BEHAVE J. Rosenberg
Internet-Draft Cisco Systems
Expires: April 26, 2007 C. Huitema
Microsoft
R. Mahy
Plantronics
D. Wing
Cisco Systems
October 23, 2006
Simple Traversal Underneath Network Address Translators (NAT) (STUN)
draft-ietf-behave-rfc3489bis-05
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Simple Traversal Underneath NATs (STUN) is a lightweight protocol
that serves as a tool for application protocols in dealing with NAT
traversal. It allows a client to determine the IP address and port
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allocated to them by a NAT and to keep NAT bindings open. It can
also serve as a check for connectivity between a client and a server
in the presence of NAT, and for the client to detect failure of the
server. STUN works with many existing NATs, and does not require any
special behavior from them. As a result, it allows a wide variety of
applications to work through existing NAT infrastructure.
Table of Contents
1. Applicability Statement . . . . . . . . . . . . . . . . . . . 5
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7
6. STUN Message Structure . . . . . . . . . . . . . . . . . . . . 11
7. STUN Transactions . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Request/Response Transactions . . . . . . . . . . . . . . 14
7.2. Indications . . . . . . . . . . . . . . . . . . . . . . . 15
8. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 15
8.2. Obtaining a Shared Secret . . . . . . . . . . . . . . . . 16
8.3. Request/Response Transactions . . . . . . . . . . . . . . 17
8.3.1. Formulating the Request Message . . . . . . . . . . . 17
8.3.2. Processing Responses . . . . . . . . . . . . . . . . . 19
8.3.3. Timeouts . . . . . . . . . . . . . . . . . . . . . . . 22
8.4. Indication Transactions . . . . . . . . . . . . . . . . . 22
9. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 23
9.1. Request/Response Transactions . . . . . . . . . . . . . . 23
9.1.1. Receive Request Message . . . . . . . . . . . . . . . 23
9.1.2. Constructing the Response . . . . . . . . . . . . . . 26
9.1.3. Sending the Response . . . . . . . . . . . . . . . . . 27
9.2. Indication Transactions . . . . . . . . . . . . . . . . . 27
10. Demultiplexing of STUN and Application Traffic . . . . . . . . 28
11. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 29
11.1. MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . . . 29
11.2. USERNAME . . . . . . . . . . . . . . . . . . . . . . . . 30
11.3. PASSWORD . . . . . . . . . . . . . . . . . . . . . . . . 31
11.4. MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . . 31
11.5. FINGERPRINT . . . . . . . . . . . . . . . . . . . . . . . 31
11.6. ERROR-CODE . . . . . . . . . . . . . . . . . . . . . . . 31
11.7. REALM . . . . . . . . . . . . . . . . . . . . . . . . . . 33
11.8. NONCE . . . . . . . . . . . . . . . . . . . . . . . . . . 33
11.9. UNKNOWN-ATTRIBUTES . . . . . . . . . . . . . . . . . . . 33
11.10. XOR-MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . 34
11.11. SERVER . . . . . . . . . . . . . . . . . . . . . . . . . 35
11.12. ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . . 35
11.13. REFRESH-INTERVAL . . . . . . . . . . . . . . . . . . . . 35
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12. STUN Usages . . . . . . . . . . . . . . . . . . . . . . . . . 36
12.1. Binding Discovery . . . . . . . . . . . . . . . . . . . . 36
12.1.1. Applicability . . . . . . . . . . . . . . . . . . . . 36
12.1.2. Client Discovery of Server . . . . . . . . . . . . . . 37
12.1.3. Server Determination of Usage . . . . . . . . . . . . 38
12.1.4. New Requests or Indications . . . . . . . . . . . . . 38
12.1.5. New Attributes . . . . . . . . . . . . . . . . . . . . 38
12.1.6. New Error Response Codes . . . . . . . . . . . . . . . 38
12.1.7. Client Procedures . . . . . . . . . . . . . . . . . . 38
12.1.8. Server Procedures . . . . . . . . . . . . . . . . . . 38
12.1.9. Security Considerations for Binding Discovery . . . . 38
12.2. NAT Keepalives . . . . . . . . . . . . . . . . . . . . . 39
12.2.1. Applicability . . . . . . . . . . . . . . . . . . . . 39
12.2.2. Client Discovery of Server . . . . . . . . . . . . . . 39
12.2.3. Server Determination of Usage . . . . . . . . . . . . 39
12.2.4. New Requests or Indications . . . . . . . . . . . . . 39
12.2.5. New Attributes . . . . . . . . . . . . . . . . . . . . 39
12.2.6. New Error Response Codes . . . . . . . . . . . . . . . 40
12.2.7. Client Procedures . . . . . . . . . . . . . . . . . . 40
12.2.8. Server Procedures . . . . . . . . . . . . . . . . . . 40
12.2.9. Security Considerations for NAT Keepalives . . . . . . 40
12.3. Short-Term Password . . . . . . . . . . . . . . . . . . . 41
12.3.1. Applicability . . . . . . . . . . . . . . . . . . . . 41
12.3.2. Client Discovery of Server . . . . . . . . . . . . . . 41
12.3.3. Server Determination of Usage . . . . . . . . . . . . 41
12.3.4. New Requests or Indications . . . . . . . . . . . . . 42
12.3.5. New Attributes . . . . . . . . . . . . . . . . . . . . 42
12.3.6. New Error Response Codes . . . . . . . . . . . . . . . 42
12.3.7. Client Procedures . . . . . . . . . . . . . . . . . . 43
12.3.8. Server Procedures . . . . . . . . . . . . . . . . . . 43
12.3.9. Security Considerations for Short-Term Password . . . 44
13. Security Considerations . . . . . . . . . . . . . . . . . . . 45
13.1. Attacks on STUN . . . . . . . . . . . . . . . . . . . . . 45
13.1.1. Attack I: DDoS Against a Target . . . . . . . . . . . 45
13.1.2. Attack II: Silencing a Client . . . . . . . . . . . . 46
13.1.3. Attack III: Assuming the Identity of a Client . . . . 46
13.1.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . . 46
13.2. Launching the Attacks . . . . . . . . . . . . . . . . . . 46
13.2.1. Approach I: Compromise a Legitimate STUN Server . . . 47
13.2.2. Approach II: DNS Attacks . . . . . . . . . . . . . . . 47
13.2.3. Approach III: Rogue Router or NAT . . . . . . . . . . 47
13.2.4. Approach IV: Man in the Middle . . . . . . . . . . . . 48
13.2.5. Approach V: Response Injection Plus DoS . . . . . . . 48
13.2.6. Approach VI: Duplication . . . . . . . . . . . . . . . 49
13.3. Countermeasures . . . . . . . . . . . . . . . . . . . . . 50
13.4. Residual Threats . . . . . . . . . . . . . . . . . . . . 51
14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 51
14.1. Problem Definition . . . . . . . . . . . . . . . . . . . 51
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14.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 52
14.3. Brittleness Introduced by STUN . . . . . . . . . . . . . 52
14.4. Requirements for a Long Term Solution . . . . . . . . . . 53
14.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 54
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 55
15.1. STUN Methods Registry . . . . . . . . . . . . . . . . . . 55
15.2. STUN Attribute Registry . . . . . . . . . . . . . . . . . 55
16. Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . . 56
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 57
18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 57
18.1. Normative References . . . . . . . . . . . . . . . . . . 57
18.2. Informational References . . . . . . . . . . . . . . . . 58
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 60
Intellectual Property and Copyright Statements . . . . . . . . . . 61
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1. Applicability Statement
This protocol is not a cure-all for the problems associated with NAT.
It is a tool that is typically used in conjunction with other
protocols, such as Interactive Connectivity Establishment (ICE) [13]
for a more complete solution. The binding discovery usage, defined
by this specification, can be used by itself with numerous
application protocols as a solution for NAT traversal. However, when
used in that way, STUN will not work with applications that require
incoming TCP connections through NAT. It will allow incoming UDP
packets through NAT, but only through a subset of existing NAT types.
In particular, the STUN binding usage by itself does not enable
incoming UDP packets through NATs whose mapping property is address
dependent or address and port dependent [14]. Furthermore, the
binding usage, when used by itself, does not work when a client is
communicating with a peer which happens to be behind the same NAT.
Nor will it work when the STUN server is not in a common shared
address realm.
The STUN relay usage, defined in [16], allows a client to obtain an
IP address and port that actually reside on the STUN server. The
STUN relay usage, when used by itself, eliminates all of the
limitations of using the binding usage by itself, as described above.
However, it requires a server to act as a relay for application
traffic, which can be expensive to provide, operate, and manage.
For multimedia protocols based on the offer/answer model [22],
including the Session Initiation Protocol (SIP) [11], Interactive
Connectivity Establishment (ICE) uses both the binding usage and
relay usage, and furthermore defines a connectivity check usage to
help determine which transport address to use.
Implementers should be aware of the specific deployment scenarios and
the specific protocol (SIP, etc) being used to determine whether NAT
traversal can be facilitated by STUN and which STUN usages are
required.
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 [20] 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
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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.
Many existing proprietary protocols, such as those for online games
(such as the games described in RFC3027 [21]) and Voice over IP, have
developed tricks that allow them to operate through NATs without
changing those NATs and without relying on ALG behavior in the NATs.
This document takes some of those ideas and codifies them into an
interoperable protocol that can meet the needs of many applications.
The protocol described here, Simple Traversal Underneath NAT (STUN),
provides a toolkit of functions. These functions allow entities
behind a NAT to learn the address bindings allocated by the NAT and
to keep those bindings open. 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 and receives STUN responses.
Clients can also generate STUN indications.
STUN Server: A STUN Server (also just referred to as a server) is an
entity that receives STUN requests and sends STUN responses.
Servers also send STUN indications.
Transport Address: The combination of an IP address and (UDP or TCP)
port.
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Reflexive Transport Address: A transport address learned by a client
that identifies that client as seen by another host on an IP
network, typically a STUN server. When there is an intervening
NAT between the client and the other host, the reflexive transport
address represents the binding allocated to the client on the
public side of the NAT. Reflexive transport addresses are learned
from the mapped address attribute (MAPPED-ADDRESS or XOR-MAPPED-
ADDRESS) in STUN responses.
Mapped Address: The source IP address and port of the STUN Binding
Request packet received by the STUN server and inserted into the
mapped address attribute (MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) of
the Binding Response message.
Long Term Credential: A username and associated password that
represent a shared secret between client and server. Long term
credentials are generally granted to the client when a subscriber
enrolles in a service and persist until the subscriber leaves the
service or explicitly changes the credential.
Long Term Password: The password from a long term credential.
Short Term Credential: A temporary username and associated password
which represent a shared secret between client and server. A
short term credential has an explicit temporal scope, which may be
based on a specific amount of time (such as 5 minutes) or on an
event (such as termination of a SIP dialog). The specific scope
of a short term credential is defined by the application usage. A
short term credential can be obtained from a Shared Secret
request, though other mechanisms are possible.
Short Term Password: The password component of a short term
credential.
5. 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: Typical STUN Server Configuration
The typical STUN configuration is shown in Figure 1. A STUN client
is connected to private network 1. This network connects to private
network 2 through NAT 1. Private network 2 connects to the public
Internet through NAT 2. The STUN server resides on the public
Internet.
STUN is a simple client-server protocol. It supports two types of
transactions. One is a request/response transaction in which client
sends a request to a server, and the server returns a response. The
second are indications that are initiated by the server or the client
and do not elicit a response. There are two types of requests
defined in this specification - Binding Requests and Shared Secret
Requests. There are no indications defined by this specification.
Binding Requests are sent from the client towards the server. When
the Binding Request arrives at the STUN server, it may have passed
through one or more NATs between the STUN client and the STUN server
(in Figure 1, there were two such NATs). As a result, the source
transport 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 transport address into a STUN Binding
Response and sends it back to the source transport address of the
STUN request. Every type of NAT will route that response so that it
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arrives at the STUN client. From this response, the client knows its
transport address allocated by the outermost NAT towards the STUN
server.
STUN provides several mechanisms for authentication and message
integrity. The client and server can share a pre-provisioned shared
secret, which is used for a digest challenge/response authentication
operation. This is known as a long-term credential or long-term
shared secret.
Alternatively, if the shared secret is obtained by some out-of-bands
means and has a lifetime that is temporally scoped, a simple HMAC is
provided, without a challenge operation. This is known as a short
term credential or short term password. Short-term passwords are
useful when there is no long-term relationship with a STUN server and
thus no long-term password is shared between the STUN client and STUN
server. Even if there is a long-term password, the issuance of a
short-term password is useful to prevent dictionary attacks.
STUN itself provides a mechanism for obtaining such short term
credentials, using the Shared Secret Request. Shared Secret requests
are sent over TLS [5] over TCP. Shared Secret Requests ask the
server to return a temporary username and password that can be used
in subsequent STUN requests.
There are many ways in which these basic mechanisms can be used to
accomplish a specific task. As a result, STUN has the notion of a
usage. A usage is a specific use case for the STUN protocol. The
usage will define what the client does with the mapped address it
receives, defines when the client would send Binding requests and
why, and would constrain the set of authentication mechanisms or
attributes that get used in that usage. STUN usages can also define
new attributes and message types, if needed. This specification
defines three STUN usages - binding discovery, NAT keepalives, and
short-term password.
The binding discovery usage is sometimes referred to as 'classic
STUN,' since it is the usage originally envisioned in RFC 3489 [15],
the predecessor to this specification. The purpose of the binding
discovery usage is for the client to obtain a transport address at
which it is reachable. The client can include these transport
addresses in application layer signaling messages such as the Session
Description Protocol (SDP) [19] (present in the body of SIP
messages), where it indicates where the client wants to receive Real
Time Transport Protocol (RTP [17]) traffic. In this usage, the STUN
server is typically located on the public Internet and run by the
service provider offering the application service (such as a SIP
provider), though this need not be the case. The client would
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utilize the STUN request just prior to sending a protocol message
(such as a SIP INVITE request or 200 OK response) that requires the
client to embed its transport address.
In the binding keepalive usage, a client sends an application
protocol message (such as a SIP REGISTER message) to a server. The
server notes the source transport address of the request, and
remembers it. Later on, if it needs to reach the client, it sends a
message to that transport address. However, this message will only
be received by the client if the binding in the NAT is still alive.
Since bindings allocated by NAT expire unless refreshed, the client
must generate keepalive messages toward the server to refresh the
binding. Rather than use expensive application layer messages, a
STUN binding request is sent by the client to the server, and is sent
to the exact same transport address used by the server for the
application protocol. In the case of SIP, this would typically mean
port 5060 or 5061. This has the effect of keeping the bindings in
the NAT alive. The STUN binding responses also inform the client
that the server is still responsive, and also inform the client if
its transport address towards the server have changed (its reflexive
transport address), in which case it may need application layer
protocol messaging to update its transport address as seen by the
server. The binding keepalive usage is used by the SIP outbound
mechanism, for example [18].
These two usages all utilize the same Binding Request message, and
all require the same basic processing on the server. They differ
only in where the server is (a standalone server in the network, or
embedded in an application layer server), when the Binding Request is
used and what the client does with the mapped address that is
returned.
The short-term password usage makes use of the Shared Secret request
and response, and allows a client to obtain a temporary set of
credentials to authenticate itself with the STUN server. The
credentials obtained from this usage can be used in requests for any
other usage.
Some usages (such as the binding keepalive) require STUN messages to
be sent on the same transport address as some application protocol,
such as RTP or SIP. To facilitate the demultiplexing of the two,
STUN defines a special field in the message called the magic cookie,
which is a fixed 32 bit value that identifies STUN traffic. STUN
requests also contain a fingerprint, which is a cryptographic hash of
the message, that allow for validation that the message was a STUN
request and not a data packet that happened to have the same 32 bit
value in the right place in the message.
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STUN servers can be discovered through DNS, though this is not
necessary in all usages. For those usages where it is needed, STUN
makes use of SRV records [3] to facilitate discovery. This discovery
allows for different transport addresses to be found for different
usages.
6. STUN Message Structure
STUN messages are TLV (type-length-value) encoded using big endian
(network ordered) binary. 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 RFC791 [2]. Unless otherwise noted, numeric
constants are in decimal (base 10). All STUN messages start with a
single 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, magic cookie,
transaction ID, and length. The length indicates the total length of
the STUN payload, not including the 20-byte header.
There are two types of transactions in STUN - request/response
transactions, which utilize a request message and a response message,
and indication transactions, which utilizes a single indication
message. Furthermore, responses are broken into two types - success
responses and error responses. Two bits in the message type field of
the STUN header indicate the class of the message - whether the
message is a request, a success response, an indication, or a failure
response. An additional 12 bits in the message type indicate the
method, which is the primary function of the message. This
specification defines two methods, Binding and Shared Secret.
STUN Requests are sent reliably. STUN can run over UDP, TCP or TCP/
TLS. When run over UDP, STUN requests are retransmitted in order to
achieve reliability. The transaction ID is used to correlate
requests and responses.
An indication message can be sent from the client to the server, or
from the server to the client. Indication messages can be sent over
TCP or UDP. STUN itself does not provide reliability for these
messages, though they will be delivered reliably when sent over TCP.
The transaction ID is used to distinguish indication messages.
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All STUN messages consist of a 20 byte header:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0| STUN Message Type | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Magic Cookie |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Transaction ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format of STUN Message Header
The most significant two bits of every STUN message are both zeroes.
This, combined with the magic cookie and the fingerprint attribute,
aid in differentiating STUN packets from other protocols when STUN is
multiplexed with other protocols on the same port.
The message type field is decomposed further into the following
structure:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|M|M|M|M|C|M|M|M|C|M|M|M|M|
|1|1|9|8|7|1|6|5|4|0|3|2|2|0|
|1|0| | | | | | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Format of STUN Message Type Field
M11 through M0 represent a 12-bit encoding of the method. C1 through
C0 represent a 2 bit encoding of the class. A class of 0 is a
Request, a class of 1 is an indication, a class of 2 is a success
response, and a class of 3 is an error response. This specification
defines two methods, Binding and Shared Secret. Their method values
are enumerated in Section 15.
The message length is the size, in bytes, of the message not
including the 20 byte STUN header.
The magic cookie is a fixed value, 0x2112A442. In the previous
version of this specification [15] this field was part of the
transaction ID. This fixed value is used as part of the
identification of a STUN message when STUN is multiplexed with other
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protocols on the same port, as is done for example in [13] and [18].
The magic cookie additionally indicates the STUN client is compliant
with this specification. The magic cookie is present in all STUN
messages -- requests, success responses, error responses and
indications.
The transaction ID is a 96 bit identifier. STUN transactions are
identified by their unique 96-bit transaction ID. For request/
response transactions, the transaction ID is chosen by the STUN
client and MUST be unique for each new STUN transaction generated by
that STUN client. The transaction ID MUST be uniformly and randomly
distributed between 0 and 2**96 - 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. A
reponse to the STUN request, whether it be a success or error
response, carries the same transaction ID as the request.
Indications are also identified by their transaction ID. The
transaction ID there MUST also be uniformly and randomly distributed
between 0 and 2**96 - 1.As with requests, the value is chosen by the
server and MUST be unique for each unique indication generated by the
server. Unless a request or indication is bit-wise identical to a
previous request, and was sent to the same server from the same
transport address, a client MUST choose a new transaction ID for it.
After the STUN header are zero or more attributes. Each attribute is
TLV encoded, with a 16 bit type, 16 bit length, and variable value.
Each STUN attribute ends on a 32 bit boundary:
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 .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Format of STUN Attributes
The Length refers to the length of the actual useful content of the
Value portion of the attribute, measured in bytes. Since STUN aligns
attributes on 32 bit boundaries, attributes whose content is not a
multiple of 4 bytes are padded with 1, 2 or 3 bytes of padding so
that they are a multiple of 4 bytes. Such padding is only needed
with attributes that take freeform strings, such as USERNAME and
PASSWORD. For attributes that contain more structured data, the
attributes are constructed to align on 32 bit boundaries. The value
in the Length field refers to the length of the Value part of the
attribute prior to padding - i.e., the useful content. Consequently,
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when parsing messages, implementations will need to round up the
Length field to the nearest multiple of four in order to find the
start of the next attribute.
The attribute types defined in this specification are in Section 11 .
7. STUN Transactions
STUN defines two types of transactions - request/response
transactions and indication transactions.
7.1. Request/Response Transactions
STUN clients are allowed to pipeline STUN requests. That is, a STUN
client MAY have multiple outstanding STUN requests with different
transaction IDs and not wait for completion of a STUN request/
response exchange before sending another STUN request.
When running STUN over UDP it is possible that the STUN request or
its response might be dropped by the network. Reliability of STUN
request message types is accomplished through client retransmissions.
Clients SHOULD retransmit the request starting with an interval of
RTO, doubling after each retransmission. RTO is an estimate of the
round-trip-time, and is computed as described in RFC 2988 [8], with
two exceptions. First, the initial value for RTO SHOULD be
configurable (rather than the 3s recommended in RFC 2988). In fixed-
line access links, a value of 100ms is RECOMMENDED. Secondly, the
value of RTO MUST NOT be rounded up to the nearest second. Rather, a
1ms accuracy MUST be maintained. As with TCP, the usage of Karn's
algorithm is RECOMMENDED. When applied to STUN, it means that RTT
estimates SHOULD NOT be computed from STUN transactions which result
in the retransmission of a request.
The value for RTO SHOULD be cached by an agent after the completion
of the transaction, and used as the starting value for RTO for the
next transaction to the same host (based on equality of IP address).
The value SHOULD be considered stale and discarded after 10 minutes.
Retransmissions continue until a response is received, or a total of
7 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. A STUN transaction over UDP is also
considered failed if there has been a transport failure of some sort,
such as a fatal ICMP error. For example, assuming an RTO of 100ms,
requests would be sent at times 0ms, 100ms, 300ms, 700ms, 1500ms,
3100ms, and 6300ms. At 7900ms, the agent would consider the
transaction to have timed out if no response has been received.
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When running STUN over TCP, TCP is responsible for ensuring delivery.
The STUN application SHOULD NOT retransmit STUN requests when running
over TCP. If the client has not received a response after 7900ms, it
considers the transaction to have timed out.
Regardless of whether TCP or UDP was used for the transaction, if a
failure occurs and the client has other servers it can reach (as a
consequence of an SRV query which provides a multiplicity of STUN
servers Section 8.1, for example), the client SHOULD create a new
request, which is identical to the previous, but has a different
transaction ID (and consequently a different MESSAGE INTEGRITY and/or
FINGERPRINT attribute).
7.2. Indications
Indications are sent from the client to the server, or from the
server to the client. Though no indications are used by this
specification, they are used by the STUN relay usage [16]. When sent
over UDP, there are no retransmissions, and reliability is not
provided. When sent over TCP, reliability is provided by TCP.
Regardless of whether TCP or UDP was used for the indication, if a
failure occurs (due to a fatal ICMP error or TCP error), and the
client has other servers it can reach (as a consequence of an SRV
query which provides a multiplicity of STUN servers Section 8.1, for
example), the client SHOULD create a new indication, which is
identical to the previous, but has a different transaction ID (and
consequently a different MESSAGE INTEGRITY and/or FINGERPRINT
attribute).
8. Client Behavior
Client behavior can be broken down into several steps. The first is
discovery of the STUN server. The next is obtaining a shared secret.
For request/response transactions, the next steps are formulating the
request and processing the response. For indication transactions,
the next step is formulating the indication.
8.1. Discovery
Unless stated otherwise by a STUN usage, DNS is used to discover the
STUN server following these procedures.
The client will be configured with a domain name of the provider of
the STUN servers. This domain name is resolved to a transport
address using the SRV procedures specified in RFC2782 [3]. The
mechanism for configuring the STUN client with the domain name to
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look up is not in scope of this document.
The DNS SRV service name depends on the application usage. For the
binding usage, the service name is "stun". The protocol can be "udp"
for UDP, "tcp" for TCP and "tls" for TLS over TCP. For the short
term password application usage, the service name is "stun-pass".
The protocol is always "tls" for TLS over TCP. The binding keepalive
usage always starts with a transport address, so no DNS SRV service
names are defined for it. New STUN usages MAY define additional DNS
SRV service names. These SHOULD start with "stun".
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, RFC2782 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 for STUN are described in Section 8.3.3.
A STUN server supporting multiple usages (such as the short term
password and binding discovery usage) MAY use the same ports for
different usages, as long as ports are not needed to differentiate
the usages. Different ports are not needed to differentiate the
usages defined in this specification. Different ports SHOULD be used
for TCP and TCP/TLS, so that the server can determine whether the
first message it will receive after the TCP connection is set up is a
STUN message or a TLS message.
The default port for STUN requests is 3478, for both TCP and UDP.
There is no default port for STUN over TLS. Administrators SHOULD
use this port in their SRV records for UDP and TCP, but MAY use
others. If no SRV records were found, the client performs an A or
AAAA 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
using UDP or TCP, independent of the STUN usage. For usages that
require TLS, such as the short term password usage, lack of SRV
records is equivalent to a failure of the transaction, since the
request or indication MUST NOT be sent unless SRV records provided a
transport address specifically for TLS.
8.2. Obtaining a Shared Secret
As discussed in Section 13, there are several attacks possible on
STUN systems. Many of these attacks are prevented through integrity
protection of requests and responses. To provide that integrity,
STUN makes use of a shared secret between client and server which is
used as the keying material for the MESSAGE-INTEGRITY attribute in
STUN messages. STUN allows for the shared secret to be obtained in
any way. The application usage defines the mechanism and required
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implementation strength for shared secrets.
Some usages assume that out of band protocols are used to obtain the
necessary credentials. Other usages, such as binding keepalives,
don't use authentication, as it is not required. Others, such as the
binding discovery, allows for authentication using either a long term
shared secret or a short term shared secret. The latter can be
obtained by using the short term password usage to obtain a short
term shared secret.
Consequently, the STUN usages define rules for obtaining shared
secrets prior to sending a request.
8.3. Request/Response Transactions
8.3.1. Formulating the Request Message
The client follows the syntax rules defined in Section 6 and the
transmission rules of Section 7. The message class MUST be a
request.
The client creates a STUN message following the STUN message
structure described in Section 6. The client SHOULD add a MESSAGE-
INTEGRITY and USERNAME attribute to the Request message if the usage
employs authentication. The specific credentials to use are
described by the STUN usage, which can specify no credentials, a
short term credential, or a long term credential. The procedures for
each are:
1. If the STUN usage specifies that no credentials are used, the
message is sent without MESSAGE-INTEGRITY
2. If a short term credential is to be used, the STUN Request or
STUN Indication would contain the USERNAME and MESSAGE-INTEGRITY
attributes. The message MUST NOT contain the REALM attribute.
The key for MESSAGE-INTEGRITY is the password.
3. If a long term credential is to be used, the STUN request
contains the USERNAME, REALM, and MESSAGE-INTEGRITY attributes.
The 16-byte key for MESSAGE-INTEGRITY HMAC is formed by taking
the MD5 hash of the result of concatenating the following five
fields: (1) The username, with any quotes and trailing nulls
removed, (2) A single colon, (3) The realm, with any quotes and
trailing nulls removed, (4) A single colon, and (5) The password,
with any trailing nulls removed. For example, if the USERNAME
field were 'user', the REALM field were '"realm"', and the
PASSWORD field were 'pass', then the 16-byte HMAC key would be
the result of performing an MD5 hash on the string 'user:realm:
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pass', or 0x8493fbc53ba582fb4c044c456bdc40eb.
This format for the key was chosen so as to enable a common
authentication database for SIP, which uses digest authentication
as defined in RFC 2617 [7] and STUN, as it is expected that
credentials are usually stored in their hashed forms.
The NONCE is included in the request only if a short or long term
credential is being used, and only if the STUN request is a retry as
a consequence of a previous error response which provided the client
with a NONCE.
For TCP and TLS-over-TCP, the client opens a TCP connection to the
server. The TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite MUST be
supported at a minimum by implementers when TLS is used with STUN.
Implementers MAY also support any other ciphersuite. When it
receives the TLS Certificate message, the client SHOULD verify the
certificate and inspect the site identified by the certificate. If
the certificate is invalid, revoked, or if it does not identify the
appropriate party, the client MUST NOT send the STUN message or
otherwise proceed with the STUN transaction. 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 [4].
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 8.1 as the host portion of
the URI that has been dereferenced. If DNS was not used, the client
MUST be configured with a set of authorized domains whose
certificates will be accepted.
When STUN is being multiplexed on the same transport address as
application data, and there are valid application layer data packets
which could be confused with STUN packets (because, for example, bits
32 through 63 can contain an arbitrary binary value which might be
equal to 0x2112A442), the FINGERPRINT attribute MUST be present.
Otherwise, its inclusion is RECOMMENDED.
Next, the client sends the request. For UDP-based requests,
reliability is accomplished through client retransmissions, following
the procedure in Section 7.1. For TCP (including TLS over TCP),
there are no retransmissions.
For TCP and TLS over TCP, the client MAY send multiple requests on
the connection. When using TCP or TLS over TCP, the client SHOULD
close the connection as soon as it has received the STUN Response, if
it has no plans to send further requests.
Regardless of the transport protocol, a client MAY pipeline requests
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(that is, it can have multiple requests outstanding at the same
time).
8.3.2. Processing Responses
Once the client has received a response to its request that it did
not discard, it MUST discard any further responses for the same
request.
All responses that were not discarded, whether success responses or
error responses, MUST first be authenticated by the client.
Authentication is performed by first comparing the Transaction ID of
the response to an oustanding request. If there is no match, the
client MUST discard the response. Then the client SHOULD check the
response for a MESSAGE-INTEGRITY attribute. If not present, and the
client placed a MESSAGE-INTEGRITY attribute into the associated
request, it MUST discard the response. If MESSAGE-INTEGRITY is
present, the client computes the HMAC over the response as described
in Section 11.4. The key that is used MUST be same as used to
compute the MESSAGE-INTEGRITY attribute in the request. If the
client did not place a MESSAGE-INTEGRITY attribute into the request,
it MUST ignore the MESSAGE-INTEGRITY attribute in the response and
continue processing the response.
If the computed HMAC matches the one from the response, processing
continues.
If the response is an Error Response, the client checks the response
code from the ERROR-CODE attribute of the response. For a 400 (Bad
Request) response code, the client SHOULD display the reason phrase
to the user. For a 420 (Unknown Attribute) response code, the client
SHOULD retry the request, this time omitting any attributes listed in
the UNKNOWN-ATTRIBUTES attribute of the response.
If the client receives a 401 (Unauthorized) response and had not
included a MESSAGE-INTEGRITY attribute in the request, it is an
indication from the server that credentials are required. If the
REALM attribute was present in the response, it is a signal to the
client to use a long term shared secret and retry the request. The
client SHOULD retry the request, using the username and password
associated with the REALM (this username and password are assumed to
be pre-provisioned into the client through some other means). If the
REALM attribute was absent in the response, it is a signal to the
client to use a short term shared secret and retry the request. If
the client doesn't have a short term shared secret, it SHOULD use the
Shared Secret request to obtain one, and then retry the request with
the username and password obtained as a result.
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If the client receives a 401 (Unauthorized) response but had included
a MESSAGE-INTEGRITY attribute in the request, there has been an
unrecoverable error. This shouldn't ever happen, but if it does, the
client SHOULD NOT retry the request.
If the client receives a 432 (Missing Username) response, and the
client had omitted the USERNAME from the request but included a
MESSAGE-INTEGRITY, the client SHOULD retry the request and include
both MESSAGE-INTEGRITY and USERNAME. If the client receives a 432
(Missing Username) but had included both MESSAGE-INTEGRITY and
USERNAME in the request, there has been an unrecoverable error. This
shouldn't ever happen, but if it does, the client SHOULD NOT retry
the request.
If the client receives a 435 (Missing Nonce) response, but had
included a NONCE in the request, an unrecoverable error has occurred
and the client SHOULD NOT retry. However, if it had omitted the
NONCE in the request and received a 435, or it had included the NONCE
but received a 438, it is a request from the server to retry using
the same credential, but with a different nonce. The client SHOULD
retry the request.
If the client receives a 430 (Stale Credentials) response, it means
that the client used a short term credential that has expired. If
the client had submitted the request using a short term credential
obtained from a Shared Secret request, the client SHOULD generate a
new Shared Secret request to obtain a new short term credential and
then retry the request with that credential. Note that the Shared
Secret request may or may not go to the same server which generated
the 430 (Stale Credentials) response; the server that receives the
Shared Secret request is determined by the DNS procedures defined
above. If a 430 (Stale Credentials) response was received and the
client had used a short term credential provided through some other
means, the client SHOULD obtain a new short term credential using
that mechanism. If the client had not used a short term credential
in the request, the 430 (Stale Credentials) error is unrecoverable
and the request SHOULD NOT be retried.
For a 431 (Integrity Check Failure) response code, the client SHOULD
alert the user, and if a short term credential obtained from a Shared
Secret request had been used previously, the client MAY try the
request again after obtaining a new short term username and password.
If the client receives a 433 (Use TLS) response, and the request was
a Shared Secret request which was not sent over TLS, the client
SHOULD retry the request, and MUST send it using TLS. If this
response is received to any other request except for a Shared Secret
request, or if the client had sent the Shared Secret request over
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TLS, it is an unrecoverable error and the client SHOULD NOT retry.
If the client receives a 434 (Missing Realm) response, and had
omitted the REALM in the request, but had included MESSAGE-INTEGRITY,
it is an indication that, though a short-term credential was used for
the request, the server desires the client to use a long term
credential. The client SHOULD retry the request using the username
and password associated with the REALM. If the 434 (Missing Realm)
was received but the request had contained a REALM, and the REALM in
the response differs from the REALM in the request, the client SHOULD
retry using the username and password associated with the REALM in
the response. If the REALMS were equal, this is an unrecoverable
error and the client SHOULD NOT retry.
It the client receives a 436 (Unknown Username) response, it means
that the username it provided in the request is unknown. For usages
where the username was collected from the user, the client SHOULD
alert the user. The client SHOULD NOT retry with the same username.
If the username was obtained using the Shared Secret request, the
client SHOULD obtain a new credential and retry. However, if the
retries are repeatedly rejected with a 436 (Unknown Username), it
SHOULD cease retrying.
For error responses which can contain a NONCE, if the error response
results in a retry, the client MUST include the NONCE in a subsequent
retry. Furthermore, the client SHOULD cache the nonce, and continue
using it in subsequent requests sent to the same server, identified
by transport address.
For a 300 (Try Alternate) response code, the client SHOULD attempt a
new transaction to the server indicated in the ALTERNATE-SERVER
attribute. The client SHOULD reuse its credentials (username and
password) when retrying. This is useful for load balancing requests
across a STUN server cluster, when those requests require some amount
of resources to process. Though this specification allows the 300
(Try Alternate) response to be applied to Binding Requests, it is
generally not useful to do so, since the work of redirecting a
Binding Request is equal to, if not more than, the work of just
processing the Binding Request. Consequently, the 300 (Try
Alternate) response code is targeted for other usages of STUN, such
as the relay usage [16].
For a 500 (Server Error) response code, the client MAY wait several
seconds and then retry the request on the same server. Or, if the
server was learned through DNS SRV records, the client MAY try the
request on the next server in the list. The same username and
password MAY be used. For a 600 (Global Failure) response code,
client MUST NOT retry the request on this server, or if the server
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was learned through DNS, any other server found through the DNS
resolution procedures.
Unknown response codes between 300 and 399 are treated like a 300.
Unknown response codes between 400 and 499 are treated like a 400,
unknown response codes between 500 and 599 are treated like a 500,
and unknown response codes between 600 and 699 are treated like a
600. Any response between 100 and 299 MUST result in the cessation
of request retransmissions, but otherwise is discarded.
Unknown optional attributes in a response (greater than 0x7FFF) MUST
be ignored by the STUN client. Responses containing unknown
mandatory attributions (less than or equal to 0x7FFF) MUST be
discarded and considered immediately as a failed transaction.
For a success response, the client SHOULD cache any nonce present in
the response, and continue using it in subsequent requests sent to
the same server, identified by transport address.
8.3.3. Timeouts
If the STUN transaction times out without receipt of a response, the
client SHOULD consider it a failure and retry the request to the next
server in the list of servers from the DNS SRV response, as specified
in RFC 2782.
8.4. Indication Transactions
This section applies to client and server behavior for sending an
Indication message.
The client or server follows the syntax rules defined in Section 6
and the transmission rules of Section 7. The message class MUST be
an indication.
Indication messages cannot be challenged or rejected. Consequently,
they cannot be authenticated using long term credentials. If a STUN
usage specifies that authentication is needed for an indication
message, it can only be done using a short term credential. In that
case, the client or server MUST add a MESSAGE-INTEGRITY and USERNAME
attribute to the Request message. The key for MESSAGE-INTEGRITY is
the password.
When STUN is being multiplexed on the same transport address as
application data, and there are valid application layer data packets
which could be confused with STUN packets (because, for example, bits
32 through 63 can contain an arbitrary binary value which might be
equal to 0x2112A442), the FINGERPRINT attribute MUST be present.
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Otherwise, its inclusion is RECOMMENDED.
Typically, indication messages are sent to the same transport
address, or over the same TCP connections as a previous request
message. However, a usage can specify that indication messages are
sent based on a DNS query, in which case the discovery procedures in
Section 8.1 are followed, along with the TCP/TLS connection
establishment procedures defined in Section 8.3.1.
Indication message types are not sent reliably, do not elicit a
response from the server, and are not retransmitted.
For TCP and TLS over TCP, the client or server MAY send multiple
indications on the connection. When using TCP or TLS over TCP, the
client SHOULD close the connection as soon as it determines it has no
further messages to send to the server.
By definition, since indications do not generate a response, they can
be pipelined, regardless of the transport protocol.
9. Server Behavior
As with clients, server behavior depends on whether it is a request/
response transaction or indication.
9.1. Request/Response Transactions
9.1.1. Receive Request Message
A STUN server MUST be prepared to receive request messages on the
transport address that will be discovered by the STUN client when the
STUN client follows its discovery procedures described in
Section 8.1. Depending on the usage, the STUN server will listen for
incoming UDP STUN messages, incoming TCP STUN messages, or incoming
TLS exchanges followed by TCP STUN messages.
If the request is a retransmission of a request for which the server
has already generated a response within the last 10 seconds, the
server MUST retransmit the response. A server can do this either by
remembering the response it transmitted, or by re-processing the
request and computing the response. The latter technique can only be
applied to requests which are idempotent and would result in the same
response for the same request. This is the case for the Binding
Request, but not for the Shared Secret Request. Extensions to STUN
SHOULD state whether their request types have this property or not.
When a STUN request is received, the server determines the usage.
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The usages describe how the STUN server makes this determination.
Based on the usage, the server determines whether the request
requires any authentication and message integrity checks. It can
require none, short-term credential based authentication, or long-
term credential authentication.
If authentication is required, the server checks for the presence of
the MESSAGE-INTEGRITY attribute. If not present, the server
generates an error response with an ERROR-CODE attribute and a
response code of 401 (Unauthorized). If the server wishes the client
to use a short term credential, the REALM is omitted from the
response. If the server wishes the client to use a long term
credential, the REALM is included in the response containing a realm
from which the username and password are scoped [7].
If the MESSAGE-INTEGRITY attribute was present, the server checks for
the existence of the USERNAME attribute. If it was not present, the
server MUST generate an error response. The error response MUST
include an ERROR-CODE attribute with a response code of 432 (Missing
Username). If the server is using a long term credential for
authentication, the response MUST include a REALM. If the server is
using a short-term credential, it MUST NOT include a REALM in the
response.
If the server is using long term credentials for authentication, and
the request contained the MESSAGE-INTEGRITY and USERNAME attributes,
the server checks for the existence of the REALM attribute. If the
attribute is not present, the server MUST generate an error response.
That error response MUST include an ERROR-CODE attribute with
response code of 434 (Missing Realm). That error response MUST also
include a REALM attribute.
If the REALM attribute was present and the server is using a long
term credential for authentication, the server checks for the
existence of the NONCE attribute. If the NONCE attribute is not
present, the server MUST generate an error response. That error
response MUST include an ERROR-CODE attribute with a response code of
435 (Missing Nonce). That error response MUST include a REALM
attribute. If the NONCE was absent and the server is authenticating
with short term credentials, the server MAY generate an error
response with an ERROR-CODE attribute with a response code of 435
(Missing Nonce). This response MUST include a NONCE. If the NONCE
was present in the request, but the server has determined it is
stale, the server MUST generate an error response with an ERROR-CODE
attribute with a response code of 438 (Stale Nonce).
If the server is authenticating the request with a short term
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credential, it checks the value of the USERNAME field. If the
USERNAME was previously valid but has expired, the server generates
an error response with an ERROR-CODE attribute with a response code
of 430 (Stale Credentials). If the server is authenticating with
either short or long term credentials, it determines whether the
USERNAME contains a known entity, and in the case of a long-term
credential, known within the realm of the REALM attribute of the
request. If the USERNAME is unknown, the server generates an error
response with an ERROR-CODE attribute with a response code of 436
(Unknown Username). For authentication using long-term credentials,
that error response MUST include a REALM attribute. For
authentication using short-term credentials, it MUST NOT include a
REALM.
At this point, if the server is doing authentication, the request
contains everything needed for that purpose. The server computes the
HMAC over the request as described in Section 11.4. The key depends
on the credential. For short-term credentials, it equals the
password associated with the username. For long term credentials, it
is computed as described in Section 8.3.1.
If the computed HMAC differs from the one from the MESSAGE-INTEGRITY
attribute in the request, the server MUST generate an error response
with an ERROR-CODE attribute with a response code of 431 (Integrity
Check Failure). If long term credentials are being used for
authentication, this response MUST include a REALM attribute. If
short term credentials are being used, it MUST NOT include a REALM.
When an error response is to be generated by the server as a
consequence of authentication problems (error codes 401, 432, 434,
435, 430 and 436, and the REALM is present in the response
(signifying the usage of a long term credential), the server MUST
include a NONCE attribute in the response. The nonce includes a
random value that the server wishes the client to reflect back in a
subsequent request (and therefore include in the message integrity
computation). When the REALM is absent in the response, the server
MAY include a NONCE in the response if it wishes to use nonces along
with short-term shared secrets (with the exception of 435, where
NONCE is mandatory even for short term credentials). However, there
is little reason to do so, since the short-term password is, by
definition, short-term, and thus additional temporal scoping through
the nonce is not needed.
At this point, the request has been authentication checked and
integrity verified.
If the method of the request is unknown to the server, it MUST
generate an error response which includes an ERROR-CORE attribute
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with a 400 response code.
Next, the server MUST check for any mandatory attributes in the
request (values less than or equal to 0x7fff) which it does not
understand. If it encounters any, the server MUST generate an error
response, and it MUST include an ERROR-CODE attribute with a 420
response code. 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.
9.1.2. Constructing the Response
To construct the STUN Response the STUN server follows the message
structure described in Section 6. The message type MUST indicate
either a success response or error response class and MUST indicate
the same method as the request. The server MUST copy the transaction
ID from the request to the response.
The attributes that get added to the response depend on the type of
response. See Figure 5 for a summary.
If the response is a type which can carry either MAPPED-ADDRESS or
XOR-MAPPED-ADDRESS (the Binding Response as defined in this
specification meets that criteria), the server examines the magic
cookie in the STUN header. If it has the value 0x2112A442, it
indicates that the client supports this version of the specification.
The server MUST insert a XOR-MAPPED-ADDRESS into the response,
carrying the exclusive-or of the source transport address and magic
cookie. If the magic cookie did not have this value, it indicates
that the client supports the previous version of this specification.
The server MUST insert a MAPPED-ADDRESS attribute into the response,
carrying the souce transport address from the request. Insertion of
either XOR-MAPPED-ADDRESS or MAPPED-ADDRESS happens regardless of the
transport protocol used for the request.
XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their encoding
of the transport address. The former, as implied by the name,
encodes the transport address by exclusive-or'ing them with the magic
cookie. The latter encodes them directly in binary. RFC 3489
originally specified only MAPPED-ADDRESS. However, deployment
experience found that some NATs rewrite the 32-bit binary payloads
containing the NAT's public IP address, such as STUN's MAPPED-ADDRESS
attribute, in the well-meaning but misguided attempt at providing a
generic ALG function. Such behavior interferes with the operation of
STUN and also causes failure of STUN's message integrity checking.
If the request contained the MESSAGE-INTEGRITY attribute, the server
MUST include a MESSAGE-INTEGRITY attribute in a successful response.
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The MESSAGE-INTEGRITY attribute MUST use the same username and
password used to authenticate the request. If long term credentials
were used, the response MUST include a NONCE. For short term
credentials, a NONCE MAY be included.
The server SHOULD include a SERVER attribute in all responses,
indicating the identity of the server generating the response. This
is useful for diagnostic purposes.
When STUN is being multiplexed on the same transport address as
application data, and there are valid application layer data packets
which could be confused with STUN packets (because, for example, bits
32 through 63 can contain an arbitrary binary value which might be
equal to 0x2112A442), the FINGERPRINT attribute MUST be present in
the response. Otherwise, its inclusion is RECOMMENDED.
In cases where the server cannot handle the request, due to
exhaustion of resources, the server MAY generate a 300 response with
an ALTERNATE-SERVER attribute. This attribute identifies an
alternate server which can service the requests. It is not expected
that 300 responses or this attribute would be used by the methods
defined in this specification.
9.1.3. Sending the Response
All UDP response messages are sent to the transport address the
associated Binding Request came from, and sent from the transport
address the Binding Request was sent to. All TCP or TLS over TCP
responses messages are sent on the TCP connections that the request
arrived on.
9.2. Indication Transactions
Indication messages cause the server to change its state. Indication
message types do not cause the server to send a response message.
A STUN server MUST be prepared to receive indication messages on the
transport address that will be discovered by the STUN client when the
STUN client follows its discovery procedures described in
Section 8.1. Depending on the usage, the STUN server will listen for
incoming UDP STUN messages, incoming TCP STUN messages, or incoming
TLS exchanges followed by TCP STUN messages.
When a STUN indication is received, the server determines the usage.
The usages describe how the STUN server makes this determination.
Based on the usage, the server determines whether the indication
requires any authentication and message integrity checks. It can
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require none or short-term credential based authentication. If
short-term credentials are utilized, the server follows the same
procedures as defined in Section 9.1.1, but if those procedures
require transmission of an error response, the server MUST instead
silently discard the indication.
Once authenticated (if authentication was in use), the processing of
the indication message depends on the method. This specification
doesn't define any indication messages.
10. Demultiplexing of STUN and Application Traffic
In the binding refresh usage, STUN traffic is multiplexed on the same
transport address as application traffic, such as RTP. In order to
apply the processing described in this specification, STUN messages
must first be separated from the application packets. This
disambiguation is done identically for all message types.
First, all STUN messages start with two bits equal to zero. If STUN
is being multiplexed with application traffic where it is known that
the topmost two bits are never zeroes, the presence of these two
zeroes signals STUN traffic.
If this mechanism doesn't suffice, the magic cookie can be used. All
STUN messages have the value 0x2112A442 as the second 32 bit word.
If the application traffic can not have this value as the second 32
bit word, then any packets with this value are STUN packets. Even if
the application packet can have this value (for example, in cases
where the application packets contain random binary data), there is
only a one in 2^32 chance that an application packet will have a
value of 0x2112A442 in its second 32 bit word. If this probability
is deemed sufficiently small for the application at hand (for
example, it is considered adequate for Voice over IP applications),
then any packet with this value in its second 32 bit word is
processed as a STUN packet.
However, a mis-classification of 1 in 2^32 may still be too high for
some usages of STUN. Consequently, STUN messages can contain a
FINGERPRINT attribute. This is a cryptographic hash over the
message, covering everything prior to the attribute. This attribute
is different from MESSAGE-INTEGRITY. The latter uses a keyed HMAC,
and thus requires a shared secret. FINGERPRINT does not use a
password, and can be computed just by examining the STUN message.
Thus, if a packet appears to be a STUN message because it has a value
of 0x2112A442 in its second 32 bit word, a client or server then
assumes the message is a STUN message, and computes the value for the
fingerprint. It then looks for the FINGERPRINT attribute in the
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message, and if the value equals the computed value, the message is
considered to be a STUN message. If not, it is considered to be an
application packet.
11. STUN Attributes
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 successfully process the message unless
it understands the attribute.
The values of the message attributes are enumerated in Section 15.
The following figure 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 - means that the
attribute is not applicable to that message type.
Error
Attribute Request Response Response Indication
_______________________________________________________
MAPPED-ADDRESS - C - -
USERNAME C - - O
PASSWORD - C - -
MESSAGE-INTEGRITY O C C O
ERROR-CODE - - M -
ALTERNATE-SERVER - - C -
REALM C - C -
NONCE C - C -
UNKNOWN-ATTRIBUTES - - C -
XOR-MAPPED-ADDRESS - C - -
SERVER - O O O
REFRESH-INTERVAL - O - -
FINGERPRINT O O O O
Figure 5: Mandatory Attributes and Message Types
11.1. MAPPED-ADDRESS
The MAPPED-ADDRESS attribute indicates the mapped transport address.
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
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address family is IPv4, the address is 32 bits, in network byte
order. If the address family is IPv6, the address is 128 bits in
network byte order.
The format of the MAPPED-ADDRESS attribute 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 | Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Format of MAPPED-ADDRESS attribute
The address family can take on the following values:
0x01:IPv4
0x02:IPv6
The port is a network byte ordered representation of the port the
request arrived from.
The first 8 bits of the MAPPED-ADDRESS are ignored for the purposes
of aligning parameters on natural 32 bit boundaries.
It is possible for an IPv4 host to receive a MAPPED-ADDRESS
containing an IPv6 address, or for an IPv6 host to receive a MAPPED-
ADDRESS containing an IPv4 address. Clients MUST be prepared for
this case.
11.2. USERNAME
The USERNAME attribute is used for message integrity. It identifies
the shared secret used in the message integrity check. Consequently,
the USERNAME MUST be included in any request that contains the
MESSAGE-INTEGRITY attribute.
The USERNAME is also always present in a Shared Secret Response,
along with the PASSWORD, which informs a client of a short term
password.
The value of USERNAME is a variable length opaque value. Note that,
as described above, if the USERNAME is not a multiple of four bytes
it is padded for encoding into the STUN message, in which case the
attribute length represents the length of the USERNAME prior to
padding.
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11.3. PASSWORD
If the message type is Shared Secret Response it MUST include the
PASSWORD attribute.
The value of PASSWORD is a variable length opaque value. The
password returned in the Shared Secret Response is used as the HMAC
key in the MESSAGE-INTEGRITY attribute of a subsequent STUN
transaction. Note that, as described above, if the USERNAME is not a
multiple of four bytes it is padded for encoding into the STUN
message, in which case the attribute length represents the length of
the USERNAME prior to padding.
11.4. MESSAGE-INTEGRITY
The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [10] of the
STUN message. The MESSAGE-INTEGRITY attribute can be present in any
STUN message type. 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 is either the last attribute, or the next to last attribute
in any STUN message (depending on whether FINGERPRINT is present).
With the exception of the FINGERPRINT attribute, which appears after
MESSAGE-INTEGRITY, elements MUST ignore all other attributes that
follow MESSAGE-INTEGRITY.
The key used as input to HMAC depends on the STUN usage and the
shared secret mechanism.
11.5. FINGERPRINT
The FINGERPRINT attribute can be present in all STUN messages. It is
computed as the CRC-32 of the STUN message up to (but excluding) the
FINGERPRINT attribute itself, xor-d with the 32 bit value 0x5354554e
(the XOR helps in cases where an application packet is also using
CRC-32 in it). The 32 bit CRC is the one defined in ITU V.42 [9],
which has a generator polynomial of x32+x26+x23+x22+x16+x12+x11+x10+
x8+x7+x5+x4+x2+x+1. When present, the FINGERPRINT attribute MUST be
the last attribute in the message.
11.6. 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 [11] and
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HTTP [12]. The reason phrase is meant for user consumption, and can
be anything appropriate for the response code. 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.
If the reason phrase has a length that is not a multiple of four
bytes, it is padded for encoding into the STUN message, in which case
the attribute length represents the length of the entire ERROR-CODE
attribute (including the reason phrase) prior to padding.
The following response codes, along with their recommended reason
phrases (in brackets) are defined at this time:
300 (Try Alternate): The client should contact an alternate server
for this request.
400 (Bad Request): The request was malformed. The client should not
retry the request without modification from the previous
attempt.
401 (Unauthorized): The 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 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 request contained a MESSAGE-
INTEGRITY attribute, but the HMAC failed verification. This
could be a sign of a potential attack, or client implementation
error.
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432 (Missing Username): The request contained a MESSAGE-INTEGRITY
attribute, but not a USERNAME attribute. Both USERNAME and
MESSAGE-INTEGRITY must be present for integrity checks.
433 (Use TLS): The Shared Secret request has to be sent over TLS,
but was not received over TLS.
434 (Missing Realm): The REALM attribute was not present in the
request.
435 (Missing Nonce): The NONCE attribute was not present in the
request.
436 (Unknown Username): The USERNAME supplied in the request is not
known or is not known to the server.
438 (Stale Nonce): The NONCE attribute was present in the request
but wasn't valid.
500 (Server Error): The server has suffered a temporary error. The
client should try again.
600 (Global Failure): The server is refusing to fulfill the request.
The client should not retry.
11.7. REALM
The REALM attribute is present in requests and responses. It
contains text which meets the grammar for "realm" as described in RFC
3261 [11], and will thus contain a quoted string (including the
quotes).
Presence of the REALM attribute in a request indicates that long-term
credentials are being used for authentication. Presence in certain
error responses indicates that the server wishes the client to use a
long-term credential for authentication.
11.8. NONCE
The NONCE attribute is present in requests and in error responses.
It contains a sequence of qdtext or quoted-pair, which are defined in
RFC 3261 [11]. See RFC 2617 [7] for guidance on selection of nonce
values in a server.
11.9. UNKNOWN-ATTRIBUTES
The UNKNOWN-ATTRIBUTES attribute is present only in an error response
when the response code in the ERROR-CODE attribute is 420.
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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 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Format of UNKNOWN-ATTRIBUTES attribute
11.10. XOR-MAPPED-ADDRESS
The XOR-MAPPED-ADDRESS attribute is present in responses. It
provides the same information that would present in the MAPPED-
ADDRESS attribute but because the NAT's public IP address is
obfuscated through the XOR function, STUN messages are able to pass
through NATs which would otherwise interfere with STUN.
This attribute MUST always be present in a Binding Response and may
be used in other responses as well. Usages defining new requests and
responses should specify if XOR-MAPPED-ADDRESS is applicable to their
responses.
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)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Format of XOR-MAPPED-ADDRESS Attribute
The Family represents the IP address family, and is encoded
identically to the Family in MAPPED-ADDRESS.
X-Port is the mapped port, exclusive or'd with most significant 16
bits of the magic cookie. If the IP address family is IPv4,
X-Address is the mapped IP address exclusive or'd with the magic
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cookie. If the IP address family is IPv6, the X-Address is the
mapped IP address exclusively or'ed with the magic cookie and the 96-
bit transaction ID.
For example, using the "^" character to indicate exclusive or, if the
IP address is 192.168.1.1 (0xc0a80101) and the port is 5555 (0x15B3),
the X-Port would be 0x15B3 ^ 0x2112 = 0x34A1, and the X-Address would
be 0xc0a80101 ^ 0x2112A442 = 0xe1baa543.
It is possible for an IPv4 host to receive a XOR-MAPPED-ADDRESS
containing an IPv6 address, or for an IPv6 host to receive a XOR-
MAPPED-ADDRESS containing an IPv4 address. Clients MUST be prepared
for this case.
11.11. SERVER
The server attribute contains a textual description of the software
being used by the server, including manufacturer and version number.
The attribute has no impact on operation of the protocol, and serves
only as a tool for diagnostic and debugging purposes. The value of
SERVER is variable length. If the value of SERVER is not a multiple
of four bytes, it is padded for encoding into the STUN message, in
which case the attribute length represents the length of the USERNAME
prior to padding.
11.12. ALTERNATE-SERVER
The alternate server represents an alternate transport address for a
different STUN server to try. It is encoded in the same way as
MAPPED-ADDRESS.
This attribute MUST only appear in an error response.
11.13. REFRESH-INTERVAL
The REFRESH-INTERVAL indicates the number of milliseconds that the
server suggests the client should use between refreshes of the NAT
bindings between the client and server. Even though the server may
not know the binding lifetimes in intervening NATs, this attribute
serves as a useful configuration mechanism for suggesting a value for
use by the client. Furthermore, when the NAT Keepalive usage is
being used, the server may become overloaded with Binding Requests
that are being used for keepalives. The REFRESH-INTERVAL provies a
mechanism for the server to gradually reduce the load on itself by
pushing back on the client.
REFRESH-INTERVAL is specified as an unsigned 32 bit integer, and
represents an interval measured in millseconds. It can be present in
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Binding Responses.
12. STUN Usages
STUN is a simple request/response protocol that provides a useful
capability in several situations. In this section, different usages
of STUN are described. Each usage may differ in how STUN servers are
discovered, when the STUN requests are sent, what message types are
used, what message attributes are used, and how authentication is
performed.
This specification defines the STUN usages for binding discovery
(Section 12.1), NAT keepalives (Section 12.2) and short-term password
(Section 12.3).
New STUN usages may be defined by other standards-track documents.
New STUN usages MUST describe their applicability, client discovery
of the STUN server, how the server determines the usage, new message
types (requests or indications), new message attributes, new error
response codes, and new client and server procedures.
12.1. Binding Discovery
The previous version of this specification, RFC3489 [15], described
only this binding discovery usage.
12.1.1. Applicability
Binding discovery is used to learn reflexive addresses from servers
on the network, generally the public Internet. That is, it is used
by a client to determine its dynamically-bound 'public' UDP transport
address that is assigned by a NAT between a STUN client and a STUN
server. This transport address will be present in the mapped address
of the STUN Binding Response.
The mapped address present in the binding response can be used by
clients to facilitate traversal of NATs for many applications. NAT
traversal is problematic for applications that require a client to
insert a transport address into a message, to which subsequent
messages will be delivered by other entities in a network. Normally,
the client would insert the transport address 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 transport address information in several places, including
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the Session Description Protocol (SDP [19]) body carried by SIP. The
transport address 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 transport address. Instead of directly using a port allocated
from a local interface, the client allocates a port from the local
interface, and from that port, generates a STUN Binding Request. The
mapped address in the Binding Response (XOR-MAPPED-ADDRESS or MAPPED-
ADDRESS) provides the client with an alternative transport address
that it can then include in the protocol payload. This transport
address may be within a different address family than the local
interfaces used by the client. This is not an error condition. In
such a case, the client would use the learned IP address and port as
if the client was a host with an interface within that address
family.
In the case of SIP, to populate the SDP appropriately, a client would
generate two STUN Binding Request messages at the time a call is
initiated or answered. One is used to obtain the transport address
for RTP, and the other, for the Real Time Control Protocol
(RTCP)[17]. The client might also need to use STUN to obtain
transport addresses 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 transport 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 transport addresses in the PDU.
One of those can be the transport address determined from STUN, and
the others can include transport addresses 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 [13]).
12.1.2. Client Discovery of Server
Clients SHOULD be configured with a domain name for a STUN server to
use. In cases where the client has no explicit configuration
mechanism for STUN, but knows the domain of its service provider, the
client SHOULD use that domain (in the case of SIP, this would be the
domain from their Address-of-Record). The discovery mechanisms
defined in Section 8.1 are then applied to that domain name.
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12.1.3. Server Determination of Usage
It is anticipated that servers would advertise a specific port in the
DNS for the Binding Discovery usage. Thus, when a request arrives at
that particular port, the server knows that the binding usage is in
use. This fact is only needed for purposes of determining the
authentication and message integrity mechanism to apply.
12.1.4. New Requests or Indications
This usage does not define any new message types.
12.1.5. New Attributes
This usage does not define any new message attributes.
12.1.6. New Error Response Codes
This usage does not define any new error response codes.
12.1.7. Client Procedures
The binding discovery is utilized by a client just prior to
generating an application PDU that requires the client to include its
transport address. The client MAY first obtain a short term
credential using the short term password STUN usage. The credential
that is obtained is then using in Binding Request messages. A
Binding Request message is generated for each distinct transport
address that the client requires to formulate the application PDU.
A successful response message will carry either an XOR-MAPPED-ADDRESS
or MAPPED-ADDRESS attribute, depending on the version of the server.
A client SHOULD use the XOR-MAPPED-ADDRESS if present. If not, it
uses the MAPPED-ADDRESS.
12.1.8. Server Procedures
It is RECOMMENDED that servers utilize short term credentials,
obtained by the client from a Shared Secret request, for
authentication and message integrity. Consequently, if a Binding
Request is generated without a short term credential, the server
SHOULD challenge for one.
12.1.9. Security Considerations for Binding Discovery
There are no security considerations for this usage beyond those
described in Section 13.
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12.2. NAT Keepalives
12.2.1. Applicability
In this STUN usage, a client is connected to a server for a
particular application protocol (for example, a SIP proxy server).
The connection is long-lived, allowing for asynchronous messaging
from the server to the client. The client is connected to the server
either using TCP, in which case there is a long-lived TCP connection
from the client to the server, or using UDP, in which case the server
stores the source transport address of a message from a client (such
as SIP REGISTER), and sends messages to the client using that
transport address.
Since the connection between the client and server is very-long
lived, the bindings established by that connection need to be
maintained in any intervening NATs. Rather than implement expensive
application-layer keepalives, the keepalives can be accomplished
using STUN Binding Requests. The client will periodically send a
Binding Request to the server, using the same transport addresses
used for the application protocol. These Binding Requests are
demultiplexed at the server using the magic cookie and possibly
FINGERPRINT. The response from the server informs the client that
the server is still alive. The STUN message also keeps the binding
active in intervening NATs. The client can also examine the mapped
address in the Binding Response. If it has changed, the client can
re-initiate application layer procedures to inform the server of its
new transport address.
12.2.2. Client Discovery of Server
In this usage, the STUN server and the application protocol are using
the same fixed port.
12.2.3. Server Determination of Usage
The server multiplexes both STUN and its application protocol on the
same port. The server knows it is has this usage because the URI
that gets resolved to this port indicates the server supports this
multiplexing.
12.2.4. New Requests or Indications
This usage does not define any new message types.
12.2.5. New Attributes
This usage does not define any new message attributes.
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12.2.6. New Error Response Codes
This usage does not define any new error response codes.
12.2.7. Client Procedures
If the STUN Response indicates the client's mapped address has
changed from the client's expected mapped address, the client SHOULD
inform other applications of its new mapped address. For example, a
SIP client could use the binding discovery usage to obtain a new
mapped address, and then register it using SIP registration
procedures.
The client SHOULD NOT include a MESSAGE-INTEGRITY attribute unless
prompted for one by the server, since authentication is not generally
used with this STUN usage.
12.2.8. Server Procedures
The server SHOULD NOT authenticate the client or look for a MESSAGE-
INTEGRITY attribute. Since the keepalives come with some regularity,
and will come for each client that is connected to the server, the
processing cost associated with authenticating each request is very
high. Consequently, authentication should only be used by small
servers, for whom the processing cost is not an issue, or when used
with application protocols where the consequences of a fake response
are very significant.
12.2.9. Security Considerations for NAT Keepalives
This STUN usage does not recommend the usage of message integrity or
authentication. This is because the client never actually uses the
mapped address from the STUN response. It merely treats a change in
that address as a hint that the client should re-apply application
layer procedures for connection establishment and registration.
An attacker could attempt to inject faked responses, or modify
responses in transit. Such an attack would require the attacker to
be on-path in order to determine the transaction ID. In the worst
case, the attack would cause the client to see a change in IP address
or port, and then perform an application layer re-registration. Such
a re-registration would not use the transport address obtained from
the Binding Response. Thus, the worst that the attacker can do is
cause the client to re-register every half minute or so, when it
otherwise wouldn't need to. Given the difficulty in launching this
attack (it requires the attacker to be on-path and to disrupt the
actual response from the server) compared to the benefit, there is
little motivation for authentication or integrity mechanisms.
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When used with application protocols where the cost of "re-
registration" is in fact high, the keepalive usage can still be used
without authentication. However, the usage would serve ONLY to keep
NAT bindings alive; it would not be useful for detecting failures of
the server or of intervening NAT. In such a case, the client would
not perform any application layer processing based on the STUN
response, even if it indicated a change in transport address.
12.3. Short-Term Password
In order to ensure interoperability, this usage describes a TLS-based
mechanism to obtain a short-term credential. The usage makes use of
the Shared Secret Request and Response messages. It is defined as a
separate usage in order to allow it to run on a separate port, and to
allow it to be more easily separated from the different STUN usages,
only some of which require this mechanism.
12.3.1. Applicability
To thwart some on-path attacks described in Section 13, it is
necessary for the STUN client and STUN server to integrity protect
the information they exchange over UDP. In the absence of a long-
term secret (password) that is shared between them, a short-term
password can be obtained using the usage described in this section.
The username and password returned in the STUN Shared Secret Response
are valid for use in subsequent STUN transactions for nine (9)
minutes with any applicable hosts as described in Section 12.3.2.
The username and password obtained with this usage are used as the
USERNAME and in the HMAC for the MESSAGE-INTEGRITY in a subsequent
STUN message, respectively.
12.3.2. Client Discovery of Server
The client follows the procedures in Section 8.1. The SRV protocol
is "tls" and the service name "stun-pass".
For example a client would look up "_stun-pass._tls.example.com" in
DNS.
12.3.3. Server Determination of Usage
The server advertises this port in the DNS as capable of receiving
TLS over TCP connections, along with the Shared Secret messages that
run over it. The server MAY also advertise this same port in DNS for
other TLS over TCP usages if the server is capable of multiplexing
those different usages. For example, the server could advertise the
short-term password and binding discovery usages on the same TLS/TCP
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port.
12.3.4. New Requests or Indications
The message type Shared Secret Request and its associated Shared
Secret Response and Shared Secret Error Response are defined in this
section. Their values are enumerated in Section 15.
The following figure indicates which attributes are present in the
Shared Secret Request, Response, and Error Response. 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 - means that the attribute is not applicable to that
message type. Attributes not listed are not applicable to Shared
Secret Request, Response, or Error Response.
Shared Shared Shared
Secret Secret Secret
Attribute Request Response Error
Response
_________________________________________________
USERNAME O M -
PASSWORD - M -
MESSAGE-INTEGRITY O O O
ERROR-CODE - - M
ALTERNATE-SERVER - - C
UNKNOWN-ATTRIBUTES - - C
SERVER - O O
REALM C - C
NONCE C - C
The Shared Secret requests, like other STUN requests, can be
authenticated. However, since its purpose is to obtain a short-term
credential, the Shared Secret request itself cannot be authenticated
with a short-term credential. However, it can be authenticated with
a long-term credential.
12.3.5. New Attributes
No new attributes are defined by this usage.
12.3.6. New Error Response Codes
This usage defines the 433 error response. Only the MESSAGE-
INTEGRITY, ERROR-CODE and SERVER attributes are applicable to this
response.
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12.3.7. Client Procedures
Shared Secret requests are formed like other STUN requests, with the
following additions. Clients MUST NOT use a short-term credential
with a Shared Secret request. They SHOULD send the request with no
credentials (omitting MESSAGE-INTEGRITY and USERNAME).
Processing of the Shared Secret response follows that of any other
STUN response. Note that clients MUST be prepared to be challenged
for a long-term credential.
If the response was a Shared Secret Response, it will contain a short
lived username and password, encoded in the USERNAME and PASSWORD
attributes, respectively. A client SHOULD use these credentials
whenever short term credentials are needed for any server discovered
using the same domain name as was used to discover the one which
returned those credentials. For example, if a client used a domain
name of example.com, it would have looked up _stun-
pass._tls.example.com in DNS, found a server, and sent a Shared
Secret request that provided a credential to the client. The client
would use this credential with a server discovered by looking up
_stun._udp.example.com in the DNS.
If the response was a Shared Secret Error Response, and ERROR-CODE
attribute was present with a response code of 433, and the client had
not sent the request over TLS, the client SHOULD establish a TLS
connection to the server and retry the request over that connection.
If the client had used TLS, this error response is unrecoverable and
the client SHOULD NOT retry.
12.3.8. Server Procedures
The procedures for general processing of STUN requests apply to
Shared Secret requests. Servers MAY challenge the client for a long-
term credential if one was not provided in a request. However, they
MUST NOT challenge the request for a short-term credential.
If the Shared Secret Request did not arrive over a TLS connection,
the server MUST generate a Shared Secret Error response with an
ERROR-CODE attribute that has a response code of 433.
If the request is valid and authenticated (assuming the server is
performing authentication), the server MUST create a short term
credential for the user. This credential consists of a username and
password. The credentials MUST be valid for a duration of at least
nine minutes, and SHOULD NOT be valid for a duration of longer than
thirty minutes. The username MUST be distinct, with extremely high
probabilities, from all usernames that have been handed out across
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all servers that are returned from DNS SRV queries for the same
domain name. Extremely high probability means that the likelihood of
collision SHOULD be better than 1 in 2**64. The password for each
username MUST be cryptographically random with at least 128 bits of
entropy.
12.3.9. Security Considerations for Short-Term Password
The security considerations in Section 13 do not apply to the Shared
Secret request and response, since these messages do not make use of
mapped addresses, which is the primary source of security
consideration discussed there. Rather, shared secret requests are
used to obtain short term credentials that are used in the
authentication of other messages.
Because the Shared Secret response itself carries a credential, in
the form of a username and password, it must be sent encrypted. For
this reason, STUN servers MUST reject any Shared Secret request that
has not arrived over a TLS connection.
Malicious clients could generate a multiplicity of Shared Secret
requests, each of which causes the server to allocate shared secrets,
each of which might consume memory and processing resources. If
shared secret requests are not being authenticated, this leads to a
possible denial-of-service attack. Indeed, even if the requestor is
authenticated, attacks are still possible.
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, servers SHOULD allocate only a small number of shared
secrets to a host with a particular source transport address.
Requests from the same transport address which exceed this limit
SHOULD be rejected with a 600 response. Servers SHOULD also limit
the total number of shared secrets they will provide at a time across
all clients, based on the number of users and expected loads during
normal peak usage. If a Shared Secret request arrives and the server
has exceeded its limit, it SHOULD reject the request with a 500
response.
Furthermore, for servers that are not authenticating shared secret
requests, it is RECOMMENDED that short-term credentials be
constructed in a way such that they do not require memory or disk to
store.
This can be done by intelligently computing the username and
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password. One approach is to construct the USERNAME as:
USERNAME = <prefix,rounded-time,hmac>
Where prefix is some random text string (different for each shared
secret request), rounded-time is the current time modulo 20 minutes,
and hmac is an HMAC [13] over the prefix and rounded-time, using a
server private key.
The password is then computed as:
password = <hmac(USERNAME,anotherprivatekey)>
With this structure the server can verify that the username was not
tampered with using the hmac present in the username.
13. Security Considerations
Attacks on STUN systems vary depending on the usage. The short term
password usage is quite different from the other usages defined here,
and its security considerations are unique to it and discussed as
part of the usage definition. However, all of the other usages are
very similar and share a similar set of security considerations as a
consequence of their usage of the mapped address from STUN Binding
Responses. Consequently, these security considerations apply to
usage of the mapped address.
13.1. 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. The attacks of greater interest
are those in which the STUN server and client are used to launch
denial of service (DoS) attacks against other entities, including the
client itself. Many of the attacks require the attacker to generate
a response to a legitimate STUN request, in order to provide the
client with a faked mapped address. The attacks that can be launched
using such a technique include:
13.1.1. 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
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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.
13.1.2. Attack II: Silencing a Client
In this attack, the attacker seeks to deny a client access to
services enabled by STUN (for example, a client using STUN to enable
SIP-based multimedia traffic). To do that, the attacker provides
that client with a faked mapped address. The mapped address it
provides is a transport 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.
13.1.3. 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.
13.1.4. 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.
13.2. 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 man in the middle for such attacks). This is because
STUN requests contain a transaction identifier, selected by the
client, which is random with 96 bits of entropy. The server echoes
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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 the transaction ID of the request. The
large amount of randomness, combined with the need to know when the
client sends a request and the transport addresses used for that
request, precludes attacks that involve guessing the transaction ID.
Since all of the above attacks rely on this one primitive - injecting
a response with a faked mapped address - preventing the attacks is
accomplished by preventing this one operation. To prevent it, we
need to consider the various ways in which it can be accomplished.
There are several:
13.2.1. 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.
13.2.2. 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. Clearly, this attack is only applicable for usages which
discover servers through DNS.
13.2.3. 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 transport 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
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the responses back to the source transport address of the request.
With a modified source transport address, the only way they can reach
the client is if the compromised router directs them there.
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
transport 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.
13.2.4. Approach IV: Man in the Middle
As an alternative to approach III (Section 13.2.3), 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 Approach III
(Section 13.2.3).
13.2.5. 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
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client, and the client uses the first response, and ignores the
second (even though it's different).
13.2.6. Approach VI: Duplication
This approach is similar to approach V (Section 13.2.5). The
attacker listens on the network for a STUN request. When it sees
one, 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, this approach is subject to the same limitations
documented in Approach III (Section 13.2.3), which limit the range of
mapped addresses the attacker can cause the STUN server to generate.
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13.3. Countermeasures
STUN provides mechanisms to counter the approaches described above,
and additional, non-STUN techniques can be used as well.
First off, it is RECOMMENDED that networks with STUN clients
implement ingress source filtering [6]. This is particularly
important for the NATs themselves. As Section 13.2.3 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 who purchase NATs to ensure
that this capability is present and enabled.
Secondly, for usages where the STUN server is not co-located with
some kind of application (such as the binding discovery usage), 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 13.2.1).
Thirdly, to prevent the DNS attack of Section 13.2.2, Section 8.2
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 [23].
The above three techniques are non-STUN mechanisms. STUN itself
provides several countermeasures.
Approaches IV (Section 13.2.4), when generating the response locally,
and V (Section 13.2.5) require an attacker to generate a faked
response. A faked response must match the 96-bit transaction ID of
the request. The attack is further prevented by using the message
integrity mechanism provided in STUN, described in Section 11.4.
Approaches III (Section 13.2.3), IV (Section 13.2.4), when using the
relaying technique, and VI (Section 13.2.6), however, are not
preventable through server signatures. These three approaches are
functional when the attacker modifies nothing but the source address
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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.
13.4. Residual Threats
None of the countermeasures listed above can prevent the attacks
described in Section 13.2.3 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 13.1.1. Note that any host which exists in the
correct topological relationship can be DDOSed. It need not be using
STUN.
14. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing"
(UNSAF), 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 (RFC3424 [24]).
STUN is an example of a protocol that performs this type of function
for the binding discovery usage. The IAB has mandated that any
protocols developed for this purpose document a specific set of
considerations. This section meets those requirements for the
binding discovery usage.
14.1. Problem Definition
From RFC3424 [24], any UNSAF proposal must provide:
Precise definition of a specific, limited-scope problem that is to
be solved with the UNSAF proposal. A short term fix should not be
generalized to solve other problems; this is why "short term fixes
usually aren't".
The specific problem being solved by STUN is to provide the
functionality necessary to describe how to connect two endpoints
regardless of the location of type of NATs in the topology.
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14.2. Exit Strategy
From RFC3424 [24], 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
[13]), that 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.
14.3. Brittleness Introduced by STUN
From RFC3424 [24], any UNSAF proposal must provide:
Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.
STUN introduces brittleness into the system in several ways:
o Transport addresses discovered by STUN in the Binding Discovery
usage will only be useful for receiving packets from a peer if the
NAT does not have address or address and port dependent mapping
properties. When this usage is used in isolation, this makes STUN
brittle, since its effectiveness depends on the type of NAT. This
brittleness is eliminated when the Binding Discovery usage is used
in concert with mechanisms which can verify the transport address
and use others if it doesn't work. ICE is an example of such a
mechanism.
o Transport addresses discovered by STUN in the Binding Discovery
usage will only be useful for receiving packets from a peer if the
STUN server subtends the address realm of the peer. 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
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both - in this case, the public Internet. When this usage is used
in isolation, this makes STUN brittle, since its effectiveness
depends on the topological placement of the STUN server. This
brittleness is eliminated when the Binding Discovery usage is used
in concert with mechanisms which can verify the transport address
and use others if it doesn't work. ICE is an example of such a
mechanism.
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 in the Binding Discovery usage 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 may result in an
increase in latency for applications.
o Transport addresses discovered by STUN in the Binding Discovery
usage will only be useful for receiving packets from a peer behind
the same NAT if the STUN server supports hairpinning [14]. When
this usage is used in isolation, this makes STUN brittle, since
its effectiveness depends on the topological placement of the STUN
server. This brittleness is eliminated when the Binding Discovery
usage is used in concert with mechanisms which can verify the
transport address and use others if it doesn't work. ICE is an
example of such a mechanism.
o Most significantly, STUN introduces potential security threats
which cannot be eliminated through cryptographic means. These
security problems are described fully in Section 13.
14.4. Requirements for a Long Term Solution
From RFC3424 [24], any UNSAF proposal must provide:
Identify requirements for longer term, sound technical solutions
-- contribute to the process of finding the right longer term
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solution.
Our experience with STUN has led to the following requirements for a
long term solution to the NAT problem:
o 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, or knowing what parameters the NAT will
use.
o 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.
o 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.
o Simplicity is Paramount. The control protocol will need to be
implemented in very simple clients. The servers will need to
support extremely high loads. The protocol will need to be
extremely robust, being the precursor to a host of application
protocols. As such, simplicity is key.
14.5. Issues with Existing NAPT Boxes
From RFC3424 [24], any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports.
Originally, RFC 3489 was developed as a standalone solution for NAT
traversal for several types of applications, including VoIP.
However, practical experience found that the limitations of its usage
in isolation made it impractical as a complete solution. There were
too many NATs which didn't support hairpinning or which had address
and port dependent mapping properties.
Consequently, STUN was revised to produce this specification, which
turns STUN into a tool that is used as part of a broader solution.
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For multimedia communications protocols, this broader solution is
ICE. ICE uses the binding discovery usage and defines its own
connectivity check usage, and then utilizes them together. When done
this way, ICE eliminates almost all of the brittleness and issues
found with RFC 3489 alone.
15. IANA Considerations
IANA is hereby requested to create two new registries - STUN methods
and STUN Attributes. IANA must assign the following values to both
registries before publication of this document as an RFC. New values
for both STUN methods and STUN attributes are assigned through the
IETF consensus process via RFCs approved by the IESG [25].
15.1. STUN Methods Registry
The initial STUN methods are:
0x001:Binding
0x002:Shared Secret
15.2. STUN Attribute Registry
STUN attributes values above 0x7FFF are considered optional
attributes; attributes equal to 0x7FFF or below are considered
mandatory attributes. The STUN client and STUN server process
optional and mandatory attributes differently. IANA should assign
values based on the RFC consensus process.
The initial STUN Attributes are:
0x0001: MAPPED-ADDRESS
0x0006: USERNAME
0x0007: PASSWORD
0x0008: MESSAGE-INTEGRITY
0x0009: ERROR-CODE
0x000A: UNKNOWN-ATTRIBUTES
0x0014: REALM
0x0015: NONCE
0x0020: XOR-MAPPED-ADDRESS
0x8023: FINGERPRINT
0x8022: SERVER
0x8023: ALTERNATE-SERVER
0x8024: REFRESH-INTERVAL
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16. Changes Since RFC 3489
This specification updates RFC3489 [15]. This specification differs
from RFC3489 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. Removed the RESPONSE-ADDRESS, CHANGED-ADDRESS,
CHANGE-REQUEST, SOURCE-ADDRESS, and REFLECTED-FROM attributes.
o Added a fixed 32-bit magic cookie and reduced length of
transaction ID by 32 bits. The magic cookie begins at the same
offset as the original transaction ID.
o Added the XOR-MAPPED-ADDRESS attribute, which is included in
Binding Responses if the magic cookie is present in the request.
Otherwise the RFC3489 behavior is retained (that is, Binding
Response includes MAPPED-ADDRESS). See discussion in XOR-MAPPED-
ADDRESS regarding this change.
o Introduced formal structure into the Message Type header field,
with an explicit pair of bits for indication of request, response,
error response or indication. Consequently, the message type
field is split into the class (one of the previous four) and
method.
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 long-term credential-based authentication.
o Added the SERVER, REALM, NONCE, and ALTERNATE-SERVER attributes.
o Removed recommendation to continue listening for STUN Responses
for 10 seconds in an attempt to recognize an attack.
o Introduced the concept of STUN usages and defined three usages -
Binding Discovery, NAT Keepalive, and Short term password.
o Changed transaction timers to be more TCP friendly.
o Removed the STUN example that centered around the separation of
the control and media planes. Instead, provided more information
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on using STUN with protocols.
17. Acknowledgements
The authors would like to thank Cedric Aoun, Pete Cordell, Cullen
Jennings, Bob Penfield, Xavier Marjou, Bruce Lowekamp 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.
18. References
18.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
[3] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[4] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[5] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[6] 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.
[7] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication:
Basic and Digest Access Authentication", RFC 2617, June 1999.
[8] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[9] International Telecommunications Union, "Error-correcting
Procedures for DCEs Using Asynchronous-to-Synchronous
Conversion", ITU-T Recommendation V.42, 1994.
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18.2. Informational References
[10] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[11] 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.
[12] 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.
[13] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for
Offer/Answer Protocols", draft-ietf-mmusic-ice-11 (work in
progress), October 2006.
[14] Audet, F. and C. Jennings, "NAT Behavioral Requirements for
Unicast UDP", draft-ietf-behave-nat-udp-08 (work in progress),
October 2006.
[15] 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.
[16] Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal
Underneath NAT (STUN)", draft-ietf-behave-turn-02 (work in
progress), October 2006.
[17] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, July 2003.
[18] Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-04 (work in progress), June 2006.
[19] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[20] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
[21] Holdrege, M. and P. Srisuresh, "Protocol Complications with the
IP Network Address Translator", RFC 3027, January 2001.
[22] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
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Session Description Protocol (SDP)", RFC 3264, June 2002.
[23] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[24] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
[25] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
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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
Plantronics
345 Encinal Street
Santa Cruz, CA 95060
US
Email: rohan@ekabal.com
Dan Wing
Cisco Systems
771 Alder Drive
San Jose, CA 95035
US
Email: dwing@cisco.com
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