How would I go about doing this?

["Zane C. B." <zanecb@midwest-connections.com>]

I have two routers. The main router than everything goes through and a
second router that I want to route some traffic through depending on the
originating IP#.

The second router has filtering and the like running on it.

For traffic coming from a specific IP# and hitting the main router, I
want it then to be routed out through the second router.

Currently I got it working for port 80, using iptables, since squid is
setup transparently on the filtering router. The command I am using is
this... iptables -t nat -A PREROUTING -s <machine to filter for> -p tcp
--destination-port 80 -j DNAT --to-destination <filtering router>. This
works, but only for like web or the like which has a transparent squid
setup to take care of it.

That works for right now, but what I want to accomplish is to have the
main router kick packets, from specified IP#, out to the secondary
router.