Those postmasters in the UK may have seen the recent short thread regarding
a UK site having mail problems (Coventry). It was pointed that the site has
an MX record which is a DNS CNAME entry (i.e. an alias for another system),
and implying that this was not correct (albeit that that was not the cause
of the problem anyway).
My question is whether using an alias (CNAME) in an MX record is correct or
not. Eg:
site.com. MX 10 mail.site.com.
mail.site.com. CNAME mailhub.site.com.
mailhub.site.com. A 10.0.0.1
According to RFC974 (Mail routing and the Domain system, page 4) an MTA
should check for MX records and repeat the query using the alias to find the
canonical name. So it seems acceptable. However, the O'Reilly DNS and BIND
book makes a caveat that CNAME's should not be used within MX records due to
possible mail loops. The implication is that the MTA will *always* use the
canonical name and thus there will be a mismatch between the hosts canonical
(true) name and its alias. Admittedly they refer to sendmail, but I'd like
to know what others think. I assume that exim follows the RFC974 scenario
and repeats DNS queries for aliases so as to obtain the true name of the
mail exchanger?
Whilst we use the canonical name for the University's MX record, we do use
CNAME's for internal servers. We have had no problems with this.
Thanks,
John.
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John Horne, University of Plymouth, UK Tel: +44 (0)1752 233914
E-mail: jhorne@???
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