How does router know where box with static IP is?

From: Robert Oschler (Oschler@earthlink.net)
Date: 07/03/02 

From: "Robert Oschler" <Oschler@earthlink.net>
Date: Wed, 3 Jul 2002 14:45:13 -0400

This question is part of a debugging session I'm in getting port forwarding
to work with my LinkSys router and my Linux box. I have my Linux box setup
with a static IP and it can access the Internet through my router without
problem. I have an entry in my LinkSys router Port forwarding table that
forwards the desired port to the static IP address belonging to my Linux
box running SUSE 7.3 Pro. When I try to access the router's WAN IP in
combination with the desired port # from an external client, the connection
times out with a winsock 10060 error (the test system is a Windoze box).

I checked the LinkSys router's incoming message log, and it does show the
connection attempt to the forwarded port. But my server program running on
the Linux box does not see the connection attempt. I know the server
program and the Linux box's static IP address are valid, because other
siblilng stations behind the router can connect to it.

What I'd like to know is, how does my SUSE Linux box tell the router where
it is? Where does this happen in the network initialization chain and what
programs/daemons/conf files are involved? I know the route I established
from the Linux box to the default gateway (my router) is valid, because I
can access the Internet from that station. But now I'm hypothesizing that
perhaps I need another route table entry that tells the router "the road
back" to my Linux box?

thx

P.S. - Note, I'm having trouble with running tcpdump and ethereal, that
latter due to a library it can't find or is missing. When I get them
running I'll post the results of running those utilities.

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