DNS name pointer

Barry Margolin barmar at alum.mit.edu
Wed Apr 26 05:11:36 UTC 2006

In article <e2m8t1$2fkg$1 at sf1.isc.org>,
 Kevin Darcy <kcd at daimlerchrysler.com> wrote:

> Rohit Tandon wrote:
> >Hi All,
> >
> >I was wondering if the NAME pointers that are used during compression
> >of a DNS packet can actually point to another pointer within the DNS
> >message. Is there any restriction that pointers should point to only
> >NAME or part of a NAME?
> >
> Offhand, I don't see any explicit restriction on that in the RFCs, but 
> what would be the value of adding the extra level(s) of indirection? I 
> see only risks, since probably most DNS implementations wouldn't expect 
> it and might handle it badly, and no benefits.

You're right that there's not much point to pointing directly to another 
pointer.  However, a variant case is certainly possible where a pointer 
points to a name that contains another pointer.  E.g. you could have the 
following names in a packet:


These could be encoded as

bar.<pointer to foo.com>
x.<pointer to bar.foo.com>

The code necessary to handle this would *probably* handle the OP's case 
properly, since expansion should be a simple recursive process.

Barry Margolin, barmar at alum.mit.edu
Arlington, MA
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