On 14. 02. 24 16:45, Shumon Huque wrote:
On Wed, Feb 14, 2024 at 7:46 AM Edward Lewis <edward.le...@icann.org
<mailto:edward.le...@icann.org>> wrote:
On 2/14/24, 04:40, "DNSOP on behalf of Petr Špaček"
<dnsop-boun...@ietf.org <mailto:dnsop-boun...@ietf.org> on behalf of
pspa...@isc.org <mailto:pspa...@isc.org>> wrote:
> In my mind this is good enough reason to outlaw keytag
collisions -
> without them it would be _much_ easier to implement reasonable
limits
> without risk of breaking legitimate clients.
That would make key tags meaningful. ;--)
The question is how, in a multi-signer friendly way.
Yes, enforcing non-colliding keytags in a multi-signer configuration is
more challenging, since coordination across multiple independent parties
may be needed. But a process could be developed to deal with that.
But I'm not sure how worried I am about it, as a practical matter. Even
if by some remarkable coincidence all keys collided in a 2 party KSK+ZSK
multi-signer configuration, Unbound with its 4-keytag limit would still
be able to deal with it.( I guess some additional room for pre-published
rollover keys may be needed if they also collided).
What colliding keytag limits are other resolver implementers placing?
Right now BIND tolerates 1 validation failure before hard-failing. This
counter is not limited to colliding key tags.
In generic terms, the amount of work scales like:
#DNSKEYs * #RRSIGs
(for matching key tags)
Ed's example in the other thread with 1 DNSKEY and 10 bad RRSIGs is
certainly a problem - thus the limit on number of validation failures.
The high-level trouble is that (currently legally) colliding keys make
it hard to find a nice threshold which does not break legal setups and
at the same time prevents resolvers from doing excessive work.
Of course even with perfectly valid RRSIGs and just one key the amount
of work can be large - CNAME chains etc. etc. For that reason we
currently also limit number of validations to 16 per fetch.
And that leaves us more or less with basic random subdomain attack, so
we are back where we started :-)
If you think colliding keys should be allowed, please propose your own
limits for sensible behavior. I will take popcorn and watch.
--
Petr Špaček
Internet Systems Consortium
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