The question: Given a net server running squid, bind and sendmail (I
think that's about it), I want all "web" traffic (tcp ports 80 and 443)
to go to a router other than the "default" and return by the same path
(there's NAT on there, so that makes things a little more difficult).
All other traffic should ideally go undisturbed.  The host has three
ethernet interfaces, one internal, another to the external network on
which our primary external router sits and a third that could be used
for the alternative router, if necessary.  I'd mildly prefer the two
routers to reside on the same external network, but I have made
provision otherwise.

I thought Darren's ipfilters would do the trick, but I haven't yet found
the right combination of simple rules to get it right (I have tried a
few alternatives that looked promising in theory).  Since then, I've
considered using lo0 as the "default" interface and the IP filters
between it and the two routers (I can place them on distinct physical
networks, if necessary) but the experimenting is impractical until next
week and I have a feeling I don't really know that what I'm suggesting
makes any sense.

There no doubt is an easier approach, but I lack the understanding to
identify it without help.  Anyone done this already?  How?

Ideally, I'd want a squid appliance between the internal network and the
ADSL link, but I'm not aware that such devices exist, so if anyone knows
of one such, please point me to that.

++L