yep, its easy
basically you want your linux box setup as a router, now Im fairly new to linux, installed redhat on a p60 to act as a router/firewall and rpogressed from there, had a friend help me set it up.
Ipchains is the way to go, you dont say how much you know about linux so its a bit hard for me to say, basically you need a script, either manual or starting on boot to forward the incoming packets to the other systems, in redhat 7.1 you can do it through kde/gnome, im not sure about mandrake, but probly easiest to just write a script manually, Im at work now and have limited net access so cant find you any sites, but go to google and search for "ip chains how to" and you will get loads of info, if you haveing probs, post back and I'll post a basic script here.
Next time you wave - use all your fingers