full-disclosure - Re: DNS and NAT (was: DNS and CheckPoint)

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Date: Fri, 11 Jul 2008 11:01:33 -0400
From: Thomas Cross <tcross@...ibm.com>
To: "Riad S. Wahby" <rsw@...t.org>
Cc: full-disclosure@...ts.grok.org.uk
Subject: Re: DNS and NAT (was: DNS and CheckPoint)

Riad,

    Thanks for testing this. A number of other readers wrote me privately
confirming your result with linux ipchains. I'm not sure what ipchains does
when it encounters a collision, but in general I think this is a good
strategy. You'd have to have many thousands of simultaneous UDP
transactions in order for randomly selected source ports to be colliding
frequently enough for it to present a substantial problem.
    On the other hand, I've also been contacted by readers who confirm that
other devices besides the one imipack mentioned share it's behavior. There
appears to be room for some research here into what collision avoidance
strategies are employed by different NAT devices, what happens to those
devices under high load, and what the security implications are.
Fortunately, Linux appears to do a good job with this right now, and
provides an example approach that NAT vendors can look to.
    I'll post more if I have time to dig into this in further detail.

Regards,
Tom Cross
IBM X-Force

             "Riad S. Wahby"                                               
             <rsw@...t.org>                                                
                                                                        To 
             07/10/2008 11:06          Thomas Cross/Atlanta/IBM@...US      
             PM                                                         cc 
                                       full-disclosure@...ts.grok.org.uk   
                                                                   Subject 
                                       Re: DNS and NAT (was: DNS and       
                                       CheckPoint)                         

Thomas Cross <tcross@...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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