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Re: DNS and NAT (was: DNS and CheckPoint)
From: Thomas Cross <tcross () us ibm com>
Date: Mon, 14 Jul 2008 16:13:28 -0400

Huzeyfe ONAL wrote me to mention that he had tested OpenBSD's pf and found
that it was assigning random ports for every new connection. Some
references [1], [2] seem to confirm this. The interesting thing about this
approach is that it may protect vulnerable DNS servers from attack if they
are placed behind it.

Also, a coworker directed me to this really excellent Internet Draft on
port randomization:

[2] http://www.openbsd.org/faq/pf/nat.html (note the mention of source port

             "Riad S. Wahby"                                           
             <rsw () jfet org>                                            
             07/10/2008 11:06          Thomas Cross/Atlanta/IBM () IBMUS  
             PM                                                         cc
                                       full-disclosure () lists grok org uk
                                       Re: DNS and NAT (was: DNS and   

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
   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

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.


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