Sendmail, MTU, firewall, and Earthlink

[Roger Beever]

-----Original Message-----
From: fedora-list-admin at redhat.com
[mailto:fedora-list-admin at redhat.com]On Behalf Of Steven Stern
Sent: 25 November 2003 23:03
To: fedora-list at redhat.com
Subject: Sendmail, MTU, firewall, and Earthlink


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.

mmmmm my Netgear adsl router has 1458 (depends if you increased or decreased
yours as to whether that would be better) which is the default (it was
changed slightly in the last firmware upgrade).
Regards Roger
PS I learned to set up my aliases file correctly today.
I'm just running all my Linux related mail on my own server for now.
oooops not all the fedora list is still via my isp.
Thought can the ISP be pop3ed and the result sent to sendmail? (I will
accept a simple yes or no for now)
Roger