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
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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