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.