FW: DNS Question

Arthur Chance freebsd at qeng-ho.org
Sun Oct 25 18:16:52 UTC 2009

Sean Cavanaugh wrote:
> The other interesting side would be reverse DNS lookups. Only one
 > record would be returned, and most likely would be the original A
 > record. A nice example of this is doing a basic "ping -a ww.yahoo.com"
 > which you get back that it is resolving "www-real.wa1.b.yahoo.com".

As a comment on reverse DNS lookups, although the example Sean gave 
should have a single PTR entry as the result of a reverse lookup, in 
general reverse DNS lookups can return *multiple* values. If multiple A 
records pointing at the same numeric address exist, the reverse lookup 
on that address must (if correct) return the corresponding multiple names.

RFC 2181 (Clarifications to the DNS Specification), section 10.2:

10.2. PTR records

    Confusion about canonical names has lead to a belief that a PTR
    record should have exactly one RR in its RRSet.  This is incorrect,
    the relevant section of RFC1034 (section 3.6.2) indicates that the
    value of a PTR record should be a canonical name.  That is, it should
    not be an alias.  There is no implication in that section that only
    one PTR record is permitted for a name.  No such restriction should
    be inferred.

    Note that while the value of a PTR record must not be an alias, there
    is no requirement that the process of resolving a PTR record not
    encounter any aliases.  The label that is being looked up for a PTR
    value might have a CNAME record.  That is, it might be an alias.  The
    value of that CNAME RR, if not another alias, which it should not be,
    will give the location where the PTR record is found.  That record
    gives the result of the PTR type lookup.  This final result, the
    value of the PTR RR, is the label which must not be an alias.

OK, there's a couple of big questions, which is how many DNS configs 
actually obey this and how much application code allows for it, but 
that's the spec.

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