Linux NAT

If you are running a recent 2.6 Linux Kernel this four step process should work for you. This has been specifically tested on Fedora Core 3, 4, 5, and 6, but should work on any modern Linux distribution. All of these commands must be executed as the root user. First you need to tell your kernel that you want to allow IP forwarding.

echo 1 > /proc/sys/net/ipv4/ip_forward

Then you'll need to configure iptables to forward the packets from your internal network, on /dev/eth1, to your external network on /dev/eth0. You do this will the following commands:

/sbin/iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
/sbin/iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
/sbin/iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

You should now be NATing. You can test this by pinging an external address from one of your internal hosts. The last step is to ensure that this setup survives over a reboot. Obviously you should only do these last two steps if your test is a success.