T_ANY

[Tony Finch]

On Sat, 20 Mar 2010, Glenn English wrote:
>
> Just why qmail reports a T_ANY failure as a CNAME failure, I also don't
> know.

This is a bug in qmail. It tries to canonicalize domains in the SMTP
envelope of outgoing messages. It originally did this by performing CNAME
queries on each domain, but this caused interop problems with BIND 4. The
CNAME query was changed to an ANY query, which is buggy in a number of
ways: ANY queries are much more likely to trigger qmail's undersized DNS
buffer bug; and ANY queries are supposed to be used for debugging not for
normal operations.

The correct way to canonicalize a mail domain is to do an MX query (or
failing that A and AAAA queries) and if the domain is not canonical the
response will include the CNAME record.

The reason for canonicalization was conformance with RFC 1123 section
5.2.2. However when qmail was written this was not necessary for
interoperability and the requirement was removed in the DRUMS process
that yielded RFC 2821.

So the way to actually fix the bug is to delete the canonicalization code.

Tony.
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