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Versions: (draft-rosenberg-midcom-stun) 00 01 02 03 04 RFC 3489

Internet Engineering Task Force                                MIDCOM WG
Internet Draft                                              J. Rosenberg
                                                             dynamicsoft
                                                           J. Weinberger
                                                             dynamicsoft
                                                              C. Huitema
                                                               Microsoft
                                                                 R. Mahy
                                                                   Cisco
draft-ietf-midcom-stun-04.txt
December 9, 2002
Expires: June 2003


         STUN - Simple Traversal of UDP Through Network Address
                              Translators

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   To view the list Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.


Abstract

   Simple Traversal of UDP Through NATs (STUN) is a lightweight protocol
   that allows applications to discover the presence and types of
   Network Address Translators (NATs) and firewalls between them and the
   public Internet. It also provides the ability for applications to
   determine the public IP addresses allocated to them by the NAT. STUN
   works with many existing NATs, and does not require any special
   behavior from them. As a result, it allows a wide variety of



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   applications to work through existing NAT infrastructure.


















































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                           Table of Contents



   1          Applicability Statement .............................    5
   2          Introduction ........................................    5
   3          Terminology .........................................    6
   4          Definitions .........................................    6
   5          NAT Variations ......................................    7
   6          Overview of Operation ...............................    7
   7          Message Overview ....................................   10
   8          Server Behavior .....................................   12
   8.1        Binding Requests ....................................   12
   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
   10         Use Cases ...........................................   20
   10.1       Discovery Process ...................................   21
   10.2       Binding Lifetime Discovery ..........................   22
   10.3       Binding Acquisition .................................   24
   11         Protocol Details ....................................   25
   11.1       Message Header ......................................   26
   11.2       Message Attributes ..................................   26
   11.2.1     MAPPED-ADDRESS ......................................   27
   11.2.2     RESPONSE-ADDRESS ....................................   28
   11.2.3     CHANGED-ADDRESS .....................................   29
   11.2.4     CHANGE-REQUEST ......................................   29
   11.2.5     SOURCE-ADDRESS ......................................   29
   11.2.6     USERNAME ............................................   29
   11.2.7     PASSWORD ............................................   30
   11.2.8     MESSAGE-INTEGRITY ...................................   30
   11.2.9     ERROR-CODE ..........................................   30
   11.2.10    UNKNOWN-ATTRIBUTES ..................................   31
   11.2.11    REFLECTED-FROM ......................................   32
   12         Security Considerations .............................   32
   12.1       Attacks on STUN .....................................   32
   12.1.1     Attack I: DDOS Against a Target .....................   32
   12.1.2     Attack II: Silencing a Client .......................   33
   12.1.3     Attack III: Assuming the Identity of a Client .......   33
   12.1.4     Attack IV: Eavesdropping ............................   33
   12.2       Launching the Attacks ...............................   33
   12.2.1     Approach I: Compromise a Legitimate STUN Server .....   34



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   12.2.2     Approach II: DNS Attacks ............................   34
   12.2.3     Approach III: Rogue Router or NAT ...................   34
   12.2.4     Approach IV: MITM ...................................   35
   12.2.5     Approach V: Response Injection Plus DoS .............   35
   12.2.6     Approach VI: Duplication ............................   36
   12.3       Countermeasures .....................................   37
   12.4       Residual Threats ....................................   38
   13         IANA Considerations .................................   38
   14         IAB Considerations ..................................   38
   14.1       Problem Definition ..................................   39
   14.2       Exit Strategy .......................................   39
   14.3       Brittleness Introduced by STUN ......................   40
   14.4       Requirements for a Long Term Solution ...............   42
   14.5       Issues with Existing NAPT Boxes .....................   43
   14.6       In Closing ..........................................   44
   15         Acknowledgments .....................................   44
   16         Authors Addresses ...................................   44
   17         Normative References ................................   45
   18         Informative References ..............................   45
































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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's discovery procedures are based on
   assumptions on NAT treatment of UDP; such assumptions may prove
   invalid down the road as new NAT devices are deployed. 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 14.

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 [9] 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 [10]. MIDCOM allows an application entity, such as an
   end client or network server of some sort (like a Session Initiation
   Protocol (SIP) proxy [11]) 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



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   applications. As a result, the incentive to upgrade these devices
   will be low in many cases. Consider, for example, an airport Internet
   lounge that provides access with a NAT. A user connecting to the
   natted 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 [12]) and Voice over IP,
   have developed tricks that allow them to operate through NATs without
   changing those NATs. This draft 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 first discover the presence
   of a NAT and the type of NAT, and then to learn the addresses
   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 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)



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             is an entity that receives STUN requests, and sends STUN
             responses. STUN servers are generally attached to the
             public Internet.

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.

   Determining the type of NAT is important in many cases. Depending on
   what the application wants to do, it may need to take the particular
   behavior into account.

6 Overview of Operation

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




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




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



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




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



   Figure 1: STUN Configuration



   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



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   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. There are attributes for
   providing message integrity and authentication.

   The trick is using STUN to discover the presence of NAT, and to learn
   and use the bindings they allocate.

   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) [13] 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 presence of a NAT,
   and 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. In the case of a full-
   cone NAT, the IP address and port in the body of the STUN response



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   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, and send the IP address and port learned
   in the STUN response to hosts that wish to communicate with it.

   Of course, the host may not be behind a full-cone NAT. Indeed, it
   doesn't yet know what type of NAT it is behind. To determine that,
   the client uses additional STUN Binding Requests. The exact procedure
   is flexible, but would generally work as follows. The client would
   send a second STUN Binding Request, this time to a different IP
   address, but from the same source IP address and port. If the IP
   address and port in the response are different from those in the
   first response, the client knows it is behind a symmetric NAT. To
   determine if its behind a full-cone NAT, the client can send a STUN
   Binding Request with flags that tell the STUN server to send a
   response from a different IP address and port than the request was
   received on. In other words, if the client sent a Binding Request to
   IP address/port A/B using a source IP address/port of X/Y, the STUN
   server would send the Binding Response to X/Y using source IP
   address/port C/D. If the client receives this response, it knows it
   is behind a full cone NAT.

   STUN also allows the client to ask the server to send the Binding
   Response from the same IP address the request was received on, but
   with a different port. This can be used to detect whether the client
   is behind a port restricted cone NAT or just a restricted cone NAT.

   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 publically routable address, that the server reside on
   the public Internet.

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



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   TLS over TCP).

   Several STUN attributes are defined for usage in Binding Requests and
   Binding Responses. 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. Its 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.
   The "change IP" and "change port" flags are useful for determining
   whether the client is behind a restricted cone NAT or restricted port
   cone NAT. 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. It is useful for detecting
   twice NAT configurations.

   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 Binding
   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. 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



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   which were unknown. The eleventh attribute is the REFLECTED-FROM
   attribute, which is present in Binding Responses. It indicates the IP
   address of the sender of a Binding Request, used for traceability
   purposes to prevent certain denial-of-service attacks.

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

   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 11.2.8. 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.



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   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
   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".

   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 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 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.



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                Flags          Source Address  Source Port
                none           Da              Dp
                Change IP      Ca              Dp
                Change port    Da              Cp
                Change IP and
                Change port    Ca              Cp


   Table 1: Impact of Flags on Packet Source


   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. These are Ca and Cp, respectively.

   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
   [14]. 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
   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



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   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 [5]
   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 not, it discards the
   request.

   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.

   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
   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



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   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 [14] 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 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, and process the Binding
   Responses.

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



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   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 the request does not
   solicit a response after 30 seconds. If a failure occurs, the client
   SHOULD create a new request, which is identical to the previous, but
   has a different transaction ID. 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.


        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 12, 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 [15]). 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 [6].
   Those procedures assume the client is derefencing a URI. For purposes
   of usage with this specification, the client treats the domain name



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   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 Shared Secret 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, the it will contain a
   short lived username and password, encoded in the USERNAME and
   PASSWORD 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 11. Any two requests that are not bit-wise
   identical, or 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 chosen 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



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   IP address and port than the one the request was sent from. This is
   useful for determining whether the client is behind a firewall, and
   for applications that have separated control and data components. See
   Section 10.3 for more details. The CHANGE-REQUEST attribute is also
   optional. Whether it is present depends on what the application is
   trying to accomplish. See Section 10 for some example uses.

   The client SHOULD add a MESSAGE-INTEGRITY and USERNAME attribute to
   the Binding Request. This MESSAGE-INTEGRITY attribute contains an
   HMAC [14]. 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, at which time the client SHOULD
   give up.

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



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   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 11.2.8. The key to use depends
   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 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) or
   different MAPPED-ADDRESSes, it is an indication of a possible attack.
   The client MUST NOT use the MAPPED-ADDRESS from any of those
   responses, 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 from any of those responses, and SHOULD alert the
   user about a potential attack.

   If the Binding Response is authenticated, and the MAPPED-ADDRESS was
   not discarded because of a potential attack, the CLIENT MAY use the
   MAPPED-ADDRESS and SOURCE-ADDRESS attributes.

10 Use Cases

   The rules of Sections 8 and 9 describe exactly how a client and
   server interact to send requests and get responses. However, they do



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   not dictate how the STUN protocol is used to accomplish useful tasks.
   That is at the discretion of the client. Here, we provide some useful
   scenarios for applying STUN.

10.1 Discovery Process

   In this scenario, a user is running a multimedia application which
   needs to determine which of the following scenarios applies to it:

        o On the open Internet

        o Firewall that blocks UDP

        o Firewall that allows UDP out, and responses have to come back
          to the source of the request (like a symmetric NAT, but no
          translation. We call this a symmetric UDP Firewall)

        o Full-cone NAT

        o Symmetric NAT

        o Restricted cone or restricted port cone NAT

   Which of the six scenarios applies can be determined through the flow
   chart described in Figure 2. The chart refers only to the sequence of
   Binding Requests; Shared Secret Requests will, of course, be needed
   to authenticate each Binding Request used in the sequence.


   The flow makes use of three tests. In test I, the client sends a STUN
   Binding Request to a server, without any flags set in the CHANGE-
   REQUEST attribute, and without the RESPONSE-ADDRESS attribute. This
   causes the server to send the response back to the address and port
   that the request came from. In test II, the client sends a Binding
   Request with both the "change IP" and "change port" flags from the
   CHANGE-REQUEST attribute set. In test III, the client sends a Binding
   Request with only the "change port" flag set.

   The client begins by initiating test I. If this test yields no
   response, the client knows right away that it is not capable of UDP
   connectivity. If the test produces a response, the client examines
   the MAPPED-ADDRESS attribute. If this address and port are the same
   as the local IP address and port of the socket used to send the
   request, the client knows that it is not natted. It executes test II.
   If a response is received, the client knows that it has open access
   to the Internet (or, at least, its behind a firewall that behaves
   like a full-cone NAT, but without the translation). If no response is
   received, the client knows its behind a symmetric UDP firewall.



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   In the event that the IP address and port of the socket did not match
   the MAPPED-ADDRESS attribute in the response to test I, the client
   knows that it is behind a NAT. It performs test II. If a response is
   received, the client knows that it is behind a full-cone NAT. If no
   response is received, it performs test I again, but this time, does
   so to the address and port from the CHANGED-ADDRESS attribute from
   the response to test I. If the IP address and port returned in the
   MAPPED-ADDRESS attribute are not the same as the ones from the first
   test I, the client knows its behind a symmetric NAT. If the address
   and port are the same, the client is either behind a restricted or
   port restricted NAT. To make a determination about which one it is
   behind, the client initiates test III. If a response is received, its
   behind a restricted NAT, and if no response is received, its behind a
   port restricted NAT.

   This procedure yields substantial information about the operating
   condition of the client application. In the event of multiple NATs
   between the client and the Internet, the type that is discovered will
   be the type of the most restrictive NAT between the client and the
   Internet. The types of NAT, in order of restrictiveness, from most to
   least, are symmetric, port restricted cone, restricted cone, and full
   cone.

   Typically, a client will re-do this discovery process periodically to
   detect changes, or look for inconsistent results. It is important to
   note that when the discovery process is redone, it should not
   generally be done from the same local address and port used in the
   previous discovery process. If the same local address and port are
   reused, bindings from the previous test may still be in existence,
   and these will invalidate the results of the test. Using a different
   local address and port for subsequent tests resolves this problem. An
   alternative is to wait sufficiently long to be confident that the old
   bindings have expired (half an hour should more than suffice).

10.2 Binding Lifetime Discovery

   STUN can also be used to discover the lifetimes of the bindings
   created by the NAT. In many cases, the client will need to refresh
   the binding, either through a new STUN request, or an application
   packet, in order for the application to continue to use the binding.
   By discovering the binding lifetime, the client can determine how
   frequently it needs to refresh.

   To determine the binding lifetime, the client first sends a Binding
   Request to the server from a particular socket, X. This creates a
   binding in the NAT. The response from the server contains a MAPPED-
   ADDRESS attribute, providing the public address and port on the NAT.
   Call this Pa and Pp, respectively. The client then starts a timer



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                       +--------+
                       |  Test  |
                       |   I    |
                       +--------+
                            |
                            |
                            V
                           /\               /\
                        N /  \ Y           /  \ Y            +--------+
         UDP     <-------/Resp\---------->/ IP \------------>|  Test  |
         Blocked         \ ?  /           \Same/             |   II   |
                          \  /             \? /              +--------+
                           \/               \/                    |
                                            | N                   |
                                            |                     V
                                            V                    /\
                                        +--------+  Sym.      N /  \
                                        |  Test  |  UDP    <---/Resp\
                                        |   II   |  Firewall   \ ?  /
                                        +--------+              \  /
                                            |                    \/
                                            V                     |Y
                 /\                         /\                    |
  Symmetric  N  /  \       +--------+   N  /  \                   V
     NAT  <--- / IP \<-----|  Test  |<--- /Resp\               Open
               \Same/      |   I    |     \ ?  /               Internet
                \? /       +--------+      \  /
                 \/                         \/
                 |                           |Y
                 |                           |
                 |                           V
                 |                           Full
                 |                           Cone
                 V              /\
             +--------+        /  \ Y
             |  Test  |------>/Resp\---->Restricted
             |   III  |       \ ?  /
             +--------+        \  /
                                \/
                                 |N
                                 |       Port
                                 +------>Restricted



   Figure 2: Flow for type discovery process


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   with a value of T seconds. When this timer fires, the client sends
   another Binding Request to the server, using the same destination
   address and port, but from a different socket, Y. This request
   contains a RESPONSE-ADDRESS address attribute, set to (Pa,Pp). This
   will create a new binding on the NAT, and cause the STUN server to
   send a Binding Response that would match the old binding, if it still
   exists. If the client receives the Binding Response on socket X, it
   knows that the binding has not expired. If the client receives the
   Binding Response on socket Y (which is possible if the old binding
   expired, and the NAT allocated the same public address and port to
   the new binding), or receives no response at all, it knows that the
   binding has expired.

   The client can find the value of the binding lifetime by doing a
   binary search through T, arriving eventually at the value where the
   response is not received for any timer greater than T, but is
   received for any timer less than T.

   This discovery process takes quite a bit of time, and is something
   that will typically be run in the background on a device once it
   boots.

   It is possible that the client can get inconsistent results each time
   this process is run. For example, if the NAT should reboot, or be
   reset for some reason, the process may discover a lifetime than is
   shorter than the actual one. For this reason, implementations are
   encouraged to run the test numerous times, and be prepared to get
   inconsistent results.

10.3 Binding Acquisition

   Consider once more the case of a VoIP phone. It used the discovery
   process above when it started up, to discover its environment. Now,
   it wants to make a call. As part of the discovery process, it
   determined that it was behind a full-cone NAT.

   Consider further that this phone consists of two logically separated
   components - a control component that handles signaling, and a media
   component that handles the audio, video, and RTP [13]. Both are
   behind the same NAT. Because of this separation of control and media,
   we wish to minimize the communication required between them. In fact,
   they may not even run on the same host.

   In order to make a voice call, the phone needs to obtain an IP
   address and port that it can place in the call setup message as the
   destination for receiving audio.

   To obtain an address, the control component sends a Shared Secret



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   Request to the server, obtains a shared secret, and then sends a
   Binding Request to the server. No CHANGE-REQUEST attribute is present
   in the Binding Request, and neither is the RESPONSE-ADDRESS
   attribute. The Binding Response contains a mapped address. The
   control component then formulates a second Binding Request. This
   request contains a RESPONSE-ADDRESS, which is set to the mapped
   address learned from the previous Binding Response. This Binding
   Request is passed to the media component, along with the IP address
   and port of the STUN server. The media component sends the Binding
   Request. The request goes to the STUN server, which sends the Binding
   Response back to the control component. The control component
   receives this, and now has learned an IP address and port that will
   be routed back to the media component that sent the request.

   The client will be able to receive media from anywhere on this mapped
   address.

   In the case of silence suppression, there may be periods where the
   client receives no media. In this case, the UDP bindings could
   timeout (UDP bindings in NATs are typically short). To deal with
   this, the application can periodically retransmit the query in order
   to keep the binding fresh.

   It is possible that both participants in the multimedia session are
   behind the same NAT. In that case, both will repeat this procedure
   above, and both will obtain public address bindings. When one sends
   media to the other, the media is routed to the NAT, and then turns
   right back around to come back into the enterprise, where it is
   translated to the private address of the recipient. This is not
   particularly efficient, and unfortunately, does not work in many
   commercial NATs. In such cases, the clients may need to retry using
   private addresses.

11 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



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   [7]. Unless otherwise noted, numeric constants are in decimal (base
   10).

11.1 Message Header

   All STUN messages consist of a 20 byte header:


    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



   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.

11.2 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:




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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Type                  |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Value                             ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   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



   Extensions, documented in standards track IETF RFCs, MAY define new
   attributes. Attributes with values greater than 0x7fff are optional,
   and those less than or equal to 0x7fff are mandatory to understand.

   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.

   Table 2 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.


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

11.2.1 MAPPED-ADDRESS




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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


   Table 2: Summary of Attributes

   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.


    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                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   The port is a network byte ordered representation of the mapped port.
   The address family is always 0x02, corresponding to IPv4. The first 8
   bits of the MAPPED-ADDRESS are ignored, for the purposes of aligning
   parameters on natural boundaries. The IPv4 address is 32 bits.

11.2.2 RESPONSE-ADDRESS

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




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11.2.3 CHANGED-ADDRESS

   The CHANGED-ADDRESS attribute indicates the IP address and port where
   responses will be sent from if the "change IP" and "change port"
   flags were 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.

11.2.4 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.

11.2.5 SOURCE-ADDRESS

   The SOURCE-ADDRESS attribute is present in Binding Responses. It
   indicates the source IP address and port that the server is sending
   the response from. Its syntax is identical to that of MAPPED-ADDRESS.

11.2.6 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.



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   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.

11.2.7 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.

11.2.8 MESSAGE-INTEGRITY

   The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [14] 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. 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.

11.2.9 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, and is consistent in its
   code assignments and semantics with SIP [11] and HTTP [16]. 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. 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)                                ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   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.

        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.

11.2.10 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.



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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Attribute 1 Type           |     Attribute 2 Type        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Attribute 3 Type           |     Attribute 4 Type    ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



11.2.11 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.

12 Security Considerations

12.1 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
   MAPPED-ADDRESS. The attacks that can be launched using such a
   technique include:

12.1.1 Attack I: DDOS Against a Target



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   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.

12.1.2 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 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.

12.1.3 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.

12.1.4 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.

12.2 Launching the Attacks



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

12.2.1 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.

12.2.2 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.

12.2.3 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



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   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
   between it and the STUN server), the attacker will not be able to
   force the server to generate arbitrary MAPPED-ADRESSes 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.

12.2.4 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 12.2.3.

12.2.5 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



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   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 its
   different).

12.2.6 Approach VI: Duplication

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



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   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 12.2.3, which limit the range of MAPPED-
   ADDRESSes the attacker can cause the STUN server to generate.

12.3 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 [8]). This is
   particularly important for the NATs themselves. As Section 12.2.3
   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 12.2.1.

   Thirdly, to prevent the DNS attack of Section 12.2.2, 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 [17].

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

   Approaches IV (Section 12.2.4), when generating the response locally,
   and V (Section 12.2.5) require an attacker to generate a faked
   response. This attack is prevented using the server signature scheme
   provided in STUN, described in Section 8.1.



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   Approaches III (Section 12.2.3) IV (Section 12.2.4), when using the
   relaying technique, and VI (12.2.6), 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, 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.

12.4 Residual Threats

   None of the countermeasures listed above can prevent the attacks
   described in Section 12.2.3 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 12.1.1. Note that any host which exists in the correct
   topological relationship can be DDOSed. It need not be using STUN.

13 IANA Considerations

   There are no IANA considerations associated with this specification.

14 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 [18]). 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



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   specific set of considerations. This section meets those
   requirements.

14.1 Problem Definition

   From RFC 3424 [18], 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 usually aren't".

   The specific problems being solved by STUN are:

        o Provide a means for a client to detect the presence of one or
          more NATs between it and a server run by a service provider on
          the public Internet. The purpose of such detection is to
          determine additional steps that might be necessary in order to
          receive service from that particular provider.

        o Provide a means for a client to detect the presence of one or
          more NATs between it and another client, where the second
          client is reachable from the first, but it is not known
          whether the second client resides on the public Internet.

        o 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.

14.2 Exit Strategy

   From [18], 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 comes with its own built in exit strategy. This strategy is the
   detection operation that is performed as a precursor to the actual
   UNSAF address-fixing operation. This discovery operation, documented
   in Section 10.1, attempts to discover the existence of, and type of,
   any NATS between the client and the service provider network. Whilst



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   the detection of the specific type of NAT may be brittle, the
   discovery of the existence of NAT is itself quite robust. As NATs are
   phased out through the deployment of IPv6, the discovery operation
   will return immediately with the result that there is no NAT, and no
   further operations are required. Indeed, the discovery operation
   itself can be used to help motivate deployment of IPv6; if a user
   detects a NAT between themselves and the public Internet, they can
   call up their access provider and complain about it.

   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.

14.3 Brittleness Introduced by STUN

   From [18], any UNSAF proposal must provide:


        Discussion of specific issues that may render systems more
        "brittle". For example, approaches that involve using data
        at 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 discovery process assumes a certain classification of
          devices based on their treatment of UDP. There could be other
          types of NATs that are deployed that would not fit into one of
          these molds. Therefore, future NATs may not be properly
          detected by STUN. STUN clients (but not servers) would need to
          change to accommodate that.

        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.



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          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 The discovery of binding lifetimes is prone to error. It
          assumes that the same lifetime will exist for all bindings.
          This may not be true if the NAT uses dynamic binding lifetimes
          to handle overload, or if the NAT itself reboots during the
          discovery process.

        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:

          - 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



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            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 most 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 12.

14.4 Requirements for a Long Term Solution

   From [18], 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.

        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



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             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.

14.5 Issues with Existing NAPT Boxes

   From [18], 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 [13] 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 [19].
   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. This
   can be mitigated by sending occasional silence packets to keep the
   binding alive. However, the result is additional brittleness; proper



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   operation depends on the 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.

14.6 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.

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 Authors Addresses



   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   email: jdrosen@dynamicsoft.com

   Joel Weinberger
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   email: jweinberger@dynamicsoft.com

   Christian Huitema
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052-6399
   email: huitema@microsoft.com



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   Rohan Mahy
   Cisco Systems
   170 West Tasman Dr, MS: SJC-21/3
   Phone: +1 408 526 8570
   Email: rohan@cisco.com




17 Normative References

   [1] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.

   [2] T. Dierks and C. Allen, "The TLS protocol version 1.0," RFC 2246,
   Internet Engineering Task Force, Jan. 1999.

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

   [14] H. Krawczyk, M. Bellare, and R. Canetti, "HMAC: keyed-hashing
   for message authentication," RFC 2104, Internet Engineering Task
   Force, Feb. 1997.

   [5] P. Chown, "Advanced encryption standard (AES) ciphersuites for
   transport layer security (TLS)," RFC 3268, Internet Engineering Task
   Force, June 2002.

   [6] E. Rescorla, "HTTP over TLS," RFC 2818, Internet Engineering Task
   Force, May 2000.

   [7] J. Postel, "Internet protocol," RFC 791, Internet Engineering
   Task Force, Sept. 1981.

   [8] P. Ferguson and D. Senie, "Network ingress filtering: Defeating
   denial of service attacks which employ IP source address spoofing,"
   RFC 2827, Internet Engineering Task Force, May 2000.

18 Informative References

   [9] D. Senie, "Network address translator (nat)-friendly application
   design guidelines," RFC 3235, Internet Engineering Task Force, Jan.
   2002.

   [10] P. Srisuresh, J. Kuthan, J. Rosenberg, A. Molitor, and A.
   Rayhan, "Middlebox communication architecture and framework," RFC
   3303, Internet Engineering Task Force, Aug. 2002.



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   [11] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
   Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
   initiation protocol," RFC 3261, Internet Engineering Task Force, June
   2002.

   [12] M. Holdrege and P. Srisuresh, "Protocol complications with the
   IP network address translator," RFC 3027, Internet Engineering Task
   Force, Jan. 2001.

   [13] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP:
   a transport protocol for real-time applications," RFC 1889, Internet
   Engineering Task Force, Jan. 1996.

   [14] H. Krawczyk, M. Bellare, and R. Canetti, "HMAC: keyed-hashing
   for message authentication," RFC 2104, Internet Engineering Task
   Force, Feb. 1997.

   [15] J. Kohl and C. Neuman, "The kerberos network authentication
   service (V5)," RFC 1510, Internet Engineering Task Force, Sept. 1993.

   [16] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P.
   Leach, and T. Berners-Lee, "Hypertext transfer protocol -- HTTP/1.1,"
   RFC 2616, Internet Engineering Task Force, June 1999.

   [17] M. Baugher et al.  , "The secure real-time transport protocol,"
   Internet Draft, Internet Engineering Task Force, June 2002.  Work in
   progress.

   [18] "IAB considerations for UNilateral self-address fixing (UNSAF)
   across network address translation," RFC 3424, Internet Engineering
   Task Force, Nov. 2002.

   [19] C. Huitema, "RTCP attribute in SDP," Internet Draft, Internet
   Engineering Task Force, Nov. 2002.  Work in progress.


   Full Copyright Statement

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   or assist in its implementation may be prepared, copied, published
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   Internet organizations, except as needed for the purpose of
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