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Versions: (RFC 3489) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 5389

BEHAVE                                                      J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: August 22, 2005                                      C. Huitema
                                                               Microsoft
                                                                 R. Mahy
                                                                Airspace
                                                       February 21, 2005


   Simple Traversal of UDP Through Network Address Translators (NAT)
                                 (STUN)
                    draft-ietf-behave-rfc3489bis-01

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 22, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   Simple Traversal of UDP Through NATs (STUN) is a lightweight protocol
   that provides the ability for applications to determine the public IP
   addresses allocated to them by the NAT.  These addresses can be



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   placed into protocol payloads where a client needs to provide a
   publically routable IP address.  STUN works with many existing NATs,
   and does not require any special behavior from them.  As a result, it
   allows a wide variety of applications to work through existing NAT
   infrastructure.

Table of Contents

   1.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  NAT Variations . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  6
   7.  Message Overview . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  Server Behavior  . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1   Binding Requests . . . . . . . . . . . . . . . . . . . . . 10
     8.2   Shared Secret Requests . . . . . . . . . . . . . . . . . . 14
   9.  Client Behavior  . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1   Discovery  . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.2   Obtaining a Shared Secret  . . . . . . . . . . . . . . . . 17
     9.3   Formulating the Binding Request  . . . . . . . . . . . . . 18
     9.4   Processing Binding Responses . . . . . . . . . . . . . . . 19
     9.5   Using the Mapped Address . . . . . . . . . . . . . . . . . 21
   10.   Protocol Details . . . . . . . . . . . . . . . . . . . . . . 22
     10.1  Message Header . . . . . . . . . . . . . . . . . . . . . . 22
     10.2  Message Attributes . . . . . . . . . . . . . . . . . . . . 23
       10.2.1  MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . . 25
       10.2.2  RESPONSE-ADDRESS . . . . . . . . . . . . . . . . . . . 26
       10.2.3  CHANGED-ADDRESS  . . . . . . . . . . . . . . . . . . . 26
       10.2.4  CHANGE-REQUEST . . . . . . . . . . . . . . . . . . . . 26
       10.2.5  SOURCE-ADDRESS . . . . . . . . . . . . . . . . . . . . 26
       10.2.6  USERNAME . . . . . . . . . . . . . . . . . . . . . . . 27
       10.2.7  PASSWORD . . . . . . . . . . . . . . . . . . . . . . . 27
       10.2.8  MESSAGE-INTEGRITY  . . . . . . . . . . . . . . . . . . 27
       10.2.9  ERROR-CODE . . . . . . . . . . . . . . . . . . . . . . 27
       10.2.10   UNKNOWN-ATTRIBUTES . . . . . . . . . . . . . . . . . 29
       10.2.11   REFLECTED-FROM . . . . . . . . . . . . . . . . . . . 29
       10.2.12   XOR-MAPPED-ADDRESS . . . . . . . . . . . . . . . . . 29
       10.2.13   XOR-ONLY . . . . . . . . . . . . . . . . . . . . . . 30
       10.2.14   SERVER . . . . . . . . . . . . . . . . . . . . . . . 30
   11.   Security Considerations  . . . . . . . . . . . . . . . . . . 31
     11.1  Attacks on STUN  . . . . . . . . . . . . . . . . . . . . . 31
       11.1.1  Attack I: DDOS Against a Target  . . . . . . . . . . . 31
       11.1.2  Attack II: Silencing a Client  . . . . . . . . . . . . 31
       11.1.3  Attack III: Assuming the Identity of a Client  . . . . 32
       11.1.4  Attack IV: Eavesdropping . . . . . . . . . . . . . . . 32
     11.2  Launching the Attacks  . . . . . . . . . . . . . . . . . . 32



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       11.2.1  Approach I: Compromise a Legitimate STUN Server  . . . 33
       11.2.2  Approach II: DNS Attacks . . . . . . . . . . . . . . . 33
       11.2.3  Approach III: Rogue Router or NAT  . . . . . . . . . . 33
       11.2.4  Approach IV: MITM  . . . . . . . . . . . . . . . . . . 34
       11.2.5  Approach V: Response Injection Plus DoS  . . . . . . . 34
       11.2.6  Approach VI: Duplication . . . . . . . . . . . . . . . 34
     11.3  Countermeasures  . . . . . . . . . . . . . . . . . . . . . 35
     11.4  Residual Threats . . . . . . . . . . . . . . . . . . . . . 37
   12.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 37
   13.   IAB Considerations . . . . . . . . . . . . . . . . . . . . . 37
     13.1  Problem Definition . . . . . . . . . . . . . . . . . . . . 37
     13.2  Exit Strategy  . . . . . . . . . . . . . . . . . . . . . . 38
     13.3  Brittleness Introduced by STUN . . . . . . . . . . . . . . 38
     13.4  Requirements for a Long Term Solution  . . . . . . . . . . 40
     13.5  Issues with Existing NAPT Boxes  . . . . . . . . . . . . . 41
     13.6  In Closing . . . . . . . . . . . . . . . . . . . . . . . . 42
   14.   Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . 42
   15.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 43
   16.   References . . . . . . . . . . . . . . . . . . . . . . . . . 43
   16.1  Normative References . . . . . . . . . . . . . . . . . . . . 43
   16.2  Informative References . . . . . . . . . . . . . . . . . . . 43
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 45
       Intellectual Property and Copyright Statements . . . . . . . . 46




























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1.  Applicability Statement

   This protocol is not a cure-all for the problems associated with NAT.
   It does not enable incoming TCP connections through NAT.  It allows
   incoming UDP packets through NAT, but only through a subset of
   existing NAT types.  In particular, STUN does not enable incoming UDP
   packets through symmetric NATs (defined below), which are common in
   large enterprises.  STUN does not work when it is used to obtain an
   address to communicate with a peer which happens to be behind the
   same NAT.  STUN does not work when the STUN server is not in a common
   shared address realm.  For a more complete discussion of the
   limitations of STUN, see Section 13.

2.  Introduction

   Network Address Translators (NATs), while providing many benefits,
   also come with many drawbacks.  The most troublesome of those
   drawbacks is the fact that they break many existing IP applications,
   and make it difficult to deploy new ones.  Guidelines have been
   developed [8] that describe how to build "NAT friendly" protocols,
   but many protocols simply cannot be constructed according to those
   guidelines.  Examples of such protocols include almost all peer-to-
   peer protocols, such as multimedia communications, file sharing and
   games.

   To combat this problem, Application Layer Gateways (ALGs) have been
   embedded in NATs.  ALGs perform the application layer functions
   required for a particular protocol to traverse a NAT.  Typically,
   this involves rewriting application layer messages to contain
   translated addresses, rather than the ones inserted by the sender of
   the message.  ALGs have serious limitations, including scalability,
   reliability, and speed of deploying new applications.  To resolve
   these problems, the Middlebox Communications (MIDCOM) protocol is
   being developed [9].  MIDCOM allows an application entity, such as an
   end client or network server of some sort (like a Session Initiation
   Protocol (SIP) proxy [10]) to control a NAT (or firewall), in order
   to obtain NAT bindings and open or close pinholes.  In this way, NATs
   and applications can be separated once more, eliminating the need for
   embedding ALGs in NATs, and resolving the limitations imposed by
   current architectures.

   Unfortunately, MIDCOM requires upgrades to existing NAT and
   firewalls, in addition to application components.  Complete upgrades
   of these NAT and firewall products will take a long time, potentially
   years.  This is due, in part, to the fact that the deployers of NAT
   and firewalls are not the same people who are deploying and using
   applications.  As a result, the incentive to upgrade these devices
   will be low in many cases.  Consider, for example, an airport



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   Internet lounge that provides access with a NAT.  A user connecting
   to the NATed network may wish to use a peer-to-peer service, but
   cannot, because the NAT doesn't support it.  Since the administrators
   of the lounge are not the ones providing the service, they are not
   motivated to upgrade their NAT equipment to support it, using either
   an ALG, or MIDCOM.

   Another problem is that the MIDCOM protocol requires that the agent
   controlling the middleboxes know the identity of those middleboxes,
   and have a relationship with them which permits control.  In many
   configurations, this will not be possible.  For example, many cable
   access providers use NAT in front of their entire access network.
   This NAT could be in addition to a residential NAT purchased and
   operated by the end user.  The end user will probably not have a
   control relationship with the NAT in the cable access network, and
   may not even know of its existence.

   Many existing proprietary protocols, such as those for online games
   (such as the games described in RFC 3027 [11]) and Voice over IP,
   have developed tricks that allow them to operate through NATs without
   changing those NATs.  This document is an attempt to take some of
   those ideas, and codify them into an interoperable protocol that can
   meet the needs of many applications.

   The protocol described here, Simple Traversal of UDP Through NAT
   (STUN), allows entities behind a NAT to learn the address bindings
   allocated by the NAT.  STUN requires no changes to NATs, and works
   with an arbitrary number of NATs in tandem between the application
   entity and the public Internet.

3.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
   [1] and indicate requirement levels for compliant STUN
   implementations.

4.  Definitions

   STUN Client: A STUN client (also just referred to as a client) is an
      entity that generates STUN requests.  A STUN client can execute on
      an end system, such as a user's PC, or can run in a network
      element, such as a conferencing server.

   STUN Server: A STUN Server (also just referred to as a server) is an
      entity that receives STUN requests, and sends STUN responses.
      STUN servers are generally attached to the public Internet.



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5.  NAT Variations

   It is assumed that the reader is familiar with NATs.  It has been
   observed that NAT treatment of UDP varies among implementations.  The
   four treatments observed in implementations are:

   Full Cone: A full cone NAT is one where all requests from the same
      internal IP address and port are mapped to the same external IP
      address and port.  Furthermore, any external host can send a
      packet to the internal host, by sending a packet to the mapped
      external address.

   Restricted Cone: A restricted cone NAT is one where all requests from
      the same internal IP address and port are mapped to the same
      external IP address and port.  Unlike a full cone NAT, an external
      host (with IP address X) can send a packet to the internal host
      only if the internal host had previously sent a packet to IP
      address X.

   Port Restricted Cone: A port restricted cone NAT is like a restricted
      cone NAT, but the restriction includes port numbers.
      Specifically, an external host can send a packet, with source IP
      address X and source port P, to the internal host only if the
      internal host had previously sent a packet to IP address X and
      port P.

   Symmetric: A symmetric NAT is one where all requests from the same
      internal IP address and port, to a specific destination IP address
      and port, are mapped to the same external IP address and port.  If
      the same host sends a packet with the same source address and
      port, but to a different destination, a different mapping is used.
      Furthermore, only the external host that receives a packet can
      send a UDP packet back to the internal host.


6.  Overview of Operation

   This section is descriptive only.  Normative behavior is described in
   Section 8 and Section 9.












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                               /-----\
                             // STUN  \\
                            |   Server  |
                             \\       //
                               \-----/


                          +--------------+             Public Internet
          ................|     NAT 2    |.......................
                          +--------------+


                          +--------------+             Private NET 2
          ................|     NAT 1    |.......................
                          +--------------+

                               /-----\
                             // STUN  \\
                            |   Client  |
                             \\       //               Private NET 1
                               \-----/

                                Figure 1

   The typical STUN configuration is shown in Figure 1.  A STUN client
   is connected to private network 1.  This network connects to private
   network 2 through NAT 1.  Private network 2 connects to the public
   Internet through NAT 2.  The STUN server resides on the public
   Internet.

   STUN is a simple client-server protocol.  A client sends a request to
   a server, and the server returns a response.  There are two types of
   requests - Binding Requests, sent over UDP, and Shared Secret
   Requests, sent over TLS [2] over TCP.  Shared Secret Requests ask the
   server to return a temporary username and password.  This username
   and password are used in a subsequent Binding Request and Binding
   Response, for the purposes of authentication and message integrity.

   Binding requests are used to determine the bindings allocated by
   NATs.  The client sends a Binding Request to the server, over UDP.
   The server examines the source IP address and port of the request,
   and copies them into a response that is sent back to the client.
   There are some parameters in the request that allow the client to ask
   that the response be sent elsewhere, or that the server send the
   response from a different address and port.  The flags allow for STUN
   to be used in diagnostic applications.  There are attributes for
   providing message integrity and authentication.




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   The STUN client is typically embedded in an application which needs
   to obtain a public IP address and port that can be used to receive
   data.  For example, it might need to obtain an IP address and port to
   receive Real Time Transport Protocol (RTP) [12] traffic.  When the
   application starts, the STUN client within the application sends a
   STUN Shared Secret Request to its server, obtains a username and
   password, and then sends it a Binding Request.  STUN servers can be
   discovered through DNS SRV records [3], and it is generally assumed
   that the client is configured with the domain to use to find the STUN
   server.  Generally, this will be the domain of the provider of the
   service the application is using (such a provider is incented to
   deploy STUN servers in order to allow its customers to use its
   application through NAT).  Of course, a client can determine the
   address or domain name of a STUN server through other means.  A STUN
   server can even be embedded within an end system.

   The STUN Binding Request is used to discover the public IP address
   and port mappings generated by the NAT.  Binding Requests are sent to
   the STUN server using UDP.  When a Binding Request arrives at the
   STUN server, it may have passed through one or more NATs between the
   STUN client and the STUN server.  As a result, the source address of
   the request received by the server will be the mapped address created
   by the NAT closest to the server.  The STUN server copies that source
   IP address and port into a STUN Binding Response, and sends it back
   to the source IP address and port of the STUN request.  For all of
   the NAT types above, this response will arrive at the STUN client.

   When the STUN client receives the STUN Binding Response, it compares
   the IP address and port in the packet with the local IP address and
   port it bound to when the request was sent.  If these do not match,
   the STUN client is behind one or more NATs.  The IP address and port
   in the body of the STUN response are public, and can be used by any
   host on the public Internet to send packets to the application that
   sent the STUN request.  An application need only listen on the IP
   address and port from which the STUN request was sent.  Packets sent
   by a host on the public Internet to the public address and port
   learned by STUN will be received by the application, so long as
   conditions permit.  The conditions in which these packets will not be
   received by the client are described in Section 1.

   It should be noted that the configuration in Figure 1 is not the only
   permissible configuration.  The STUN server can be located anywhere,
   including within another client.  The only requirement is that the
   STUN server is reachable by the client, and if the client is trying
   to obtain a publicly routable address, that the server reside on the
   public Internet.





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7.  Message Overview

   STUN messages are TLV (type-length-value) encoded using big endian
   (network ordered) binary.  All STUN messages start with a STUN
   header, followed by a STUN payload.  The payload is a series of STUN
   attributes, the set of which depends on the message type.  The STUN
   header contains a STUN message type, transaction ID, and length.  The
   message type can be Binding Request, Binding Response, Binding Error
   Response, Shared Secret Request, Shared Secret Response, or Shared
   Secret Error Response.  The transaction ID is used to correlate
   requests and responses.  The length indicates the total length of the
   STUN payload, not including the header.  This allows STUN to run over
   TCP.  Shared Secret Requests are always sent over TCP (indeed, using
   TLS over TCP).

   Several STUN attributes are defined.  The first is a MAPPED-ADDRESS
   attribute, which is an IP address and port.  It is always placed in
   the Binding Response, and it indicates the source IP address and port
   the server saw in the Binding Request.  There is also a RESPONSE-
   ADDRESS attribute, which contains an IP address and port.  The
   RESPONSE-ADDRESS attribute can be present in the Binding Request, and
   indicates where the Binding Response is to be sent.  It's optional,
   and when not present, the Binding Response is sent to the source IP
   address and port of the Binding Request.

   The third attribute is the CHANGE-REQUEST attribute, and it contains
   two flags to control the IP address and port used to send the
   response.  These flags are called "change IP" and "change port"
   flags.  The CHANGE-REQUEST attribute is allowed only in the Binding
   Request.  They instruct the server to send the Binding Responses from
   a different source IP address and port.  The CHANGE-REQUEST attribute
   is optional in the Binding Request.

   The fourth attribute is the CHANGED-ADDRESS attribute.  It is present
   in Binding Responses.  It informs the client of the source IP address
   and port that would be used if the client requested the "change IP"
   and "change port" behavior.

   The fifth attribute is the SOURCE-ADDRESS attribute.  It is only
   present in Binding Responses.  It indicates the source IP address and
   port where the response was sent from.

   The RESPONSE-ADDRESS, CHANGE-REQUEST, CHANGED-ADDRESS and
   SOURCE-ADDRESS attributes are primarily useful for diagnostic
   applications that use STUN in order to determine information about
   the type of NAT.  The usage of these attributes for such purposes is
   outside the scope of this specification.




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   The sixth attribute is the USERNAME attribute.  It is present in a
   Shared Secret Response, which provides the client with a temporary
   username and password (encoded in the PASSWORD attribute).  The
   USERNAME is also present in Binding Requests, serving as an index to
   the shared secret used for the integrity protection of the Binding
   Request.  The seventh attribute, PASSWORD, is only found in Shared
   Secret Response messages.  The eight attribute is the MESSAGE-
   INTEGRITY attribute, which contains a message integrity check over
   the Binding Request or Binding Response.

   The ninth attribute is the ERROR-CODE attribute.  This is present in
   the Binding Error Response and Shared Secret Error Response.  It
   indicates the error that has occurred.  The tenth attribute is the
   UNKNOWN-ATTRIBUTES attribute, which is present in either the Binding
   Error Response or Shared Secret Error Response.  It indicates the
   mandatory attributes from the request which were unknown.  The
   eleventh attribute is the REFLECTED-FROM attribute, which is present
   in Binding Responses.  It indicates the IP address and port of the
   sender of a Binding Request, used for traceability purposes to
   prevent certain denial-of-service attacks.

   The twelfth attribute is XOR-MAPPED-ADDRESS.  Like MAPPED-ADDRESS, it
   is present in the Binding Response, and tells the client the source
   IP address and port where the Binding Request came from.  However, it
   is encoded using an Exclusive Or (XOR) operation with the transaction
   ID.  Some NAT devices have been found to rewrite binary encoded IP
   addresses present in protocol PDUs.  Such behavior interferes with
   the operation of STUN.  Clients use XOR-MAPPED-ADDRESS instead of
   MAPPED-ADDRESS whenever both are present in a Binding Response.
   Using XOR-MAPPED-ADDRESS protects the client from such interfering
   NAT devices.

   The last attribute is XOR-ONLY.  It can be present in the Binding
   Request.  It tells the server to only send a XOR-MAPPED-ADDRESS in
   the Binding Response.

8.  Server Behavior

   The server behavior depends on whether the request is a Binding
   Request or a Shared Secret Request.

8.1  Binding Requests

   A STUN server MUST be prepared to receive Binding Requests on four
   address/port combinations - (A1, P1), (A2, P1), (A1, P2), and (A2,
   P2).  (A1, P1) represent the primary address and port, and these are
   the ones obtained through the client discovery procedures below.
   Typically, P1 will be port 3478, the default STUN port.  A2 and P2



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   are arbitrary.  A2 and P2 are advertised by the server through the
   CHANGED-ADDRESS attribute, as described below.

      OPEN ISSUE: Experience has shown that the usage of a dynamic port
      for P2 has been problematic.  This is because firewall
      administrators have opened up port 3478 to permit STUN, but
      disallowed the dynamic port used by the server.  This causes the
      diagnostic techniques to fail.  This can be fixed through
      allocation of a second port number from IANA.  Does that belong in
      this specification or in the diagnostic specification? I think it
      has to go here.

   It is RECOMMENDED that the server check the Binding Request for a
   MESSAGE-INTEGRITY attribute.  If not present, and the server requires
   integrity checks on the request, it generates a Binding Error
   Response with an ERROR-CODE attribute with response code 401.  If the
   MESSAGE-INTEGRITY attribute was present, the server computes the HMAC
   over the request as described in Section 10.2.8.  The key to use
   depends on the shared secret mechanism.  If the STUN Shared Secret
   Request was used, the key MUST be the one associated with the
   USERNAME attribute present in the request.  If the USERNAME attribute
   was not present, the server MUST generate a Binding Error Response.
   The Binding Error Response MUST include an ERROR-CODE attribute with
   response code 432.  If the USERNAME is present, but the server
   doesn't remember the shared secret for that USERNAME (because it
   timed out, for example), the server MUST generate a Binding Error
   Response.  The Binding Error Response MUST include an ERROR-CODE
   attribute with response code 430.  If the server does know the shared
   secret, but the computed HMAC differs from the one in the request,
   the server MUST generate a Binding Error Response with an ERROR-CODE
   attribute with response code 431.  The Binding Error Response is sent
   to the IP address and port the Binding Request came from, and sent
   from the IP address and port the Binding Request was sent to.

   Assuming the message integrity check passed, processing continues.
   The server MUST check for any attributes in the request with values
   less than or equal to 0x7fff which it does not understand.  If it
   encounters any, the server MUST generate a Binding Error Response,
   and it MUST include an ERROR-CODE attribute with a 420 response code.

   That response MUST contain an UNKNOWN-ATTRIBUTES attribute listing
   the attributes with values less than or equal to 0x7fff which were
   not understood.  The Binding Error Response is sent to the IP address
   and port the Binding Request came from, and sent from the IP address
   and port the Binding Request was sent to.

   Assuming the request was correctly formed, the server MUST generate a
   single Binding Response.  The Binding Response MUST contain the same



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   transaction ID contained in the Binding Request.  The length in the
   message header MUST contain the total length of the message in bytes,
   excluding the header.  The Binding Response MUST have a message type
   of "Binding Response".

   If the XOR-ONLY attribute was not present in the request, the server
   MUST add a MAPPED-ADDRESS attribute to the Binding Response.  The IP
   address component of this attribute MUST be set to the source IP
   address observed in the Binding Request.  The port component of this
   attribute MUST be set to the source port observed in the Binding
   Request.  If the XOR-ONLY attribute was present in the request, the
   server MUST NOT include the MAPPED-ADDRESS attribute in the Binding
   Response.

   The server MUST add a XOR-MAPPED-ADDRESS attribute to the Binding
   Response.  This attribute has the same information content as
   MAPPED-ADDRESS (in particular, it conveys the IP address and port
   observed in the source IP and source port fields of the STUN
   request), but is encoded by performing an XOR operation between the
   transaction ID and the IP address and port.  The details on the
   encoding can be found in Section 10.2.12.

   The server SHOULD add a SERVER attribute to any Binding Response or
   Binding Error Response it generates, and its value SHOULD indicate
   the manufacturer of the software and a software version or build
   number.

   If the RESPONSE-ADDRESS attribute was absent from the Binding
   Request, the destination address and port of the Binding Response
   MUST be the same as the source address and port of the Binding
   Request.  Otherwise, the destination address and port of the Binding
   Response MUST be the value of the IP address and port in the
   RESPONSE-ADDRESS attribute.

   The source address and port of the Binding Response depend on the
   value of the CHANGE-REQUEST attribute and on the address and port the
   Binding Request was received on, and are summarized in Table 1.

   Let Da represent the destination IP address of the Binding Request
   (which will be either A1 or A2), and Dp represent the destination
   port of the Binding Request (which will be either P1 or P2).  Let Ca
   represent the other address, so that if Da is A1, Ca is A2.  If Da is
   A2, Ca is A1.  Similarly, let Cp represent the other port, so that if
   Dp is P1, Cp is P2.  If Dp is P2, Cp is P1.  If the "change port"
   flag was set in CHANGE-REQUEST attribute of the Binding Request, and
   the "change IP" flag was not set, the source IP address of the
   Binding Response MUST be Da and the source port of the Binding
   Response MUST be Cp.  If the "change IP" flag was set in the Binding



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   Request, and the "change port" flag was not set, the source IP
   address of the Binding Response MUST be Ca and the source port of the
   Binding Response MUST be Dp.  When both flags are set, the source IP
   address of the Binding Response MUST be Ca and the source port of the
   Binding Response MUST be Cp.  If neither flag is set, or if the
   CHANGE-REQUEST attribute is absent entirely, the source IP address of
   the Binding Response MUST be Da and the source port of the Binding
   Response MUST be Dp.


   Flags          Source Address  Source Port   CHANGED-ADDRESS
   none           Da              Dp            Ca:Cp
   Change IP      Ca              Dp            Ca:Cp
   Change port    Da              Cp            Ca:Cp
   Change IP and
     Change port  Ca              Cp            Ca:Cp

                                Figure 2

   The server MUST add a SOURCE-ADDRESS attribute to the Binding
   Response, containing the source address and port used to send the
   Binding Response.

   The server MUST add a CHANGED-ADDRESS attribute to the Binding
   Response.  This contains the source IP address and port that would be
   used if the client had set the "change IP" and "change port" flags in
   the Binding Request.  As summarized in Table 1, these are Ca and Cp,
   respectively, regardless of the value of the CHANGE-REQUEST flags.

   If the Binding Request contained both the USERNAME and MESSAGE-
   INTEGRITY attributes, the server MUST add a MESSAGE-INTEGRITY
   attribute to the Binding Response.  The attribute contains an HMAC
   [13] over the response, as described in Section 10.2.8.  The key to
   use depends on the shared secret mechanism.  If the STUN Shared
   Secret Request was used, the key MUST be the one associated with the
   USERNAME attribute present in the Binding Request.

   If the Binding Request contained a RESPONSE-ADDRESS attribute, the
   server MUST add a REFLECTED-FROM attribute to the response.  If the
   Binding Request was authenticated using a username obtained from a
   Shared Secret Request, the REFLECTED-FROM attribute MUST contain the
   source IP address and port where that Shared Secret Request came
   from.  If the username present in the request was not allocated using
   a Shared Secret Request, the REFLECTED-FROM attribute MUST contain
   the source address and port of the entity which obtained the
   username, as best can be verified with the mechanism used to allocate
   the username.  If the username was not present in the request, and
   the server was willing to process the request, the REFLECTED-FROM



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   attribute SHOULD contain the source IP address and port where the
   request came from.

   The server SHOULD NOT retransmit the response.  Reliability is
   achieved by having the client periodically resend the request, each
   of which triggers a response from the server.

8.2  Shared Secret Requests

   Shared Secret Requests are always received on TLS connections.  When
   the server receives a request from the client to establish a TLS
   connection, it MUST proceed with TLS, and SHOULD present a site
   certificate.  The TLS ciphersuite TLS_RSA_WITH_AES_128_CBC_SHA [4]
   SHOULD be used.  Client TLS authentication MUST NOT be done, since
   the server is not allocating any resources to clients, and the
   computational burden can be a source of attacks.

   If the server receives a Shared Secret Request, it MUST verify that
   the request arrived on a TLS connection.  If it did not receive the
   request over TLS, it MUST generate a Shared Secret Error Response,
   and it MUST include an ERROR-CODE attribute with a 433 response code.
   The destination for the error response depends on the transport on
   which the request was received.  If the Shared Secret Request was
   received over TCP, the Shared Secret Error Response is sent over the
   same connection the request was received on.  If the Shared Secret
   Request was receive over UDP, the Shared Secret Error Response is
   sent to the source IP address and port that the request came from.

   The server MUST check for any attributes in the request with values
   less than or equal to 0x7fff which it does not understand.  If it
   encounters any, the server MUST generate a Shared Secret Error
   Response, and it MUST include an ERROR-CODE attribute with a 420
   response code.  That response MUST contain an UNKNOWN-ATTRIBUTES
   attribute listing the attributes with values less than or equal to
   0x7fff which were not understood.  The Shared Secret Error Response
   is sent over the TLS connection.

   All Shared Secret Error Responses MUST contain the same transaction
   ID contained in the Shared Secret Request.  The length in the message
   header MUST contain the total length of the message in bytes,
   excluding the header.  The Shared Secret Error Response MUST have a
   message type of "Shared Secret Error Response" (0x0112).

   Assuming the request was properly constructed, the server creates a
   Shared Secret Response.  The Shared Secret Response MUST contain the
   same transaction ID contained in the Shared Secret Request.  The
   length in the message header MUST contain the total length of the
   message in bytes, excluding the header.  The Shared Secret Response



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   MUST have a message type of "Shared Secret Response".  The Shared
   Secret Response MUST contain a USERNAME attribute and a PASSWORD
   attribute.  The USERNAME attribute serves as an index to the
   password, which is contained in the PASSWORD attribute.  The server
   can use any mechanism it chooses to generate the username.  However,
   the username MUST be valid for a period of at least 10 minutes.
   Validity means that the server can compute the password for that
   username.  There MUST be a single password for each username.  In
   other words, the server cannot, 10 minutes later, assign a different
   password to the same username.  The server MUST hand out a different
   username for each distinct Shared Secret Request.  Distinct, in this
   case, implies a different transaction ID.  It is RECOMMENDED that the
   server explicitly invalidate the username after ten minutes.  It MUST
   invalidate the username after 30 minutes.  The PASSWORD contains the
   password bound to that username.  The password MUST have at least 128
   bits.  The likelihood that the server assigns the same password for
   two different usernames MUST be vanishingly small, and the passwords
   MUST be unguessable.  In other words, they MUST be a
   cryptographically random function of the username.

   These requirements can still be met using a stateless server, by
   intelligently computing the USERNAME and PASSWORD.  One approach is
   to construct the USERNAME as:


     USERNAME = <prefix,rounded-time,clientIP,hmac>

   Where prefix is some random text string (different for each shared
   secret request), rounded-time is the current time modulo 20 minutes,
   clientIP is the source IP address where the Shared Secret Request
   came from, and hmac is an HMAC [13] over the prefix, rounded-time,
   and client IP, using a server private key.

   The password is then computed as:


     password = <hmac(USERNAME,anotherprivatekey)>

   With this structure, the username itself, which will be present in
   the Binding Request, contains the source IP address where the Shared
   Secret Request came from.  That allows the server to meet the
   requirements specified in Section 8.1 for constructing the
   REFLECTED-FROM attribute.  The server can verify that the username
   was not tampered with, using the hmac present in the username.

   The server SHOULD include a SERVER attribute in any Shared Secret
   Response or Shared Secret Error response it generates, and its value
   SHOULD indicate the manufacturer of the software and a software



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   version or build number.

   The Shared Secret Response is sent over the same TLS connection the
   request was received on.  The server SHOULD keep the connection open,
   and let the client close it.

9.  Client Behavior

   The behavior of the client is very straightforward.  Its task is to
   discover the STUN server, obtain a shared secret, formulate the
   Binding Request, handle request reliability, process the Binding
   Responses, and use the resulting addresses.

9.1  Discovery

   Generally, the client will be configured with a domain name of the
   provider of the STUN servers.  This domain name is resolved to an IP
   address and port using the SRV procedures specified in RFC 2782 [3].

   Specifically, the service name is "stun".  The protocol is "udp" for
   sending Binding Requests, or "tcp" for sending Shared Secret
   Requests.  The procedures of RFC 2782 are followed to determine the
   server to contact.  RFC 2782 spells out the details of how a set of
   SRV records are sorted and then tried.  However, it only states that
   the client should "try to connect to the (protocol, address,
   service)" without giving any details on what happens in the event of
   failure.  Those details are described here for STUN.

   For STUN requests, failure occurs if there is a transport failure of
   some sort (generally, due to fatal ICMP errors in UDP or connection
   failures in TCP).  Failure also occurs if the transaction fails due
   to timeout.  This occurs 9.5 seconds after the first request is sent,
   for both Shared Secret Requests and Binding Requests.  See Section
   9.3 for details on transaction timeouts for Binding Requests.  If a
   failure occurs, the client SHOULD create a new request, which is
   identical to the previous, but has a different transaction ID and
   MESSAGE INTEGRITY attribute (the HMAC will change because the
   transaction ID has changed).  That request is sent to the next
   element in the list as specified by RFC 2782.

   The default port for STUN requests is 3478, for both TCP and UDP.
   Administrators SHOULD use this port in their SRV records, but MAY use
   others.

   If no SRV records were found, the client performs an A record lookup
   of the domain name.  The result will be a list of IP addresses, each
   of which can be contacted at the default port.




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   This would allow a firewall admin to open the STUN port, so hosts
   within the enterprise could access new applications.  Whether they
   will or won't do this is a good question.

9.2  Obtaining a Shared Secret

   As discussed in Section 11, there are several attacks possible on
   STUN systems.  Many of these are prevented through integrity of
   requests and responses.  To provide that integrity, STUN makes use of
   a shared secret between client and server, used as the keying
   material for an HMAC used in both the Binding Request and Binding
   Response.  STUN allows for the shared secret to be obtained in any
   way (for example, Kerberos [14]).  However, it MUST have at least 128
   bits of randomness.  In order to ensure interoperability, this
   specification describes a TLS-based mechanism.  This mechanism,
   described in this section, MUST be implemented by clients and
   servers.

   First, the client determines the IP address and port that it will
   open a TCP connection to.  This is done using the discovery
   procedures in Section 9.1.  The client opens up the connection to
   that address and port, and immediately begins TLS negotiation [2].
   The client MUST verify the identity of the server.  To do that, it
   follows the identification procedures defined in Section 3.1 of RFC
   2818 [5].  Those procedures assume the client is dereferencing a URI.
   For purposes of usage with this specification, the client treats the
   domain name or IP address used in Section 9.1 as the host portion of
   the URI that has been dereferenced.

   Once the connection is opened, the client sends a Shared Secret
   request.  This request has no attributes, just the header.  The
   transaction ID in the header MUST meet the requirements outlined for
   the transaction ID in a binding request, described in Section 9.3
   below.  The server generates a response, which can either be a Shared
   Secret Response or a Shared Secret Error Response.

   If the response was a Shared Secret Error Response, the client checks
   the response code in the ERROR-CODE attribute.  Interpretation of
   those response codes is identical to the processing of Section 9.4
   for the Binding Error Response.

   If a client receives a Shared Secret Response with an attribute whose
   type is greater than 0x7fff, the attribute MUST be ignored.  If the
   client receives a Shared Secret Response with an attribute whose type
   is less than or equal to 0x7fff, the response is ignored.

   If the response was a Shared Secret Response, it will contain a short
   lived username and password, encoded in the USERNAME and PASSWORD



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   attributes, respectively.

   The client MAY generate multiple Shared Secret Requests on the
   connection, and it MAY do so before receiving Shared Secret Responses
   to previous Shared Secret Requests.  The client SHOULD close the
   connection as soon as it has finished obtaining usernames and
   passwords.

   Section 9.3 describes how these passwords are used to provide
   integrity protection over Binding Requests, and Section 8.1 describes
   how it is used in Binding Responses.

9.3  Formulating the Binding Request

   A Binding Request formulated by the client follows the syntax rules
   defined in Section 10.  Any two requests that are not bit-wise
   identical, and not sent to the same server from the same IP address
   and port, MUST carry different transaction IDs.  The transaction ID
   MUST be uniformly and randomly distributed between 0 and 2**128 - 1.
   The large range is needed because the transaction ID serves as a form
   of randomization, helping to prevent replays of previously signed
   responses from the server.  The message type of the request MUST be
   "Binding Request".

   The RESPONSE-ADDRESS attribute is optional in the Binding Request.
   It is used if the client wishes the response to be sent to a
   different IP address and port than the one the request was sent from.
   The CHANGE-REQUEST attribute is also optional.  It tells the server
   to send the response from a different address or port.  Both
   RESPONSE-ADDRESS and CHANGE-REQUEST are primarily useful in
   diagnostic operations for analyzing the behavior of a NAT.  Under
   normal usage, neither of these attributes will be present.

   The client SHOULD add a MESSAGE-INTEGRITY and USERNAME attribute to
   the Binding Request.  This MESSAGE-INTEGRITY attribute contains an
   HMAC [13].  The value of the username, and the key to use in the
   MESSAGE-INTEGRITY attribute depend on the shared secret mechanism.
   If the STUN Shared Secret Request was used, the USERNAME must be a
   valid username obtained from a Shared Secret Response within the last
   nine minutes.  The shared secret for the HMAC is the value of the
   PASSWORD attribute obtained from the same Shared Secret Response.

   Once formulated, the client sends the Binding Request.  Reliability
   is accomplished through client retransmissions.  Clients SHOULD
   retransmit the request starting with an interval of 100ms, doubling
   every retransmit until the interval reaches 1.6s.  Retransmissions
   continue with intervals of 1.6s until a response is received, or a
   total of 9 requests have been sent.  If no response is received by



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   1.6 seconds after the last request has been sent, the client SHOULD
   consider the transaction to have failed.  In other words, requests
   would be sent at times 0ms, 100ms, 300ms, 700ms, 1500ms, 3100ms,
   4700ms, 6300ms, and 7900ms.  At 9500ms, the client considers the
   transaction to have failed if no response has been received.

9.4  Processing Binding Responses

   The response can either be a Binding Response or Binding Error
   Response.  Binding Error Responses are always received on the source
   address and port the request was sent from.  A Binding Response will
   be received on the address and port placed in the RESPONSE-ADDRESS
   attribute of the request.  If none was present, the Binding Responses
   will be received on the source address and port the request was sent
   from.

   If the response is a Binding Error Response, the client checks the
   response code from the ERROR-CODE attribute of the response.  For a
   400 response code, the client SHOULD display the reason phrase to the
   user.  For a 420 response code, the client SHOULD retry the request,
   this time omitting any attributes listed in the UNKNOWN-ATTRIBUTES
   attribute of the response.  For a 430 response code, the client
   SHOULD obtain a new shared secret, and retry the Binding Request with
   a new transaction.  For 401 and 432 response codes, if the client had
   omitted the USERNAME or MESSAGE-INTEGRITY attribute as indicated by
   the error, it SHOULD try again with those attributes.  For a 431
   response code, the client SHOULD alert the user, and MAY try the
   request again after obtaining a new username and password.  For a 500
   response code, the client MAY wait several seconds and then retry the
   request.  For a 600 response code, the client MUST NOT retry the
   request, and SHOULD display the reason phrase to the user.  Unknown
   attributes between 400 and 499 are treated like a 400, unknown
   attributes between 500 and 599 are treated like a 500, and unknown
   attributes between 600 and 699 are treated like a 600.  Any response
   between 100 and 399 MUST result in the cessation of request
   retransmissions, but otherwise is discarded.

   If a client receives a response with an attribute whose type is
   greater than 0x7fff, the attribute MUST be ignored.  If the client
   receives a response with an attribute whose type is less than or
   equal to 0x7fff, request retransmissions MUST cease, but the entire
   response is otherwise ignored.

   If the response is a Binding Response, the client SHOULD check the
   response for a MESSAGE-INTEGRITY attribute.  If not present, and the
   client placed a MESSAGE-INTEGRITY attribute into the request, it MUST
   discard the response.  If present, the client computes the HMAC over
   the response as described in Section 10.2.8.  The key to use depends



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   on the shared secret mechanism.  If the STUN Shared Secret Request
   was used, the key MUST be same as used to compute the MESSAGE-
   INTEGRITY attribute in the request.  If the computed HMAC differs
   from the one in the response, the client SHOULD determine if the
   integrity check failed due to a NAT rewriting the MAPPED-ADDRESS.  To
   perform this check, the client compares the IP address and port in
   the MAPPED-ADDRESS with the IP address and port extracted from
   XOR-MAPPED-ADDRESS (extraction involves xor'ing the contents of
   X-port and X-value with the transaction ID, as described in Section
   10).  If the two IP addresses and ports differ, the client MUST
   discard the response, but then it SHOULD retry the Binding Request
   with the XOR-ONLY attribute included.  This tells the server not to
   include a MAPPED-ADDRESS in the Binding Response.

   If there is no XOR-MAPPED-ADDRESS, or if there is, but there are no
   differences between the two IP addresses and ports, the client MUST
   discard the response and SHOULD alert the user about a possible
   attack.

   If the computed HMAC matches the one from the response, processing
   continues.

   Reception of a response (either Binding Error Response or Binding
   Response) to a Binding Request will terminate retransmissions of that
   request.  However, clients MUST continue to listen for responses to a
   Binding Request for 10 seconds after the first response.  If it
   receives any responses in this interval with different message types
   (Binding Responses and Binding Error Responses, for example),
   different MAPPED-ADDRESSes, or different XOR-MAPPED-ADDRESSes, it is
   an indication of a possible attack.  The client MUST NOT use the
   MAPPED-ADDRESS or XOR-MAPPED-ADDRESS from any of the responses it
   received (either the first or the additional ones), and SHOULD alert
   the user.

   Furthermore, if a client receives more than twice as many Binding
   Responses as the number of Binding Requests it sent, it MUST NOT use
   the MAPPED-ADDRESS or XOR-MAPPED-ADDRESS from any of those responses,
   and SHOULD alert the user about a potential attack.

   If the Binding Response is authenticated, and the MAPPED-ADDRESS or
   XOR-MAPPED-ADDRESS was not discarded because of a potential attack,
   the CLIENT MAY use the information in the Binding Response.  In
   particular, the client SHOULD used the IP address and port from the
   XOR-MAPPED-ADDRESS instead of the information from the
   MAPPED-ADDRESS, assuming XOR-MAPPED-ADDRESS was present in the
   Binding Response.  Servers compliant to RFC 3489 [19] will not
   generate XOR-MAPPED-ADDRESS, so a client MUST be prepared to handle
   the case where only MAPPED-ADDRESS is present.  In such a case, the



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   information from MAPPED-ADDRESS is used.

   It is also possible for an IPv4 host to receive a XOR-MAPPED-ADDRESS
   or MAPPED-ADDRESS containing an IPv6 address, or for an IPv6 host to
   receive a XOR-MAPPED-ADDRESS or MAPPED-ADDRESS containing an IPv4
   address.  Clients MUST be prepared for this case.

   The next section provides additional details on how the mapped
   address information is used.

9.5  Using the Mapped Address

   The mapped address present in the XOR-MAPPED-ADDRESS attribute (or
   MAPPED-ADDRESS if not present) of the binding response can be used by
   clients to facilitate UDP traversal of NATs for many applications.

   NAT traversal is problematic for applications which require a client
   to insert an IP address and port into a message, to which subsequent
   messages will be delivered by other entities in a network.  Normally,
   the client would insert the IP address and port from a local
   interface into the message.  However, if the client is behind a NAT,
   this local interface will be a private address.  Clients within other
   address realms will not be able to send messages to that address.

   An example of a such an application is SIP, which requires a client
   to include IP address and port information in several places,
   including the Session Description Protocol (SDP) body [20] carried by
   SIP.  The IP address and port present in the SDP is used for receipt
   of media.

   To use STUN as a technique for traversal of SIP and other protocols,
   when the client wishes to send a protocol message, it figures out the
   places in the protocol data unit where it is supposed to insert its
   own IP address along with a port.  Instead of directly using a port
   allocated from a local interface, the client allocates a port from
   the local interface, and from that port, initiates the STUN
   procedures described above.  The XOR-MAPPED-ADDRESS (or
   MAPPED-ADDRESS if not present) in the STUN Binding Response provides
   the client with an alternative IP address and port which it can then
   include in the protocol PDU.  This IP address and port may be within
   a different address family than the local interfaces used by the
   client.  This is not an error condition.  In such a case, the client
   would use the learned IP address and port as if the client was a host
   with an interface within that address family.

   In the case of SIP, to populate the SDP appropriately, a client would
   generate two STUN Binding Request messages at the time a call is
   initiated or answered.  One is used to obtain the IP address and port



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   for RTP, and the other, for the Real Time Control Protocol (RTCP)
   [12].  The client might also need to use STUN to obtain IP addresses
   and ports for usage in other parts of the SIP message.  The detailed
   usage of STUN to facilitate SIP NAT traversal is outside the scope of
   this specification.

   As discussed above, the addresses learned by STUN may not be usable
   with all entities with whom a client might wish to communicate.  The
   way in which this problem is handled depends on the application
   protocol.  The ideal solution is for a protocol to allow a client to
   include a multiplicity of addresses and ports in the PDU.  One of
   those can be the address and port determined from STUN, and the
   others can include addresses and ports learned from other techniques.
   The application protocol would then provide a means for dynamically
   detecting which one works.  An example of such an an approach is
   Interactive Connectivity Establishment (ICE) [21].

10.  Protocol Details

   This section presents the detailed encoding of a STUN message.

   STUN is a request-response protocol.  Clients send a request, and the
   server sends a response.  There are two requests, Binding Request,
   and Shared Secret Request.  The response to a Binding Request can
   either be the Binding Response or Binding Error Response.  The
   response to a Shared Secret Request can either be a Shared Secret
   Response or a Shared Secret Error Response.

   STUN messages are encoded using binary fields.  All integer fields
   are carried in network byte order, that is, most significant byte
   (octet) first.  This byte order is commonly known as big-endian.  The
   transmission order is described in detail in Appendix B of RFC 791
   [6].  Unless otherwise noted, numeric constants are in decimal (base
   10).

10.1  Message Header

   All STUN messages consist of a 20 byte header:













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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      STUN Message Type        |         Message Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                            Transaction ID
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Message Types can take on the following values:


     0x0001  :  Binding Request
     0x0101  :  Binding Response
     0x0111  :  Binding Error Response
     0x0002  :  Shared Secret Request
     0x0102  :  Shared Secret Response
     0x0112  :  Shared Secret Error Response

   It is important to note that the most significant two bits of every
   STUN message are equal to 0b00.  This aids in differentiating STUN
   packets from RTP packets, in the case that both are sent to the same
   IP address and port, as is done with ICE.

   The message length is the count, in bytes, of the size of the
   message, not including the 20 byte header.

   The transaction ID is a 128 bit identifier.  It also serves as salt
   to randomize the request and the response.  All responses carry the
   same identifier as the request they correspond to.

10.2  Message Attributes

   After the header are 0 or more attributes.  Each attribute is TLV
   encoded, with a 16 bit type, 16 bit length, and variable value:


   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Type                  |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Value                             ....



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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The following types are defined:


   0x0001: MAPPED-ADDRESS
   0x0002: RESPONSE-ADDRESS
   0x0003: CHANGE-REQUEST
   0x0004: SOURCE-ADDRESS
   0x0005: CHANGED-ADDRESS
   0x0006: USERNAME
   0x0007: PASSWORD
   0x0008: MESSAGE-INTEGRITY
   0x0009: ERROR-CODE
   0x000a: UNKNOWN-ATTRIBUTES
   0x000b: REFLECTED-FROM
   0x0020: XOR-MAPPED-ADDRESS
   0x0021: XOR-ONLY
   0x0022: SERVER

   To allow future revisions of this specification to add new attributes
   if needed, the attribute space is divided into optional and mandatory
   ones.  Attributes with values greater than 0x7fff are optional, which
   means that the message can be processed by the client or server even
   though the attribute is not understood.  Attributes with values less
   than or equal to 0x7fff are mandatory to understand, which means that
   the client or server cannot process the message unless it understands
   the attribute.

   The MESSAGE-INTEGRITY attribute MUST be the last attribute within a
   message.  Any attributes that are known, but are not supposed to be
   present in a message (MAPPED-ADDRESS in a request, for example) MUST
   be ignored.

   Figure 9 indicates which attributes are present in which messages.
   An M indicates that inclusion of the attribute in the message is
   mandatory, O means its optional, C means it's conditional based on
   some other aspect of the message, and N/A means that the attribute is
   not applicable to that message type.












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                                        Binding  Shared  Shared  Shared
                      Binding  Binding  Error    Secret  Secret  Secret
   Att.                Req.     Resp.    Resp.    Req.    Resp.   Error
                                                                  Resp.
   _____________________________________________________________________
   MAPPED-ADDRESS      N/A      M        N/A      N/A     N/A     N/A
   RESPONSE-ADDRESS    O        N/A      N/A      N/A     N/A     N/A
   CHANGE-REQUEST      O        N/A      N/A      N/A     N/A     N/A
   SOURCE-ADDRESS      N/A      M        N/A      N/A     N/A     N/A
   CHANGED-ADDRESS     N/A      M        N/A      N/A     N/A     N/A
   USERNAME            O        N/A      N/A      N/A     M       N/A
   PASSWORD            N/A      N/A      N/A      N/A     M       N/A
   MESSAGE-INTEGRITY   O        O        N/A      N/A     N/A     N/A
   ERROR-CODE          N/A      N/A      M        N/A     N/A     M
   UNKNOWN-ATTRIBUTES  N/A      N/A      C        N/A     N/A     C
   REFLECTED-FROM      N/A      C        N/A      N/A     N/A     N/A
   XOR-MAPPED-ADDRESS  N/A      M        N/A      N/A     N/A     N/A
   XOR-ONLY            O        N/A      N/A      N/A     N/A     N/A
   SERVER              N/A      O        O        N/A     O       O

                                Figure 9

   The length refers to the length of the value element, expressed as an
   unsigned integral number of bytes.

10.2.1  MAPPED-ADDRESS

   The MAPPED-ADDRESS attribute indicates the mapped IP address and
   port.  It consists of an eight bit address family, and a sixteen bit
   port, followed by a fixed length value representing the IP address.
   If the address family is IPv4, the address is 32 bits.  If the
   address family is IPv6, the address is 128 bits.


   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |x x x x x x x x|    Family     |           Port                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Address  (variable)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The port is a network byte ordered representation of the mapped port.
   The address family can take on the following values:

   0x01: IPv4





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   0x02: IPv6

   The first 8 bits of the MAPPED-ADDRESS are ignored, for the purposes
   of aligning parameters on natural boundaries.

10.2.2  RESPONSE-ADDRESS

   The RESPONSE-ADDRESS attribute indicates where the response to a
   Binding Request should be sent.  Its syntax is identical to MAPPED-
   ADDRESS.

10.2.3  CHANGED-ADDRESS

   The CHANGED-ADDRESS attribute indicates the IP address and port where
   responses would have been sent from if the "change IP" and "change
   port" flags had been set in the CHANGE-REQUEST attribute of the
   Binding Request.  The attribute is always present in a Binding
   Response, independent of the value of the flags.  Its syntax is
   identical to MAPPED-ADDRESS.

10.2.4  CHANGE-REQUEST

   The CHANGE-REQUEST attribute is used by the client to request that
   the server use a different address and/or port when sending the
   response.  The attribute is 32 bits long, although only two bits (A
   and B) are used:


   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 A B 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The meaning of the flags is:

   A: This is the "change IP" flag.  If true, it requests the server to
      send the Binding Response with a different IP address than the one
      the Binding Request was received on.

   B: This is the "change port" flag.  If true, it requests the server
      to send the Binding Response with a different port than the one
      the Binding Request was received on.


10.2.5  SOURCE-ADDRESS

   The SOURCE-ADDRESS attribute is present in Binding Responses.  It



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   indicates the source IP address and port that the server is sending
   the response from.  Its syntax is identical to that of MAPPED-
   ADDRESS.

10.2.6  USERNAME

   The USERNAME attribute is used for message integrity.  It serves as a
   means to identify the shared secret used in the message integrity
   check.  The USERNAME is always present in a Shared Secret Response,
   along with the PASSWORD.  It is optionally present in a Binding
   Request when message integrity is used.

   The value of USERNAME is a variable length opaque value.  Its length
   MUST be a multiple of 4 (measured in bytes) in order to guarantee
   alignment of attributes on word boundaries.

10.2.7  PASSWORD

   The PASSWORD attribute is used in Shared Secret Responses.  It is
   always present in a Shared Secret Response, along with the USERNAME.

   The value of PASSWORD is a variable length value that is to be used
   as a shared secret.  Its length MUST be a multiple of 4 (measured in
   bytes) in order to guarantee alignment of attributes on word
   boundaries.

10.2.8  MESSAGE-INTEGRITY

   The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [13] of the
   STUN message.  It can be present in Binding Requests or Binding
   Responses.  Since it uses the SHA1 hash, the HMAC will be 20 bytes.
   The text used as input to HMAC is the STUN message, including the
   header, up to and including the attribute preceding the MESSAGE-
   INTEGRITY attribute.  That text is then padded with zeroes so as to
   be a multiple of 64 bytes.  As a result, the MESSAGE-INTEGRITY
   attribute MUST be the last attribute in any STUN message.  The key
   used as input to HMAC depends on the context.

10.2.9  ERROR-CODE

   The ERROR-CODE attribute is present in the Binding Error Response and
   Shared Secret Error Response.  It is a numeric value in the range of
   100 to 699 plus a textual reason phrase encoded in UTF-8, and is
   consistent in its code assignments and semantics with SIP [10] and
   HTTP [15].  The reason phrase is meant for user consumption, and can
   be anything appropriate for the response code.  The lengths of the
   reason phrases MUST be a multiple of 4 (measured in bytes).  This can
   be accomplished by added spaces to the end of the text, if necessary.



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   Recommended reason phrases for the defined response codes are
   presented below.

   To facilitate processing, the class of the error code (the hundreds
   digit) is encoded separately from the rest of the code.


   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   0                     |Class|     Number    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Reason Phrase (variable)                                ..
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The class represents the hundreds digit of the response code.  The
   value MUST be between 1 and 6.  The number represents the response
   code modulo 100, and its value MUST be between 0 and 99.

   The following response codes, along with their recommended reason
   phrases (in brackets) are defined at this time:

   400 (Bad Request): The request was malformed.  The client should not
      retry the request without modification from the previous attempt.

   401 (Unauthorized): The Binding Request did not contain a
      MESSAGE-INTEGRITY attribute.

   420 (Unknown Attribute): The server did not understand a mandatory
      attribute in the request.

   430 (Stale Credentials): The Binding Request did contain a
      MESSAGE-INTEGRITY attribute, but it used a shared secret that has
      expired.  The client should obtain a new shared secret and try
      again.

   431 (Integrity Check Failure): The Binding Request contained a
      MESSAGE-INTEGRITY attribute, but the HMAC failed verification.
      This could be a sign of a potential attack, or client
      implementation error.

   432 (Missing Username): The Binding Request contained a MESSAGE-
      INTEGRITY attribute, but not a USERNAME attribute.  Both must be
      present for integrity checks.

   433 (Use TLS): The Shared Secret request has to be sent over TLS, but
      was not received over TLS.




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   500 (Server Error): The server has suffered a temporary error.  The
      client should try again.

   600 (Global Failure): The server is refusing to fulfill the request.
      The client should not retry.


10.2.10  UNKNOWN-ATTRIBUTES

   The UNKNOWN-ATTRIBUTES attribute is present only in a Binding Error
   Response or Shared Secret Error Response when the response code in
   the ERROR-CODE attribute is 420.

   The attribute contains a list of 16 bit values, each of which
   represents an attribute type that was not understood by the server.
   If the number of unknown attributes is an odd number, one of the
   attributes MUST be repeated in the list, so that the total length of
   the list is a multiple of 4 bytes.


   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Attribute 1 Type           |     Attribute 2 Type        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Attribute 3 Type           |     Attribute 4 Type    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


10.2.11  REFLECTED-FROM

   The REFLECTED-FROM attribute is present only in Binding Responses,
   when the Binding Request contained a RESPONSE-ADDRESS attribute.  The
   attribute contains the identity (in terms of IP address) of the
   source where the request came from.  Its purpose is to provide
   traceability, so that a STUN server cannot be used as a reflector for
   denial-of-service attacks.

   Its syntax is identical to the MAPPED-ADDRESS attribute.

10.2.12  XOR-MAPPED-ADDRESS

   The XOR-MAPPED-ADDRESS attribute is only present in Binding
   Responses.  It provides the same information that is present in the
   MAPPED-ADDRESS attribute.  However, the information is encoded by
   performing an exclusive or (XOR) operation between the mapped address
   and the transaction ID.  Unfortunately, some NAT devices have been
   found to rewrite binary encoded IP addresses and ports that are



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   present in protocol payloads.  This behavior interferes with the
   operation of STUN.  By providing the mapped address in an obfuscated
   form, STUN can continue to operate through these devices.

   The format of the XOR-MAPPED-ADDRESS is:


   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |x x x x x x x x|    Family     |         X-Port                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                X-Address (Variable)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Family represents the IP address family, and is encoded
   identically to the Family in MAPPED-ADDRESS.

   X-Port is equal to the port in MAPPED-ADDRESS, exclusive or'ed with
   most significant 16 bits of the transaction ID.  If the IP address
   family is IPv4, X-Address is equal to the IP address in
   MAPPED-ADDRESS, exclusive or'ed with the most significant 32 bits of
   the transaction ID.  If the IP address family is IPv6, the X-Address
   is equal to the IP address in MAPPED-ADDRESS, exclusive or'ed with
   the entire 128 bit transaction ID.

10.2.13  XOR-ONLY

   This attribute is present in a Binding Request.  It is used by a
   client to request that a server compliant to this specification omit
   the MAPPED-ADDRESS from a Binding Response, and include only the
   XOR-MAPPED-ADDRESS.  This is necessary in cases where a Binding
   Response is failing integrity checks because a NAT is rewriting the
   contents of a MAPPED-ADDRESS in the Binding Response.

   This attribute has a length of zero, and therefore contains no other
   information past the common attribute header.

10.2.14  SERVER

   The server attribute contains a textual description of the software
   being used by the server, including manufacturer and version number.
   The attribute has no impact on operation of the protocol, and serves
   only as a tool for diagnostic and debugging purposes.

   The value of SERVER is variable length.  Its length MUST be a
   multiple of 4 (measured in bytes) in order to guarantee alignment of
   attributes on word boundaries.



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11.  Security Considerations

11.1  Attacks on STUN

   Generally speaking, attacks on STUN can be classified into denial of
   service attacks and eavesdropping attacks.  Denial of service attacks
   can be launched against a STUN server itself, or against other
   elements using the STUN protocol.

   STUN servers create state through the Shared Secret Request
   mechanism.  To prevent being swamped with traffic, a STUN server
   SHOULD limit the number of simultaneous TLS connections it will hold
   open by dropping an existing connection when a new connection request
   arrives (based on an Least Recently Used (LRU) policy, for example).
   Similarly, it SHOULD limit the number of shared secrets it will
   store, in the event that the server is storing the shared secrets.

   The attacks of greater interest are those in which the STUN server
   and client are used to launch DOS attacks against other entities,
   including the client itself.

   Many of the attacks require the attacker to generate a response to a
   legitimate STUN request, in order to provide the client with a faked
   XOR-MAPPED-ADDRESS or MAPPED-ADDRESS.  In the sections below, we
   refer to either the XOR-MAPPED-ADDRESS or MAPPED-ADDRESS as just the
   mapped address (note the lower case).  The attacks that can be
   launched using such a technique include:

11.1.1  Attack I: DDOS Against a Target

   In this case, the attacker provides a large number of clients with
   the same faked mapped address that points to the intended target.
   This will trick all the STUN clients into thinking that their
   addresses are equal to that of the target.  The clients then hand out
   that address in order to receive traffic on it (for example, in SIP
   or H.323 messages).  However, all of that traffic becomes focused at
   the intended target.  The attack can provide substantial
   amplification, especially when used with clients that are using STUN
   to enable multimedia applications.

11.1.2  Attack II: Silencing a Client

   In this attack, the attacker seeks to deny a client access to
   services enabled by STUN (for example, a client using STUN to enable
   SIP-based multimedia traffic).  To do that, the attacker provides
   that client with a faked mapped address.  The mapped address it
   provides is an IP address that routes to nowhere.  As a result, the
   client won't receive any of the packets it expects to receive when it



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   hands out the mapped address.

   This exploitation is not very interesting for the attacker.  It
   impacts a single client, which is frequently not the desired target.
   Moreover, any attacker that can mount the attack could also deny
   service to the client by other means, such as preventing the client
   from receiving any response from the STUN server, or even a DHCP
   server.

11.1.3  Attack III: Assuming the Identity of a Client

   This attack is similar to attack II.  However, the faked mapped
   address points to the attacker themself.  This allows the attacker to
   receive traffic which was destined for the client.

11.1.4  Attack IV: Eavesdropping

   In this attack, the attacker forces the client to use a mapped
   address that routes to itself.  It then forwards any packets it
   receives to the client.  This attack would allow the attacker to
   observe all packets sent to the client.  However, in order to launch
   the attack, the attacker must have already been able to observe
   packets from the client to the STUN server.  In most cases (such as
   when the attack is launched from an access network), this means that
   the attacker could already observe packets sent to the client.  This
   attack is, as a result, only useful for observing traffic by
   attackers on the path from the client to the STUN server, but not
   generally on the path of packets being routed towards the client.

11.2  Launching the Attacks

   It is important to note that attacks of this nature (injecting
   responses with fake mapped addresses) require that the attacker be
   capable of eavesdropping requests sent from the client to the server
   (or to act as a MITM for such attacks).  This is because STUN
   requests contain a transaction identifier, selected by the client,
   which is random with 128 bits of entropy.  The server echoes this
   value in the response, and the client ignores any responses that
   don't have a matching transaction ID.  Therefore, in order for an
   attacker to provide a faked response that is accepted by the client,
   the attacker needs to know what the transaction ID in the request
   was.  The large amount of randomness, combined with the need to know
   when the client sends a request, precludes attacks that involve
   guessing the transaction ID.

   Since all of the above attacks rely on this one primitive - injecting
   a response with a faked mapped address - preventing the attacks is
   accomplished by preventing this one operation.  To prevent it, we



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   need to consider the various ways in which it can be accomplished.
   There are several:

11.2.1  Approach I: Compromise a Legitimate STUN Server

   In this attack, the attacker compromises a legitimate STUN server
   through a virus or Trojan horse.  Presumably, this would allow the
   attacker to take over the STUN server, and control the types of
   responses it generates.

   Compromise of a STUN server can also lead to discovery of open ports.
   Knowledge of an open port creates an opportunity for DoS attacks on
   those ports (or DDoS attacks if the traversed NAT is a full cone
   NAT).  Discovering open ports is already fairly trivial using port
   probing, so this does not represent a major threat.

11.2.2  Approach II: DNS Attacks

   STUN servers are discovered using DNS SRV records.  If an attacker
   can compromise the DNS, it can inject fake records which map a domain
   name to the IP address of a STUN server run by the attacker.  This
   will allow it to inject fake responses to launch any of the attacks
   above.

11.2.3  Approach III: Rogue Router or NAT

   Rather than compromise the STUN server, an attacker can cause a STUN
   server to generate responses with the wrong mapped address by
   compromising a router or NAT on the path from the client to the STUN
   server.  When the STUN request passes through the rogue router or
   NAT, it rewrites the source address of the packet to be that of the
   desired mapped address.  This address cannot be arbitrary.  If the
   attacker is on the public Internet (that is, there are no NATs
   between it and the STUN server), and the attacker doesn't modify the
   STUN request, the address has to have the property that packets sent
   from the STUN server to that address would route through the
   compromised router.  This is because the STUN server will send the
   responses back to the source address of the request.  With a modified
   source address, the only way they can reach the client is if the
   compromised router directs them there.  If the attacker is on the
   public Internet, but they can modify the STUN request, they can
   insert a RESPONSE-ADDRESS attribute into the request, containing the
   actual source address of the STUN request.  This will cause the
   server to send the response to the client, independent of the source
   address the STUN server sees.  This gives the attacker the ability to
   forge an arbitrary source address when it forwards the STUN request.

   If the attacker is on a private network (that is, there are NATs



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   between it and the STUN server), the attacker will not be able to
   force the server to generate arbitrary mapped addresses in responses.
   They will only be able force the STUN server to generate mapped
   addresses which route to the private network.  This is because the
   NAT between the attacker and the STUN server will rewrite the source
   address of the STUN request, mapping it to a public address that
   routes to the private network.  Because of this, the attacker can
   only force the server to generate faked mapped addresses that route
   to the private network.  Unfortunately, it is possible that a low
   quality NAT would be willing to map an allocated public address to
   another public address (as opposed to an internal private address),
   in which case the attacker could forge the source address in a STUN
   request to be an arbitrary public address.  This kind of behavior
   from NATs does appear to be rare.

11.2.4  Approach IV: MITM

   As an alternative to approach III, if the attacker can place an
   element on the path from the client to the server, the element can
   act as a man-in-the-middle.  In that case, it can intercept a STUN
   request, and generate a STUN response directly with any desired value
   of the mapped address field.  Alternatively, it can forward the STUN
   request to the server (after potential modification), receive the
   response, and forward it to the client.  When forwarding the request
   and response, this attack is subject to the same limitations on the
   mapped address described in Section 11.2.3.

11.2.5  Approach V: Response Injection Plus DoS

   In this approach, the attacker does not need to be a MITM (as in
   approaches III and IV).  Rather, it only needs to be able to
   eavesdrop onto a network segment that carries STUN requests.  This is
   easily done in multiple access networks such as ethernet or
   unprotected 802.11.  To inject the fake response, the attacker
   listens on the network for a STUN request.  When it sees one, it
   simultaneously launches a DoS attack on the STUN server, and
   generates its own STUN response with the desired mapped address
   value.  The STUN response generated by the attacker will reach the
   client, and the DoS attack against the server is aimed at preventing
   the legitimate response from the server from reaching the client.
   Arguably, the attacker can do without the DoS attack on the server,
   so long as the faked response beats the real response back to the
   client, and the client uses the first response, and ignores the
   second (even though it's different).

11.2.6  Approach VI: Duplication

   This approach is similar to approach V.  The attacker listens on the



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   network for a STUN request.  When it sees it, it generates its own
   STUN request towards the server.  This STUN request is identical to
   the one it saw, but with a spoofed source IP address.  The spoofed
   address is equal to the one that the attacker desires to have placed
   in the mapped address of the STUN response.  In fact, the attacker
   generates a flood of such packets.  The STUN server will receive the
   one original request, plus a flood of duplicate fake ones.  It
   generates responses to all of them.  If the flood is sufficiently
   large for the responses to congest routers or some other equipment,
   there is a reasonable probability that the one real response is lost
   (along with many of the faked ones), but the net result is that only
   the faked responses are received by the STUN client.  These responses
   are all identical and all contain the mapped address that the
   attacker wanted the client to use.

   The flood of duplicate packets is not needed (that is, only one faked
   request is sent), so long as the faked response beats the real
   response back to the client, and the client uses the first response,
   and ignores the second (even though it's different).

   Note that, in this approach, launching a DoS attack against the STUN
   server or the IP network, to prevent the valid response from being
   sent or received, is problematic.  The attacker needs the STUN server
   to be available to handle its own request.  Due to the periodic
   retransmissions of the request from the client, this leaves a very
   tiny window of opportunity.  The attacker must start the DoS attack
   immediately after the actual request from the client, causing the
   correct response to be discarded, and then cease the DoS attack in
   order to send its own request, all before the next retransmission
   from the client.  Due to the close spacing of the retransmits (100ms
   to a few seconds), this is very difficult to do.

   Besides DoS attacks, there may be other ways to prevent the actual
   request from the client from reaching the server.  Layer 2
   manipulations, for example, might be able to accomplish it.

   Fortunately, Approach IV is subject to the same limitations
   documented in Section 11.2.3, which limit the range of mapped
   addresses the attacker can cause the STUN server to generate.

11.3  Countermeasures

   STUN provides mechanisms to counter the approaches described above,
   and additional, non-STUN techniques can be used as well.

   First off, it is RECOMMENDED that networks with STUN clients
   implement ingress source filtering (RFC 2827 [7]).  This is
   particularly important for the NATs themselves.  As Section 11.2.3



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   explains, NATs which do not perform this check can be used as
   "reflectors" in DDoS attacks.  Most NATs do perform this check as a
   default mode of operation.  We strongly advise people that purchase
   NATs to ensure that this capability is present and enabled.

   Secondly, it is RECOMMENDED that STUN servers be run on hosts
   dedicated to STUN, with all UDP and TCP ports disabled except for the
   STUN ports.  This is to prevent viruses and Trojan horses from
   infecting STUN servers, in order to prevent their compromise.  This
   helps mitigate Approach I (Section 11.2.1).

   Thirdly, to prevent the DNS attack of Section 11.2.2, Section 9.2
   recommends that the client verify the credentials provided by the
   server with the name used in the DNS lookup.

   Finally, all of the attacks above rely on the client taking the
   mapped address it learned from STUN, and using it in application
   layer protocols.  If encryption and message integrity are provided
   within those protocols, the eavesdropping and identity assumption
   attacks can be prevented.  As such, applications that make use of
   STUN addresses in application protocols SHOULD use integrity and
   encryption, even if a SHOULD level strength is not specified for that
   protocol.  For example, multimedia applications using STUN addresses
   to receive RTP traffic would use secure RTP [16].

   The above three techniques are non-STUN mechanisms.  STUN itself
   provides several countermeasures.

   Approaches IV (Section 11.2.4), when generating the response locally,
   and V (Section 11.2.5) require an attacker to generate a faked
   response.  This attack is prevented using the message integrity
   mechanism provided in STUN, described in Section 8.1.

   Approaches III (Section 11.2.3) IV (Section 11.2.4), when using the
   relaying technique, and VI (Section 11.2.6), however, are not
   preventable through server signatures.  Both approaches are most
   potent when the attacker can modify the request, inserting a
   RESPONSE-ADDRESS that routes to the client.  Fortunately, such
   modifications are preventable using the message integrity techniques
   described in Section 9.3.  However, these three approaches are still
   functional when the attacker modifies nothing but the source address
   of the STUN request.  Sadly, this is the one thing that cannot be
   protected through cryptographic means, as this is the change that
   STUN itself is seeking to detect and report.  It is therefore an
   inherent weakness in NAT, and not fixable in STUN.  To help mitigate
   these attacks, Section 9.4 provides several heuristics for the client
   to follow.  The client looks for inconsistent or extra responses,
   both of which are signs of the attacks described above.  However,



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   these heuristics are just that - heuristics, and cannot be guaranteed
   to prevent attacks.  The heuristics appear to prevent the attacks as
   we know how to launch them today.  Implementors should stay posted
   for information on new heuristics that might be required in the
   future.  Such information will be distributed on the IETF MIDCOM
   mailing list, midcom@ietf.org.

11.4  Residual Threats

   None of the countermeasures listed above can prevent the attacks
   described in Section 11.2.3 if the attacker is in the appropriate
   network paths.  Specifically, consider the case in which the attacker
   wishes to convince client C that it has address V.  The attacker
   needs to have a network element on the path between A and the server
   (in order to modify the request) and on the path between the server
   and V so that it can forward the response to C.  Furthermore, if
   there is a NAT between the attacker and the server, V must also be
   behind the same NAT.  In such a situation, the attacker can either
   gain access to all the application-layer traffic or mount the DDOS
   attack described in Section 11.1.1.  Note that any host which exists
   in the correct topological relationship can be DDOSed.  It need not
   be using STUN.

12.  IANA Considerations

   STUN cannot be extended.  Changes to the protocol are made through a
   standards track revision of this specification.  As a result, no IANA
   registries are needed.  Any future extensions will establish any
   needed registries.

13.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing",
   which is the general process by which a client attempts to determine
   its address in another realm on the other side of a NAT through a
   collaborative protocol reflection mechanism (RFC 3424 [17]).  STUN is
   an example of a protocol that performs this type of function.  The
   IAB has mandated that any protocols developed for this purpose
   document a specific set of considerations.  This section meets those
   requirements.

13.1  Problem Definition

   From RFC 3424 [17], any UNSAF proposal must provide:

      Precise definition of a specific, limited-scope problem that is to
      be solved with the UNSAF proposal.  A short term fix should not be
      generalized to solve other problems; this is why "short term fixes



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      usually aren't".

   The specific problem being solved by STUN is to provide a means for a
   client to obtain an address on the public Internet from a
   non-symmetric NAT, for the express purpose of receiving incoming UDP
   traffic from another host, targeted to that address.

   STUN does not address TCP, either incoming or outgoing, and does not
   address outgoing UDP communications.

13.2  Exit Strategy

   From [17], any UNSAF proposal must provide:

      Description of an exit strategy/transition plan.  The better short
      term fixes are the ones that will naturally see less and less use
      as the appropriate technology is deployed.

   STUN by itself does not provide an exit strategy.  This is provided
   by techniques, such as Interactive Connectivity Establishment (ICE)
   [21], which allow a client to determine whether addresses learned
   from STUN are needed, or whether other addresses, such as the one on
   the local interface,  will work when communicating with another host.
   With such a detection technique, as a client finds that the addresses
   provided by STUN are never used, STUN queries can cease to be made,
   thus allowing them to phase out.

   STUN can also help facilitate the introduction of midcom.  As
   midcom-capable NATs are deployed, applications will, instead of using
   STUN (which also resides at the application layer), first allocate an
   address binding using midcom.  However, it is a well-known limitation
   of midcom that it only works when the agent knows the middleboxes
   through which its traffic will flow.  Once bindings have been
   allocated from those middleboxes, a STUN detection procedure can
   validate that there are no additional middleboxes on the path from
   the public Internet to the client.  If this is the case, the
   application can continue operation using the address bindings
   allocated from midcom.  If it is not the case, STUN provides a
   mechanism for self-address fixing through the remaining midcom-
   unaware middleboxes.  Thus, STUN provides a way to help transition to
   full midcom-aware networks.

13.3  Brittleness Introduced by STUN

   From [17], any UNSAF proposal must provide:

      Discussion of specific issues that may render systems more
      "brittle".  For example, approaches that involve using data at



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      multiple network layers create more dependencies, increase
      debugging challenges, and make it harder to transition.

   STUN introduces brittleness into the system in several ways:

   o  The binding acquisition usage of STUN does not work for all NAT
      types.  It will work for any application for full cone NATs only.
      For restricted cone and port restricted cone NAT, it will work for
      some applications depending on the application.  Application
      specific processing will generally be needed.  For symmetric NATs,
      the binding acquisition will not yield a usable address.  The
      tight dependency on the specific type of NAT makes the protocol
      brittle.

   o  STUN assumes that the server exists on the public Internet.  If
      the server is located in another private address realm, the user
      may or may not be able to use its discovered address to
      communicate with other users.  There is no way to detect such a
      condition.

   o  The bindings allocated from the NAT need to be continuously
      refreshed.  Since the timeouts for these bindings is very
      implementation specific, the refresh interval cannot easily be
      determined.  When the binding is not being actively used to
      receive traffic, but to wait for an incoming message, the binding
      refresh will needlessly consume network bandwidth.

   o  The use of the STUN server as an additional network element
      introduces another point of potential security attack.  These
      attacks are largely prevented by the security measures provided by
      STUN, but not entirely.

   o  The use of the STUN server as an additional network element
      introduces another point of failure.  If the client cannot locate
      a STUN server, or if the server should be unavailable due to
      failure, the application cannot function.

   o  The use of STUN to discover address bindings will result in an
      increase in latency for applications.  For example, a Voice over
      IP application will see an increase of call setup delays equal to
      at least one RTT to the STUN server.

   o  STUN imposes some restrictions on the network topologies for
      proper operation.  If client A obtains an address from STUN server
      X, and sends it to client B, B may not be able to send to A using
      that IP address.  The address will not work if any of the
      following is true:




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      *  The STUN server is not in an address realm that is a common
         ancestor (topologically) of both clients A and B.  For example,
         consider client A and B, both of which have residential NAT
         devices.  Both devices connect them to their cable operators,
         but both clients have different providers.  Each provider has a
         NAT in front of their entire network, connecting it to the
         public Internet.  If the STUN server used by A is in A's cable
         operator's network, an address obtained by it will not be
         usable by B.  The STUN server must be in the network which is a
         common ancestor to both - in this case, the public Internet.

      *  The STUN server is in an address realm that is a common
         ancestor to both clients, but both clients are behind the same
         NAT connecting to that address realm.  For example, if the two
         clients in the previous example had the same cable operator,
         that cable operator had a single NAT connecting their network
         to the public Internet, and the STUN server was on the public
         Internet, the address obtained by A would not be usable by B.
         That is because some NATs will not accept an internal packet
         sent to a public IP address which is mapped back to an internal
         address.  To deal with this, additional protocol mechanisms or
         configuration parameters need to be introduced which detect
         this case.

   o  Most significantly, STUN introduces potential security threats
      which cannot be eliminated.  This specification describes
      heuristics that can be used to mitigate the problem, but it is
      provably unsolvable given what STUN is trying to accomplish.
      These security problems are described fully in Section 11.


13.4  Requirements for a Long Term Solution

   From [17], any UNSAF proposal must provide:

      Identify requirements for longer term, sound technical solutions
      -- contribute to the process of finding the right longer term
      solution.

   Our experience with STUN has led to the following requirements for a
   long term solution to the NAT problem:

   Requests for bindings and control of other resources in a NAT need to
   be explicit.  Much of the brittleness in STUN derives from its
      guessing at the parameters of the NAT, rather than telling the NAT
      what parameters to use.





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   Control needs to be in-band.  There are far too many scenarios in
      which the client will not know about the location of middleboxes
      ahead of time.  Instead, control of such boxes needs to occur
      in-band, traveling along the same path as the data will itself
      travel.  This guarantees that the right set of middleboxes are
      controlled.  This is only true for first-party controls;
      third-party controls are best handled using the midcom framework.

   Control needs to be limited.  Users will need to communicate through
      NATs which are outside of their administrative control.  In order
      for providers to be willing to deploy NATs which can be controlled
      by users in different domains, the scope of such controls needs to
      be extremely limited - typically, allocating a binding to reach
      the address where the control packets are coming from.

   Simplicity is Paramount.  The control protocol will need to be
      implement in very simple clients.  The servers will need to
      support extremely high loads.  The protocol will need to be
      extremely robust, being the precursor to a host of application
      protocols.  As such, simplicity is key.


13.5  Issues with Existing NAPT Boxes

   From [17], any UNSAF proposal must provide:

      Discussion of the impact of the noted practical issues with
      existing, deployed NA[P]Ts and experience reports.

   Several of the practical issues with STUN involve future proofing -
   breaking the protocol when new NAT types get deployed.  Fortunately,
   this is not an issue at the current time, since most of the deployed
   NATs are of the types assumed by STUN.  The primary usage STUN has
   found is in the area of VoIP, to facilitate allocation of addresses
   for receiving RTP [12] traffic.  In that application, the periodic
   keepalives are provided by the RTP traffic itself.  However, several
   practical problems arise for RTP.  First, RTP assumes that RTCP
   traffic is on a port one higher than the RTP traffic.  This pairing
   property cannot be guaranteed through NATs that are not directly
   controllable.  As a result, RTCP traffic may not be properly
   received.  Protocol extensions to SDP have been proposed which
   mitigate this by allowing the client to signal a different port for
   RTCP [18].  However, there will be interoperability problems for some
   time.

   For VoIP, silence suppression can cause a gap in the transmission of
   RTP packets.  This could result in the loss of a binding in the
   middle of a call, if that silence period exceeds the binding timeout.



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   This can be mitigated by sending occasional silence packets to keep
   the binding alive.  However, the result is additional brittleness;
   proper operation depends on the silence suppression algorithm in use,
   the usage of a comfort noise codec, the duration of the silence
   period, and the binding lifetime in the NAT.

13.6  In Closing

   The problems with STUN are not design flaws in STUN.  The problems in
   STUN have to do with the lack of standardized behaviors and controls
   in NATs.  The result of this lack of standardization has been a
   proliferation of devices whose behavior is highly unpredictable,
   extremely variable, and uncontrollable.  STUN does the best it can in
   such a hostile environment.  Ultimately, the solution is to make the
   environment less hostile, and to introduce controls and standardized
   behaviors into NAT.  However, until such time as that happens, STUN
   provides a good short term solution given the terrible conditions
   under which it is forced to operate.

14.  Changes Since RFC 3489

   This specification updates RFC 3489 [19].  This specification differs
   from RFC 3489 in the following ways:

   o  Removed the usage of STUN for NAT type detection and binding
      lifetime discovery.  These techniques have proven overly brittle
      due to wider variations in the types of NAT devices than described
      in this document.  The protocol semantics used for NAT type
      detection remain, however, to provide backwards compatibility, and
      to allow for the NAT type detection to occur in purely diagnostic
      applications.

   o  Removed the STUN example that centered around the separation of
      the control and media planes.  Instead, provided more information
      on using STUN with protocols.

   o  Added the XOR-MAPPED-ADDRESS attribute, which clients prefer to
      the MAPPED-ADDRESS when both are present in a Binding Response.
      XOR-MAPPED-ADDRESS is obfuscated so that NATs which try to "help"
      by rewriting binary IP addresses they find in protocols will not
      interfere with the operation of STUN.

   o  Added the XOR-ONLY attribute, which clients can use to request
      that the server send a response with only the XOR-MAPPED-ADDRESS.
      This is necessary in case a Binding Response fails integrity
      checks due to a NAT that rewrites the MAPPED-ADDRESS.

   o  Explicitly point out that the most significant two bits of STUN



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      are 0b00, allowing easy differentiation with RTP packets when used
      with ICE.

   o  Added support for IPv6.  Made it clear that an IPv4 client could
      get a v6 mapped address, and vice-a-versa.

   o  Added the SERVER attribute.


15.  Acknowledgments

   The authors would like to thank Cedric Aoun, Pete Cordell, Cullen
   Jennings, Bob Penfield and Chris Sullivan for their comments, and
   Baruch Sterman and Alan Hawrylyshen for initial implementations.
   Thanks for Leslie Daigle, Allison Mankin, Eric Rescorla, and Henning
   Schulzrinne for IESG and IAB input on this work.

16.  References

16.1  Normative References

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

   [2]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
        2246, January 1999.

   [3]  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
        specifying the location of services (DNS SRV)", RFC 2782,
        February 2000.

   [4]  Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for
        Transport Layer Security (TLS)", RFC 3268, June 2002.

   [5]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [6]  Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.

   [7]  Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating
        Denial of Service Attacks which employ IP Source Address
        Spoofing", BCP 38, RFC 2827, May 2000.

16.2  Informative References

   [8]   Senie, D., "Network Address Translator (NAT)-Friendly
         Application Design Guidelines", RFC 3235, January 2002.

   [9]   Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A. and A.



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         Rayhan, "Middlebox communication architecture and framework",
         RFC 3303, August 2002.

   [10]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [11]  Holdrege, M. and P. Srisuresh, "Protocol Complications with the
         IP Network Address Translator", RFC 3027, January 2001.

   [12]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", RFC
         3550, July 2003.

   [13]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997.

   [14]  Kohl, J. and B. Neuman, "The Kerberos Network Authentication
         Service (V5)", RFC 1510, September 1993.

   [15]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
         Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
         HTTP/1.1", RFC 2616, June 1999.

   [16]  Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K.
         Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
         3711, March 2004.

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

   [18]  Huitema, C., "Real Time Control Protocol (RTCP) attribute in
         Session Description Protocol (SDP)", RFC 3605, October 2003.

   [19]  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.

   [20]  Handley, M. and V. Jacobson, "SDP: Session Description
         Protocol", RFC 2327, April 1998.

   [21]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network  Address Translator (NAT) Traversal for
         Multimedia Session Establishment Protocols",
         draft-ietf-mmusic-ice-03 (work in progress), October 2004.





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Authors' Addresses

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   EMail: jdrosen@cisco.com
   URI:   http://www.jdrosen.net


   Christian Huitema
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   US

   EMail: huitema@microsoft.com


   Rohan Mahy
   Airspace

   EMail: rohan@ekabal.com

























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