This should be fairly simple to set up, depending on your Linux distro... Mandriva has an option in a config file that you need to modify in order to activate ip forwarding (just add FORWARD_IPV4=yes in /etc/sysconfig/network)
However, this would mean no firewall, no protection, no advanced thingies. I've been using shorewall as a firewall for a few years now and it is pretty simple to configure, there is extensive documentation on their site (
www.shorewall.net), and works with every distro, provided the requirements are met. Configuring ip forwarding is just a matter of setting IP_FORWARDING to On in /etc/shorewall/shorewall.conf.
Read here for some more info about setting up networking:
http://www.linux.se/doc/HOWTO/mini/H...i-HOWTO-3.html