MX vedouci na CNAME
Pavel Kankovsky
peak na argo.troja.mff.cuni.cz
Pátek Září 27 11:13:14 CEST 2002
On Wed, 25 Sep 2002 houstek na karlin.mff.cuni.cz wrote:
> Prosim o nazor ci nakopnuti. Rad bych pochopil, co je na $SUBJ spatneho.
> Na internetu jsem nasel nekolik utrzku, ale nic pochopitelneho a
> srozumitelneho.
RFC 2181, Clarifications to the DNS Specification
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.
--Pavel Kankovsky aka Peak
"Welcome to the Czech Republic. Bring your own lifeboats."
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