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Re: DNS and NAT (was: DNS and CheckPoint)
From: "Riad S. Wahby" <rsw () jfet org>
Date: Thu, 10 Jul 2008 23:06:03 -0400

Thomas Cross <tcross () us ibm com> wrote:
   We've also been wondering whether NAT devices ought to randomly assign
   UDP source ports, although no NAT vendor that wea**re aware of has done
   this to date.

Some quick testing implies that ipchains MASQUERADE-based NAT doesn't
suffer this problem because it preserves the source port.

My test setup is as follows: call the computer inside the NAT Alice, and
the computer outside Bob.  Alice contacts Bob via Trent, a linux-based
router, in my case a DLink DSL-2540B DSL modem / router combo.  On
Alice, I run the following:

( for j in $(seq 1 100); do i=$RANDOM; /bin/echo -n "$i "; echo $i | nc -q 0 -vv -p $i -u <Bob> 5555; sleep 1; done ) 
&> foo.Alice

On Bob, I run 

( while true; do nc -vv -l -u -p 5555 -q 0 </dev/null; done ) &> foo.Bob

At the end, I compare the actual source port in foo.Alice to the
apparent source port in foo.Bob.  In my setup, they are always
identical.

Obviously it is impossible to guarantee that this will always be the
case; in order to identify dangerous corner cases one would have to
consult the ipchains code, but given the relative frailty of the
randomized source port / randomized sequence number solution, for a
small number of computers behind a NAT (e.g., home users) I claim that's
a second-order danger at best.

In a large production environment where there is a huge amount of NAT
traffic being generated one would do well to consider a solution like
Thomas's suggestion that the servers be moved outside the firewall.

-=rsw

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