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A  U  S  C  E  R  T                                           A  L  E  R  T

                       AL-2008.0083 -- AUSCERT ALERT
                     [Win][UNIX/Linux][Juniper][Cisco]
 DNS cache poisoning fix may be ineffective with an intervening NAT device
                               23 July 2008

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        AusCERT Alert Summary
        ---------------------

Product:              DNS
Operating System:     UNIX variants (UNIX, Linux, OSX)
                      Windows
                      Juniper
                      Cisco
Impact:               Provide Misleading Information
Access:               Remote/Unauthenticated
Member content until: Wednesday, August 20 2008

Ref:                  AL-2008.0080

OVERVIEW

        The source-port randomisation fix for the recent DNS cache
        poisoning vulnerability [1] may be made ineffective by a NAT
        device which does not perform source port randomisation in the
        query path [2].


IMPACT

        DNS software behind such a NAT device would be vulnerable to cache
        poisoning as if the source-port randomisation fix had not been
        applied.


DETAILS

        To understand how an affected NAT device reverses the fix, we can
        examine the data flow for a DNS request.

        First, the patched DNS server sends out a request on a random port.
        This request reaches the affected NAT device which then makes its
        own request on behalf of the DNS server. The NAT device, not having
        source port randomisation, sends this request from a fixed port and
        a forged reply is able to be returned. The NAT device accepts this
        malicious reply and then responds to the DNS server with that
        information. The DNS server, thinking it has received a secure
        response, accepts this information, caching valid records contained
        in the malicious query response.


MITIGATION

        Affected DNS servers should be set to forward to a non-vulnerable
        DNS server and to stop caching. Information on how to do this with
        BIND 9 can be found on ISCs "Setting up BIND to forward to patched
        name server" page [3].

        If a router or other NAT device is being used as a DNS server and
        that device does not randomise ports the only solution is to
        either upgrade the firmware on that device to a version that does
        randomise ports (if available) or replace it with a device that
        is not vulnerable. If possible, forwarding to a DNS resolver
        outside the NAT device (and thereby bypassing it) could also solve
        this problem.

        To find out whether your DNS servers are vulnerable to cache
        poisoning, the following commands can be used. For UNIX or Linux
        users:

          dig porttest.dns-oarc.net in txt

        or for Windows users:

          nslookup -class=in -type=txt porttest.dns-oarc.net

        The result of these commands should include a line like:

           [SERVER ADDRESS] is GOOD: X queries in Y seconds from Z ports...

        A status of GOOD or FAIR is considered acceptable. Any other
        status should be considered a failure.


REFERENCES

        [1] AusCERT Alert AL-2008.0080
            http://www.auscert.org.au/9546

        [2] Paul Vixie's Blog - Not a Guessing Game
            http://www.circleid.com/posts/87143_dns_not_a_guessing_game/

        [3] ISC - BIND Forwarding
            http://www.isc.org/sw/bind/docs/forwarding.php


AusCERT has made every effort to ensure that the information contained
in this document is accurate.  However, the decision to use the information
described is the responsibility of each user or organisation. The decision to
follow or act on information or advice contained in this security bulletin is
the responsibility of each user or organisation, and should be considered in
accordance with your organisation's site policies and procedures. AusCERT
takes no responsibility for consequences which may arise from following or
acting on information or advice contained in this security bulletin.

If you believe that your computer system has been compromised or attacked in 
any way, we encourage you to let us know by completing the secure National IT 
Incident Reporting Form at:

        http://www.auscert.org.au/render.html?it=3192

===========================================================================
Australian Computer Emergency Response Team
The University of Queensland
Brisbane
Qld 4072

Internet Email: auscert@auscert.org.au
Facsimile:      (07) 3365 7031
Telephone:      (07) 3365 4417 (International: +61 7 3365 4417)
                AusCERT personnel answer during Queensland business hours
                which are GMT+10:00 (AEST).
                On call after hours for member emergencies only.
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