Re: 2 forwarded messages...DNSEXT discussion of Day and Kaminsky

On Fri, Feb 13, 2009 at 12:54 AM, Dean Anderson <[email protected]> wrote:
> You simply didn't follow the math of a birthday attack with 200 ports
> out of 65k.

Sorry, I followed the math fine.

> Each port of 200 used has a unique QID. The attacker has to
> get BOTH the port AND the QID correct.

Right.  But in this scenario, a single forged response packet has
potentially 200 chances of success (because it might match any
port/qid pair).  With the qmerge patches applied, a single forged
response packet can never match more than one outstanding query.

> With exactly one port, that port might be
> predictable, or they might be able to find it easilly with a quick scan
> for open/closed ports.

You keep alluding to your knowledge of cryptography.  You understand
security reduction proofs, right?

So you understand that if an attacker can carry out an attack against
dnscache with a single outstanding UDP query, then he can apply the
same technique when dnscache has 200 outstanding UDP queries and have
the same (or better) chance of success, right?  I.e., you understand
that any attack that an attacker can carry out with the qmerge patches
applied, he can still carry out without it applied, right?

Here's a homework assignment for you if you still think the qmerge
patches are bad: describe an attack that has a better chance of
success when the qmerge patch is applied than when it is not.
"Success" means dnscache accepts the forged response packet and
processes it as though it were legitimate.

In fact, I'll offer $100 to anyone who can describe such an attack.
(This offer only covers attacks against the idea of the qmerge patch;
i.e., modifying dnscache to merge duplicate outbound queries.  I
haven't yet thoroughly read the patch to look for implementation bugs,
so this offer doesn't cover those; once I've had a chance to, however,
I may extend the offer.)