Duane,

Wessels, Duane:
> 
> This draft proposes a technique and new RR type for calculating and verifying 
> a message digest over the contents of a zone file.  Using this technique, the 
> recipient of a zone containing the new RR type can verify it for completeness 
> and correctness, especially so when the zone is signed.  We welcome your 
> feedback on this document.

I think this is a great idea. Certainly one of the things that we were
missing in the Yeti project was a way to confirm that the contents of
the root zone transfered from any particular master were actually the
correct version, so this fills a real need.

One issue is that the algorithm proposed requires that the recipient who
is generating a digest has to store basically the entire zone before
beginning the digest calculation, since it has no way to know if the
zone will be delivered in any kind of canonical order. It might be nice
to have some way to signal that. I'm not sure exactly how, and possibly
that belongs in a separate document (maybe we can tack it onto MIXFR?),
but it seems like a potential problem as it adds extra work (possibly
re-reading the entire zone an extra time along with the associated I/O,
memory consumption, and unneeded latency due to checking).

As for specific questions in the draft:

* I think that it should be allowed for unsigned zones. If nothing else,
  it provides the equivalent of checksum to help prevent truncations or
  not properly updated records (missing deletions or additions).

* I don't see much benefit in multiple ZONEMD. We generally expect
  masters & slaves to co-operate, so they should be able to find
  software that has an algorithm that both sides support.

* I think the serial in ZONEMD is helpful. Doesn't it also help prevent
  replay attacks?

* I would rather not special-case ZONEMD regarding responding. Even
  though it is bigger than most RDATA, large responses seem like a
  general problem. OTOH, I won't object too loudly if someone feels very
  strongly that this should be an option.

A final possible concern is that generating a digest on a large zone
might be computationally quite expensive. Indeed it could be used as an
attack vector on a hosting provider by using small IXFR to cause large
zone digests to be repeatedly calculated. Is it worth exploring the
possibility of including multiple ZONEMD in a zone at different names,
which digest the part of the zone up to that point? So something like:

a.example.com ...
 .
 .
 .
n.example.com ...
n.example.com ... ZONEMD ...  ; covers a-n
 .
 .
 .
z.example.com ...
example.com ... ZONEMD ... ; covers o-z

(Sorry this needs to be more carefully thought through, but that's the
basic idea.)

Cheers,

--
Shane

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