chapters of the HOWTO. Thanks to Thomas Zajic for bringing this to my attention. 

• 10/23/05 − Updated the dynamic IP FAQ section to give complete examples on how to re−run the 

rc.firewall−* scripts for various different DHCP clients 

• 10/19/05 − Updated the HOWTO to be very clear on loading the various rc.firewall−* rulesets (there 

are 6 of them in this HOWTO both simple and stronger versions for IPTABLES, IPCHAINS, and 

IPFWADM) files vs. loading a generic rc.firewall file. I also updated the troubleshooting section to 

reflect this possibly confusing point. 

• 05/27/05 − Updated the Multiple NAT situation to include ProxyARP solutions 

• 05/26/05 − Clarified the section for IPMASQ on multiple internal LAN segments 



• 05/22/05 − Updated the rc.firewall−iptables−stronger ruleset to 0.87s. Removed the unused 

drop−and−logit chain as it was only later being deleted anyway. Thanks to Matthew Concannon for 


this one. 

• 05/21/05 − Updated the Multiple−IPs FAQ entry a bit 


• 05/03/05 − Updated the rc.firewall−iptables−stronger ruleset to fix a typo 


• 04/30/05 − Updated the IP address for unc.metalab.org and published the HOWTO to the web. 

• 12/18/04 − Added some comments in the IPTABLES, IPCHAINS, and IPFWADM rulesets why the 

default policy is ACCEPT and not something like DROP. 

• 07/24/04: Renamed the rc.firewall−2.4/2.2/2.0−* rulesets to rc.firewall−iptables/ipchains/ipfwadm−*. 

This change better reflects that these rulesets can run on different kernel versions (such as 2.6.x). 

Updated the rc.firewall−iptables−stronger ruleset to 0.85s to fix an improper /24 netmask for the 

INTIP variable. 

• 04/10/04: Updated the rc.firewall−2.4−stronger ruleset to use the 192.16.0.x network instead of 

192.168.1.x network to better align with the rest of the HOWTO 

• 04/04/04: Added that Redhat9 supports IPMASQ 


• 03/18/04: Added a sub−section for supporting multiple internal networks for IPTABLES 

• 02/02/04: Updated some old jhardin rubyriver to impsec.org URLs 

• 01/10/04: Updated the rc.firewall−2.4−stronger and 2.2 rulesets to make placement of PORTFW 

configs more obvious 

• 01/01/04: Some systems require that the /etc/rc.d/init.d/firewall−2.* files be executable. Fixed. 

Thanks to Chris Carter and others for the nudge. 

• 01/01/04: Some systems require that the /etc/rc.d/init.d/firewall−2.* files be executable. Fixed. 

Thanks to Chris Carter and others for the nudge. 

• 01/01/04: Added an additional chkconfig check on Redhat systems to make sure that the firewall will 

load upon init level change. Thanks to Chris Carter for the idea. 

• 12/19/03: Updated the rc.firewall−2.4−stronger ruleset to 0.82. This new ruleset has a special ICMP 

filter to work around a Netfilter bug. Also, the drop−and−log−it chain has been renamed to 

reject−and−log−it since that's actually what it's doing. Thanks to Bart Martens for the 

recommendations. 

• 12/13/03: Fixed some minor grammar issues. Thanks to Lawrence Berlinsk for pointing them out. 

• 11/30/03: Updated the rc.firewall−2.4−stronger ruleset to 0.81s, the rc−firewall−2.2−stronger ruleset 

to 0.72s, and updated the rc.firewall−2.0−stronger ruleset to 0.72s (never had a version # before). 

These changes reflect either the ruleset not having strong enough comments or allowing all traffic 

destined to the MASQ server itself from being protected. It's recommend that if you want to enable 

access to servers running on the MASQ server itself (http, ssh, etc.), selectively enable them under the 

OPTIONAL INPUT section. 

• 11/03/03: Updated the rc.firewall−2.2−stronger ruleset where an INTLAN rule that was allowing 

traffic from ANY IP address instead of the proper INTIP IP address only. This aligns the IPCHAINS 

ruleset with the IPTABLES and IPFWADM ruleset examples 

• 11/10/03: Deleted all kernelnotes.org URLS (juanjox URLs) 



• 10/25/03: Fixed a dead RFC1918 URL in section 3.3. Thanks to Mark Sobell for the report. 


• 07/07/03: Added the "reducing−masq−log" FAQ entry to help people reduce the size of their firewall 

logs. 

• 06/27/03: Updated the rc.firewall−2.4−stronger ruleset to 0.80s. Added a DISABLED ip_nat_irc 

kernel module section, changed the default of the ip_conntrack_irc to NOT load by default, and added 

additional kernel module comments. 

• 06/27/03: Updated the rc.firewall−2.4 ruleset to 0.75. Added additional iptables kernel module 

comments. 

• 06/24/03: Added Debian 3.0 to the supported distro list 

• 06/23/03: Change the PMTU URLs to point to Phil's primary www site 


• 06/22/03: Updated the various Indyramp MASQ email URLs again as things seemed to have changed. 

Again. 

• 06/21/03: Rewrote the MTU FAQ section to be more clear, include specific information of the 

problems, and also fixed a bad typo for PPPoE users who were trying to configure 

"−−clamp−mss−to−mtu" when it should have been "−−clamp−mss−to−pmtu" (missing the p in pmtu). 

• 06/13/03: Added kernel info for Mandrake 8.1 

• 06/02/03: Fixed a typo where extended 2.2.x kernel checks for IPMASQ functionality was using "cat" 

and not "ls" 



• 05/26/03: updated the firewall rulesets: rc.firewall−2.4 (to 0.74), rc.firewall−2.2 (to 1.22), 

rc.firewall−2.4−stronger (to 0.79s), and rc.firewall−2.2−strongerw (to 0.71s) to use modprobe instead 

of insmod. 

• 05/26/03: Added how to dump IPTABLES MASQ entries in the Accounting FAQ section 

• 05/26/03: Added Clamp−MSS recommendations to the MTU faq section 

• 05/26/03: Added additional troubleshooting steps in Section 5 when the MASQ client cannot ping the 

MASQ server. 

• 05/26/03: Added additional traffic shaping / traffic limiter URLs to the SHAPING FAQ entry 

• 05/26/03: Renamed the "IPROUTE2" FAQ entry to "Souce Routing"; Added IPTABLES examples to 

the section; fixed an incorrect IP address of 62123.123.123.123 

• 05/25/03: Fixed a SGML script that was improperly converting ampersands for the downloadable 

firewall−* and rc.firewall−* scripts. Also caught a SGML ampersand bug in a comment section of the 

rc.firewall−2.0 file 

• 05/25/03: Deleted several dead links: ftp.gts.cz, novell.com LWP5, Old Juanjox mirror (geocities), 

old ipmasq2.webhop.net URL, old zzdmacka NAT information URL, old linux.org/uk/VERSION url, 

old netfilter.samba.org URLs (no longer a netfilter mirror − redirect), old Activision BattleZone DLL 

url, old iproute2 (rpms, ras.ru, donlug, dontsk, tusur, waaug, etc.) urls, old rlynch ipautofw mirror 

• 05/25/03: Updated several URLs: suse/proxy_suite/, www.indyramp.net URLs, several urls with " ~ " 

in it became ~732 for some reason, updated all of the jhardin URls to point from wolfnet.com to 

impsec.org, updated all LDP urls (linuxdoc.org to tldp.org), IPCHAINS patches for 2.0.x kernels, 

metalab to tldp.org, winfiles.com to download.com, Microsoft technet article 172227, Oidentd, 

mumford LooseUDP URL, 2.2.x PORT−FTP URL, IRQTUNE url, midentd URL 

• 05/25/03: Pending updates from remote webmasters: Indyramp EQL URL, insecurity.net sidentd 

• 05/25/03: Lots of little updates like:: updated the Intro section verbage a little to reflect BETA kernels 

and not OLD kernels; Updated the Forward section (not PORTFW) to be a little more generic; Added 

a link in the Forward to the IPMASQ email list; Updated the dates in the copyright notice; 

• 05/24/03: Updated the "Current Status" to add the remark that some programs have been updated to 

use NAT−friendly protocols and thus special NAT modules are no longer required 


• 05/24/03: Updated the 2.4 Requirements section: deleted a duplicate line (true 1:1 NAT); cleaned 

some addition things up; Added CuSeeme to the 2.4 ported list 

• 05/24/03: Updated the 2.2 / 2.0 Requirements section: Deleted the reference to the obsoltele IPMASQ 

ICQ module; Updated the link for the LooseUDP URL; 

• 05/24/03: Updated the Compiling Linux 2.2.x / 2.0.x section: Deleted the recommendations to load 

the rc.firewall ruleset via rc.local. This should come later in the HOWTO and offer other methods for 

different Linux distributions 

• 05/24/03: Updated the ICQ Application section to say that these steps are /not/ required for modern 

ICQ clients. I've left this section in the HOWTO to demonstrate a large PORTFW example 

• 05/24/03: Made some of the FAQ entries more kernel version generic and also deleted the 2.0.x 

"upgrades−cont.html" FAQ entry as it was basically a duplicate 

• 05/24/03: Updated the LooseUDP game section to explain how it works, explain how much of this 

was properly solved under the stateful IPTABLES systtem, and also say that it is NOT available for 

the 2.4.x kernels. If IPTABLES's stateful UDP tracking doesn't work for, you're probably out of luck. 

• 05/24/03: Mentioned in the FAQ section that MASQ timers are NOT adjustable under IPTABLES 

• 05/24/03: Vastly expanded the packet firewall log FAQ entry and finally added a IPTABLES packet 

log description section. I also aligned the IPCHAINS example to match the IPFWADM entry 

• 04/11/03: Fixed a incorrect echo statement saying the IPTABLES policy was being set to REJECT 

and not DROP. 


• 04/08/03: Added additional formatting and the "ip_masquerade" /proc entry into Section 3.2. This 

helps users determine if their kernel is MASQ−ready. 

• 03/08/03: Added the EXTIP variable to the 2.4.x PORTFW example as several people were trying to 

use this with the BASIC ruleset and I had assumed they were using the STRONGER ruleset. Thanks 

to Greg Lukins for bringing this to my attention. 

• 03/08/03: Added Distros to the MASQ compatibility list: Mandrake, Gentoo 

• 02/08/03: Forgot to update the VERSION number for the rc.firewall−2.4−stronger rulese. Added 

some additional formatting 

• 02/01/03: Added additional checking in the kernel compiling section to understand if your kernel 

supports IPMASQ via modules or by being statically compiled in. 



• 01/31/03: Doh. I should have read my own comments. I've reversed the 2.4.x. policy settings from 

REJECT back to DROP. REJECT, for some lame reason, is not a legal policy. The recommended 

REJECT action is still carried out via the "drop−and−log−it" user chain. 

• 01/30/03: Updated the Multiple−IPs FAQ entry to better describe how users that want to put external 

IPs behind a Linux router. Added additional URLs and cleaned up the text a bit too. 

• 01/30/03: Updated the 2.4.x requirement section to reflect more of the pros of IPTABLES as well as 

updated the update status of some old legacy 2.2.x modules 

• 01/30/03: Added an additional FAQ entry that clearly explains what the ipchains.o module can and 

CANNOT do on 2.4.x. kernels 

• 01/28/03: Extensively updated the 2.4.x kernel compilation section to reflect a 2.4.20 kernel with 

IPTABLES 1.2.7a. The section also reflects the new methods to compile IPTABLES, apply 

Patch−O−Matic patches, and also included lots of example output too. 

• 01/28/03: Updated the kernel compiling section to be a little more clear on how different Linux 

distros can have different kernels (modules vs. monolithic) 

• 01/17/03: Fixed a major issue where the rc.firewall−2.2−stronger ruleset was referencing missing 

executable variables. This was taken from the 2.4−stronger ruleset but I guess I forgot to finish it off. 


Fixed. Thanks to Samuel Kim for catching this! 

• 01/17/03: Fixed an issue where the rc.firewall−2.2−stronger's commented HTTP section was missing 

the "−p tcp" option. Thanks to Samuel Kim for catching this! 

• 01/16/03: Updated the URL for DJSF's ICQ module 

• 01/16/03: Changed the default policy and drop chain from DENY to REJECT on both IPTABLES 

rulesets and on the advanced IPFWADM rulset. Thanks to Jonathan Hutchins for bringing this to my 

attention. 

• 01/16/03: Fixed a typo in the commented out HTTPd OUTPUT section of the rc.firewall−2.2−s 

ruleset 

• 01/13/03: Updated the IPMASQ www site URL from ipmasq.cjb.net to ipmasq.webhop.net. CJB 

started to change their policies so we switched. 

• 01/13/03: Added to the 2.4.x Requirements section that IPTABLES v1.2.7a is out and recommended. 

• 01/13/03: Added an additional test item to the "Test Section − Section 5" for versions of IPTABLES 

that are too old. I also cleaned up this section to read easier. 

• 01/13/03: Updated the rc.firewall−2.4−stronger ruleset to include commented rules to allow in HTTP 

traffic to a local HTTP server. Also added a rule comment in the FORWARD section to help users 

know where to put PORTFW commands. 

• 01/13/03: Updated the rc.firewall−2.2−stronger ruleset to include commented rules to allow in HTTP 

traffic to a local HTTP server. Also added a rule comment in the FORWARD section to help users 

know where to put PORTFW commands. 

• 01/13/03: Clarified the PORTFW section to help users better understand where the PORTFW 

commands should go in the rc.firewall rulesets. I also cleaned up this section to read a little better. 


• 01/03/03: Added Redhat 7.3 and 8.0 to the compatibility chart. 

• 01/03/03: Fixed various typos. Thanks to Gabriel Withington for the sharp eye. 

• 12/22/02: Updated the 2.2.x H.323 kernel patch URL. Thanks to Maxime Plante for pointing this out. 

• 12/22/02: Updated the 2.4.x kernel compiling section to let users know that most modern kernels don't 

need IPTABLES Patch−o−matic patches to be applied except to fix bugs or add additional 

functionality. 


• 11/27/02: Fixed the init.d scripts to point the header to the correct config file. This must be due to 

newer versions of "chkconfig" doing better checking. Please note that this might still be a problem for 

the rc.firewall−2.?−stronger rulesets. Thanks to Joris Van Puyenbroeck for the heads up. 

• 11/25/02: Updated all the firewall comments to reflect that PPPoE users need to user the "ppp0" 

logical interface as their external interface instead of the physical interface such as "eth0". Thanks to 

Meng Cheah for the nudge. 

• 11/13/02: Updated the URL for the Donald Becker based NIC drivers. Thanks to Bruce Gorgon for 

the heads up. 

• 11/01/02: Added a new FAQ section that covers redirection of local INTERNAL traffic to internal 

PORTFWed servers 

• 11/01/02: Updated the PORTFW section to be a little more clear. 



• 09/29/02: Fixed a stray incorrect IP address pointing to metalab.unc.edu 

• 08/29/02: Fixed a typo in the firewall−2.2 startup script which was starting the 2.4 firewall and not the 

2.2. version. Thanks to Jean−Marc Vanel for catching this. 



• 08/25/02: Updated the rc.firewall−2.2−stronger and rc.firewall−2.2 scripts to use shell environment 

variables. 

• 07/09/02: Updated the FTP PORTFW section to be more readible 

• 07/06/02: Replaced all the filewatcher.org URLs with netfilter.org URLs 

• 06/12/02: Changed some of the formatting to try and help newbies better understand that the "\" 

character is used as a continuation of the previous line. 

• 06/12/02: Updated the IP address of metalab.unc.edu in Section 5. Thanks to Pete Trachy for bringing 

this to my attention but please note that even major sites like Metalab change their IPs, subnets, or 

even ISPs from time to time. 

• 06/02/02: Updated the rc.firewall−2.4 ruleset to include a commented option for NATing IRC DCCs, 

added the use of more environment vars, and added additional formatting. 

• 05/18/02: Added some extra # lines the commented section of the the rc.firewall−2.4−stronger ruleset 

to better serve Cut and Paste users. 

• 05/04/02: − Updated the various PPTP MASQ links to point to a valid URL. Also updated the 

HOWTO to reflect that PPTP is now supported on the 2.4.x kernels. 

• 05/03/02: − Updated the 2.4.x kernel requirements section to point out that IPCHAINS compatibility 

under 2.4.x kernels isn't very good. If you want to use IPMASQ under a 2.4.x kernel, you should use 

IPTABLES rules only. 

Changes from 01/05/02 to 04/19/02 − v2.00.041902 pubsished to the LDP 

• 04/01/02: − Updated the rc.firewall−2.4−stronger ruleset to denote and disable internal DHCP server 

support on the OUTPUT rules 

• 02/09/02: − Added Redhat−style init.d scripts to start the rc.firewall files 

• 02/09/02: − Updated all the various chapters to use human readable file names vs. things like 

x2623.html. 

• 02/09/02: − Expanded the IPMASQ accounting section 

• 02/04/02: − Deleted an extra "$" from the PORTFW variable in section 6.7.1 

• 01/31/02: − Updated the URLs for the PPPd and Diald homepages 

• 01/26/02: − Fixed some typos and added a LooseUDP clarification to tell users to read the example 

rc.firewall−2.2 ruleset comments on how to enable LooseUDP. 

• 01/08/02: − Made some slight clarifications to IP Alias support 

Changes from 11/19/01 to 01/05/02 − 010502 pubsished to the LDP 

• 01/05/02: − Added disabled rules to the rc.firewall−2.4−stronger ruleset to support INTERNAL 

DHCP server and EXTERNAL access to a WWW server running on the MASQ machine. 

• 01/05/02: − Added required changes to the loading of the ip_conntrack_ftp module if people 

PORTFW to non−standard FTP ports. 

• 01/05/02: − Added an example in the 2.4.x PORTFW section on how to REDIRECT internal traffic 

back to an INTERNAL server. This is the same as running REDIR under 2.2.x and 2.0.x kernels. 

• 01/05/02: − Added Juanjox mirror URLs to the HOWTO. 

• 01/04/02: − Clarified and cleaned up the ICQ PORTFW section; Added thoughts on the ip_masq_icq, 

PORTFW, and SOCKS solutions 

• 01/05/02: − Added Slackware 8.0 to the supported list. 

• 01/04/02: − Fixed some spelling mistakes in the 2.4 and 2.2 rulesets. Thanks to Michael Ott for the 

sharp eye. 

• 12/19/01: − Fixed a minor comment typo in the rc.firewall−2.4 file. Thanks to Bruno Negrao for this 

one. 

• 12/02/01: − Fixed some minor version typos in the 2.4.x rc.firewall ruleset; Added a missing 

$PORTFWIF variable for the 2.4.x PORTFW example. Thanks to Neil Bunn for the errata. 


• 11/25/01: − Expanded on the ipchains module conflict error messages in Section 5 

• 11/23/01: − Updated the HOWTO to reflect a new PPTP kernel module for the 2.4.x kernels 

• 11/19/01: − Clarified the PPTP supports for 2.4.x kernels 

Changes from 08/26/01 to 11/18/01 − 111801 published to the LDP 

• 11/12/01: − updated various comments to reflect new versions:linux 2.4.14, iptables 1.2.4, and linux 

2.2.20. 

• 11/12/01: − Added the rc.firewall−2.4−stronger ruleset to the HOWTO, updated the 2.4.x kernel and 

IPTABLES compiling steps to reflect 2.4.14 and 1.2.4. 

• 11/10/01: − Added the directly downloadable versions of the 2.4, 2.4−stronger, 2.2, 2.2−stronger, 2.0, 

and and 2.0.x−stronger rulesets to the WWW. 

• 11/10/01: − Updated the 2.4.x PORTW example to add the missing FORWARD option. 

• 11/10/01: − Updated the DSL−HOWTO link in the HOWTO 

• 10/27/01: − Updated the network diagram in section 2.5 to be a little more verbose. 

• 09/18/01: − Fixed some broken reference links pointing to the respective 2.4.x, 2.2.x, and 2.0.x kernel 

compiling recommendations. 

• 09/16/01: − Cleaned up and updated the PORTFW section to also include PREROUTING examples 

for 2.4.x kernels. 

• 09/13/01: − Updated the IPTABLES simple rc.firewall ruleset to 0.62. This fixed a typo on the 

MASQ enable line that used eth0 instead of $EXTIF. Thanks to Hafi for reporting this. 

• 09/07/01: − It seems that most people who are getting IPCHAINS and IPTABLES conflicts are 

running Redhat 7.1. I have updated section 5 on how to fix this. Thanks to Jason Wenzel for helping 

me with this. 

• 09/07/01: − Noted that IPTABLES v1.2.3 is current version. All versions less than v1.2.3 have an 

FTP module bug that can bypass strong firewall rulesets. Please upgrade your copy of IPTABLES 

now. 

• 09/07/01: − Created version numbers for the simple rc.firewall rulesets (IPTABLES − v0.61) 

(IPCHAINS − v1.01) (IPFWADM − v2.01). and cleaned up some of the comments in each section. 

• 09/07/01: − Added rules to the simple rc.firewall rulesets to flush the various tables. In addition to 

this, I have added the use of environment variables and more echo statements in the rulesets to make 

things easier to edit and monitor. Thanks to Ian Bishop for the good idea. 

• 09/07/01: − Added the use of EXTIF and INTIF interface variables in each of the rc.firewall and 

partial firewall rulesets for better clarity (similar to how TrinityOS has been doing for a while now). 

Thanks to Sean McKeon for the nudge. 

• 09/07/01: − Fixed a typo in the UNIX client configuration section where the network broadcast was 

192.168.0.25 instead of .255. 

Changes from 2.01 to 2.05 − 08/26/01 

• 08/19/01: − Added an additional testing step in Section5 to make sure the rc.firewall file loads ok. 

Thanks to Steven Levis for the good idea. 

• 08/15/01: − Change the reference for the /etc/hosts file from RFC952 to RFC1035. Thanks to Michael 

F. Maggard for the correction. 



• 08/12/01: − Updated the basic IPTABLES ruleset to 0.60 which fixed a major issue where all 

MASQed packets were being dropped. Ultimately, I forgot to add a rule to ACCEPT correct packets 

through the forwarding chain. 

• − Added an additional rule to log all bogus FORWARD packets 


• − Load the FTP nat modules now by default 

• − Changed the load order of some of the kernel modules to not create bogus error messages 

• − Added an IPTABLES section on how to MASQ specific hosts vs. an entire subnet 

• − Added more MASQ−client compatible operating systems 

• 07/19/01: − The advanced IPCHAINS example for forwarding between multiple interfaces was 

missing the critital "−j ACCEPT" to actually let the packets flow. Thanks to Shingo Yamaguchi for 

catching this. 


• 06/21/01: Updated Section 5 (Testing Section) to add an additional test to help users troubleshoot 

their MASQ setup. There are now a total of −11− tests. 06/16/01: Updated the intro History section at 

the beginning of the HOWTO. 06/14/01: Added mirror Netfilter and IPCHAINs mirror URLs 

06/13/01: Updated the H.323 URL 

• 06/10/01: Double DOH! The simple rc.firewall script for the 2.4 kernels had two major errors in it. 

The new version is far more informative and even works! I am continuing to go through the HOWTO 

and cleaning things up but I'm not done quite yet. 

• 06/02/01: Updated the lists of known compatible MASQ'ed operating systems (Windows M3, Linux 

2.3, 2.4, etc) Made more references to DHCP and DNS in the various different MASQ client 

configuration guides. 

• 04/12/01: Thanks to the Joshua X and the other people at Command Prompt, Inc. for the port of the 

HOWTO from LinuxDoc to DocBook. Add email list URL to line 126 



• A BIG thanks to the Joshua X and the other people at Command Prompt, Inc. for the port of the 

HOWTO from LinuxDoc to DocBook. 

• Added a quick upfront notice in the intro that running a SINGLE NIC in MASQ mutliple ethernet 

segments is NOT recommended and linked to the relivant FAQ entry. Thanks to Daniel Chudnov for 

helping the HOWTO be more clear. 

• Added a pointer in the Intro section to the FAQ section for users looking for how MASQ is different 

from NAT and Proxy services. 

• Reordered the Kernel requirements sections to be 2.2.x, 2.4.x, 2.0.x 

• Expanded the kernel testing in Section 3 to see if a given kernel already supports MASQ or not. 

• Reversed the order of the displayed simple MASQ ruleset examples (2.2.x and 2.0.x) 

• Cleaned up some formatting issues in the 2.0.x and 2.2.x rc.firewall files 

• Noted in the 2.2.x rc.firewall that the defrag option is gone in some distro's proc (Debian, 

TurboLinux, etc) 

• Added a NOTE #3 to the rc.firewall scripts to include instructions for Pump. Thanks to Ross Johnson 

for this one. 

• Cleaned up the simple MASQ ruleset examples for both the 2.2.x and 2.2.x kernels 

• Updated the simple and stronger IPCHAINS and IPFWADM rulesets to include the external interface 

names (IPCHAINS is −i; IPFWADM is −W) to avoid some internal traffic MASQing issues. 

• Vastly expanded the Section 5 (testing) with even more testing steps with added complete examples 

of what the output of the testing commands should look like. 

• Moved the H.323 application documentation from NOT supported to Supported. :−) 

• Reordered the Multiple LAN section examples (2.2.x then 2.0.x) 

• Made some additional clarifications to the Multiple LAN examples 

• Fixed a critical typo with multiple NIC MASQing where the network examples had the specified 

networks reversed. Thanks to Matt Goheen for catching this. 



• Added a little intro to MFW in the PORTFW section. 

• Reveresed the 2.0.x and 2.2.x sections for PORTFW 

• Updated the news regarding PORTFWing FTP traffic for 2.2.x kernels 

NOTE: At this time, there *IS* a BETA level IP_MASQ_FTP module 

for PORT Forwarding FTP connections 2.2.x kernels which also supports 

adding additional PORTFW FTP ports on the fly without the requirement 

of unloading and reloading the IP_MASQ_FTP module and thus breaking 

any existing FTP transfers. 

• Added a top level note about PORTFWed FTP support 

• Added a noted to the 2.0.x PORTFW'ed FTP example why users DON'T need to PORTFW port 20. 

• Updated the PORTFW section to also mention that users can use FTP proxy applications like the one 

from SuSe to support PORTFWed FTP−like functionality. Thanks to Stephen Graham for this one. 

• Updated the example for how to enable PORTFWed FTP to also include required configurations on 

how the ip_masq_ftp module is loaded for users who use multiple PORTs to contact multiple internal 

FTP servers. Thanks to Bob Britton for reminding me about this one. 

• Added a FAQ entry for users who have embedded ^Ms in their rc.firewall file 

• Expanded the FAQ entry talking about how MASQ is different from NAT and Proxy to include some 

informative URLs. 

• Updated the explanation of the MASQ MTU issue and described the two main explanations for the 

issue. 

• Clarified that the RFC, PPPoE should only require an MTU of 1492 though some ISPs require a 

setting of 1460. Because of this, I have updated the example to show an MTU of 1492. 

• Broke out the Windows 9x sections into Win95 and Win98 as they use different settings (DWORD 

vs. STRING). I also updated the sections to be clearer and the Registry backup methods have been 

updated. 

• Fixed a typo where the NT 4.0 Registry entries were backwards (Tcpip/Parameters vs. 

Parameters/Tcpip). 

• Fixed an issue where the WinNT entry should have been a DWORD and not a STRING. 

• A serious thanks goes out to Geoff Mottram for his various PPPoE and various Windows Registry 

entry fixes. 

• Added an explict URL for Oident in the IRC FAQ entry 

• Updated the FAQ section regarding some broken "netstat" versions 

• Added new FAQ sections for MASQ accounting ideas and traffic shaping 

• Expanded the IPROUTE2 FAQ entry on what Policy−routing is. 

• Moved the IPROUTE2 URLs to the 2.2.x Kernel requirements section and also added a few more 

URLs as well. 

• Corrected the "intnet" varible in the stronger IPCHAINS ruleset to reflect the 192.168.0.0 network to 

be consistent with the rest of the example. Thanks to Ross Johnson for this one. 

• Added a new FAQ section for users asking about forwarding problems between multiple internal 

MASQed LANs. 

• Added a new FAQ section about users wanting to PORTFW all ports from multiple external IP 

addresses to internal ones. I also touched on users who were trying to PORTFW all ports on multiple 

IP ALIASed interfaces and also noted the Bridge+Firewall HOWTO for DSL and Cablemodem users 

who have multiple IPs in a non−routed environment. 

• Added Mandrake 7.1, Mandrake 7.2, and Slackware 7.1 to the supported list 

• Added Redhat 7.0 to the MASQ supported distros. Thanks to Eugene Goldstein for this one. 

• Fixed a mathematical error in the "Maximum Throughput" calculation in the FAQ section. Thanks to 

Joe White @ ip255@msn.com for this one. 

• Fixed the Windows9x MTU changes to be a STRING change and not a DWORD change to the 

registry. Thanks to jmoore@sober.com for this one. 


• Updated the comments in the 2.0.x rc.firewall script to note that the ip_defrag option is for both 2.0 

and 2.2 kernels. Thanks to pumilia@est.it for this clarification. 



• Updated the URL for TrinityOS to reflect its newest layout 

• Caught a typo in the IPCHAINS rulesets where I was setting "ip_ip_always_defrag" instead of 

"ip_always_defrag" 

• The URL to Taro Fukunaga was invaild since it was using "mail:" instead of "mailto:" 

• Added some clarification to the "Masqing multiple internal interfaces" where some users didn't 

understand why eth0 was referenced multiple times. 

• Fixed another "space after the EXTIP variable" bug in the stronger IPCHAINS section. I guess I 

missed one. 

• In Test #7 of Section 5, I referred users to go back to step #4. That should have been step #6. 

• Updated the kernel versions that came with SuSe 5.2 and 6.0 

• Fixed a typo (or vs. of) in Section 7.2 

• Added Item #9 to the Testing MASQ section to refer users who are still haing MASQ problems to 

read the MTU entry in the FAQ 

• Improved the itemization in Section 5 

• Updated the IPCHAINS syntax to show the MASQ/FORWARD table. Before, it was valid to run 

"ipchains −F −L" but now only "ipchains −M −L" works. 

• Updated the LooseUDP documentation to reflect the new LooseUDP behavior in 2.2.16+ kernels. 

Before, it was always enabled, now, it defaults to OFF due to a possible MASQed UDP port scanning 

vulnerability. I updated the BASIC and SEMI−STRONG IPCHAINS rulesets to reflect this option. 

• Updated the recommended 2.2.x kernel to be 2.2.16+ since there is a TCP root exploit vulnerability 

on all lesser versions. 

• Added Redhat 6.2 to the MASQ supported list 

• Updated the link for Sonny Parlin's FWCONFIG to point to fBuilder. 

• Updated the various examples of IP addresses from 111.222.333.444 to be 111.222.121.212 and 

within a valid IP address range 

• Updated the URL for the BETA H.323 MASQ module 

• Finally updated the MTU FAQ section to help out PPPoE DSL and Cablemodem users. Basically, 

Section 7.15 now reflects the fact that users can also change the MTU settings of all of their 

INTERNAL machines to solve the dreaded MASQ MTU issue. 

• Added a clarification to the PORTFW section that PORTFWed connections which work for 

EXTERNAL clients but will not work for INTERNAL clients. If you also need INTERNAL portfw, 

you will need to also implement the REDIR tool as well. I also noted that this issue is fixed in the 

2.4.x kernels with Netfilter. 

• I also added a technical explanation from Juanjo to the end of the PORTFW section to why this 

senario doesn't work properly. 

• Updated all of the IPCHAINS URLs to point to Paul Rusty's new site at 

http://www.netfilter.org/ipchains/ 

• Updated Paul Rustys email address 

• Added a new FAQ section for users whose connections remain idle for a long period of time and 

PORTFWed connections no longer work. 

• Updated all the URLs to the LDP that pointed to metalab.unc.edu to the new site of linuxdoc.org 

• Updated the Netfilter URLs to point to renamed HOWTOs, etc. 

• I also updated the status of the 2.4.x support to note that I *will* add full Netfilter support to this 

HOWTO and if the time comes, then split that support off into a different HOWTO. 

• Updated the 2.4.x Requirements section to reflect how NetFilter has changed compared to 

IPFWADM and IPCHAINS and gave a PROs/CONs list of new features and changes to old 


ehaviors. 

• Added a TCP/IP math example to the "My MASQ connection is slow" FAQ entry to better explain 

what a user should expect performance wise. 

• Updated the HOWTO to reflect that newer versions of the "pump" DHCP client now can run scripts 

upon bringup, lease renew, etc. 

• Updated the PORTFWing of FTP to reflect that several users say they can successfully forward FTP 

traffic to internal machines without the need of a special ip_masq_ftp module. I have made the 

HOWTO reflect that users should try it without the modified module first and then move to the patch 

if required. 


• Ambrose Au's name has been taken off the title page as David Ranch has been the primary maintainer 

for the HOWTO for over a year. Ambrose will still be involved with the WWW site though. 

• Deleted a stray SPACE in section 6.4 

• Re−ordered the compatible MASQ'ed OS section and added instructions for setting up a AS/400 

system running on OS/400. Thanks to jaco@libero.it for the notes. 

• Added an additional PORFW−FTP patch URL for FTP access if HTTP access fails. 

• Updated the kernel versions for Redhat 5.1 & 6.1 in the FAQ 

• Added FloppyFW to the list of MASQ−enabled Linux distros 

• Fixed an issue in the Stronger IPFWADM rule set where there were spaces between "ppp_ip" and the 

"=". 

• In the kernel compiling section for 2.2.x kernels, I removed the reference to enable 

"CONFIG_IP_ALWAYS_DEFRAG". This option was removed from the compiling section and 

enabled by default with MASQ enabled in 2.2.12. 

• Because of the above change in the kernel behavior, I added the enabling of ip_always_defrag to all 

the rc.firewall examples. 

• Updated the status of support for H.323. There are now ALPHA versions of modules to support 

H.323 on both 2.0.x and 2.2.x kernels. 

• Added Debian v2.2 to the supported MASQ distributions list 

• Fixed a long standing issue where the section that covered explict filtering of IP addresses for 

IPCHAINS had old IPFWADM syntax. I've also cleaned this section up a little and made it 

understandable. 

• Doh! Added Juan Ciarlante's URL to the important MASQ resources section. Man.. you guys need to 

make me more honest than this!! 

• Updated the HOWTO to reflect kernels 2.0.38 and 2.2.15 

• Reversed the order shown to compile kernels to show 2.2.x kernels first as 2.0.x is getting pretty old. 

• Updated the 2.2.x kernel compiling section to reflect the changed options for the latter 2.2.x kernels. 

• Added a a possible solution for users that fail to get past MASQ test #5. 



• Added a missing subsection for /proc/sys/net/ipv4/ip_dynaddr in the stronger IPCHAINS ruleset. 

Section 6.5 

• Changed the IP Masq support for Debian 2.1 to YES 

• Reorganized and updated the "Masq is slow" FAQ section to include fixing Ethernet speed and 

duplex issues. 

• Added a link to Donald Becker's MII utilities for Ethernet NIC cards 

• Added a missing ")" for the 2.2.x section (previously fixed it only for the 2.0.x version) to the ICQ 

portfw script and changed the evaluation from −lt to −le 

• Added Caldera eServer v2.3 to the MASQ supported list 


• Added Mandrake 6.0, 6.1, 7.0 to the MASQ supported list 

• Added Slackware v7.0 to the MASQ supported list 

• Added Redhat 6.1 to the MASQ supported list 

• Added TurboLinux 4.0 Lite to the MASQ supported list 

• Added SuSe 6.3 to the MASQ supported list 

• Updated the recommended stable 2.2.x kernel to be anything newer than 2.2.11 

• In section 3.3, the HOWTO forgot how to tell the user how to load the /etc/rc.d/rc.firewall upon each 

reboot. This has now been covered for Redhat (and Redhat−based distros) and Slackware. 

• Added clarification in the Windows WFWG v3.x and NT setup sections why users should NOT 

configure the DHCP, WINS, and Forwarding options. 

• Added a FAQ section on how to fix FTP problems with MASQed machines. 

• Fixed a typo in the Stronger firewall rulesets. The "extip" variabl cannot have the SPACE between the 

variable name and the "=" sign. Thanks to johnh@mdscomp.com for the sharp eye. 

• Updated the compatibly section: Mandrake 7.0 is based on 2.2.14 and TurboLinux v6.0 runs 2.2.12 


• Updated the ICQ section to reflect that the new ICQ Masq module supports file transfer and real−time 

chat. The 2.0.x module still has those limitations. 

• Updated Steven E. Grevemeyer's email address. He is the maintainer of the IP Masq Applications 

page. 

• Fixed a few lines that were missing the work AREN'T for the "setsockopt" errors. 

• Updated a error the strong IPCHAINS ruleset where it was using the variable name "ppp_ip" instead 

of "extip". 

• Fixed a "." vs a "?" typo in section 3.3.1 in the DHCP comment section. 

• Added a missing ")" to the ICQ portfw script and changed the evaluation from −lt to −le 

• Updated the Quake Module syntax to NOT use the "ports=" verbage 


• Fixed a space typo when setting the "ppp_ip" address. 

• Fixed a typo in the simple IPCHAINS ruleset. "deny" to "DENY" 

• Updated the URLs for Bjorn's "modutils" for Linux 

• Added verbage about NetFilter and IPTables and gave URLs until it is added to this HOWTO or a 

different HOWTO. 

• Updated the simple /etc/rc.d/rc.firewall examples to notify users about the old Quake module bug. 

• Updated the STRONG IPFWADM /etc/rc.d/rc.firewall to clarify users about dynamic IP addresses 

(PPP & DHCP), newer DHCPCD syntax, and the old Quake module bug. 

• Updated the STRONG IPCHAINS /etc/rc.d/rc.firewall to ADD a missing section on dynamic IP 

addresses (PPP & DHCP) and the old Quake module bug. 

• Added a note in the "Applications that DO NOT work" section that there IS a beta module for 

Microsoft NetMeeting (H.323 based) v2.x on 2.0.x kernels. There is NO versions available for 

Netmeeting 3.x and/or 2.2.x kernels as of yet. 


• Updated the HOWTO name to reflect that it isn't a MINI anymore! 



• Fixed a typo in "Section 6.6 − Multiple Internal Networks" where the −a policy was ommited. 


• Deleted the 2.2.x kernel configure option "Drop source routed frames" since it is now enabled by 

default and the kernel compile option was removed. 

• Updated the 2.2.x and all other IPCHAINS sections to notify users of the IPCHAINS fragmentation 

bug. 

• Updated all of the URLs pointing at Lee Nevo's old IP Masq Applications page to Seg's new page. 


• Fixed a typo in the Port fowarding section that used "ipmasqadm ipportfw −C" instead of "ipmasqadm 

portfw −f" 


• Updated the "ipfwadm: setsockopt failed: Protocol not available" message in the FAQ to be clearer 

instead of making the user hunt for the answer in the Forwarders section. 

• Fixed incorrect syntax in section 6.7 for IPMASQADM and "portfw" 



• Fixed the quake module port setup order for the weak IPFWADM & IPCHAINS ruleset and the 

strong IPFWADM ruleset as well. 

• Added a user report about port forwarding ICQ 4000 directly in and using ICQ's default settings 

WITHOUT enabling the "Non−Sock" proxy setup. 

• Updated the URLs for the IPMASQADM tool 

• Added references to Taro Fukunaga, tarozax@earthlink.net for his MkLinux port of the HOWTO 

• Updated the blurb about Sonny Parlin's FWCONFIG tool to note new IPCHAINS support 

• Noted that Fred Vile's patch for portfw'ed FTP access is ONLY available for the 2.0.x kernels 

• Updated the 2.2.x kernel step with a few clarifications on the Experiemental tag 

• Added Glen Lamb's name to the credits for the LooseUDP patch 

• Added a clarification on installing the LooseUDP patch that it should use "cat" for non−compressed 

patches. 

• Fixed a typo in the IPAUTO FAQ section 

• I had the DHCP client port numbers reversed for the IPFWADM and IPCHAINS rulesets. The order I 

had was if your Linux server was a DHCP SERVER. 

• Added explict /sbin path to all weak and strong ruleset examples. 

• Made some clarifications in the strong IPFWADM section regarding Dynamic IP addresses for PPP 

and DHCP users. I also noted that the strong rulesets should be re−run when PPP comes up or when a 

DHCP lease is renewed. 

• Added references in the 2.2.x requirements, updated the ICQ FAQ section, and added Andrew 

Deryabin to the credits section for his ICQ MASQ module. 

• Added some clarifcations to the FAQ section explaining why the 2.1.x and 2.2.x kernels went to 

IPCHAINS. 

• Added a little FAQ section on Microsoft File/Print/Domain services (Samba) through a MASQ server. 

I also added an URL to a Microsoft Knowledge based document for more details. 

• Added clarifications to the FAQ section that NO Debian distribution supports IP masq out of the box. 

• Updated the supported MASQ distributions in the FAQ section. 

• Added to the Aliased NIC section of the FAQ that you CANNOT masq out of an aliased interface. 

• Wow.. never caught this before but the "ppp−ip" variable in the strong ruleset section is an invalid 

variable name! It has been renamed to "ppp_ip" 

• In both the IPFWADM and IPCHAINS simple ruleset setup areas, I had a commented out section on 

enabling DHCP traffic. Problem is, it was below the final reject line! Doh! I moved both up a section. 



• In the simple IPCHAINS setup, the #d out line for DHCP users, I was using the IPFWADM "−W" 

command instead of IPCHAINS's "−i" parameter. 

• Added a little blurb to the Forwarders section the resolution to the famous "ipfwadm: setsockopt 

failed: Protocol not available" error. This also includes a little /proc test to let users confirm if 

IPPORTFW is enabled in the kernel. I also added this error to a FAQ section for simple searching. 

• Added a Strong IPCHAINS ruleset to the HOWTO 

• Added a FAQ section explaining the "kernel: ip_masq_new(proto=UDP): no free ports." error. 

• Added an example of scripting IPMASQADM PORTFW rules 

• Updated a few of the Linux Documentation Project (LDP) URLs 

• Added Quake III support in the module loading sections of all the rc.firewall rulesets. 

• Fixed the IPMASQADM forwards for ICQ 

• 1.72 − 4/14/99 − Dranch: Added a large list of Windows NAT/Proxy alternatives with rough pricing 

and URLs to the FAQ. 

• 1.71 − 4/13/99 − Dranch: Added IPCHAINS setups for multiple internal MASQed networks. 

Changed the ICQ setup to use ICQ's default 60 second timeout and changed IPFWADM/IPCHAINS 

timeout to 160 seconds. Updated the MASQ and MASQ−DEV email list and archive subscription 

instructions. 

• 1.70 − 3/30/99 − Dranch: Added two new FAQ sections that cover SMTP/POP−3 timeout problems 

and how to masquerade multiple internal networks out onto different external IP addresses with 

IPROUTE2. 

• 1.65 − 3/29/99 − Dranch: Typo fixes, clarifications of required 2.2.x kernel options, added dynamic 

PPP IP address support to the strong firewall section, additional quake II module ports, noted that the 

LooseUDP patch is built into later 2.2.x kernels and its from Glenn Lamb and not Dan Kegel, added 

more game info in the compatibility section. 

• 1.62 − Dranch: Make the final first−draft changes to the doc and now announce it in the MASQ email 

list. 

• 1.61 − Dranch: Made editorial changes, cleaned things up and fixed some errors in the Windows95 

and NT setups. 

• 1.58 − Dranch: Addition of the port forwarding sections; LooseUDP setup; Ident servers for IRC 

users, how to read firewall logs, deleted the CuSeeme Mini−HOWTO since it is rarely used. 

• 1.55 − Dranch: Complete overhaul, feature and FAQ addition, and editing sweep of the v1.50 

HOWTO. Completed the 2.2.x kernel and IPCHAINS configurations. Did a conversion from 

IPAUTOFW to IPPORTFW for the examples that applied. Added many URLs to various other 

documentation and utility sites. There are so many changes.. I hope everyone likes it. Final publishing 

of this new rev of the HOWTO to the LDP project won't happen until the doc is looked over and 

approved by the IP MASQ email list (then v2.00). 

• 1.50 − Ambrose: A serious update to the HOWTO and the initial addition of the 2.2.0 and IPCHAINS 

configurations. 

• 1.20 − Ambrose: One of the more recent HOWTO versions that solely dealt with < 2.0.x kernels and 

IPFWADM.

Linux IP Masquerade HOWTO - The Linux Documentation Project

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