Archive for July, 2008

How to Really Fix Your DNS

July 25, 2008

Obviously the first thing everyone should be doing is to apply the patches that the major vendors rolled out, and do it quickly.  It is no longer the time for debate in regard to whether or not you really do need to patch… the answer to that question is quite clear; Yes.  Yes you do. Stop reading this, go to your vendor right now, and get the patches. Then apply them.  This will still be here when you get back…

Unfortunately, the existing patch doesn’t really fix the problem, it just makes it much harder to attack, which is a good thing.  If you still aren’t patched, you obviously didn’t follow my instructions in the first paragraph, so I’ll reiterate: Stop reading this, go to your vendor right now, and get the patches. Then apply them.

The patches that most major vendors rolled out when this vulnerability was announced, albeit with no technical details, primarily revolves around randomizing the source port that the nameserver makes it’s queries from.  Without this randomization, the only other piece of random information in the DNS packet is the transaction ID, which DNS servers use to correlate queries and replies, and also helps prevent reply-spoofing attacks by requiring that the attacker correctly guess this value.  Given the randomized hostname exploitation technique used in this attack, the attacker can force the nameserver to do as many queries as they like, which provides a birthday attack scenario for guessing the transaction ID value and succeeding in spoofing the reply.  The search space of the transaction ID is 16 bits, which provides possible values of 0-65535 within which the attacker has to guess correctly.  Given as many attempts as the attacker likes, this can take anywhere from a few seconds to a couple of minutes.  By adding the source port randomization to the picture, this adds around another 16 bits to the equation (minus source ports already used, privileged source port range, etc.), making the time it takes to correctly guess much longer, but still not impossible.

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