Configure a custom hardware firewall so that servers connected to it have an internet connection via the firewall

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http://serverfault.com – i want to setup a custom hardware Firewall using debian. The hardware firewall is behind a switch which is behind a router. Behind the firewall is my server. The internet connection works fine except from the point server to firewall. My server is not able to connect to the internet. I tested alot, configured the firewall as gateway, but still got no connect. The server has a lan cable which goes in the 2nd network card(eth1) inside the firewall. The firewall itself has internet over 1st network card (eth0). I would be happy if somebody could give me some tips on how to configure my fire (HowTos)