gordanbobich.net
Date: Thu Nov 01 2007 - 08:35:46 CDT

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Hi,

I'm seeing some strange behaviour with my Postfix setup. It occasionally
doesn't try the MX records in the correct order. It will try a low
priority one, and if the machine is on a local subnet but not running,
it'll get back a "no route to host", and get stuck. It'll keep retrying
this one quite a few times, before it gives up and rolls over to the high
priority MX-es which it should have tried first.

Is this a known bug?

One thing that could be causing it is that I am, unfortunately, relying on
a partially broken PowerDNS setup, which truncates the additional section
of the response down to 512 bytes so it can respond via UDP. This leads to
it reporting a complete list of, say, 10 MX records, but only reports IPs
for 3 in the additional section (and these 3 seem to rotate around all the
time).

Does Postfix try to cut a corner here and use the MX with the IP returned
in the additional section, instead of going purely based on the MX list
and trying the records in the priority order as it should?

I cannot think of any other explanation. My logs show that it never even
bothered trying to connect to the high priority (low number) MX-es before
it tried a low priority one (which, as it happens, wasn't working). It
then got stuck retrying that non-working MX for hours instead of retrying
them in the correct order. But that's less of a problem - the serious
problem is that it didn't try connecting to the MX-es in the correct
order.

This only seems to happen on a small number of emails. Most of the time,
it seems to do the right thing.

Many thanks.

Gordan

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