Multiple PTR records

Kevin Darcy kcd at daimlerchrysler.com
Thu Jun 7 19:39:15 UTC 2001


To clarify: there is nothing in the DNS protocol to stop you from creating multiple
PTR records with the same name, however no app is known to actually look beyond 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

Marc Thach Xuan Ky wrote:

> On 06/06/2001 20:47:10 bind-users-bounce wrote:
> > Yes. But you may have to make a choice about where to point the reverse
> records.
> >
> >
> > - Kevin
> >
> > Peter Apockotos wrote:
> >
> > > So then it is ok to give the same IP addresses that the testdomain.com has
> > > to the testdomain.net ?
> > >
> >
>
> Kevin implies here that only one PTR record should be defined for one IP address
> (in the presence of the number of A records using that address), whereas the
> other day, he and Brad were having a laugh and incidentally referring to
> mul;tiple PTRs as below:
>
> > >                                                                   Multiple
> > >  A records can point to the same address, true, but my (facetious)
> assumption
> > >  was that every A record had a corresponding PTR record:
> >
> >         Do you mean that multiple names could point to the same A record?
> > If so, yes that is certainly true.
>
> After reading too many RFCs I decided to have a policy to apply by default PTR
> records for each A record in my database.  So when I saw these mails I had a
> look through the archives and found a thread where Cricket (Feb 99) had said:
>
> > However, a resolver that retrieves all those PTR records may not do
> > anything intelligent with them.  At best, it'll probably just take the
> > first one in the list and ignore the rest.
>
> Barry Margolin notes:
>
> > While this is true, I generally recommend *against* multiple PTR records if
> > you don't have a good reason for them.  They're rarely all needed, and they
> > don't really service much purpose.  Because of round-robin, they'll result
> > in non-deterministic output from programs that translate addresses to
> > names.  And if you're using hostnames in access files (e.g. .rhosts) you'll
> > need to list all the names.
> >
> > It's important that every PTR record have a corresponding A record, but
> > it's not usually necessary for every A record to have a corresponding PTR
> > record.
>
> Cricket:
>
> > PTR records aren't round robinned, though--at least in BIND.  Here's
> > the salient section of ns_resp.c:
> >
> > ................
>
> Then there was a similar thread in Dec 99, and a few references in Oct 99.  The
> consensus seems to be that it's best not to add multiple PTR records.  All well
> and good but how do I select which domain name to use in the single PTR record?
> Nobody seemed to touch on this.  It needs to be a pragmatic decision, but in the
> absense of mail servers, where I'm not expecting anybody to be using
> r-utilities, what other criteria should I be looking at?
> Rgds to all,
> Marc TXK
>
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