kernel 2.4.18 -> 2.4.20 subtle upgrade problem

[Graeme Tank <graemet@umich.edu>]

I've been happy with my self-compiled 2.4.18 kernel on a stable/testing
Woody system that is mostly stable.

Recently, I installed kernel-source-2.4.20-6 and used my 2.4.18 config
file to compile 2.4.20. I compile my kernels like so:

$ cd /usr/local/source/kernel-source-2.4.20
$ cp /boot/config-2.4.18-gat.05 .config
$ make menuconfig          // didn't change anything but did save
$ make-kpkg clean
$ VER=gat.05
$ make-kpkg -rootcmd fakeroot --append-to-version -${VER}
--revision=${VER} kernel_image
# dpkg -i ../kernel-image-2.4.20-gat.05_gat.05_i386.deb

Now the problem. Using the 2.4.20 kernel I can access every website I
could with the 2.4.18 kernel except for http://www.wash.k12.mi.us and
http://www.remc.org. This is interesting in and of itself, but this is
also the only way my wife can access her webmail. She's also largely
responsible for the content of both sites.

I've tested the site with

$ telnet www.wash.k12.mi.us 80

many times over the past several days from the different kernels with
always the same result -- immediate access with the 2.4.18 kernel and a
three minute timeout with the 2.4.20 kernel.

My config files can be found at:
http://www-personal.umich.edu/~graemet/config-2.4.18-gat.05
http://www-personal.umich.edu/~graemet/config-2.4.20-gat.05
(Yeah, I know there's a lot of stuff in them, but I was modeling them
after the 2.4.18-bf2.4 kernel which I knew did not have this problem.)

Yes, I'm running a firewall using the ipmasq package on this machine.
Disabling the firewall 

# /etc/init.d/ipmasq stop
# iptables -P INPUT ACCEPT
# iptables -P OUTPUT ACCEPT
# iptables -P FORWARD ACCEPT
# iptables -F
# iptables -L

does not change this problem.

When I first began troubleshooting this problem, I thought perhaps my ip
had gotten blacklisted by the website. (Not that I was doing anything
nasty, just looking for possible causes) However, booting back into the
2.4.18 kernel fixes the problem, and dhcp3 reacquires the same ip on
each reboot, independent of which kernel is running.

Of course, this may not be a problem with the kernel but with the
website. The IT staff at the website tells me that this is the first
time they've heard of such a problem.

Any help would be appreciated. It might be interesting to see if those
of you running the 2.4.20 kernel have a similar problem.

Graeme