[RFCI-Discuss] about CNAME

[Derek J. Balling]

RFC2181, Section 10.3

10.3. MX and NS records

    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.  Not only is
    the specification clear on this point, but using an alias in either
    of these positions neither works as well as might be hoped, nor well
    fulfills the ambition that may have led to this approach.  This
    domain name must have as its value one or more address records.
    Currently those will be A records, however in the future other  
record
    types giving addressing information may be acceptable.  It can also
    have other RRs, but never a CNAME RR.

    Searching for either NS or MX records causes "additional section
    processing" in which address records associated with the value of  
the
    record sought are appended to the answer.  This helps avoid needless
    extra queries that are easily anticipated when the first was made.

    Additional section processing does not include CNAME records, let
    alone the address records that may be associated with the canonical
    name derived from the alias.  Thus, if an alias is used as the value
    of an NS or MX record, no address will be returned with the NS or MX
    value.  This can cause extra queries, and extra network burden, on
    every query.  It is trivial for the DNS administrator to avoid this
    by resolving the alias and placing the canonical name directly in  
the
    affected record just once when it is updated or installed.  In some
    particular hard cases the lack of the additional section address
    records in the results of a NS lookup can cause the request to fail.




On Mar 9, 2006, at 5:59 AM, Jeff Pang wrote:

> hello,list,
>
> I'm a little confused that why the CNAME for MX records is  
> disobedient to RFC?
> For example,some a domain is 'ddd.com',whois MX is  
> 'mx.ddd.com',while the 'mx.ddd.com' is aliased to another A record  
> as 'split.ccc.com'.
> Why this case breach the RFC? Thanks.
>
> --
> Jeff Pang
> NetEase AntiSpam Team
> http://corp.netease.com
> _______________________________________________
> RFCI-Discuss mailing list
> RFCI-Discuss at lists.megacity.org
> http://lists.megacity.org/mailman/listinfo/rfci-discuss

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