I'm just now starting to mess with setting up my linux box as a router. I have it up and runnig now, but I don't entirely understand how. The question I have is, what causes a packet to go to the FORWARD chain? My default policy for INPUT is ACCEPT and I have no rules. Same for OUTPUT. All the rules are on the FORWARD chain, but I don't see what causes packets to jump to that chain. Help?

FORWARD is used for packets that must pass through the box to get to their destination.

For local packets, the destination ip address is somewhere in the Internet, so the routing table decides they have to go thru' FORWARD. This is called using the box as a Gateway.

For incoming packets, the nat PREROUTING table needs to give the packets an address that is in the local network before they go thru' the FORWARD chain. This is called DNAT, 'destination network address translation'.

Have a read of his tutorial for extra indepth info.

I don't think I understand yet.

When a packet is received, the first chain it hits is the INPUT chain, right? And if there are no rules on the INPUT chain and the default policy for that chain is ACCEPT, then what happens after it hits the default policy? There's no jump involved. Does it automatically go to the FORWARD chain at that point or what?

Also, how do you indicate that all incoming packets on a particular interface are destined for the local machine, and not to be routed? I want my linux machine to run a few servers (HTTP, VNC, FTP, SSH, etc), and it has to respond to incoming requests rather than dropping or forwarding them.

Okay, wow I'm an idiot. I totally understand how the FORWARD chain is used now. I just was misreading your post in a terrible way.

Now I just need to know how to indicate that incoming requests on certain protocols need to be handled by the linux box (on which the firewall runs).