From: Will Yardley (william+)
Date: Thu Nov 01 2001 - 02:13:10 EST
Hamish Marson wrote:
> Why did you lookup a CNAME for taxsaver.com.au? Sureley the algorithm
> is to lookup an MX record first, then lookup a CNAME or A record for
> the RESULT of the MX record... Why would you ever try to lookup a
> CNAME for the original domain if an MX record existing that pinted at
> another host?
>
> FWIW The Cricket book recommends using only canonical names in MX
> records, never aliases... But either will (usually) work.
having an MX point to a CNAME is prohibited by several RFCs, and bind,
at least, won't load a zone properly if it contains an MX record
pointing to a CNAME.
from rfc974:
Note that the algorithm to delete irrelevant RRs breaks if LOCAL has
a alias and the alias is listed in the MX records for REMOTE. (E.g.
REMOTE has an MX of ALIAS, where ALIAS has a CNAME of LOCAL). This
can be avoided if aliases are never used in the data section of MX
RRs.
and from rfc2181:
The domain name used as the value of a NS resource record, or part of
the value of a MX resource record must not be an alias.
(there is further explanation of this in the text under 10.3; there are
a couple of other RFCs that mention that you shouldn't do this as well)
i personally think it's a good idea to avoid CNAMEs unless you know
enough about DNS to understand when it's a good time to use them. in
most circumstances, using an A record is preferable, and not really much
more difficult.
w
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