[Preston] strange mail server problem

[Geoff King]

Matthew T. Atkinson wrote:

<snip>

> Most messages come in through my server, as they should, but every so
> often one doesn't get through -- instead it goes to my backup MX (the
> headers show this).  After the person who helped me set it up did
> some tests, he informed me that sometimes the connection is not
> getting made at the TCP level (and is therefore unrelated to the mail
> server software).

Hello, I'm new here :) But anyway, it sounds like it might be the ECN
support. Have you tried disabling it?

echo 0 > /proc/sys/net/ipv4/tcp_ecn

For those of you interested, heres why (stolen from kerneltrap.org):

ECN is one of the more controversial features in the 2.4.x kernel
series. Basically, it reduces congestion. In theory. However, there are
quite a few broken routers out there that will just drop ECN packets.
So, how is this related to Debian? It's included in the default woody
kernel-image. Read on for a more detailed explanation.
ECN (Explicit Congestion Notification) is a new feature in linux-2.4.x.
A special flag in the IP packages helps routers to balance the traffic
and increase network performance.

That was the theory. In real life, many routers and firewalls are
broken. If they receive a ECN packages, they corrupt them or drop them.
So if ECN is enabled, you cannot connect to some hosts.

--
-Geoff

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