MX pointing to CNAME

Josh Littlefield joshl at
Tue May 30 17:05:46 UTC 2006

Peter Dambier wrote:
> Tom Del Rosso wrote:
>> Who's right?  The ISP says MX records must point to a canonical name, but
>> says "CNAMEs are prohibited in MX records".

These statements are not in conflict, but are in complete agreement. 
The left side name (owner name) of a CNAME RR is not a canonical name, 
it is an alias.  The right side name (data) of a CNAME RR is supposed to 
be a canonical name (not another CNAME), however, CNAME "chaining" is 
generally tolerated so some unspecified depth.

A canonical name is a name which owns real data, as opposed to an alias 
which owns a CNAME record.  See RFC 1034, sec. 3.6.2.  The reason the 
CNAME record is called that (for "canonical name") is that, like all 
other RRs, it refers to what is found on the data side of the name, not 
the owner side.


Josh Littlefield                                  Cisco Systems, Inc.
joshl at                             1414 Massachusetts Avenue
tel: 978-936-1379  fax: 978-936-2226       Boxborough, MA  01719-2205

More information about the bind-users mailing list