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Re: DNS and NAT (was: DNS and CheckPoint)
From: "Riad S. Wahby" <rsw () jfet org>
Date: Fri, 11 Jul 2008 13:04:29 -0400

Valdis.Kletnieks () vt edu wrote:
With 64K source ports, you'll have collisions over 1% of the time at only 1024
in use. With 8K in use, you're hitting collisions 12% of the time.

Good point.  When collisions occur, as Thomas and I have pointed out,
the behavior of the NAT implementation is going to be crucial.

In the case of linux netfilter NATs, a quick look at the code reveals
that when netfilter selects a source port it does so with the
net_random() function call, which is #define'd (in include/linux/net.h)
as random32().  random32() comes from the kernel's lib/random32.c, and
according to the file header:

  This is a maximally equidistributed combined Tausworthe generator
  based on code from GNU Scientific Library 1.5 (30 Jun 2004)

   x_n = (s1_n ^ s2_n ^ s3_n)

   s1_{n+1} = (((s1_n & 4294967294) <<12) ^ (((s1_n <<13) ^ s1_n) >>19))
   s2_{n+1} = (((s2_n & 4294967288) << 4) ^ (((s2_n << 2) ^ s2_n) >>25))
   s3_{n+1} = (((s3_n & 4294967280) <<17) ^ (((s3_n << 3) ^ s3_n) >>11))

Even if this random number generator is prone to sequence prediction
attacks, Mallory does not know when the NAT is rewriting Alice's port
selection.  Beyond that, our scenario requires that there are collisions
happening, meaning that NAT's RNG state is being advanced by an unknown
number of other machines.  This mixing of Alice's random source ports
with Trent's random source ports and Carol's colliding source ports
would seem to make exploitation impractical even in the case that
Trent's RNG is weak against sequence prediction.


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