Setting up IP-Masquerading

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System Administration

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original in en Guido Socher

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Guido is a long time Linux fan. His Linux home page can be found at

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IP-Masquerading provides the possibility to connect several computers to the Internet using a computer running Linux with just one public IP address. This means you can connect a whole private network to the Internet, and your Internet Service provider thinks you are still connecting just one single computer. This article will explain how to configure IP-Masquerading with a 2.2.x Kernel. It does not explain how to build a network. To get a general idea about computer networks I recommend to read also the January 2000 issue article about home networks.

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To use IP-Masquerading as explained in this article you need at least one Linux box with a 2.2.x Kernel. This machine is used to set up the connection to the Internet. Using Linux as your connection-sharing box doesn't mean that you have run Linux in your internal network. In fact, Linux works well with Windows, Macs, and other flavours of Unix.
This connection-sharing box is what we are concerned about in this article. It connects on one side to the Internet and on the other side to your private network. The machine has therefore at least 2 interfaces and also at least 2 IP addresses. One of the IP addresses is a public IP address which can be routed in the Internet. This IP address is usually assigned to you by your Internet Service Provider the very moment you setup your modem connection (or what ever you use). The other IP addresses is a private address which you can assign from one of these ranges:

This article does not explain how to set up your network. I assume that your private network is already in place and configured.

IP-Masquerading principles

Basically IP-Masquerading translates internal IP addresses into external IP addresses. This is called network address translation and Linux does this by using something called port-numbers. From the outside world, all connections will seem to be originating from your Linux box. You can find a detailed explanation in January issue.

Sometimes, IP packets are special in nature and IP-Masquerading may not work for all applications, but it works in most cases. There are modules for ICQ, ftp, and quake that need to be inserted in the Kernel in order for those special applications to run correctly from the internal network. In general though, anything that uses only the HTTP (web browsers), telnet, ssh, or smtp (email) will work fine.

Setting up the Kernel

People who use an out of the box Kernel from one of the major Linux distributions (Redhat, Mandrake, Debian, Suse...) can skip this chapter as their kernel is already prepared to use IP-Masquerading.

I usually make a backup of /usr/src/linux/.config after I have compiled a successfully working Kernel. Next time I need to compile a Kernel I just load this configuration and I have already the configuration of my previous Kernel in place. It is then relatively simple to configure minor changes such as IP-Masquerading.

To use IP-Masquerading say yes to the following when configuring the Kernel. These are just the components you need for IP Masquerade, select whatever other options you need for your specific setup.

Configure IP-Masquerading

We will write a little script to automate IP-Masquerading configuration. You should put the the following script into /etc/rc.d/init.d/ and call it ipmasq. Change permissions with chmod 755 ipmasq to make it executable. The script below assumes that you have used the static IP address on the interface towards your internal network (ifconfig eth0 netmask Please change the script if you are using something else. This picture shows the network plan of the network that we are using.
echo "Setting up IP masquerading ..."
# People still using windows to surf the web must convert this
# to a UNIX text file before using it.
# Support masquerading of FTP file transfer.
/sbin/modprobe ip_masq_ftp
# Note: the modules below are commented out from loading. Remove the
# comment sign if you want to use the corresponding applications form
# one of the computers inside your internal network.
# Support masquerading of RealAudio over UDP. 
#/sbin/modprobe ip_masq_raudio
# Supports the masquerading of IRC DCC file transfers
#/sbin/modprobe ip_masq_irc
# Support masquerading of Quake and QuakeWorld 
# Quake I / QuakeWorld (ports 26000 and 27000)
#/sbin/modprobe ip_masq_quake
# Quake I/II/III / QuakeWorld (ports 26000, 27000, 27910, 27960)
#/sbin/modprobe ip_masq_quake ports=26000,27000,27910,27960
# Support masquerading of the CuSeeme video conferencing software
#/sbin/modprobe ip_masq_cuseeme
#Support masquerading of the VDO-live video conferencing software
#/sbin/modprobe ip_masq_vdolive
# Important: Enable IP forwarding. It is disabled by default in
# the 2.2.x Kernels
echo "1" > /proc/sys/net/ipv4/ip_forward
# NOTE: This is an example for an internal Network address of
# 192.168.0.x The sub netmask is or "24" bit
# Please change this if you use different internal IP addresses.
/sbin/ipchains -P forward DENY
/sbin/ipchains -A forward -s -j MASQ
#--- end of file
To test it make sure you have configured all your computers in your private network to have a default gateway route (!!!) to (the masquerading Linux host). Then run the script as root on the masquerading host. After that send a ping from one machine inside your network to a host in the Internet (e.g ping
If this works then masquerading works. Try also ping This should give the same results as the above ping. If it does not work then check the /etc/resolv.conf file on your clients. It should exist on every computer inside your network and should list the DNS server of your ISP.
With Windows 9x-Clients you need to bind the network configuration of the TCP/IP-Stack to the Networkcard and DNS needs to be activated by adding the DNS server of the ISP under 'search order for DNS Server'.
Once the pings work everything else (e.g web browsing) will also work.

Now it is time to change your configuration such that your /etc/rc.d/init.d/ipmasq script will be executed automatically every time you boot your Linux connection-box. The best way to do this is in my opinion to edit the /etc/rc.d/init.d/network file (this file should already exist) and execute /etc/rc.d/init.d/ipmasq AT THE END of the start section in the init.d/network file. Look for a case statement and then for the "start)".


As you saw it is not difficult to setup IP-Masquerading. It is basically just ip forwarding enabled and 2 ipchains commands. IP-Masquerading is a very powerful application for small home networks schools small business networks etc....

For further and more detailed information I recommend reading the IP-Masquerading mini howto and the IPCHAINS-HOWTO.

To use IP-Masquerading you need to have of course a correctly configured network. This is not covered by this article. The Home networking article will give you some vocabulary and the Net 3 howto or the The Network Administrator Guide explains in detail how to build a network. I plan also to write another article on networking later on in the year but you might not want to wait that long :-).