[dns-operations] forward compression pointers?

Roy Arends roy at dnss.ec
Wed Sep 2 10:00:08 UTC 2009

On Aug 30, 2009, at 10:59 PM, Robert Edmonds wrote:

> are there any DNS responders that generate forward compression  
> pointers,
> or DNS initiators that reject them?

I haven't seen one in the wild that create them. I have build a test- 
server thinghy that responds with forward pointers to see how  
resolvers behave. There are resolvers that reject them, and that  
follow them., and that go completely bezerk.

> i note RFC 5625 section 6.3 which could be construed to imply that  
> forward
> pointers may be dropped, since forward pointers "might cause a  
> parsing loop":
>   Examples of malformed packets that MAY be dropped include:
>   o  invalid compression pointers (i.e., those that point outside of
>      the current packet or that might cause a parsing loop)

I can't construe from that sentence alone that forward pointers may be  
dropped, for instance, a pointer to the exact location of that pointer  
is not a forward pointer, but can still cause loops.

> but 5625 is BCP, not standards track.  c.f.,
>    http://blacka.com/david/2008/03/09/internet-draft-ideas-dns- 
> related/
>    Idea #3: DNS Name Compression Standards
>    A draft mandating the DNS name compression only be done in one
>    direction. Virtually all (or perhaps even actually all)
>    implementations have DNS compression pointers only pointing to
>    earlier in the message. This draft would propose that
>    forward-pointing compression pointers should be treated as format
>    errors. This would accomplish two things:
>        1. Simplify what implementers need to support when parsing
>        messages, and
>        2. outlaw any possibility of having to deal with a compression
>        pointer loop.
>    And, in the process, effectively codify standard practice.

IMHO, this BCP does not violate any [proposed] STD.


More information about the dns-operations mailing list