MIDCOM WG                                                    C. Jennings
Internet-Draft                                             Cisco Systems
Expires: January 14, 2005                                  July 16, 2004

                 NAT Classification Results using STUN

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   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   IETF has several groups that are considering the impact of NATs on
   various protocols.  Having a classification of the types of NATs that
   are being developed and deployed is useful in gauging the impact of
   various solutions.  This draft records the results of classifying
   NATs using the STUN protocol.

1.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   document are to be interpreted as described in RFC-2119 [2].

   In this document, the term NAT means port address translation.  This
   is an unfortunate use of the terminology but is what NAT has come to

2.  Introduction

   A major issue in working with NAT traversal solutions for various
   protocols is that NATs behave in many different ways.  RFC 3489
   (STUN) classifies these and provides a method to test them.  This
   draft describes the results of testing several residential style

   Several NATs attempt to use the same external port number as the
   internal host used.  This is referred to as port preservation.  On
   the NATs that did this, some were found to have different
   characteristics depending on whether the port was already in use or
   not.  This was tested by running the STUN tests from a particular
   port on one internal IP address and then running them again from the
   same port on a different internal IP address.  The results from the
   first interface, where the port was preserved are referred to as the
   primary type while the results from the second interface, which did
   not manage to get the same external port because it was already in
   use, is referred to as the secondary type.  On most NATs the
   secondary type is the same as the primary but on some it is
   different; these are referred to as nondeterministic NATs, since a
   client with a single internal IP address can not figure out what the
   type of the NAT is.

   There are several NATs that would be detected as address restricted
   by the STUN tests but are not.  These NATs always use the same
   external port as the internal port and store the IP address of the
   most recent internal host to send a packet on that port.  The NATs
   then forward any traffic arriving to the external interface of the
   NAT on this port to the most recent internal host to use it.  These
   NATs are labeled of type "Bad" in the result table since they do not
   meet the definitions of NAPT in RFC 3022.  Interestingly, as long as
   the clients behind the NAT choose random port numbers, they often do
   work.  STUN detects these NATs as address restricted although they
   are really not address restricted NATs.  This type of NAT is easily
   detected by sending a STUN packet from the same port on two different
   internal IP addresses and looking at the mapped port in the return.
   If both packets were mapped to the same external port, the NAT is of
   the Bad type.

   Another important aspect of a NAT for some applications is whether it
   can send media from one internal host back to another host behind the

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   same NAT.  This is referred to as supporting hairpin media.

   Some NATs were rumored to exist that looked in arbitrary packets for
   either the NATs' external IP address or for the internal host IP
   address - either in binary or dotted decimal form - and rewrote it to
   something else.  STUN could be extended to test for exactly this type
   of behavior by echoing arbitrary client data and the mapped address
   but sending the bits inverted so these evil NATs did not mess with
   them.  NATs that do this will break integrity detection on payloads.

   To help organize the NATs by what types of applications they can
   support, the following groups are defined.  The application of using
   a SIP phone with a TLS connection for signaling and using STUN for
   media ports is considered.  It is assumed the RTP/RTCP media is on
   random port pairs as recommended for RTP.

      Group A: NATs that are deterministic, not symmetric, and support
      hairpin media.  These NATs would work with many phones behind
      Group B: NATs that are not symmetric on the primary mapping.  This
      group would work with many IP phones as long as the media ports
      did not conflict.  This is unlikely to happen often but will
      occasionally happen.  Because they may not support hairpin media,
      a call from one phone behind a NAT to another phone behind the
      same NAT may not work.
      Group D: NATs of the type Bad.  These have the same limitations of
      group B but when the ports conflict, media gets delivered to a
      random phone behind the NAT.
      Group F: These NATs are symmetric and phones will not work.

3.  Results

   The following table shows the results from several NATs.  This
   includes some random NATs the author had lying around as well as
   every NAT that could be purchased in February 2004 in the San Jose
   Fry's, Best Buy, CompUSA, and Circuit City.  Clearly this is not a
   very good approximation to a random sample.  It is clear that the
   NATs widely purchased in the US are different from what are available
   in Japan or in Europe.

   In the following table the Prim column indicates the primary type of
   the NAT.  A value of Port indicates port restricted, Cone is a full
   cone, Bad is described in the next section, Symm is Symmetric, and
   Addr is Address restricted.  The Hair column value of Y or N
   indicates whether the NAT will hairpin media.  The Pres column
   indicates whether the NAT attempts to preserve port numbers.  The Sec
   column indicates the secondary type of the NAT, and a value of Same
   indicates it is the same as the primary type.  The Grp indicates the

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   group that this NAT falls into.

   Vendor      Model       Fimware            Prim  Sec  Hair Pres Grp

   Airlink     ASOHO4P     V1.01.0095         Port  Symm  N    Y    B
   Apple       Air Base StaV5.2               Cone  Same  Y    N    A
   Belkin      F5D5321     V1.13              Port  Same  N    N    B
   Cisco       IOS                            Port  Symm            -
   Cisco       PIX                            Port  Same            -
   Corega      BAR Pro2    R1.00 Feb 21 2003  Cone                  -
   DLink       DI-604      2.0 Jun 2002       Cone  Same  N    N    B
   DLink       DI-704P     2.61 build 2       Cone  Same  Y    N    A
   Dlink       DI-804      .30, Tue,Jun 24 20 Cone  Same  Y    N    A
   Hawkings    FR24        6.26.02h Build 004 Bad   Same  Y    Y    D
   Linksys     BEFSR11                        Port                  B
   Linksys     BEFSR11 V2  1.42.7, Apr 02 200 Port                  B
   Linksys     BEFSR41     v1.44.2            Port                  B
   Linksys     BEFSR81 June 2002 Addr  Same  N    Y    B
   Linksys     BEFSRU31                       Port                  B
   Linksys     BEFSX41     1.44.3, Dec 24 200 Port                  B
   Linksys     BEFVP41     1.41.1, Sep 04 200 Port                  B
   Linksys     BEFW11S4    1.45.3, Jul 1 2003 Port                  B
   Linksys     WRT54G      1.42.2             Port  Symm  N    Y    B
   Linksys     WRT55AG     1.04, Jun.30, 2003 Port                  B
   Linksys     WRV54G      2.03               Port  Same  N    Y    B
   Microsoft   MN-700      Cone  Same  N    N    B
   Netgear     FVS318      V1.4 Jul. 15 2003  Port  Same  N    N    B
   Netgear     RP114       3.26(CD.0) 8/17/20 Cone                  -
   Netgear     RP614       4.00 April 2002    Cone  Same  Y    N    A
   NetworkEver NR041       Version 1.0 Rel 10 Symm  Same  N    N    F
   NetworkEver NR041       Version 1.2 Rel 03 Bad   Same  Y    Y    D
   SMC         2804WBRP-G  v1.00 Oct 14 2003  Port  Symm  Y    Y    B
   SMC         7004ABR     V1.42.003          Port  Same  N    N    B
   SMC         7004VBR     v1.03 Jun 12, 2002 Cone                  -
   Toshiba     WRC-1000    1.07.03a-C024a     Port  Cone  N    Y    B
   umax        ugate-3000  2.06h              Port                  -
   US Robotics USR8003     1.04 08            Cone  Same  N    N    B
   ZOT         BR1014      Unknown            Bad   Same  N    Y    D

   Since the time this testing was done, some addition testing has
   provided the following results.

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   Vendor      Model       Fimware            Prim  Sec  Hair Pres Grp

   Netgear          MR814v2     Version 5.01       Bad   Same  Y    Y    D
   Cisco            PIX 515     6.3(3)             Port  Same  N    N    B
   Dynex            DX-E401     1.03               Cone  Same  Y    N    A
   Asante           FR1004      R1.13 V2           Cone  Same  N    N    B
   Linksys          BEFSR81           Addr Note 1 N    Y    B

   The NAT with a secondary type of "Note 1" is particularly weird.  The
   primary connection is address restricted.  If a second host uses this
   same port, it also gets an Address Restricted but when a third host
   uses this same port, it get Symmetric.

   Another good source of information for behavior of various NATs is
   the NATCECK [6] web page.

4.  Discussion

   It is clear from discussions with various vendors and watching how
   tests have changed over the years that symmetric is becoming less
   common.  This change is being driven primarily by the desire to make
   online gaming work; many games use methods similar to STUN for NAT
   traversal.  The only symmetric NAT found was an old device.  More
   recent version of the software on the same device were not symmetric.
   It is clear that other symmetric NATs are deployed, but it is hard to
   find them.

5.  Security Concerns

   It is often assumed that symmetric NATs are more secure than port
   restricted NATs.  This is not true - they are identical from a
   security point of view.  They both only allow a packet to come inside
   the NAT if the host inside has previously sent to the exact same
   external IP and port.  One can argue that cone is less secure than
   port restricted, but this is not true if the attacker can spoof the
   IP address, which is fairly easy to do in many cases.  What level of
   security can be expected from NATs at all is a strange and curious
   topic.  With all the NATs, if you allow packets out, packets can come
   in, so don't be surprised if NATs provide less security that

6.  Open Issues

   The hairpin media tests were done by having a single host use STUN to
   find a public address on the NAT and then send media to itself and
   see if it was received.  It is possible that NATs might not hairpin
   media to the same host but would hairpin media to another host behind
   the same NAT.  It is possible that because of this, the hairpin

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   results reported here might be wrong.

   This sample set of NATs is very US-centric: D-Link, Lynksys, and
   Netgear dominate the US consumer market.  It would be good to get
   more results from other places.

   These test results should be verified by another group.  This has not
   been done yet.

   This draft should be moved to use the terminology in [7].

7.  Acknowledgments

   Many people and several mailing lists have contributed to the
   material on understanding NATs in this document.  Many thanks to
   Larry Metzger, Dan Wing, and Rohan Mahy.  The STUN server and client
   is open source and available at http://sourceforge.net/projects/stun
   and thank you to Jason Fischl who runs the public STUN server at
   larry.gloo.net.  Thanks to Yutaka Takeda who tested and found bugs
   and Christian Stredicke for getting people thinking.  Thanks to
   Francois AUDET for catching mistakes, verifying several results, and
   finding the very strange non-deterministic nature in the BEFSR81.

8.  References

8.1  Normative References

   [1]  Rosenberg, J., Weinberger, J., Huitema, C. and R. Mahy, "STUN -
        Simple Traversal of User Datagram Protocol (UDP) Through Network
        Address Translators (NATs)", RFC 3489, March 2003.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

8.2  Informative References

   [3]  Daigle, L. and IAB, "IAB Considerations for UNilateral
        Self-Address Fixing (UNSAF) Across Network Address Translation",
        RFC 3424, November 2002.

   [4]  Srisuresh, P. and K. Egevang, "Traditional IP Network Address
        Translator (Traditional NAT)", RFC 3022, January 2001.

   [5]  Srisuresh, P. and M. Holdrege, "IP Network Address Translator
        (NAT) Terminology and Considerations", RFC 2663, August 1999.

   [6]  Ford, B. and D. Andersen, "Nat Check Web Site:
        http://midcom-p2p.sourceforge.net", June 2004.

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   [7]  AUDET, F. and C. Jennings, "NAT/Firewall Behavioral
        Requirements", July 2004.

Author's Address

   Cullen Jennings
   Cisco Systems
   170 West Tasman Drive
   Mailstop SJC-21/2
   San Jose, CA  95134

   Phone: +1 408 421 9990
   EMail: fluffy@cisco.com

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