Multiple PTR records
Kevin Darcy
kcd at daimlerchrysler.com
Fri Jun 8 00:24:38 UTC 2001
Mark.Andrews at nominum.com wrote:
> >
> > To clarify: there is nothing in the DNS protocol to stop you from creating mu
> > ltiple
> > PTR records with the same name, however no app is known to actually look beyo
> > nd the
> > first PTR in a response, and because of this fact BIND suppresses RR sorting
> > for
> > PTR records. So essentially all PTRs beyond the first one are "invisible" and
> > a
> > waste of packet space (if the response overflows the 512-byte limit, then it
> > may
> > also waste TCP retransmissions too).
> >
> >
> > - Kevin
>
> Some site even go so far adding PTR records that they exceed
> the protocols ability to send them in a response. I wonder
> about sites that do this and how much else they don't know
> about.
>
> You could even use multiple PTR records as a filtering
> critera when selecting web hosting providers. If they list
> multiple PTR records then they most probable don't know
> what they are doing and you should shy away from them.
>
> It sound like you are trying to learn what to do which is
> good. Good luck and keep up the learning.
I wonder if this would be good BCP material (?). RFC 2181 (not a BCP of course but
Standards Track) almost seems to *encourage* multiple PTRs by "clarifying" that it
is supported in the protocol. Now that the cat is out of the bag, perhaps there
should be a BCP stating that, while multiple PTRs are technically possible, they
are generally undesirable and when taken to extremes can in fact cause problems.
I would not volunteer to write such a document, of course, given my
even-more-radical view that reverse DNS should probably go away or its use be
severely limited (and I don't think keeping reverse DNS around solely as a sort of
"ISP intelligence test" is really a strong argument, even when couched in terms of
spam-prevention).
- Kevin
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