Subject: Re: NSEC3 Issue 9: Potential DoS on Resolvers

From: Wes Hardaker <[email-masked]>
Date: Thu, 23 Feb 2006 13:12:59 -0800

>>>>> On Wed, 22 Feb 2006 23:43:39 +0000, Geoffrey Sisson <[email-masked]> said:

Geoffrey> One thought, though: perhaps max iterations could be registered as
Geoffrey> an IANA protocol parameter rather than in a formal document.

Or...  wait for it....  wait for it...

A new RRTYPE at the zone (thus should always exist for nsec3 enabled
zones) containing the number of iterations (or a range) that the zone
uses for it's nsec3 records.  Implementations could choose to query
for that record if the number advertised in a return nsec3 record was
greater than what it wanted to calculate without verifying it has to.

I'm about 90% kidding, but it would help.

Geoffrey> Another consideration: there will presumably have to be
Geoffrey> distinct max iterations values for each NSEC3 hash function
Geoffrey> in use.

True.

Another related attack is actually not for people forging NSEC3
records and returning them to an attacker, but is actually much worse:
Consider a registered zone with huge nsec3 iterations in it.  Then
consider sending mail from that zone to any mail server that performs
dns lookups for spam checking or other filtering reasons.  Now imagine
sending 10000 messages or more.

-- 
Wes Hardaker
Sparta, Inc.

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From: Ben Laurie <[email-masked]>
Date: Fri, 24 Feb 2006 12:15:01 +0000

> I'm about 90% kidding, but it would help.

I don't understand the point of this, since the number of iterations is
available in every NSEC3 record.

> records and returning them to an attacker, but is actually much worse:

Forgery is not an issue, since the signature won't check.

> sending 10000 messages or more.

This is indeed an example of the attack we mean.

Cheers,

Ben.

-- 
http://www.apache-ssl.org/ben.html           http://www.links.org/

"There is no limit to what a man can do or how far he can go if he
doesn't mind who gets the credit." - Robert Woodruff

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From: Geoffrey Sisson <[email-masked]>
Date: Fri, 24 Feb 2006 18:11:07 +0000

> I'm about 90% kidding, but it would help.

The problem with the 10% non-kidding part is that a malicious/ignorant
zone operator could put the value in this RR arbitrarily high, too.
The underlying issue -- how to get all (or most) servers and resolvers
to agree on a palatable maximum value that may change over time -- is
unsolved.

Geoff