BEHAVE J. Rosenberg
Internet-Draft Cisco Systems
Expires: January 18, 2006 C. Huitema
Microsoft
R. Mahy
Airspace
July 17, 2005
Simple Traversal of UDP Through Network Address Translators (NAT) (STUN)
draft-ietf-behave-rfc3489bis-02
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Simple Traversal of UDP Through NATs (STUN) is a lightweight protocol
that provides the ability for applications to determine the public IP
addresses allocated to them by the NAT. These addresses can be
placed into protocol payloads where a client needs to provide a
publically routable IP address. STUN works with many existing NATs,
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and does not require any special behavior from them. As a result, it
allows a wide variety of applications to work through existing NAT
infrastructure.
Table of Contents
1. Applicability Statement . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. NAT Variations . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Overview of Operation . . . . . . . . . . . . . . . . . . . . 6
7. Message Overview . . . . . . . . . . . . . . . . . . . . . . . 9
8. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 10
8.1 Binding Requests . . . . . . . . . . . . . . . . . . . . . 10
8.2 Shared Secret Requests . . . . . . . . . . . . . . . . . . 14
9. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 16
9.1 Discovery . . . . . . . . . . . . . . . . . . . . . . . . 16
9.2 Obtaining a Shared Secret . . . . . . . . . . . . . . . . 17
9.3 Formulating the Binding Request . . . . . . . . . . . . . 18
9.4 Processing Binding Responses . . . . . . . . . . . . . . . 19
9.5 Using the Mapped Address . . . . . . . . . . . . . . . . . 21
10. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 22
10.1 Message Header . . . . . . . . . . . . . . . . . . . . . . 22
10.2 Message Attributes . . . . . . . . . . . . . . . . . . . . 23
10.2.1 MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . . 25
10.2.2 RESPONSE-ADDRESS . . . . . . . . . . . . . . . . . . . 26
10.2.3 CHANGED-ADDRESS . . . . . . . . . . . . . . . . . . . 26
10.2.4 CHANGE-REQUEST . . . . . . . . . . . . . . . . . . . . 26
10.2.5 SOURCE-ADDRESS . . . . . . . . . . . . . . . . . . . . 27
10.2.6 USERNAME . . . . . . . . . . . . . . . . . . . . . . . 27
10.2.7 PASSWORD . . . . . . . . . . . . . . . . . . . . . . . 27
10.2.8 MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . 27
10.2.9 ERROR-CODE . . . . . . . . . . . . . . . . . . . . . . 27
10.2.10 UNKNOWN-ATTRIBUTES . . . . . . . . . . . . . . . . . 29
10.2.11 REFLECTED-FROM . . . . . . . . . . . . . . . . . . . 29
10.2.12 XOR-MAPPED-ADDRESS . . . . . . . . . . . . . . . . . 29
10.2.13 XOR-ONLY . . . . . . . . . . . . . . . . . . . . . . 30
10.2.14 SERVER . . . . . . . . . . . . . . . . . . . . . . . 30
11. Security Considerations . . . . . . . . . . . . . . . . . . 31
11.1 Attacks on STUN . . . . . . . . . . . . . . . . . . . . . 31
11.1.1 Attack I: DDOS Against a Target . . . . . . . . . . . 31
11.1.2 Attack II: Silencing a Client . . . . . . . . . . . . 31
11.1.3 Attack III: Assuming the Identity of a Client . . . . 32
11.1.4 Attack IV: Eavesdropping . . . . . . . . . . . . . . . 32
11.2 Launching the Attacks . . . . . . . . . . . . . . . . . . 32
11.2.1 Approach I: Compromise a Legitimate STUN Server . . . 33
11.2.2 Approach II: DNS Attacks . . . . . . . . . . . . . . . 33
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11.2.3 Approach III: Rogue Router or NAT . . . . . . . . . . 33
11.2.4 Approach IV: MITM . . . . . . . . . . . . . . . . . . 34
11.2.5 Approach V: Response Injection Plus DoS . . . . . . . 34
11.2.6 Approach VI: Duplication . . . . . . . . . . . . . . . 35
11.3 Countermeasures . . . . . . . . . . . . . . . . . . . . . 35
11.4 Residual Threats . . . . . . . . . . . . . . . . . . . . . 37
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . 37
13. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 37
13.1 Problem Definition . . . . . . . . . . . . . . . . . . . . 37
13.2 Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 38
13.3 Brittleness Introduced by STUN . . . . . . . . . . . . . . 38
13.4 Requirements for a Long Term Solution . . . . . . . . . . 40
13.5 Issues with Existing NAPT Boxes . . . . . . . . . . . . . 41
13.6 In Closing . . . . . . . . . . . . . . . . . . . . . . . . 42
14. Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . 42
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 43
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 43
16.1 Normative References . . . . . . . . . . . . . . . . . . . 43
16.2 Informative References . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 45
Intellectual Property and Copyright Statements . . . . . . . . 46
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1. Applicability Statement
This protocol is not a cure-all for the problems associated with NAT.
It does not enable incoming TCP connections through NAT. It allows
incoming UDP packets through NAT, but only through a subset of
existing NAT types. In particular, STUN does not enable incoming UDP
packets through symmetric NATs (defined below), which are common in
large enterprises. STUN does not work when it is used to obtain an
address to communicate with a peer which happens to be behind the
same NAT. STUN does not work when the STUN server is not in a common
shared address realm. For a more complete discussion of the
limitations of STUN, see Section 13.
2. Introduction
Network Address Translators (NATs), while providing many benefits,
also come with many drawbacks. The most troublesome of those
drawbacks is the fact that they break many existing IP applications,
and make it difficult to deploy new ones. Guidelines have been
developed [8] that describe how to build "NAT friendly" protocols,
but many protocols simply cannot be constructed according to those
guidelines. Examples of such protocols include almost all peer-to-
peer protocols, such as multimedia communications, file sharing and
games.
To combat this problem, Application Layer Gateways (ALGs) have been
embedded in NATs. ALGs perform the application layer functions
required for a particular protocol to traverse a NAT. Typically,
this involves rewriting application layer messages to contain
translated addresses, rather than the ones inserted by the sender of
the message. ALGs have serious limitations, including scalability,
reliability, and speed of deploying new applications. To resolve
these problems, the Middlebox Communications (MIDCOM) protocol is
being developed [9]. MIDCOM allows an application entity, such as an
end client or network server of some sort (like a Session Initiation
Protocol (SIP) proxy [10]) to control a NAT (or firewall), in order
to obtain NAT bindings and open or close pinholes. In this way, NATs
and applications can be separated once more, eliminating the need for
embedding ALGs in NATs, and resolving the limitations imposed by
current architectures.
Unfortunately, MIDCOM requires upgrades to existing NAT and
firewalls, in addition to application components. Complete upgrades
of these NAT and firewall products will take a long time, potentially
years. This is due, in part, to the fact that the deployers of NAT
and firewalls are not the same people who are deploying and using
applications. As a result, the incentive to upgrade these devices
will be low in many cases. Consider, for example, an airport
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Internet lounge that provides access with a NAT. A user connecting
to the NATed network may wish to use a peer-to-peer service, but
cannot, because the NAT doesn't support it. Since the administrators
of the lounge are not the ones providing the service, they are not
motivated to upgrade their NAT equipment to support it, using either
an ALG, or MIDCOM.
Another problem is that the MIDCOM protocol requires that the agent
controlling the middleboxes know the identity of those middleboxes,
and have a relationship with them which permits control. In many
configurations, this will not be possible. For example, many cable
access providers use NAT in front of their entire access network.
This NAT could be in addition to a residential NAT purchased and
operated by the end user. The end user will probably not have a
control relationship with the NAT in the cable access network, and
may not even know of its existence.
Many existing proprietary protocols, such as those for online games
(such as the games described in RFC 3027 [11]) and Voice over IP,
have developed tricks that allow them to operate through NATs without
changing those NATs. This document is an attempt to take some of
those ideas, and codify them into an interoperable protocol that can
meet the needs of many applications.
The protocol described here, Simple Traversal of UDP Through NAT
(STUN), allows entities behind a NAT to learn the address bindings
allocated by the NAT. STUN requires no changes to NATs, and works
with an arbitrary number of NATs in tandem between the application
entity and the public Internet.
3. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[1] and indicate requirement levels for compliant STUN
implementations.
4. Definitions
STUN Client: A STUN client (also just referred to as a client) is an
entity that generates STUN requests. A STUN client can execute on
an end system, such as a user's PC, or can run in a network
element, such as a conferencing server.
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STUN Server: A STUN Server (also just referred to as a server) is an
entity that receives STUN requests, and sends STUN responses.
STUN servers are generally attached to the public Internet.
5. NAT Variations
It is assumed that the reader is familiar with NATs. It has been
observed that NAT treatment of UDP varies among implementations. The
four treatments observed in implementations are:
Full Cone: A full cone NAT is one where all requests from the same
internal IP address and port are mapped to the same external IP
address and port. Furthermore, any external host can send a
packet to the internal host, by sending a packet to the mapped
external address.
Restricted Cone: A restricted cone NAT is one where all requests from
the same internal IP address and port are mapped to the same
external IP address and port. Unlike a full cone NAT, an external
host (with IP address X) can send a packet to the internal host
only if the internal host had previously sent a packet to IP
address X.
Port Restricted Cone: A port restricted cone NAT is like a restricted
cone NAT, but the restriction includes port numbers.
Specifically, an external host can send a packet, with source IP
address X and source port P, to the internal host only if the
internal host had previously sent a packet to IP address X and
port P.
Symmetric: A symmetric NAT is one where all requests from the same
internal IP address and port, to a specific destination IP address
and port, are mapped to the same external IP address and port. If
the same host sends a packet with the same source address and
port, but to a different destination, a different mapping is used.
Furthermore, only the external host that receives a packet can
send a UDP packet back to the internal host.
6. Overview of Operation
This section is descriptive only. Normative behavior is described in
Section 8 and Section 9.
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/-----\
// STUN \\
| Server |
\\ //
\-----/
+--------------+ Public Internet
................| NAT 2 |.......................
+--------------+
+--------------+ Private NET 2
................| NAT 1 |.......................
+--------------+
/-----\
// STUN \\
| Client |
\\ // Private NET 1
\-----/
Figure 1
The typical STUN configuration is shown in Figure 1. A STUN client
is connected to private network 1. This network connects to private
network 2 through NAT 1. Private network 2 connects to the public
Internet through NAT 2. The STUN server resides on the public
Internet.
STUN is a simple client-server protocol. A client sends a request to
a server, and the server returns a response. There are two types of
requests - Binding Requests, sent over UDP, and Shared Secret
Requests, sent over TLS [2] over TCP. Shared Secret Requests ask the
server to return a temporary username and password. This username
and password are used in a subsequent Binding Request and Binding
Response, for the purposes of authentication and message integrity.
Binding requests are used to determine the bindings allocated by
NATs. The client sends a Binding Request to the server, over UDP.
The server examines the source IP address and port of the request,
and copies them into a response that is sent back to the client.
There are some parameters in the request that allow the client to ask
that the response be sent elsewhere, or that the server send the
response from a different address and port. The flags allow for STUN
to be used in diagnostic applications. There are attributes for
providing message integrity and authentication.
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The STUN client is typically embedded in an application which needs
to obtain a public IP address and port that can be used to receive
data. For example, it might need to obtain an IP address and port to
receive Real Time Transport Protocol (RTP) [12] traffic. When the
application starts, the STUN client within the application sends a
STUN Shared Secret Request to its server, obtains a username and
password, and then sends it a Binding Request. STUN servers can be
discovered through DNS SRV records [3], and it is generally assumed
that the client is configured with the domain to use to find the STUN
server. Generally, this will be the domain of the provider of the
service the application is using (such a provider is incented to
deploy STUN servers in order to allow its customers to use its
application through NAT). Of course, a client can determine the
address or domain name of a STUN server through other means. A STUN
server can even be embedded within an end system.
The STUN Binding Request is used to discover the public IP address
and port mappings generated by the NAT. Binding Requests are sent to
the STUN server using UDP. When a Binding Request arrives at the
STUN server, it may have passed through one or more NATs between the
STUN client and the STUN server. As a result, the source address of
the request received by the server will be the mapped address created
by the NAT closest to the server. The STUN server copies that source
IP address and port into a STUN Binding Response, and sends it back
to the source IP address and port of the STUN request. For all of
the NAT types above, this response will arrive at the STUN client.
When the STUN client receives the STUN Binding Response, it compares
the IP address and port in the packet with the local IP address and
port it bound to when the request was sent. If these do not match,
the STUN client is behind one or more NATs. The IP address and port
in the body of the STUN response are public, and can be used by any
host on the public Internet to send packets to the application that
sent the STUN request. An application need only listen on the IP
address and port from which the STUN request was sent. Packets sent
by a host on the public Internet to the public address and port
learned by STUN will be received by the application, so long as
conditions permit. The conditions in which these packets will not be
received by the client are described in Section 1.
It should be noted that the configuration in Figure 1 is not the only
permissible configuration. The STUN server can be located anywhere,
including within another client. The only requirement is that the
STUN server is reachable by the client, and if the client is trying
to obtain a publicly routable address, that the server reside on the
public Internet.
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7. Message Overview
STUN messages are TLV (type-length-value) encoded using big endian
(network ordered) binary. All STUN messages start with a STUN
header, followed by a STUN payload. The payload is a series of STUN
attributes, the set of which depends on the message type. The STUN
header contains a STUN message type, transaction ID, and length. The
message type can be Binding Request, Binding Response, Binding Error
Response, Shared Secret Request, Shared Secret Response, or Shared
Secret Error Response. The transaction ID is used to correlate
requests and responses. The length indicates the total length of the
STUN payload, not including the header. This allows STUN to run over
TCP. Shared Secret Requests are always sent over TCP (indeed, using
TLS over TCP).
Several STUN attributes are defined. The first is a MAPPED-ADDRESS
attribute, which is an IP address and port. It is always placed in
the Binding Response, and it indicates the source IP address and port
the server saw in the Binding Request. There is also a RESPONSE-
ADDRESS attribute, which contains an IP address and port. The
RESPONSE-ADDRESS attribute can be present in the Binding Request, and
indicates where the Binding Response is to be sent. It's optional,
and when not present, the Binding Response is sent to the source IP
address and port of the Binding Request.
The third attribute is the CHANGE-REQUEST attribute, and it contains
two flags to control the IP address and port used to send the
response. These flags are called "change IP" and "change port"
flags. The CHANGE-REQUEST attribute is allowed only in the Binding
Request. They instruct the server to send the Binding Responses from
a different source IP address and port. The CHANGE-REQUEST attribute
is optional in the Binding Request.
The fourth attribute is the CHANGED-ADDRESS attribute. It is present
in Binding Responses. It informs the client of the source IP address
and port that would be used if the client requested the "change IP"
and "change port" behavior.
The fifth attribute is the SOURCE-ADDRESS attribute. It is only
present in Binding Responses. It indicates the source IP address and
port where the response was sent from.
The RESPONSE-ADDRESS, CHANGE-REQUEST, CHANGED-ADDRESS and SOURCE-
ADDRESS attributes are primarily useful for diagnostic applications
that use STUN in order to determine information about the type of
NAT. The usage of these attributes for such purposes is outside the
scope of this specification.
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The sixth attribute is the USERNAME attribute. It is present in a
Shared Secret Response, which provides the client with a temporary
username and password (encoded in the PASSWORD attribute). The
USERNAME is also present in Binding Requests, serving as an index to
the shared secret used for the integrity protection of the Binding
Request. The seventh attribute, PASSWORD, is only found in Shared
Secret Response messages. The eight attribute is the MESSAGE-
INTEGRITY attribute, which contains a message integrity check over
the Binding Request or Binding Response.
The ninth attribute is the ERROR-CODE attribute. This is present in
the Binding Error Response and Shared Secret Error Response. It
indicates the error that has occurred. The tenth attribute is the
UNKNOWN-ATTRIBUTES attribute, which is present in either the Binding
Error Response or Shared Secret Error Response. It indicates the
mandatory attributes from the request which were unknown. The
eleventh attribute is the REFLECTED-FROM attribute, which is present
in Binding Responses. It indicates the IP address and port of the
sender of a Binding Request, used for traceability purposes to
prevent certain denial-of-service attacks.
The twelfth attribute is XOR-MAPPED-ADDRESS. Like MAPPED-ADDRESS, it
is present in the Binding Response, and tells the client the source
IP address and port where the Binding Request came from. However, it
is encoded using an Exclusive Or (XOR) operation with the transaction
ID. Some NAT devices have been found to rewrite binary encoded IP
addresses present in protocol PDUs. Such behavior interferes with
the operation of STUN. Clients use XOR-MAPPED-ADDRESS instead of
MAPPED-ADDRESS whenever both are present in a Binding Response.
Using XOR-MAPPED-ADDRESS protects the client from such interfering
NAT devices.
The last attribute is XOR-ONLY. It can be present in the Binding
Request. It tells the server to only send a XOR-MAPPED-ADDRESS in
the Binding Response.
8. Server Behavior
The server behavior depends on whether the request is a Binding
Request or a Shared Secret Request.
8.1 Binding Requests
A STUN server MUST be prepared to receive Binding Requests on four
address/port combinations - (A1, P1), (A2, P1), (A1, P2), and (A2,
P2). (A1, P1) represent the primary address and port, and these are
the ones obtained through the client discovery procedures below.
Typically, P1 will be port 3478, the default STUN port. A2 and P2
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are arbitrary. A2 and P2 are advertised by the server through the
CHANGED-ADDRESS attribute, as described below.
OPEN ISSUE: Experience has shown that the usage of a dynamic port
for P2 has been problematic. This is because firewall
administrators have opened up port 3478 to permit STUN, but
disallowed the dynamic port used by the server. This causes the
diagnostic techniques to fail. This can be fixed through
allocation of a second port number from IANA. Does that belong in
this specification or in the diagnostic specification? I think it
has to go here.
It is RECOMMENDED that the server check the Binding Request for a
MESSAGE-INTEGRITY attribute. If not present, and the server requires
integrity checks on the request, it generates a Binding Error
Response with an ERROR-CODE attribute with response code 401. If the
MESSAGE-INTEGRITY attribute was present, the server computes the HMAC
over the request as described in Section 10.2.8. The key to use
depends on the shared secret mechanism. If the STUN Shared Secret
Request was used, the key MUST be the one associated with the
USERNAME attribute present in the request. If the USERNAME attribute
was not present, the server MUST generate a Binding Error Response.
The Binding Error Response MUST include an ERROR-CODE attribute with
response code 432. If the USERNAME is present, but the server
doesn't remember the shared secret for that USERNAME (because it
timed out, for example), the server MUST generate a Binding Error
Response. The Binding Error Response MUST include an ERROR-CODE
attribute with response code 430. If the server does know the shared
secret, but the computed HMAC differs from the one in the request,
the server MUST generate a Binding Error Response with an ERROR-CODE
attribute with response code 431. The Binding Error Response is sent
to the IP address and port the Binding Request came from, and sent
from the IP address and port the Binding Request was sent to.
Assuming the message integrity check passed, processing continues.
The server MUST check for any attributes in the request with values
less than or equal to 0x7fff which it does not understand. If it
encounters any, the server MUST generate a Binding Error Response,
and it MUST include an ERROR-CODE attribute with a 420 response code.
That response MUST contain an UNKNOWN-ATTRIBUTES attribute listing
the attributes with values less than or equal to 0x7fff which were
not understood. The Binding Error Response is sent to the IP address
and port the Binding Request came from, and sent from the IP address
and port the Binding Request was sent to.
Assuming the request was correctly formed, the server MUST generate a
single Binding Response. The Binding Response MUST contain the same
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transaction ID contained in the Binding Request. The length in the
message header MUST contain the total length of the message in bytes,
excluding the header. The Binding Response MUST have a message type
of "Binding Response".
If the XOR-ONLY attribute was not present in the request, the server
MUST add a MAPPED-ADDRESS attribute to the Binding Response. The IP
address component of this attribute MUST be set to the source IP
address observed in the Binding Request. The port component of this
attribute MUST be set to the source port observed in the Binding
Request. If the XOR-ONLY attribute was present in the request, the
server MUST NOT include the MAPPED-ADDRESS attribute in the Binding
Response.
The server MUST add a XOR-MAPPED-ADDRESS attribute to the Binding
Response. This attribute has the same information content as MAPPED-
ADDRESS (in particular, it conveys the IP address and port observed
in the source IP and source port fields of the STUN request), but is
encoded by performing an XOR operation between the transaction ID and
the IP address and port. The details on the encoding can be found in
Section 10.2.12.
The server SHOULD add a SERVER attribute to any Binding Response or
Binding Error Response it generates, and its value SHOULD indicate
the manufacturer of the software and a software version or build
number.
If the RESPONSE-ADDRESS attribute was absent from the Binding
Request, the destination address and port of the Binding Response
MUST be the same as the source address and port of the Binding
Request. Otherwise, the destination address and port of the Binding
Response MUST be the value of the IP address and port in the
RESPONSE-ADDRESS attribute.
The source address and port of the Binding Response depend on the
value of the CHANGE-REQUEST attribute and on the address and port the
Binding Request was received on, and are summarized in Table 1.
Let Da represent the destination IP address of the Binding Request
(which will be either A1 or A2), and Dp represent the destination
port of the Binding Request (which will be either P1 or P2). Let Ca
represent the other address, so that if Da is A1, Ca is A2. If Da is
A2, Ca is A1. Similarly, let Cp represent the other port, so that if
Dp is P1, Cp is P2. If Dp is P2, Cp is P1. If the "change port"
flag was set in CHANGE-REQUEST attribute of the Binding Request, and
the "change IP" flag was not set, the source IP address of the
Binding Response MUST be Da and the source port of the Binding
Response MUST be Cp. If the "change IP" flag was set in the Binding
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Request, and the "change port" flag was not set, the source IP
address of the Binding Response MUST be Ca and the source port of the
Binding Response MUST be Dp. When both flags are set, the source IP
address of the Binding Response MUST be Ca and the source port of the
Binding Response MUST be Cp. If neither flag is set, or if the
CHANGE-REQUEST attribute is absent entirely, the source IP address of
the Binding Response MUST be Da and the source port of the Binding
Response MUST be Dp.
Flags Source Address Source Port CHANGED-ADDRESS
none Da Dp Ca:Cp
Change IP Ca Dp Ca:Cp
Change port Da Cp Ca:Cp
Change IP and
Change port Ca Cp Ca:Cp
Figure 2
The server MUST add a SOURCE-ADDRESS attribute to the Binding
Response, containing the source address and port used to send the
Binding Response.
The server MUST add a CHANGED-ADDRESS attribute to the Binding
Response. This contains the source IP address and port that would be
used if the client had set the "change IP" and "change port" flags in
the Binding Request. As summarized in Table 1, these are Ca and Cp,
respectively, regardless of the value of the CHANGE-REQUEST flags.
If the Binding Request contained both the USERNAME and MESSAGE-
INTEGRITY attributes, the server MUST add a MESSAGE-INTEGRITY
attribute to the Binding Response. The attribute contains an HMAC
[13] over the response, as described in Section 10.2.8. The key to
use depends on the shared secret mechanism. If the STUN Shared
Secret Request was used, the key MUST be the one associated with the
USERNAME attribute present in the Binding Request.
If the Binding Request contained a RESPONSE-ADDRESS attribute, the
server MUST add a REFLECTED-FROM attribute to the response. If the
Binding Request was authenticated using a username obtained from a
Shared Secret Request, the REFLECTED-FROM attribute MUST contain the
source IP address and port where that Shared Secret Request came
from. If the username present in the request was not allocated using
a Shared Secret Request, the REFLECTED-FROM attribute MUST contain
the source address and port of the entity which obtained the
username, as best can be verified with the mechanism used to allocate
the username. If the username was not present in the request, and
the server was willing to process the request, the REFLECTED-FROM
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attribute SHOULD contain the source IP address and port where the
request came from.
The server SHOULD NOT retransmit the response. Reliability is
achieved by having the client periodically resend the request, each
of which triggers a response from the server.
8.2 Shared Secret Requests
Shared Secret Requests are always received on TLS connections. When
the server receives a request from the client to establish a TLS
connection, it MUST proceed with TLS, and SHOULD present a site
certificate. The TLS ciphersuite TLS_RSA_WITH_AES_128_CBC_SHA [4]
SHOULD be used. Client TLS authentication MUST NOT be done, since
the server is not allocating any resources to clients, and the
computational burden can be a source of attacks.
If the server receives a Shared Secret Request, it MUST verify that
the request arrived on a TLS connection. If it did not receive the
request over TLS, it MUST generate a Shared Secret Error Response,
and it MUST include an ERROR-CODE attribute with a 433 response code.
The destination for the error response depends on the transport on
which the request was received. If the Shared Secret Request was
received over TCP, the Shared Secret Error Response is sent over the
same connection the request was received on. If the Shared Secret
Request was receive over UDP, the Shared Secret Error Response is
sent to the source IP address and port that the request came from.
The server MUST check for any attributes in the request with values
less than or equal to 0x7fff which it does not understand. If it
encounters any, the server MUST generate a Shared Secret Error
Response, and it MUST include an ERROR-CODE attribute with a 420
response code. That response MUST contain an UNKNOWN-ATTRIBUTES
attribute listing the attributes with values less than or equal to
0x7fff which were not understood. The Shared Secret Error Response
is sent over the TLS connection.
All Shared Secret Error Responses MUST contain the same transaction
ID contained in the Shared Secret Request. The length in the message
header MUST contain the total length of the message in bytes,
excluding the header. The Shared Secret Error Response MUST have a
message type of "Shared Secret Error Response" (0x0112).
Assuming the request was properly constructed, the server creates a
Shared Secret Response. The Shared Secret Response MUST contain the
same transaction ID contained in the Shared Secret Request. The
length in the message header MUST contain the total length of the
message in bytes, excluding the header. The Shared Secret Response
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MUST have a message type of "Shared Secret Response". The Shared
Secret Response MUST contain a USERNAME attribute and a PASSWORD
attribute. The USERNAME attribute serves as an index to the
password, which is contained in the PASSWORD attribute. The server
can use any mechanism it chooses to generate the username. However,
the username MUST be valid for a period of at least 10 minutes.
Validity means that the server can compute the password for that
username. There MUST be a single password for each username. In
other words, the server cannot, 10 minutes later, assign a different
password to the same username. The server MUST hand out a different
username for each distinct Shared Secret Request. Distinct, in this
case, implies a different transaction ID. It is RECOMMENDED that the
server explicitly invalidate the username after ten minutes. It MUST
invalidate the username after 30 minutes. The PASSWORD contains the
password bound to that username. The password MUST have at least 128
bits. The likelihood that the server assigns the same password for
two different usernames MUST be vanishingly small, and the passwords
MUST be unguessable. In other words, they MUST be a
cryptographically random function of the username.
These requirements can still be met using a stateless server, by
intelligently computing the USERNAME and PASSWORD. One approach is
to construct the USERNAME as:
USERNAME = <prefix,rounded-time,clientIP,hmac>
Where prefix is some random text string (different for each shared
secret request), rounded-time is the current time modulo 20 minutes,
clientIP is the source IP address where the Shared Secret Request
came from, and hmac is an HMAC [13] over the prefix, rounded-time,
and client IP, using a server private key.
The password is then computed as:
password = <hmac(USERNAME,anotherprivatekey)>
With this structure, the username itself, which will be present in
the Binding Request, contains the source IP address where the Shared
Secret Request came from. That allows the server to meet the
requirements specified in Section 8.1 for constructing the REFLECTED-
FROM attribute. The server can verify that the username was not
tampered with, using the hmac present in the username.
The server SHOULD include a SERVER attribute in any Shared Secret
Response or Shared Secret Error response it generates, and its value
SHOULD indicate the manufacturer of the software and a software
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version or build number.
The Shared Secret Response is sent over the same TLS connection the
request was received on. The server SHOULD keep the connection open,
and let the client close it.
9. Client Behavior
The behavior of the client is very straightforward. Its task is to
discover the STUN server, obtain a shared secret, formulate the
Binding Request, handle request reliability, process the Binding
Responses, and use the resulting addresses.
9.1 Discovery
Generally, the client will be configured with a domain name of the
provider of the STUN servers. This domain name is resolved to an IP
address and port using the SRV procedures specified in RFC 2782 [3].
Specifically, the service name is "stun". The protocol is "udp" for
sending Binding Requests, or "tcp" for sending Shared Secret
Requests. The procedures of RFC 2782 are followed to determine the
server to contact. RFC 2782 spells out the details of how a set of
SRV records are sorted and then tried. However, it only states that
the client should "try to connect to the (protocol, address,
service)" without giving any details on what happens in the event of
failure. Those details are described here for STUN.
For STUN requests, failure occurs if there is a transport failure of
some sort (generally, due to fatal ICMP errors in UDP or connection
failures in TCP). Failure also occurs if the transaction fails due
to timeout. This occurs 9.5 seconds after the first request is sent,
for both Shared Secret Requests and Binding Requests. See
Section 9.3 for details on transaction timeouts for Binding Requests.
If a failure occurs, the client SHOULD create a new request, which is
identical to the previous, but has a different transaction ID and
MESSAGE INTEGRITY attribute (the HMAC will change because the
transaction ID has changed). That request is sent to the next
element in the list as specified by RFC 2782.
The default port for STUN requests is 3478, for both TCP and UDP.
Administrators SHOULD use this port in their SRV records, but MAY use
others.
If no SRV records were found, the client performs an A record lookup
of the domain name. The result will be a list of IP addresses, each
of which can be contacted at the default port.
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This would allow a firewall admin to open the STUN port, so hosts
within the enterprise could access new applications. Whether they
will or won't do this is a good question.
9.2 Obtaining a Shared Secret
As discussed in Section 11, there are several attacks possible on
STUN systems. Many of these are prevented through integrity of
requests and responses. To provide that integrity, STUN makes use of
a shared secret between client and server, used as the keying
material for an HMAC used in both the Binding Request and Binding
Response. STUN allows for the shared secret to be obtained in any
way (for example, Kerberos [14]). However, it MUST have at least 128
bits of randomness. In order to ensure interoperability, this
specification describes a TLS-based mechanism. This mechanism,
described in this section, MUST be implemented by clients and
servers.
First, the client determines the IP address and port that it will
open a TCP connection to. This is done using the discovery
procedures in Section 9.1. The client opens up the connection to
that address and port, and immediately begins TLS negotiation [2].
The client MUST verify the identity of the server. To do that, it
follows the identification procedures defined in Section 3.1 of RFC
2818 [5]. Those procedures assume the client is dereferencing a URI.
For purposes of usage with this specification, the client treats the
domain name or IP address used in Section 9.1 as the host portion of
the URI that has been dereferenced.
Once the connection is opened, the client sends a Shared Secret
request. This request has no attributes, just the header. The
transaction ID in the header MUST meet the requirements outlined for
the transaction ID in a binding request, described in Section 9.3
below. The server generates a response, which can either be a Shared
Secret Response or a Shared Secret Error Response.
If the response was a Shared Secret Error Response, the client checks
the response code in the ERROR-CODE attribute. Interpretation of
those response codes is identical to the processing of Section 9.4
for the Binding Error Response.
If a client receives a Shared Secret Response with an attribute that
is not understood whose type is greater than 0x7fff, the attribute
MUST be ignored. If the client receives a Shared Secret Response
with an unknown attribute whose type is less than or equal to 0x7fff,
the response is ignored.
If the response was a Shared Secret Response, it will contain a short
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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 10. Any two requests that are not bit-wise
identical, and not sent to the same server from the same IP address
and port, MUST carry different transaction IDs. The transaction ID
MUST be uniformly and randomly distributed between 0 and 2**128 - 1.
The large range is needed because the transaction ID serves as a form
of randomization, helping to prevent replays of previously signed
responses from the server. The message type of the request MUST be
"Binding Request".
The RESPONSE-ADDRESS attribute is optional in the Binding Request.
It is used if the client wishes the response to be sent to a
different IP address and port than the one the request was sent from.
The CHANGE-REQUEST attribute is also optional. It tells the server
to send the response from a different address or port. Both
RESPONSE-ADDRESS and CHANGE-REQUEST are primarily useful in
diagnostic operations for analyzing the behavior of a NAT. Under
normal usage, neither of these attributes will be present.
The client SHOULD add a MESSAGE-INTEGRITY and USERNAME attribute to
the Binding Request. This MESSAGE-INTEGRITY attribute contains an
HMAC [13]. The value of the username, and the key to use in the
MESSAGE-INTEGRITY attribute depend on the shared secret mechanism.
If the STUN Shared Secret Request was used, the USERNAME must be a
valid username obtained from a Shared Secret Response within the last
nine minutes. The shared secret for the HMAC is the value of the
PASSWORD attribute obtained from the same Shared Secret Response.
Once formulated, the client sends the Binding Request. Reliability
is accomplished through client retransmissions. Clients SHOULD
retransmit the request starting with an interval of 100ms, doubling
every retransmit until the interval reaches 1.6s. Retransmissions
continue with intervals of 1.6s until a response is received, or a
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total of 9 requests have been sent. If no response is received by
1.6 seconds after the last request has been sent, the client SHOULD
consider the transaction to have failed. In other words, requests
would be sent at times 0ms, 100ms, 300ms, 700ms, 1500ms, 3100ms,
4700ms, 6300ms, and 7900ms. At 9500ms, the client considers the
transaction to have failed if no response has been received.
9.4 Processing Binding Responses
The response can either be a Binding Response or Binding Error
Response. Binding Error Responses are always received on the source
address and port the request was sent from. A Binding Response will
be received on the address and port placed in the RESPONSE-ADDRESS
attribute of the request. If none was present, the Binding Responses
will be received on the source address and port the request was sent
from.
If the response is a Binding Error Response, the client checks the
response code from the ERROR-CODE attribute of the response. For a
400 response code, the client SHOULD display the reason phrase to the
user. For a 420 response code, the client SHOULD retry the request,
this time omitting any attributes listed in the UNKNOWN-ATTRIBUTES
attribute of the response. For a 430 response code, the client
SHOULD obtain a new shared secret, and retry the Binding Request with
a new transaction. For 401 and 432 response codes, if the client had
omitted the USERNAME or MESSAGE-INTEGRITY attribute as indicated by
the error, it SHOULD try again with those attributes. For a 431
response code, the client SHOULD alert the user, and MAY try the
request again after obtaining a new username and password. For a 500
response code, the client MAY wait several seconds and then retry the
request. For a 600 response code, the client MUST NOT retry the
request, and SHOULD display the reason phrase to the user. Unknown
attributes between 400 and 499 are treated like a 400, unknown
attributes between 500 and 599 are treated like a 500, and unknown
attributes between 600 and 699 are treated like a 600. Any response
between 100 and 399 MUST result in the cessation of request
retransmissions, but otherwise is discarded.
If a client receives a response with an unknown attribute whose type
is greater than 0x7fff, the attribute MUST be ignored. If the client
receives a response with an unknown 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
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the response as described in Section 10.2.8. The key to use depends
on the shared secret mechanism. If the STUN Shared Secret Request
was used, the key MUST be same as used to compute the MESSAGE-
INTEGRITY attribute in the request. If the computed HMAC differs
from the one in the response, the client SHOULD determine if the
integrity check failed due to a NAT rewriting the MAPPED-ADDRESS. To
perform this check, the client compares the IP address and port in
the MAPPED-ADDRESS with the IP address and port extracted from XOR-
MAPPED-ADDRESS (extraction involves xor'ing the contents of X-port
and X-value with the transaction ID, as described in Section 10). If
the two IP addresses and ports differ, the client MUST discard the
response, but then it SHOULD retry the Binding Request with the XOR-
ONLY attribute included. This tells the server not to include a
MAPPED-ADDRESS in the Binding Response.
If there is no XOR-MAPPED-ADDRESS, or if there is, but there are no
differences between the two IP addresses and ports, the client MUST
discard the response and SHOULD alert the user about a possible
attack.
If the computed HMAC matches the one from the response, processing
continues.
Reception of a response (either Binding Error Response or Binding
Response) to a Binding Request will terminate retransmissions of that
request. However, clients MUST continue to listen for responses to a
Binding Request for 10 seconds after the first response. If it
receives any responses in this interval with different message types
(Binding Responses and Binding Error Responses, for example),
different MAPPED-ADDRESSes, or different XOR-MAPPED-ADDRESSes, it is
an indication of a possible attack. The client MUST NOT use the
MAPPED-ADDRESS or XOR-MAPPED-ADDRESS from any of the responses it
received (either the first or the additional ones), and SHOULD alert
the user.
Furthermore, if a client receives more than twice as many Binding
Responses as the number of Binding Requests it sent, it MUST NOT use
the MAPPED-ADDRESS or XOR-MAPPED-ADDRESS from any of those responses,
and SHOULD alert the user about a potential attack.
If the Binding Response is authenticated, and the MAPPED-ADDRESS or
XOR-MAPPED-ADDRESS was not discarded because of a potential attack,
the CLIENT MAY use the information in the Binding Response. In
particular, the client SHOULD used the IP address and port from the
XOR-MAPPED-ADDRESS instead of the information from the MAPPED-
ADDRESS, assuming XOR-MAPPED-ADDRESS was present in the Binding
Response. Servers compliant to RFC 3489 [19] will not generate XOR-
MAPPED-ADDRESS, so a client MUST be prepared to handle the case where
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only MAPPED-ADDRESS is present. In such a case, the information from
MAPPED-ADDRESS is used.
It is also possible for an IPv4 host to receive a XOR-MAPPED-ADDRESS
or MAPPED-ADDRESS containing an IPv6 address, or for an IPv6 host to
receive a XOR-MAPPED-ADDRESS or MAPPED-ADDRESS containing an IPv4
address. Clients MUST be prepared for this case.
The next section provides additional details on how the mapped
address information is used.
9.5 Using the Mapped Address
The mapped address present in the XOR-MAPPED-ADDRESS attribute (or
MAPPED-ADDRESS if not present) of the binding response can be used by
clients to facilitate UDP traversal of NATs for many applications.
NAT traversal is problematic for applications which require a client
to insert an IP address and port into a message, to which subsequent
messages will be delivered by other entities in a network. Normally,
the client would insert the IP address and port from a local
interface into the message. However, if the client is behind a NAT,
this local interface will be a private address. Clients within other
address realms will not be able to send messages to that address.
An example of a such an application is SIP, which requires a client
to include IP address and port information in several places,
including the Session Description Protocol (SDP) body [20] carried by
SIP. The IP address and port present in the SDP is used for receipt
of media.
To use STUN as a technique for traversal of SIP and other protocols,
when the client wishes to send a protocol message, it figures out the
places in the protocol data unit where it is supposed to insert its
own IP address along with a port. Instead of directly using a port
allocated from a local interface, the client allocates a port from
the local interface, and from that port, initiates the STUN
procedures described above. The XOR-MAPPED-ADDRESS (or MAPPED-
ADDRESS if not present) in the STUN Binding Response provides the
client with an alternative IP address and port which it can then
include in the protocol PDU. This IP address and port may be within
a different address family than the local interfaces used by the
client. This is not an error condition. In such a case, the client
would use the learned IP address and port as if the client was a host
with an interface within that address family.
In the case of SIP, to populate the SDP appropriately, a client would
generate two STUN Binding Request messages at the time a call is
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initiated or answered. One is used to obtain the IP address and port
for RTP, and the other, for the Real Time Control Protocol (RTCP)
[12]. The client might also need to use STUN to obtain IP addresses
and ports for usage in other parts of the SIP message. The detailed
usage of STUN to facilitate SIP NAT traversal is outside the scope of
this specification.
As discussed above, the addresses learned by STUN may not be usable
with all entities with whom a client might wish to communicate. The
way in which this problem is handled depends on the application
protocol. The ideal solution is for a protocol to allow a client to
include a multiplicity of addresses and ports in the PDU. One of
those can be the address and port determined from STUN, and the
others can include addresses and ports learned from other techniques.
The application protocol would then provide a means for dynamically
detecting which one works. An example of such an an approach is
Interactive Connectivity Establishment (ICE) [21].
10. Protocol Details
This section presents the detailed encoding of a STUN message.
STUN is a request-response protocol. Clients send a request, and the
server sends a response. There are two requests, Binding Request,
and Shared Secret Request. The response to a Binding Request can
either be the Binding Response or Binding Error Response. The
response to a Shared Secret Request can either be a Shared Secret
Response or a Shared Secret Error Response.
STUN messages are encoded using binary fields. All integer fields
are carried in network byte order, that is, most significant byte
(octet) first. This byte order is commonly known as big-endian. The
transmission order is described in detail in Appendix B of RFC 791
[6]. Unless otherwise noted, numeric constants are in decimal (base
10).
10.1 Message Header
All STUN messages consist of a 20 byte header:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| STUN Message Type | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Transaction ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Types can take on the following values:
0x0001 : Binding Request
0x0101 : Binding Response
0x0111 : Binding Error Response
0x0002 : Shared Secret Request
0x0102 : Shared Secret Response
0x0112 : Shared Secret Error Response
It is important to note that the most significant two bits of every
STUN message are equal to 0b00. This aids in differentiating STUN
packets from RTP packets, in the case that both are sent to the same
IP address and port, as is done with ICE.
The message length is the count, in bytes, of the size of the
message, not including the 20 byte header.
The transaction ID is a 128 bit identifier. It also serves as salt
to randomize the request and the response. All responses carry the
same identifier as the request they correspond to.
10.2 Message Attributes
After the header are 0 or more attributes. Each attribute is TLV
encoded, with a 16 bit type, 16 bit length, and variable value:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value ....
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following types are defined:
0x0001: MAPPED-ADDRESS
0x0002: RESPONSE-ADDRESS
0x0003: CHANGE-REQUEST
0x0004: SOURCE-ADDRESS
0x0005: CHANGED-ADDRESS
0x0006: USERNAME
0x0007: PASSWORD
0x0008: MESSAGE-INTEGRITY
0x0009: ERROR-CODE
0x000a: UNKNOWN-ATTRIBUTES
0x000b: REFLECTED-FROM
0x8020: XOR-MAPPED-ADDRESS
0x0021: XOR-ONLY
0x8022: SERVER
To allow future revisions of this specification to add new attributes
if needed, the attribute space is divided into optional and mandatory
ones. Attributes with values greater than 0x7fff are optional, which
means that the message can be processed by the client or server even
though the attribute is not understood. Attributes with values less
than or equal to 0x7fff are mandatory to understand, which means that
the client or server cannot process the message unless it understands
the attribute.
The MESSAGE-INTEGRITY attribute MUST be the last attribute within a
message. Any attributes that are known, but are not supposed to be
present in a message (MAPPED-ADDRESS in a request, for example) MUST
be ignored.
Figure 9 indicates which attributes are present in which messages.
An M indicates that inclusion of the attribute in the message is
mandatory, O means its optional, C means it's conditional based on
some other aspect of the message, and N/A means that the attribute is
not applicable to that message type.
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Binding Shared Shared Shared
Binding Binding Error Secret Secret Secret
Att. Req. Resp. Resp. Req. Resp. Error
Resp.
_____________________________________________________________________
MAPPED-ADDRESS N/A M N/A N/A N/A N/A
RESPONSE-ADDRESS O N/A N/A N/A N/A N/A
CHANGE-REQUEST O N/A N/A N/A N/A N/A
SOURCE-ADDRESS N/A M N/A N/A N/A N/A
CHANGED-ADDRESS N/A M N/A N/A N/A N/A
USERNAME O N/A N/A N/A M N/A
PASSWORD N/A N/A N/A N/A M N/A
MESSAGE-INTEGRITY O O N/A N/A N/A N/A
ERROR-CODE N/A N/A M N/A N/A M
UNKNOWN-ATTRIBUTES N/A N/A C N/A N/A C
REFLECTED-FROM N/A C N/A N/A N/A N/A
XOR-MAPPED-ADDRESS N/A M N/A N/A N/A N/A
XOR-ONLY O N/A N/A N/A N/A N/A
SERVER N/A O O N/A O O
Figure 9
The length refers to the length of the value element, expressed as an
unsigned integral number of bytes.
10.2.1 MAPPED-ADDRESS
The MAPPED-ADDRESS attribute indicates the mapped IP address and
port. It consists of an eight bit address family, and a sixteen bit
port, followed by a fixed length value representing the IP address.
If the address family is IPv4, the address is 32 bits. If the
address family is IPv6, the address is 128 bits.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x x x x x x x x| Family | Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The port is a network byte ordered representation of the mapped port.
The address family can take on the following values:
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0x01: IPv4
0x02: IPv6
The first 8 bits of the MAPPED-ADDRESS are ignored, for the purposes
of aligning parameters on natural boundaries.
10.2.2 RESPONSE-ADDRESS
The RESPONSE-ADDRESS attribute indicates where the response to a
Binding Request should be sent. Its syntax is identical to MAPPED-
ADDRESS.
10.2.3 CHANGED-ADDRESS
The CHANGED-ADDRESS attribute indicates the IP address and port where
responses would have been sent from if the "change IP" and "change
port" flags had been set in the CHANGE-REQUEST attribute of the
Binding Request. The attribute is always present in a Binding
Response, independent of the value of the flags. Its syntax is
identical to MAPPED-ADDRESS.
10.2.4 CHANGE-REQUEST
The CHANGE-REQUEST attribute is used by the client to request that
the server use a different address and/or port when sending the
response. The attribute is 32 bits long, although only two bits (A
and B) are used:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 A B 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The meaning of the flags is:
A: This is the "change IP" flag. If true, it requests the server to
send the Binding Response with a different IP address than the one
the Binding Request was received on.
B: This is the "change port" flag. If true, it requests the server
to send the Binding Response with a different port than the one
the Binding Request was received on.
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10.2.5 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.
10.2.6 USERNAME
The USERNAME attribute is used for message integrity. It serves as a
means to identify the shared secret used in the message integrity
check. The USERNAME is always present in a Shared Secret Response,
along with the PASSWORD. It is optionally present in a Binding
Request when message integrity is used.
The value of USERNAME is a variable length opaque value. Its length
MUST be a multiple of 4 (measured in bytes) in order to guarantee
alignment of attributes on word boundaries.
10.2.7 PASSWORD
The PASSWORD attribute is used in Shared Secret Responses. It is
always present in a Shared Secret Response, along with the USERNAME.
The value of PASSWORD is a variable length value that is to be used
as a shared secret. Its length MUST be a multiple of 4 (measured in
bytes) in order to guarantee alignment of attributes on word
boundaries.
10.2.8 MESSAGE-INTEGRITY
The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [13] of the
STUN message. It can be present in Binding Requests or Binding
Responses. Since it uses the SHA1 hash, the HMAC will be 20 bytes.
The text used as input to HMAC is the STUN message, including the
header, up to and including the attribute preceding the MESSAGE-
INTEGRITY attribute. That text is then padded with zeroes so as to
be a multiple of 64 bytes. As a result, the MESSAGE-INTEGRITY
attribute MUST be the last attribute in any STUN message. The key
used as input to HMAC depends on the context.
10.2.9 ERROR-CODE
The ERROR-CODE attribute is present in the Binding Error Response and
Shared Secret Error Response. It is a numeric value in the range of
100 to 699 plus a textual reason phrase encoded in UTF-8, and is
consistent in its code assignments and semantics with SIP [10] and
HTTP [15]. The reason phrase is meant for user consumption, and can
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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) ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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433 (Use TLS): The Shared Secret request has to be sent over TLS, but
was not received over TLS.
500 (Server Error): The server has suffered a temporary error. The
client should try again.
600 (Global Failure): The server is refusing to fulfill the request.
The client should not retry.
10.2.10 UNKNOWN-ATTRIBUTES
The UNKNOWN-ATTRIBUTES attribute is present only in a Binding Error
Response or Shared Secret Error Response when the response code in
the ERROR-CODE attribute is 420.
The attribute contains a list of 16 bit values, each of which
represents an attribute type that was not understood by the server.
If the number of unknown attributes is an odd number, one of the
attributes MUST be repeated in the list, so that the total length of
the list is a multiple of 4 bytes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute 1 Type | Attribute 2 Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute 3 Type | Attribute 4 Type ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
10.2.11 REFLECTED-FROM
The REFLECTED-FROM attribute is present only in Binding Responses,
when the Binding Request contained a RESPONSE-ADDRESS attribute. The
attribute contains the identity (in terms of IP address) of the
source where the request came from. Its purpose is to provide
traceability, so that a STUN server cannot be used as a reflector for
denial-of-service attacks.
Its syntax is identical to the MAPPED-ADDRESS attribute.
10.2.12 XOR-MAPPED-ADDRESS
The XOR-MAPPED-ADDRESS attribute is only present in Binding
Responses. It provides the same information that is present in the
MAPPED-ADDRESS attribute. However, the information is encoded by
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performing an exclusive or (XOR) operation between the mapped address
and the transaction ID. Unfortunately, some NAT devices have been
found to rewrite binary encoded IP addresses and ports that are
present in protocol payloads. This behavior interferes with the
operation of STUN. By providing the mapped address in an obfuscated
form, STUN can continue to operate through these devices.
The format of the XOR-MAPPED-ADDRESS is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x x x x x x x x| Family | X-Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| X-Address (Variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Family represents the IP address family, and is encoded
identically to the Family in MAPPED-ADDRESS.
X-Port is equal to the port in MAPPED-ADDRESS, exclusive or'ed with
most significant 16 bits of the transaction ID. If the IP address
family is IPv4, X-Address is equal to the IP address in MAPPED-
ADDRESS, exclusive or'ed with the most significant 32 bits of the
transaction ID. If the IP address family is IPv6, the X-Address is
equal to the IP address in MAPPED-ADDRESS, exclusive or'ed with the
entire 128 bit transaction ID.
10.2.13 XOR-ONLY
This attribute is present in a Binding Request. It is used by a
client to request that a server compliant to this specification omit
the MAPPED-ADDRESS from a Binding Response, and include only the XOR-
MAPPED-ADDRESS. This is necessary in cases where a Binding Response
is failing integrity checks because a NAT is rewriting the contents
of a MAPPED-ADDRESS in the Binding Response.
This attribute has a length of zero, and therefore contains no other
information past the common attribute header.
10.2.14 SERVER
The server attribute contains a textual description of the software
being used by the server, including manufacturer and version number.
The attribute has no impact on operation of the protocol, and serves
only as a tool for diagnostic and debugging purposes.
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The value of SERVER is variable length. Its length MUST be a
multiple of 4 (measured in bytes) in order to guarantee alignment of
attributes on word boundaries.
11. Security Considerations
11.1 Attacks on STUN
Generally speaking, attacks on STUN can be classified into denial of
service attacks and eavesdropping attacks. Denial of service attacks
can be launched against a STUN server itself, or against other
elements using the STUN protocol.
STUN servers create state through the Shared Secret Request
mechanism. To prevent being swamped with traffic, a STUN server
SHOULD limit the number of simultaneous TLS connections it will hold
open by dropping an existing connection when a new connection request
arrives (based on an Least Recently Used (LRU) policy, for example).
Similarly, it SHOULD limit the number of shared secrets it will
store, in the event that the server is storing the shared secrets.
The attacks of greater interest are those in which the STUN server
and client are used to launch DOS attacks against other entities,
including the client itself.
Many of the attacks require the attacker to generate a response to a
legitimate STUN request, in order to provide the client with a faked
XOR-MAPPED-ADDRESS or MAPPED-ADDRESS. In the sections below, we
refer to either the XOR-MAPPED-ADDRESS or MAPPED-ADDRESS as just the
mapped address (note the lower case). The attacks that can be
launched using such a technique include:
11.1.1 Attack I: DDOS Against a Target
In this case, the attacker provides a large number of clients with
the same faked mapped address that points to the intended target.
This will trick all the STUN clients into thinking that their
addresses are equal to that of the target. The clients then hand out
that address in order to receive traffic on it (for example, in SIP
or H.323 messages). However, all of that traffic becomes focused at
the intended target. The attack can provide substantial
amplification, especially when used with clients that are using STUN
to enable multimedia applications.
11.1.2 Attack II: Silencing a Client
In this attack, the attacker seeks to deny a client access to
services enabled by STUN (for example, a client using STUN to enable
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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.
11.1.3 Attack III: Assuming the Identity of a Client
This attack is similar to attack II. However, the faked mapped
address points to the attacker themself. This allows the attacker to
receive traffic which was destined for the client.
11.1.4 Attack IV: Eavesdropping
In this attack, the attacker forces the client to use a mapped
address that routes to itself. It then forwards any packets it
receives to the client. This attack would allow the attacker to
observe all packets sent to the client. However, in order to launch
the attack, the attacker must have already been able to observe
packets from the client to the STUN server. In most cases (such as
when the attack is launched from an access network), this means that
the attacker could already observe packets sent to the client. This
attack is, as a result, only useful for observing traffic by
attackers on the path from the client to the STUN server, but not
generally on the path of packets being routed towards the client.
11.2 Launching the Attacks
It is important to note that attacks of this nature (injecting
responses with fake mapped addresses) require that the attacker be
capable of eavesdropping requests sent from the client to the server
(or to act as a MITM for such attacks). This is because STUN
requests contain a transaction identifier, selected by the client,
which is random with 128 bits of entropy. The server echoes this
value in the response, and the client ignores any responses that
don't have a matching transaction ID. Therefore, in order for an
attacker to provide a faked response that is accepted by the client,
the attacker needs to know what the transaction ID in the request
was. The large amount of randomness, combined with the need to know
when the client sends a request, precludes attacks that involve
guessing the transaction ID.
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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:
11.2.1 Approach I: Compromise a Legitimate STUN Server
In this attack, the attacker compromises a legitimate STUN server
through a virus or Trojan horse. Presumably, this would allow the
attacker to take over the STUN server, and control the types of
responses it generates.
Compromise of a STUN server can also lead to discovery of open ports.
Knowledge of an open port creates an opportunity for DoS attacks on
those ports (or DDoS attacks if the traversed NAT is a full cone
NAT). Discovering open ports is already fairly trivial using port
probing, so this does not represent a major threat.
11.2.2 Approach II: DNS Attacks
STUN servers are discovered using DNS SRV records. If an attacker
can compromise the DNS, it can inject fake records which map a domain
name to the IP address of a STUN server run by the attacker. This
will allow it to inject fake responses to launch any of the attacks
above.
11.2.3 Approach III: Rogue Router or NAT
Rather than compromise the STUN server, an attacker can cause a STUN
server to generate responses with the wrong mapped address by
compromising a router or NAT on the path from the client to the STUN
server. When the STUN request passes through the rogue router or
NAT, it rewrites the source address of the packet to be that of the
desired mapped address. This address cannot be arbitrary. If the
attacker is on the public Internet (that is, there are no NATs
between it and the STUN server), and the attacker doesn't modify the
STUN request, the address has to have the property that packets sent
from the STUN server to that address would route through the
compromised router. This is because the STUN server will send the
responses back to the source address of the request. With a modified
source address, the only way they can reach the client is if the
compromised router directs them there. If the attacker is on the
public Internet, but they can modify the STUN request, they can
insert a RESPONSE-ADDRESS attribute into the request, containing the
actual source address of the STUN request. This will cause the
server to send the response to the client, independent of the source
address the STUN server sees. This gives the attacker the ability to
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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 addresses in responses.
They will only be able force the STUN server to generate mapped
addresses which route to the private network. This is because the
NAT between the attacker and the STUN server will rewrite the source
address of the STUN request, mapping it to a public address that
routes to the private network. Because of this, the attacker can
only force the server to generate faked mapped addresses that route
to the private network. Unfortunately, it is possible that a low
quality NAT would be willing to map an allocated public address to
another public address (as opposed to an internal private address),
in which case the attacker could forge the source address in a STUN
request to be an arbitrary public address. This kind of behavior
from NATs does appear to be rare.
11.2.4 Approach IV: MITM
As an alternative to approach III, if the attacker can place an
element on the path from the client to the server, the element can
act as a man-in-the-middle. In that case, it can intercept a STUN
request, and generate a STUN response directly with any desired value
of the mapped address field. Alternatively, it can forward the STUN
request to the server (after potential modification), receive the
response, and forward it to the client. When forwarding the request
and response, this attack is subject to the same limitations on the
mapped address described in Section 11.2.3.
11.2.5 Approach V: Response Injection Plus DoS
In this approach, the attacker does not need to be a MITM (as in
approaches III and IV). Rather, it only needs to be able to
eavesdrop onto a network segment that carries STUN requests. This is
easily done in multiple access networks such as ethernet or
unprotected 802.11. To inject the fake response, the attacker
listens on the network for a STUN request. When it sees one, it
simultaneously launches a DoS attack on the STUN server, and
generates its own STUN response with the desired mapped address
value. The STUN response generated by the attacker will reach the
client, and the DoS attack against the server is aimed at preventing
the legitimate response from the server from reaching the client.
Arguably, the attacker can do without the DoS attack on the server,
so long as the faked response beats the real response back to the
client, and the client uses the first response, and ignores the
second (even though it's different).
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11.2.6 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 it's different).
Note that, in this approach, launching a DoS attack against the STUN
server or the IP network, to prevent the valid response from being
sent or received, is problematic. The attacker needs the STUN server
to be available to handle its own request. Due to the periodic
retransmissions of the request from the client, this leaves a very
tiny window of opportunity. The attacker must start the DoS attack
immediately after the actual request from the client, causing the
correct response to be discarded, and then cease the DoS attack in
order to send its own request, all before the next retransmission
from the client. Due to the close spacing of the retransmits (100ms
to a few seconds), this is very difficult to do.
Besides DoS attacks, there may be other ways to prevent the actual
request from the client from reaching the server. Layer 2
manipulations, for example, might be able to accomplish it.
Fortunately, Approach IV is subject to the same limitations
documented in Section 11.2.3, which limit the range of mapped
addresses the attacker can cause the STUN server to generate.
11.3 Countermeasures
STUN provides mechanisms to counter the approaches described above,
and additional, non-STUN techniques can be used as well.
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First off, it is RECOMMENDED that networks with STUN clients
implement ingress source filtering (RFC 2827 [7]). This is
particularly important for the NATs themselves. As Section 11.2.3
explains, NATs which do not perform this check can be used as
"reflectors" in DDoS attacks. Most NATs do perform this check as a
default mode of operation. We strongly advise people that purchase
NATs to ensure that this capability is present and enabled.
Secondly, it is RECOMMENDED that STUN servers be run on hosts
dedicated to STUN, with all UDP and TCP ports disabled except for the
STUN ports. This is to prevent viruses and Trojan horses from
infecting STUN servers, in order to prevent their compromise. This
helps mitigate Approach I (Section 11.2.1).
Thirdly, to prevent the DNS attack of Section 11.2.2, Section 9.2
recommends that the client verify the credentials provided by the
server with the name used in the DNS lookup.
Finally, all of the attacks above rely on the client taking the
mapped address it learned from STUN, and using it in application
layer protocols. If encryption and message integrity are provided
within those protocols, the eavesdropping and identity assumption
attacks can be prevented. As such, applications that make use of
STUN addresses in application protocols SHOULD use integrity and
encryption, even if a SHOULD level strength is not specified for that
protocol. For example, multimedia applications using STUN addresses
to receive RTP traffic would use secure RTP [16].
The above three techniques are non-STUN mechanisms. STUN itself
provides several countermeasures.
Approaches IV (Section 11.2.4), when generating the response locally,
and V (Section 11.2.5) require an attacker to generate a faked
response. This attack is prevented using the message integrity
mechanism provided in STUN, described in Section 8.1.
Approaches III (Section 11.2.3) IV (Section 11.2.4), when using the
relaying technique, and VI (Section 11.2.6), however, are not
preventable through server signatures. Both approaches are most
potent when the attacker can modify the request, inserting a
RESPONSE-ADDRESS that routes to the client. Fortunately, such
modifications are preventable using the message integrity techniques
described in Section 9.3. However, these three approaches are still
functional when the attacker modifies nothing but the source address
of the STUN request. Sadly, this is the one thing that cannot be
protected through cryptographic means, as this is the change that
STUN itself is seeking to detect and report. It is therefore an
inherent weakness in NAT, and not fixable in STUN. To help mitigate
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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.
11.4 Residual Threats
None of the countermeasures listed above can prevent the attacks
described in Section 11.2.3 if the attacker is in the appropriate
network paths. Specifically, consider the case in which the attacker
wishes to convince client C that it has address V. The attacker needs
to have a network element on the path between A and the server (in
order to modify the request) and on the path between the server and V
so that it can forward the response to C. Furthermore, if there is a
NAT between the attacker and the server, V must also be behind the
same NAT. In such a situation, the attacker can either gain access
to all the application-layer traffic or mount the DDOS attack
described in Section 11.1.1. Note that any host which exists in the
correct topological relationship can be DDOSed. It need not be using
STUN.
12. IANA Considerations
STUN cannot be extended. Changes to the protocol are made through a
standards track revision of this specification. As a result, no IANA
registries are needed. Any future extensions will establish any
needed registries.
13. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing",
which is the general process by which a client attempts to determine
its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism (RFC 3424 [17]). STUN is
an example of a protocol that performs this type of function. The
IAB has mandated that any protocols developed for this purpose
document a specific set of considerations. This section meets those
requirements.
13.1 Problem Definition
From RFC 3424 [17], any UNSAF proposal must provide:
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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 problem being solved by STUN is to provide a means for a
client to obtain an address on the public Internet from a non-
symmetric NAT, for the express purpose of receiving incoming UDP
traffic from another host, targeted to that address.
STUN does not address TCP, either incoming or outgoing, and does not
address outgoing UDP communications.
13.2 Exit Strategy
From [17], any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better short
term fixes are the ones that will naturally see less and less use
as the appropriate technology is deployed.
STUN by itself does not provide an exit strategy. This is provided
by techniques, such as Interactive Connectivity Establishment (ICE)
[21], which allow a client to determine whether addresses learned
from STUN are needed, or whether other addresses, such as the one on
the local interface, will work when communicating with another host.
With such a detection technique, as a client finds that the addresses
provided by STUN are never used, STUN queries can cease to be made,
thus allowing them to phase out.
STUN can also help facilitate the introduction of midcom. As midcom-
capable NATs are deployed, applications will, instead of using STUN
(which also resides at the application layer), first allocate an
address binding using midcom. However, it is a well-known limitation
of midcom that it only works when the agent knows the middleboxes
through which its traffic will flow. Once bindings have been
allocated from those middleboxes, a STUN detection procedure can
validate that there are no additional middleboxes on the path from
the public Internet to the client. If this is the case, the
application can continue operation using the address bindings
allocated from midcom. If it is not the case, STUN provides a
mechanism for self-address fixing through the remaining midcom-
unaware middleboxes. Thus, STUN provides a way to help transition to
full midcom-aware networks.
13.3 Brittleness Introduced by STUN
From [17], any UNSAF proposal must provide:
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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 binding acquisition usage of STUN does not work for all NAT
types. It will work for any application for full cone NATs only.
For restricted cone and port restricted cone NAT, it will work for
some applications depending on the application. Application
specific processing will generally be needed. For symmetric NATs,
the binding acquisition will not yield a usable address. The
tight dependency on the specific type of NAT makes the protocol
brittle.
o STUN assumes that the server exists on the public Internet. If
the server is located in another private address realm, the user
may or may not be able to use its discovered address to
communicate with other users. There is no way to detect such a
condition.
o The bindings allocated from the NAT need to be continuously
refreshed. Since the timeouts for these bindings is very
implementation specific, the refresh interval cannot easily be
determined. When the binding is not being actively used to
receive traffic, but to wait for an incoming message, the binding
refresh will needlessly consume network bandwidth.
o The use of the STUN server as an additional network element
introduces another point of potential security attack. These
attacks are largely prevented by the security measures provided by
STUN, but not entirely.
o The use of the STUN server as an additional network element
introduces another point of failure. If the client cannot locate
a STUN server, or if the server should be unavailable due to
failure, the application cannot function.
o The use of STUN to discover address bindings will result in an
increase in latency for applications. For example, a Voice over
IP application will see an increase of call setup delays equal to
at least one RTT to the STUN server.
o STUN imposes some restrictions on the network topologies for
proper operation. If client A obtains an address from STUN server
X, and sends it to client B, B may not be able to send to A using
that IP address. The address will not work if any of the
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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 residential NAT
devices. Both devices connect them to their cable operators,
but both clients have different providers. Each provider has a
NAT in front of their entire network, connecting it to the
public Internet. If the STUN server used by A is in A's cable
operator's network, an address obtained by it will not be
usable by B. The STUN server must be in the network which is a
common ancestor to both - in this case, the public Internet.
* The STUN server is in an address realm that is a common
ancestor to both clients, but both clients are behind the same
NAT connecting to that address realm. For example, if the two
clients in the previous example had the same cable operator,
that cable operator had a single NAT connecting their network
to the public Internet, and the STUN server was on the public
Internet, the address obtained by A would not be usable by B.
That is because some NATs will not accept an internal packet
sent to a public IP address which is mapped back to an internal
address. To deal with this, additional protocol mechanisms or
configuration parameters need to be introduced which detect
this case.
o Most significantly, STUN introduces potential security threats
which cannot be eliminated. This specification describes
heuristics that can be used to mitigate the problem, but it is
provably unsolvable given what STUN is trying to accomplish.
These security problems are described fully in Section 11.
13.4 Requirements for a Long Term Solution
From [17], any UNSAF proposal must provide:
Identify requirements for longer term, sound technical solutions
-- contribute to the process of finding the right longer term
solution.
Our experience with STUN has led to the following requirements for a
long term solution to the NAT problem:
Requests for bindings and control of other resources in a NAT need to
be explicit. Much of the brittleness in STUN derives from its
guessing at the parameters of the NAT, rather than telling the NAT
what parameters to use.
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Control needs to be in-band. There are far too many scenarios in
which the client will not know about the location of middleboxes
ahead of time. Instead, control of such boxes needs to occur in-
band, traveling along the same path as the data will itself
travel. This guarantees that the right set of middleboxes are
controlled. This is only true for first-party controls; third-
party controls are best handled using the midcom framework.
Control needs to be limited. Users will need to communicate through
NATs which are outside of their administrative control. In order
for providers to be willing to deploy NATs which can be controlled
by users in different domains, the scope of such controls needs to
be extremely limited - typically, allocating a binding to reach
the address where the control packets are coming from.
Simplicity is Paramount. The control protocol will need to be
implement in very simple clients. The servers will need to
support extremely high loads. The protocol will need to be
extremely robust, being the precursor to a host of application
protocols. As such, simplicity is key.
13.5 Issues with Existing NAPT Boxes
From [17], any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports.
Several of the practical issues with STUN involve future proofing -
breaking the protocol when new NAT types get deployed. Fortunately,
this is not an issue at the current time, since most of the deployed
NATs are of the types assumed by STUN. The primary usage STUN has
found is in the area of VoIP, to facilitate allocation of addresses
for receiving RTP [12] traffic. In that application, the periodic
keepalives are provided by the RTP traffic itself. However, several
practical problems arise for RTP. First, RTP assumes that RTCP
traffic is on a port one higher than the RTP traffic. This pairing
property cannot be guaranteed through NATs that are not directly
controllable. As a result, RTCP traffic may not be properly
received. Protocol extensions to SDP have been proposed which
mitigate this by allowing the client to signal a different port for
RTCP [18]. However, there will be interoperability problems for some
time.
For VoIP, silence suppression can cause a gap in the transmission of
RTP packets. This could result in the loss of a binding in the
middle of a call, if that silence period exceeds the binding timeout.
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This can be mitigated by sending occasional silence packets to keep
the binding alive. However, the result is additional brittleness;
proper operation depends on the silence suppression algorithm in use,
the usage of a comfort noise codec, the duration of the silence
period, and the binding lifetime in the NAT.
13.6 In Closing
The problems with STUN are not design flaws in STUN. The problems in
STUN have to do with the lack of standardized behaviors and controls
in NATs. The result of this lack of standardization has been a
proliferation of devices whose behavior is highly unpredictable,
extremely variable, and uncontrollable. STUN does the best it can in
such a hostile environment. Ultimately, the solution is to make the
environment less hostile, and to introduce controls and standardized
behaviors into NAT. However, until such time as that happens, STUN
provides a good short term solution given the terrible conditions
under which it is forced to operate.
14. Changes Since RFC 3489
This specification updates RFC 3489 [19]. This specification differs
from RFC 3489 in the following ways:
o Removed the usage of STUN for NAT type detection and binding
lifetime discovery. These techniques have proven overly brittle
due to wider variations in the types of NAT devices than described
in this document. The protocol semantics used for NAT type
detection remain, however, to provide backwards compatibility, and
to allow for the NAT type detection to occur in purely diagnostic
applications.
o Removed the STUN example that centered around the separation of
the control and media planes. Instead, provided more information
on using STUN with protocols.
o Added the XOR-MAPPED-ADDRESS attribute, which clients prefer to
the MAPPED-ADDRESS when both are present in a Binding Response.
XOR-MAPPED-ADDRESS is obfuscated so that NATs which try to "help"
by rewriting binary IP addresses they find in protocols will not
interfere with the operation of STUN.
o Added the XOR-ONLY attribute, which clients can use to request
that the server send a response with only the XOR-MAPPED-ADDRESS.
This is necessary in case a Binding Response fails integrity
checks due to a NAT that rewrites the MAPPED-ADDRESS.
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o Explicitly point out that the most significant two bits of STUN
are 0b00, allowing easy differentiation with RTP packets when used
with ICE.
o Added support for IPv6. Made it clear that an IPv4 client could
get a v6 mapped address, and vice-a-versa.
o Added the SERVER attribute.
15. Acknowledgments
The authors would like to thank Cedric Aoun, Pete Cordell, Cullen
Jennings, Bob Penfield and Chris Sullivan for their comments, and
Baruch Sterman and Alan Hawrylyshen for initial implementations.
Thanks for Leslie Daigle, Allison Mankin, Eric Rescorla, and Henning
Schulzrinne for IESG and IAB input on this work.
16. References
16.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[3] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[4] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for
Transport Layer Security (TLS)", RFC 3268, June 2002.
[5] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[6] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
[7] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating
Denial of Service Attacks which employ IP Source Address
Spoofing", BCP 38, RFC 2827, May 2000.
16.2 Informative References
[8] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
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[9] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A.
Rayhan, "Middlebox communication architecture and framework",
RFC 3303, August 2002.
[10] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[11] Holdrege, M. and P. Srisuresh, "Protocol Complications with the
IP Network Address Translator", RFC 3027, January 2001.
[12] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, July 2003.
[13] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[14] Kohl, J. and B. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.
[15] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[16] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[17] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
[18] Huitema, C., "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October 2003.
[19] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
- Simple Traversal of User Datagram Protocol (UDP) Through
Network Address Translators (NATs)", RFC 3489, March 2003.
[20] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[21] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for
Multimedia Session Establishment Protocols",
draft-ietf-mmusic-ice-04 (work in progress), February 2005.
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Authors' Addresses
Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
Christian Huitema
Microsoft
One Microsoft Way
Redmond, WA 98052
US
Email: huitema@microsoft.com
Rohan Mahy
Airspace
Email: rohan@ekabal.com
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