[dns-operations] Can MX be working with CNAME?

Jeroen Massar jeroen at massar.ch
Mon Oct 21 14:04:34 UTC 2013

Tony Finch wrote:
> Jeroen Massar wrote:
>>  "Don't use CNAMEs in combination with RRs which point to other names"
>> And thus CNAME -> MX -> A falls under that too.
> No, it is only names in RDATA that should not refer to CNAMEs.

CNAMES (and DNAMEs in a different form) cause all kinds of unexpected
things, this is one of them.

> In practice, this depends a lot in the RR in question. NS pointing to
> CNAME is not going to work. MX pointing to CNAME probably will work.

In practice you will find that Sendmail is a pain in a place. It will
use the CNAME to rewrite all the headers involved as I indicated.

> CNAME pointing to anything should work, except for the historical screwup
> in the way mail software handles CNAME. Note that this does not just
> affect CNAME pointing to MX, but also CNAME pointing to A and CNAME
> pointing to AAAA, when the CNAME is used as a mail domain.

Current Sendmail still does this. Hence why I highlighted this problem.

Set up a sendmail default out of the box on a Debian box for instance
and you will find this problem to be true when you use CNAMEs anywhere
in the MX (thus either 'domain CNAME somethingelse MX final' or domain
MX final CNAME somethingelse' will cause this).


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