From: Wietse Venema (wietseporcupine.org)
Date: Thu Nov 01 2007 - 10:48:41 CDT

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gordanbobich.net:
> > I have one suggestion.
> >
> > Point Postfix to a DNS server that doesn't truncate replies, and
> > show evidence that Postfix behaves incorrectly. If so, we will
> > investigate and may even ask you to turn on debug logging.
>
> The DNS is _only_ truncating the additional section. It is NOT truncating
> the main section, regardless of how big it is.
>
> > Postfix doesn't cut corners; it takes the MX list and then does
> > separate A lookups for each of them. If something is cutting corners
> > here it is more likely that DNS server that is truncating your
> > replies, and that does who knows what with the partial information.
> > I am, however, not interested in adding code to Postfix to work
> > around mis-behaving DNS servers.
>
> Agreed, this would be bad. If Postfix does separate lookups then DNS is
> not the problem, as the main section is always returned correctly.

Postfix ignores the additional A records, but the broken DNS server
will cache them.

The question then is what happens with the A records that were
chopped off. Perhaps the broken DNS server assumes that those
records don't exist? Usually, all the MX's A records are included
as additional information. If the broken DNS server assumes that
"record not present in additional information" means "record does
not exist", then we have an explanation for the erratic behavior
you observe.

You can test this by hand, by the way. First look up the MX records,
then see what A replies you get for each MX host.

        Wietse

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