I would like to setup one firewall to handle both my DSL line and my T1 line.

All of the incoming and outgoing e-mail goes through the DSL line.

All of the web traffic passes through the T1 line.

Right now, my DNS server goes through the DSL line but in the future I'd like the DNS to be able to accept queries on both the DSL and the T1 networks.

I am thinking that I'd need 3 network interfaces on my firewall. 1 for the LAN, 1 for the DSL and 1 for the T1.

Does anyone have any pointers on setting up a Linux machine with 3+ NICS?

I currently run a pair of Linux firewalls (ipchains) with 2 NICS. I setup 1 multihomed Linux machine to handle the DSL line and 1 multihomed Linux machine to handle the T1 line. I'd like to setup a single box to handle both the DSL and the T1 lines.

if i get you right: shouldn't make any difference if you use 3 nics. even your dns (bind9?) should be able to answer questions from both (or all) interfaces. you may also choose to have a parent/child dns for the two networks.
you can also work with one external nic that can take both, the t1 and dsl line with one/two networks.
there is another option: share the bandwidth over the
two links (bridge/shape) to get best reachability/troughput.

there is a vast number of possibilities you may use for your net, so i'd suggest to ask a more specific question.

sl mritch.