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

Paul Vixie paul at redbarn.org
Fri Oct 18 13:20:58 UTC 2013

Joseph S D Yao wrote:
> On 2013-10-18 04:46, Doug wrote:
>> Hello,
>> $ idig plus.google.com mx
>> plus.google.com.    1200    IN    CNAME    plus-china.l.google.com.
>> plus-china.l.google.com. 600    IN    MX    40 alt3.aspmx.l.google.com.
>> plus-china.l.google.com. 600    IN    MX    50 alt4.aspmx.l.google.com.
>> plus-china.l.google.com. 600    IN    MX    10 aspmx.l.google.com.
>> plus-china.l.google.com. 600    IN    MX    20 alt1.aspmx.l.google.com.
>> plus-china.l.google.com. 600    IN    MX    30 alt2.aspmx.l.google.com.
>> I never saw this type of MX.  Are they valid records?
> ...
> Doug,
> This is fine.  What would be bad is having a CNAME associated with the
> other side:
> mail.domain.ex.         MX      10 bad.mailer.ex.
> bad.mailer.ex.          CNAME   real.mailer.ex.
> Cf. RFC 1912, 2.4 CNAME records.

while it's true that it is worse to have a cname used to link an MX to
its target, it is not true that pointing a CNAME at an MX will nec'ily
end well. in the above example, Sendmail in its default configuration
will rewrite on the next hop the From: header so that it shows
@plus-china.l.google.com.  i think this is not the behaviour that
google's trying to achieve here. and Sendmail may or may not be wrong to
rewrite headers, but it's the default config, just the same.


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