On Tue, Sep 12, 2000 at 04:32:52PM -0400, Michael T. Babcock wrote:
> For the sake of answering the original questionner w.r.t. reasoning, from
> RFC974 (which is standard 0014):
>
> 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.
>
> cf. http://www.faqs.org/rfcs/rfc974.html
>
> This is the only mention of the non-use of CNAMEs in the mail standards.
Mail standards aren't the only standards. The origin of all this fuss
is probably RFC1034 (also part of STD 13):
3.6.2. Aliases and canonical names
...
CNAME RRs cause special action in DNS software. When a name server
fails to find a desired RR in the resource set associated with the
domain name, it checks to see if the resource set consists of a CNAME
record with a matching class. If so, the name server includes the CNAME
record in the response and restarts the query at the domain name
specified in the data field of the CNAME record. The one exception to
this rule is that queries which match the CNAME type are not restarted.
For example, suppose a name server was processing a query with for USC-
ISIC.ARPA, asking for type A information, and had the following resource
records:
USC-ISIC.ARPA IN CNAME C.ISI.EDU
C.ISI.EDU IN A 10.0.0.52
Both of these RRs would be returned in the response to the type A query,
while a type CNAME or * query should return just the CNAME.
Domain names in RRs which point at another name should always point at
the primary name and not the alias. This avoids extra indirections in
accessing information. For example, the address to name RR for the
above host should be:
52.0.0.10.IN-ADDR.ARPA IN PTR C.ISI.EDU
rather than pointing at USC-ISIC.ARPA. Of course, by the robustness
principle, domain software should not fail when presented with CNAME
chains or loops; CNAME chains should be followed and CNAME loops
signalled as an error.
Jost
--
| [EMAIL PROTECTED] Please help stamp out spam! |
| Postmaster, JAPH, resident answer machine am RZ der RUB |
| Pluralitas non est ponenda sine necessitate |
| William of Ockham (1285-1347/49) |
