How do I troubleshoot my DHCP server and eth1 configuration?

[Jason Hsu <jhsu802701@jasonhsu.com>]

My hardware setup:
Modem -> (eth0) -> Firewall/server computer (Debian Lenny, Shorewall, DHCP3 server) -> (eth1) ->
Ethernet switch -> (eth0) -> Main computer

I did successfully access the Internet before from my main computer with this setup.  I used the instructions at http://www.cyberdogtech.com/firewalls/ (with some adaptations for Lenny) to set up Debian Lenny and Shorewall.  I used the instructions at http://failserver.nl/blog/2009/03/how-to-install-a-dhcp-server-in-linux-debian-lenny/ to set up a DHCP server.

Unfortunately, I made an unsuccessful attempt to upgrade to Squeeze and had to reinstall Lenny.  Now I'm unable to connect to the Internet from my main computer with this setup.

I'm pretty sure that it's just one or just a few minor little details that are stopping my main computer from accessing to the Internet with this setup.  When I connect my main computer to the modem and restart its eth0 interface, I can connect to the Internet, so it's not an Internet connection problem.  My firewall/server computer has no difficulty accessing the Internet when connected to the modem and with eth0 properly activated, so I don't think there's a hardware issue there.  I tried uninstalling shorewall on my server computer, but that didn't solve the problem, so I don't think the firewall was blocking Internet access.  Thus, the only thing I can think of is a problem with my DHCP server settings or its eth1 settings.

How do I troubleshoot a DHCP server and the eth1 configuration?  

-- 
Jason Hsu <jhsu802701@jasonhsu.com>