[falko]

I found these links that can help you to build your own Debian router:

http://www.cyberdogtech.com/firewalls/
http://users.skynet.be/langewouters/...ling_drgw.html
http://gate-bunker.p6.msu.ru/~berk/router.html

But it will be easier to use something like IPCop ( http://www.ipcop.org/ ) as a router. This distribution is specifically designed to act as a router.