MIDCOM WG C. Jennings
Internet-Draft Cisco Systems
Expires: April 24, 2005 October 24, 2004
NAT Classification Results using STUN
draft-jennings-midcom-stun-results-02
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Abstract
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.
This work is being discussed on the ietf-behave@list.sipfoundry.org
mailing list
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1. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
mean.
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
NATs.
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
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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
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
them.
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
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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
group that this NAT falls into.
Vendor Model Firmware Prim Sec Hair Pres Grp
Airlink ASOHO4P V1.01.0095 Port Symm N Y B
Apple Air Base V5.2 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 2.42.7.1 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 02.00.07.0331 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 and two
shopping sprees in France and Taiwan, has provided the following
results.
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Vendor Model Firmware 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 2.42.7.1 Addr Note 1 N Y B
Lanner BRL-04FPU Cone Same N N
AboCom CAS3047 Port Same N Y
Lemmel LM-IS6400B Port Same N Y
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
anticipated.
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
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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
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 be consistent with the classification
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
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(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.
[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
USA
Phone: +1 408 421 9990
EMail: fluffy@cisco.com
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