Sendmail, MTU, firewall, and Earthlink

[Steven Stern]

What did I learn in Linux today?

Configured my computer through DYNDNS.ORG so I could be a real mail machine.
After a bit of doing, I was receiving mail. Test messages came in fine from
Yahoo and Hotmail, but Earthlink was timing out attempting to deliver mail.
After tinkering and poking around Google searches, I found that there's some
nasty interaction between MTU size autodiscovery and my SMC Barricade router.
Apparently, Earthlink connects and sets its TCP packets to non-fragmenting.
Due to a miscalculation by the MTU size discovery thingie, packets get
fragmented and the inbound traffic from Earthlink stalls.

It got fixed when I used 

ifconfig mtu 576 eth0

To make the change permanent, I edited

/etc/sysconfig/network/etc/sysconfig/network-scripts/ifcfg-eth0

and added MTU=576 to the file.

I'm not sure this is the optimal MTU size, but it works for now.