A Question for the Worlds Smartest Network Minds

A Question for the Worlds Smartest Network Minds

[Marc Perkel]

I hope this question isn't too far off topic but I'm having a very weird 
network problem and don't know where to ask it. Perhaps someone could 
point me in the right direction. I have an unusual packet loss problem 
that has me stumped.

Working at home with a cable modem connected to a Linksys WRT54G router. 
Behind the router are two computers. One is a Linux server and a windows 
XP notebook.

The Linux box is accessable to the outside world because I opened 3 
ports in the NAT. Port 22,25,and 53. The server acts as a name server 
for the domains I host and it also is a backup spam filtering server 
receiving many incomming connections on port 25 and sending out far less 
good email also on port 25 to various other server who I am the front 
ens spam filter for.

Linux box is running FC5 with the latest stock FC5 kernel. Some flavor 
of 2.6.16.

Here's the problem. I can ping the router gateway 192.168.2.1 fine from 
both computers with no packet loss if I turn off the Exim SMTP daemon. 
But if I turn it on then I get about 10% packet loss pinging the gateway 
on both the Linux and Windows computers, both plugged into the Linksys 
router.

Thinking it might be the quantity of traffic I tried uploading a large 
file with rsync ovser SSH which creates a lot more traffic than the SMTP 
traffic (which is about 50k or so) and no packet loss.

I only get packet loss if I have Port 25 traffic.

So - what could cause traffing on just port 25 to cause 2 computers to 
have backet loss? Could it be related to the number of connections? It 
does seem like when I turn on the SMTP that it takes a little while 
before the packet loss starts. Say 3 minutes or so.

Thanks in advance.