Help with strange web server problem

Chuck Swiger cswiger at
Wed Feb 15 20:13:45 PST 2006

Jerry Bell wrote:
[ ... ]
> I've done some more troubleshooting and some strange things have
> appeared.  First, the colo says there is NO proxy, and NO firewall in
> front of this server.

That's believable too, perhaps you simply have a NIC which is failing or is
screwing up the packet checksums in some odd case.  You would have to sniff the
traffic from another machine (perhaps a sysadmin's laptop?) and grab the full
packets ("-s 0" to tcpdump to be sure.

Have you tried swapping NICs or adding a PCI NIC card?  BTW:

> 1671172334:1671172334(0) win 64512 <mss 1260,nop,nop,sackOK> 

This is not quite enough data to tell, but this looks like maybe you're seeing
the IPv6 MSS of 1260 rather than what I get by default (1460?) under FreeBSD?
Of course, it could just be a Windows client machine or something going through
something like a VPN/PPTP tunnel which reduces the MTU...?

What happens if you reduce your interface MTU to 1260?

You ought to be looking for all traffic between your server and a test host,
BTW, sometimes the ICMP traffic, if any, is important to understanding the issue.


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