Welcome to Hell / Mysterious networking troubles on FreeBSD

[Joel V.]

Hello all,

I'm not experiencing this problem, my friend is. He's simply too pissed off
to write here and I'm afraid he's going to set his office on fire if he
doesn't solve the problem soon, so without further ado, here's the problem:

He has two fbsd boxes, main server running 6.1 and dns server running 4.3.
He has 4 public IPs which he can use and the main server is running on
x.x.x.122. He's main box is NOT acting as a gateway/NAT box in the office.
Today he noticed that net is getting awfully slow. Sometimes there would be
50% pl when pinging, sometimes pinging would be all OK, but SSH is dead-slow
and the webpages running on the main server are not displaying. E-mails are
not going through. He calls the ISP, who say that his network is showing
major uploading activity. He switches off networking services one by one in
the main box but situation does not improve. He disconnects the main server
and puts a windows xp box instead, which seems to run fine. He puts back the
freebsd box, disables all networking services again except for SSH and
connects the network: instant 100% networking slow-down. He tried to change
the switch, thinking it's faulty. He disconnect every other computer in the
office from the network: nothing. He put the public IP address on the
second, internal network NIC: same thing. Now it gets really mysterious: he
puts the old dns server with the x.x.x.122 IP and instantly it becomes slow
as death. The logical conclusion would be that someone is flooding that IP?
Only the windows xp box seemed to work fine and the ISP guy said it was
upload bandwidth that was excessive...

Netstat -a doesn't show anything interesting, arp -a doesn't show any
incomplete addresses He tried to build and install a new fresh kernel.
Nothing. This is the most creepy networking problem I've heard of. Can YOU
help? Any ideas where to start looking?

I'm not in the freebsd-hackers list, so if you want the e-mail to reach me,
send a copy to joel at spirit.ee

Thank you in advance!
Joel