Multiple links to the net

Here's one that I'm sure can be solved, I'm just not sure where to start.

I have a home network and at present have the luxury of a cable modem for
high-speed stuff and an ISDN link to a different ISP that actually manages
all the techie stuff that the cable company are particularly bad at. I've
got NAT set up for each port and all bar one machine (the one that deals
with the techie bits) use the cable link by default and the ISDN link only
for accessing that particular network.

What I'd like is to be able to run some servers on machines here that can be
accessed from either link, so the machine would need to know where stuff
came from. I can see that by binding two IP addresses to the port it would
give me that indication by NAT routeing to different addresses, but a way
to get an arbitrary machine (not necessarily a Linux box) to correctly
route to multiple external links would be good. I'm sort of guessing that a
Linux PC acting as a firewall/router for both external interfaces and a
separate internal segment should be capable of achieving what I want so any
pointers to where to start reading would be good.

Dave
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mail: da (E-Mail Removed) (without the space)
http://www.llondel.org/
So many gadgets, so little time...