Request for Comments: 3489 J. Weinberger
Category: Standards Track dynamicsoft
C. Huitema
Microsoft
R. Mahy
Cisco
March 2003
STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
Simple Traversal of User Datagram Protocol (UDP) Through Network
Address Translators (NATs) (STUN) is a lightweight protocol that
allows applications to discover the presence and types of NATs and
firewalls between them and the public Internet. It also provides the
ability for applications to determine the public Internet Protocol
(IP) addresses allocated to them by the NAT. STUN works with many
existing NATs, and does not require any special behavior from them.
As a result, it allows a wide variety of applications to work through
existing NAT infrastructure.
Table of Contents
1. Applicability Statement ................................... 3
2. Introduction .............................................. 3
3. Terminology ............................................... 4
4. Definitions ............................................... 5
5. NAT Variations ............................................ 5
6. Overview of Operation ..................................... 6
7. Message Overview .......................................... 8
8. Server Behavior ........................................... 10
8.1 Binding Requests .................................... 10
8.2 Shared Secret Requests .............................. 13
9. Client Behavior ........................................... 14
9.1 Discovery ........................................... 15
9.2 Obtaining a Shared Secret ........................... 15
9.3 Formulating the Binding Request ..................... 17
9.4 Processing Binding Responses ........................ 17
10. Use Cases ................................................. 19
10.1 Discovery Process ................................... 19
10.2 Binding Lifetime Discovery .......................... 21
10.3 Binding Acquisition ................................. 23
11. Protocol Details .......................................... 24
11.1 Message Header ...................................... 25
11.2 Message Attributes .................................. 26
11.2.1 MAPPED-ADDRESS .............................. 27
11.2.2 RESPONSE-ADDRESS ............................ 27
11.2.3 CHANGED-ADDRESS ............................. 28
11.2.4 CHANGE-REQUEST .............................. 28
11.2.5 SOURCE-ADDRESS .............................. 28
11.2.6 USERNAME .................................... 28
11.2.7 PASSWORD .................................... 29
11.2.8 MESSAGE-INTEGRITY ........................... 29
11.2.9 ERROR-CODE .................................. 29
11.2.10 UNKNOWN-ATTRIBUTES .......................... 31
11.2.11 REFLECTED-FROM .............................. 31
12. Security Considerations ................................... 31
12.1 Attacks on STUN ..................................... 31
12.1.1 Attack I: DDOS Against a Target ............. 32
12.1.2 Attack II: Silencing a Client ............... 32
12.1.3 Attack III: Assuming the Identity of a Client 32
12.1.4 Attack IV: Eavesdropping .................... 33
12.2 Launching the Attacks ............................... 33
12.2.1 Approach I: Compromise a Legitimate
STUN Server ................................. 33
12.2.2 Approach II: DNS Attacks .................... 34
12.2.3 Approach III: Rogue Router or NAT ........... 34
12.2.4 Approach IV: MITM ........................... 35
12.2.5 Approach V: Response Injection Plus DoS ..... 35
12.2.6 Approach VI: Duplication .................... 35
12.3 Countermeasures ..................................... 36
12.4 Residual Threats .................................... 37
13. IANA Considerations ....................................... 38
14. IAB Considerations ........................................ 38
14.1 Problem Definition .................................. 38
14.2 Exit Strategy ....................................... 39
14.3 Brittleness Introduced by STUN ...................... 40
14.4 Requirements for a Long Term Solution ............... 42
14.5 Issues with Existing NAPT Boxes ..................... 43
14.6 In Closing .......................................... 43
15. Acknowledgments ........................................... 44
16. Normative References ...................................... 44
17. Informative References .................................... 44
18. Authors' Addresses ........................................ 46
19. Full Copyright Statement................................... 47
1. Applicability Statement
This protocol is not a cure-all for the problems associated with NAT.
It does not enable incoming TCP connections through NAT. It allows
incoming UDP packets through NAT, but only through a subset of
existing NAT types. In particular, STUN does not enable incoming UDP
packets through symmetric NATs (defined below), which are common in
large enterprises. STUN's discovery procedures are based on
assumptions on NAT treatment of UDP; such assumptions may prove
invalid down the road as new NAT devices are deployed. STUN does not
work when it is used to obtain an address to communicate with a peer
which happens to be behind the same NAT. STUN does not work when the
STUN server is not in a common shared address realm. For a more
complete discussion of the limitations of STUN, see Section 14.
2. Introduction
Network Address Translators (NATs), while providing many benefits,
also come with many drawbacks. The most troublesome of those
drawbacks is the fact that they break many existing IP applications,
and make it difficult to deploy new ones. Guidelines have been
developed [8] that describe how to build "NAT friendly" protocols,
but many protocols simply cannot be constructed according to those
guidelines. Examples of such protocols include almost all peer-to-
peer protocols, such as multimedia communications, file sharing and
games.
To combat this problem, Application Layer Gateways (ALGs) have been
embedded in NATs. ALGs perform the application layer functions
required for a particular protocol to traverse a NAT. Typically,
this involves rewriting application layer messages to contain
translated addresses, rather than the ones inserted by the sender of
the message. ALGs have serious limitations, including scalability,
reliability, and speed of deploying new applications. To resolve
these problems, the Middlebox Communications (MIDCOM) protocol is
being developed [9]. MIDCOM allows an application entity, such as an
end client or network server of some sort (like a Session Initiation
Protocol (SIP) proxy [10]) to control a NAT (or firewall), in order
to obtain NAT bindings and open or close pinholes. In this way, NATs
and applications can be separated once more, eliminating the need for
embedding ALGs in NATs, and resolving the limitations imposed by
current architectures.
Unfortunately, MIDCOM requires upgrades to existing NAT and
firewalls, in addition to application components. Complete upgrades
of these NAT and firewall products will take a long time, potentially
years. This is due, in part, to the fact that the deployers of NAT
and firewalls are not the same people who are deploying and using
applications. As a result, the incentive to upgrade these devices
will be low in many cases. Consider, for example, an airport
Internet lounge that provides access with a NAT. A user connecting
to the NATed network may wish to use a peer-to-peer service, but
cannot, because the NAT doesn't support it. Since the administrators
of the lounge are not the ones providing the service, they are not
motivated to upgrade their NAT equipment to support it, using either
an ALG, or MIDCOM.
Another problem is that the MIDCOM protocol requires that the agent
controlling the middleboxes know the identity of those middleboxes,
and have a relationship with them which permits control. In many
configurations, this will not be possible. For example, many cable
access providers use NAT in front of their entire access network.
This NAT could be in addition to a residential NAT purchased and
operated by the end user. The end user will probably not have a
control relationship with the NAT in the cable access network, and
may not even know of its existence.
Many existing proprietary protocols, such as those for online games
(such as the games described in RFC3027 [11]) and Voice over IP,
have developed tricks that allow them to operate through NATs without
changing those NATs. This document is an attempt to take some of
those ideas, and codify them into an interoperable protocol that can
meet the needs of many applications.
The protocol described here, Simple Traversal of UDP Through NAT
(STUN), allows entities behind a NAT to first discover the presence
of a NAT and the type of NAT, and then to learn the addresses
bindings allocated by the NAT. STUN requires no changes to NATs, and
works with an arbitrary number of NATs in tandem between the
application entity and the public Internet.
3. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC2119
[1] and indicate requirement levels for compliant STUN
implementations.
4. Definitions
STUN Client: A STUN client (also just referred to as a client)
is an entity that generates STUN requests. A STUN client can
execute on an end system, such as a user's PC, or can run in a
network element, such as a conferencing server.
STUN Server: A STUN Server (also just referred to as a server)
is an entity that receives STUN requests, and sends STUN
responses. STUN servers are generally attached to the public
Internet.
5. NAT Variations
It is assumed that the reader is familiar with NATs. It has been
observed that NAT treatment of UDP varies among implementations. The
four treatments observed in implementations are:
Full Cone: A full cone NAT is one where all requests from the
same internal IP address and port are mapped to the same external
IP address and port. Furthermore, any external host can send a
packet to the internal host, by sending a packet to the mapped
external address.
Restricted Cone: A restricted cone NAT is one where all requests
from the same internal IP address and port are mapped to the same
external IP address and port. Unlike a full cone NAT, an external
host (with IP address X) can send a packet to the internal host
only if the internal host had previously sent a packet to IP
address X.
Port Restricted Cone: A port restricted cone NAT is like a
restricted cone NAT, but the restriction includes port numbers.
Specifically, an external host can send a packet, with source IP
address X and source port P, to the internal host only if the
internal host had previously sent a packet to IP address X and
port P.
Symmetric: A symmetric NAT is one where all requests from the
same internal IP address and port, to a specific destination IP
address and port, are mapped to the same external IP address and
port. If the same host sends a packet with the same source
address and port, but to a different destination, a different
mapping is used. Furthermore, only the external host that
receives a packet can send a UDP packet back to the internal host.
Determining the type of NAT is important in many cases. Depending on
what the application wants to do, it may need to take the particular
behavior into account.
6. Overview of Operation
This section is descriptive only. Normative behavior is described in
Sections 8 and 9.
/-----\
// STUN \\
| Server |
\\ //
\-----/
+--------------+ Public Internet
................| NAT 2 |.......................
+--------------+
+--------------+ Private NET 2
................| NAT 1 |.......................
+--------------+
/-----\
// STUN \\
| Client |
\\ // Private NET 1
\-----/
Figure 1: STUN Configuration
The typical STUN configuration is shown in Figure 1. A STUN client
is connected to private network 1. This network connects to private
network 2 through NAT 1. Private network 2 connects to the public
Internet through NAT 2. The STUN server resides on the public
Internet.
STUN is a simple client-server protocol. A client sends a request to
a server, and the server returns a response. There are two types of
requests - Binding Requests, sent over UDP, and Shared Secret
Requests, sent over TLS [2] over TCP. Shared Secret Requests ask the
server to return a temporary username and password. This username
and password are used in a subsequent Binding Request and Binding
Response, for the purposes of authentication and message integrity.
Binding requests are used to determine the bindings allocated by
NATs. The client sends a Binding Request to the server, over UDP.
The server examines the source IP address and port of the request,
and copies them into a response that is sent back to the client.
There are some parameters in the request that allow the client to ask
that the response be sent elsewhere, or that the server send the
response from a different address and port. There are attributes for
providing message integrity and authentication.
The trick is using STUN to discover the presence of NAT, and to learn
and use the bindings they allocate.
The STUN client is typically embedded in an application which needs
to obtain a public IP address and port that can be used to receive
data. For example, it might need to obtain an IP address and port to
receive Real Time Transport Protocol (RTP) [12] traffic. When the
application starts, the STUN client within the application sends a
STUN Shared Secret Request to its server, obtains a username and
password, and then sends it a Binding Request. STUN servers can be
discovered through DNS SRV records [3], and it is generally assumed
that the client is configured with the domain to use to find the STUN
server. Generally, this will be the domain of the provider of the
service the application is using (such a provider is incented to
deploy STUN servers in order to allow its customers to use its
application through NAT). Of course, a client can determine the
address or domain name of a STUN server through other means. A STUN
server can even be embedded within an end system.
The STUN Binding Request is used to discover the presence of a NAT,
and to discover the public IP address and port mappings generated by
the NAT. Binding Requests are sent to the STUN server using UDP.
When a Binding Request arrives at the STUN server, it may have passed
through one or more NATs between the STUN client and the STUN server.
As a result, the source address of the request received by the server
will be the mapped address created by the NAT closest to the server.
The STUN server copies that source IP address and port into a STUN
Binding Response, and sends it back to the source IP address and port
of the STUN request. For all of the NAT types above, this response
will arrive at the STUN client.
When the STUN client receives the STUN Binding Response, it compares
the IP address and port in the packet with the local IP address and
port it bound to when the request was sent. If these do not match,
the STUN client is behind one or more NATs. In the case of a full-
cone NAT, the IP address and port in the body of the STUN response
are public, and can be used by any host on the public Internet to
send packets to the application that sent the STUN request. An
application need only listen on the IP address and port from which
the STUN request was sent. Any packets sent by a host on the public
Internet to the public address and port learned by STUN will be
received by the application.
Of course, the host may not be behind a full-cone NAT. Indeed, it
doesn't yet know what type of NAT it is behind. To determine that,
the client uses additional STUN Binding Requests. The exact
procedure is flexible, but would generally work as follows. The
client would send a second STUN Binding Request, this time to a
different IP address, but from the same source IP address and port.
If the IP address and port in the response are different from those
in the first response, the client knows it is behind a symmetric NAT.
To determine if it's behind a full-cone NAT, the client can send a
STUN Binding Request with flags that tell the STUN server to send a
response from a different IP address and port than the request was
received on. In other words, if the client sent a Binding Request to
IP address/port A/B using a source IP address/port of X/Y, the STUN
server would send the Binding Response to X/Y using source IP
address/port C/D. If the client receives this response, it knows it
is behind a full cone NAT.
STUN also allows the client to ask the server to send the Binding
Response from the same IP address the request was received on, but
with a different port. This can be used to detect whether the client
is behind a port restricted cone NAT or just a restricted cone NAT.
It should be noted that the configuration in Figure 1 is not the only
permissible configuration. The STUN server can be located anywhere,
including within another client. The only requirement is that the
STUN server is reachable by the client, and if the client is trying
to obtain a publicly routable address, that the server reside on the
public Internet.
7. Message Overview
STUN messages are TLV (type-length-value) encoded using big endian
(network ordered) binary. All STUN messages start with a STUN
header, followed by a STUN payload. The payload is a series of STUN
attributes, the set of which depends on the message type. The STUN
header contains a STUN message type, transaction ID, and length. The
message type can be Binding Request, Binding Response, Binding Error
Response, Shared Secret Request, Shared Secret Response, or Shared
Secret Error Response. The transaction ID is used to correlate
requests and responses. The length indicates the total length of the
STUN payload, not including the header. This allows STUN to run over
TCP. Shared Secret Requests are always sent over TCP (indeed, using
TLS over TCP).
Several STUN attributes are defined. The first is a MAPPED-ADDRESS
attribute, which is an IP address and port. It is always placed in
the Binding Response, and it indicates the source IP address and port
the server saw in the Binding Request. There is also a RESPONSE-
ADDRESS attribute, which contains an IP address and port. The
RESPONSE-ADDRESS attribute can be present in the Binding Request, and
indicates where the Binding Response is to be sent. It's optional,
and when not present, the Binding Response is sent to the source IP
address and port of the Binding Request.
The third attribute is the CHANGE-REQUEST attribute, and it contains
two flags to control the IP address and port used to send the
response. These flags are called "change IP" and "change port"
flags. The CHANGE-REQUEST attribute is allowed only in the Binding
Request. The "change IP" and "change port" flags are useful for
determining whether the client is behind a restricted cone NAT or
restricted port cone NAT. They instruct the server to send the
Binding Responses from a different source IP address and port. The
CHANGE-REQUEST attribute is optional in the Binding Request.
The fourth attribute is the CHANGED-ADDRESS attribute. It is present
in Binding Responses. It informs the client of the source IP address
and port that would be used if the client requested the "change IP"
and "change port" behavior.
The fifth attribute is the SOURCE-ADDRESS attribute. It is only
present in Binding Responses. It indicates the source IP address and
port where the response was sent from. It is useful for detecting
twice NAT configurations.
The sixth attribute is the USERNAME attribute. It is present in a
Shared Secret Response, which provides the client with a temporary
username and password (encoded in the PASSWORD attribute). The
USERNAME is also present in Binding Requests, serving as an index to
the shared secret used for the integrity protection of the Binding
Request. The seventh attribute, PASSWORD, is only found in Shared
Secret Response messages. The eight attribute is the MESSAGE-
INTEGRITY attribute, which contains a message integrity check over
the Binding Request or Binding Response.
The ninth attribute is the ERROR-CODE attribute. This is present in
the Binding Error Response and Shared Secret Error Response. It
indicates the error that has occurred. The tenth attribute is the
UNKNOWN-ATTRIBUTES attribute, which is present in either the Binding
Error Response or Shared Secret Error Response. It indicates the
mandatory attributes from the request which were unknown. The
eleventh attribute is the REFLECTED-FROM attribute, which is present
in Binding Responses. It indicates the IP address and port of the
sender of a Binding Request, used for traceability purposes to
prevent certain denial-of-service attacks.
8. Server Behavior
The server behavior depends on whether the request is a Binding
Request or a Shared Secret Request.
8.1 Binding Requests
A STUN server MUST be prepared to receive Binding Requests on four
address/port combinations - (A1, P1), (A2, P1), (A1, P2), and (A2,
P2). (A1, P1) represent the primary address and port, and these are
the ones obtained through the client discovery procedures below.
Typically, P1 will be port 3478, the default STUN port. A2 and P2
are arbitrary. A2 and P2 are advertised by the server through the
CHANGED-ADDRESS attribute, as described below.
It is RECOMMENDED that the server check the Binding Request for a
MESSAGE-INTEGRITY attribute. If not present, and the server requires
integrity checks on the request, it generates a Binding Error
Response with an ERROR-CODE attribute with response code 401. If the
MESSAGE-INTEGRITY attribute was present, the server computes the HMAC
over the request as described in Section 11.2.8. The key to use
depends on the shared secret mechanism. If the STUN Shared Secret
Request was used, the key MUST be the one associated with the
USERNAME attribute present in the request. If the USERNAME attribute
was not present, the server MUST generate a Binding Error Response.
The Binding Error Response MUST include an ERROR-CODE attribute with
response code 432. If the USERNAME is present, but the server
doesn't remember the shared secret for that USERNAME (because it
timed out, for example), the server MUST generate a Binding Error
Response. The Binding Error Response MUST include an ERROR-CODE
attribute with response code 430. If the server does know the shared
secret, but the computed HMAC differs from the one in the request,
the server MUST generate a Binding Error Response with an ERROR-CODE
attribute with response code 431. The Binding Error Response is sent
to the IP address and port the Binding Request came from, and sent
from the IP address and port the Binding Request was sent to.
Assuming the message integrity check passed, processing continues.
The server MUST check for any attributes in the request with values
less than or equal to 0x7fff which it does not understand. If it
encounters any, the server MUST generate a Binding Error Response,
and it MUST include an ERROR-CODE attribute with a 420 response code.
That response MUST contain an UNKNOWN-ATTRIBUTES attribute listing
the attributes with values less than or equal to 0x7fff which were
not understood. The Binding Error Response is sent to the IP address
and port the Binding Request came from, and sent from the IP address
and port the Binding Request was sent to.
Assuming the request was correctly formed, the server MUST generate a
single Binding Response. The Binding Response MUST contain the same
transaction ID contained in the Binding Request. The length in the
message header MUST contain the total length of the message in bytes,
excluding the header. The Binding Response MUST have a message type
of "Binding Response".
The server MUST add a MAPPED-ADDRESS attribute to the Binding
Response. The IP address component of this attribute MUST be set to
the source IP address observed in the Binding Request. The port
component of this attribute MUST be set to the source port observed
in the Binding Request.
If the RESPONSE-ADDRESS attribute was absent from the Binding
Request, the destination address and port of the Binding Response
MUST be the same as the source address and port of the Binding
Request. Otherwise, the destination address and port of the Binding
Response MUST be the value of the IP address and port in the
RESPONSE-ADDRESS attribute.
The source address and port of the Binding Response depend on the
value of the CHANGE-REQUEST attribute and on the address and port the
Binding Request was received on, and are summarized in Table 1.
Let Da represent the destination IP address of the Binding Request
(which will be either A1 or A2), and Dp represent the destination
port of the Binding Request (which will be either P1 or P2). Let Ca
represent the other address, so that if Da is A1, Ca is A2. If Da is
A2, Ca is A1. Similarly, let Cp represent the other port, so that if
Dp is P1, Cp is P2. If Dp is P2, Cp is P1. If the "change port"
flag was set in CHANGE-REQUEST attribute of the Binding Request, and
the "change IP" flag was not set, the source IP address of the
Binding Response MUST be Da and the source port of the Binding
Response MUST be Cp. If the "change IP" flag was set in the Binding
Request, and the "change port" flag was not set, the source IP
address of the Binding Response MUST be Ca and the source port of the
Binding Response MUST be Dp. When both flags are set, the source IP
address of the Binding Response MUST be Ca and the source port of the
Binding Response MUST be Cp. If neither flag is set, or if the
CHANGE-REQUEST attribute is absent entirely, the source IP address of
the Binding Response MUST be Da and the source port of the Binding
Response MUST be Dp.
Flags Source Address Source Port CHANGED-ADDRESS
none Da Dp Ca:Cp
Change IP Ca Dp Ca:Cp
Change port Da Cp Ca:Cp
Change IP and
Change port Ca Cp Ca:Cp
Table 1: Impact of Flags on Packet Source and CHANGED-ADDRESS
The server MUST add a SOURCE-ADDRESS attribute to the Binding
Response, containing the source address and port used to send the
Binding Response.
The server MUST add a CHANGED-ADDRESS attribute to the Binding
Response. This contains the source IP address and port that would be
used if the client had set the "change IP" and "change port" flags in
the Binding Request. As summarized in Table 1, these are Ca and Cp,
respectively, regardless of the value of the CHANGE-REQUEST flags.
If the Binding Request contained both the USERNAME and MESSAGE-
INTEGRITY attributes, the server MUST add a MESSAGE-INTEGRITY
attribute to the Binding Response. The attribute contains an HMAC
[13] over the response, as described in Section 11.2.8. The key to
use depends on the shared secret mechanism. If the STUN Shared
Secret Request was used, the key MUST be the one associated with the
USERNAME attribute present in the Binding Request.
If the Binding Request contained a RESPONSE-ADDRESS attribute, the
server MUST add a REFLECTED-FROM attribute to the response. If the
Binding Request was authenticated using a username obtained from a
Shared Secret Request, the REFLECTED-FROM attribute MUST contain the
source IP address and port where that Shared Secret Request came
from. If the username present in the request was not allocated using
a Shared Secret Request, the REFLECTED-FROM attribute MUST contain
the source address and port of the entity which obtained the
username, as best can be verified with the mechanism used to allocate
the username. If the username was not present in the request, and
the server was willing to process the request, the REFLECTED-FROM
attribute SHOULD contain the source IP address and port where the
request came from.
The server SHOULD NOT retransmit the response. Reliability is
achieved by having the client periodically resend the request, each
of which triggers a response from the server.
8.2 Shared Secret Requests
Shared Secret Requests are always received on TLS connections. When
the server receives a request from the client to establish a TLS
connection, it MUST proceed with TLS, and SHOULD present a site
certificate. The TLS ciphersuite TLS_RSA_WITH_AES_128_CBC_SHA [4]
SHOULD be used. Client TLS authentication MUST NOT be done, since
the server is not allocating any resources to clients, and the
computational burden can be a source of attacks.
If the server receives a Shared Secret Request, it MUST verify that
the request arrived on a TLS connection. If it did not receive the
request over TLS, it MUST generate a Shared Secret Error Response,
and it MUST include an ERROR-CODE attribute with a 433 response code.
The destination for the error response depends on the transport on
which the request was received. If the Shared Secret Request was
received over TCP, the Shared Secret Error Response is sent over the
same connection the request was received on. If the Shared Secret
Request was receive over UDP, the Shared Secret Error Response is
sent to the source IP address and port that the request came from.
The server MUST check for any attributes in the request with values
less than or equal to 0x7fff which it does not understand. If it
encounters any, the server MUST generate a Shared Secret Error
Response, and it MUST include an ERROR-CODE attribute with a 420
response code. That response MUST contain an UNKNOWN-ATTRIBUTES
attribute listing the attributes with values less than or equal to
0x7fff which were not understood. The Shared Secret Error Response
is sent over the TLS connection.
All Shared Secret Error Responses MUST contain the same transaction
ID contained in the Shared Secret Request. The length in the message
header MUST contain the total length of the message in bytes,
excluding the header. The Shared Secret Error Response MUST have a
message type of "Shared Secret Error Response" (0x0112).
Assuming the request was properly constructed, the server creates a
Shared Secret Response. The Shared Secret Response MUST contain the
same transaction ID contained in the Shared Secret Request. The
length in the message header MUST contain the total length of the
message in bytes, excluding the header. The Shared Secret Response
MUST have a message type of "Shared Secret Response". The Shared
Secret Response MUST contain a USERNAME attribute and a PASSWORD
attribute. The USERNAME attribute serves as an index to the
password, which is contained in the PASSWORD attribute. The server
can use any mechanism it chooses to generate the username. However,
the username MUST be valid for a period of at least 10 minutes.
Validity means that the server can compute the password for that
username. There MUST be a single password for each username. In
other words, the server cannot, 10 minutes later, assign a different
password to the same username. The server MUST hand out a different
username for each distinct Shared Secret Request. Distinct, in this
case, implies a different transaction ID. It is RECOMMENDED that the
server explicitly invalidate the username after ten minutes. It MUST
invalidate the username after 30 minutes. The PASSWORD contains the
password bound to that username. The password MUST have at least 128
bits. The likelihood that the server assigns the same password for
two different usernames MUST be vanishingly small, and the passwords
MUST be unguessable. In other words, they MUST be a
cryptographically random function of the username.
These requirements can still be met using a stateless server, by
intelligently computing the USERNAME and PASSWORD. One approach is
to construct the USERNAME as:
USERNAME = <prefix,rounded-time,clientIP,hmac>
Where prefix is some random text string (different for each shared
secret request), rounded-time is the current time modulo 20 minutes,
clientIP is the source IP address where the Shared Secret Request
came from, and hmac is an HMAC [13] over the prefix, rounded-time,
and client IP, using a server private key.
The password is then computed as:
password = <hmac(USERNAME,anotherprivatekey)>
With this structure, the username itself, which will be present in
the Binding Request, contains the source IP address where the Shared
Secret Request came from. That allows the server to meet the
requirements specified in Section 8.1 for constructing the
REFLECTED-FROM attribute. The server can verify that the username
was not tampered with, using the hmac present in the username.
The Shared Secret Response is sent over the same TLS connection the
request was received on. The server SHOULD keep the connection open,
and let the client close it.
9. Client Behavior
The behavior of the client is very straightforward. Its task is to
discover the STUN server, obtain a shared secret, formulate the
Binding Request, handle request reliability, and process the Binding
Responses.
9.1 Discovery
Generally, the client will be configured with a domain name of the
provider of the STUN servers. This domain name is resolved to an IP
address and port using the SRV procedures specified in RFC2782 [3].
Specifically, the service name is "stun". The protocol is "udp" for
sending Binding Requests, or "tcp" for sending Shared Secret
Requests. The procedures of RFC2782 are followed to determine the
server to contact. RFC2782 spells out the details of how a set of
SRV records are sorted and then tried. However, it only states that
the client should "try to connect to the (protocol, address,
service)" without giving any details on what happens in the event of
failure. Those details are described here for STUN.
For STUN requests, failure occurs if there is a transport failure of
some sort (generally, due to fatal ICMP errors in UDP or connection
failures in TCP). Failure also occurs if the transaction fails due
to timeout. This occurs 9.5 seconds after the first request is sent,
for both Shared Secret Requests and Binding Requests. See Section
9.3 for details on transaction timeouts for Binding Requests. If a
failure occurs, the client SHOULD create a new request, which is
identical to the previous, but has a different transaction ID and
MESSAGE INTEGRITY attribute (the HMAC will change because the
transaction ID has changed). That request is sent to the next
element in the list as specified by RFC2782.
The default port for STUN requests is 3478, for both TCP and UDP.
Administrators SHOULD use this port in their SRV records, but MAY use
others.
If no SRV records were found, the client performs an A record lookup
of the domain name. The result will be a list of IP addresses, each
of which can be contacted at the default port.
This would allow a firewall admin to open the STUN port, so hosts
within the enterprise could access new applications. Whether they
will or won't do this is a good question.
9.2 Obtaining a Shared Secret
As discussed in Section 12, there are several attacks possible on
STUN systems. Many of these are prevented through integrity of
requests and responses. To provide that integrity, STUN makes use of
a shared secret between client and server, used as the keying
material for an HMAC used in both the Binding Request and Binding
Response. STUN allows for the shared secret to be obtained in any
way (for example, Kerberos [14]). However, it MUST have at least 128
bits of randomness. In order to ensure interoperability, this
specification describes a TLS-based mechanism. This mechanism,
described in this section, MUST be implemented by clients and
servers.
First, the client determines the IP address and port that it will
open a TCP connection to. This is done using the discovery
procedures in Section 9.1. The client opens up the connection to
that address and port, and immediately begins TLS negotiation [2].
The client MUST verify the identity of the server. To do that, it
follows the identification procedures defined in Section 3.1 of RFC
2818 [5]. Those procedures assume the client is dereferencing a URI.
For purposes of usage with this specification, the client treats the
domain name or IP address used in Section 9.1 as the host portion of
the URI that has been dereferenced.
Once the connection is opened, the client sends a Shared Secret
request. This request has no attributes, just the header. The
transaction ID in the header MUST meet the requirements outlined for
the transaction ID in a binding request, described in Section 9.3
below. The server generates a response, which can either be a Shared
Secret Response or a Shared Secret Error Response.
If the response was a Shared Secret Error Response, the client checks
the response code in the ERROR-CODE attribute. Interpretation of
those response codes is identical to the processing of Section 9.4
for the Binding Error Response.
If a client receives a Shared Secret Response with an attribute whose
type is greater than 0x7fff, the attribute MUST be ignored. If the
client receives a Shared Secret Response with an attribute whose type
is less than or equal to 0x7fff, the response is ignored.
If the response was a Shared Secret Response, it will contain a short
lived username and password, encoded in the USERNAME and PASSWORD
attributes, respectively.
The client MAY generate multiple Shared Secret Requests on the
connection, and it MAY do so before receiving Shared Secret Responses
to previous Shared Secret Requests. The client SHOULD close the
connection as soon as it has finished obtaining usernames and
passwords.
Section 9.3 describes how these passwords are used to provide
integrity protection over Binding Requests, and Section 8.1 describes
how it is used in Binding Responses.
9.3 Formulating the Binding Request
A Binding Request formulated by the client follows the syntax rules
defined in Section 11. Any two requests that are not bit-wise
identical, and not sent to the same server from the same IP address
and port, MUST carry different transaction IDs. The transaction ID
MUST be uniformly and randomly distributed between 0 and 2**128 - 1.
The large range is needed because the transaction ID serves as a form
of randomization, helping to prevent replays of previously signed
responses from the server. The message type of the request MUST be
"Binding Request".
The RESPONSE-ADDRESS attribute is optional in the Binding Request.
It is used if the client wishes the response to be sent to a
different IP address and port than the one the request was sent from.
This is useful for determining whether the client is behind a
firewall, and for applications that have separated control and data
components. See Section 10.3 for more details. The CHANGE-REQUEST
attribute is also optional. Whether it is present depends on what
the application is trying to accomplish. See Section 10 for some
example uses.
The client SHOULD add a MESSAGE-INTEGRITY and USERNAME attribute to
the Binding Request. This MESSAGE-INTEGRITY attribute contains an
HMAC [13]. The value of the username, and the key to use in the
MESSAGE-INTEGRITY attribute depend on the shared secret mechanism.
If the STUN Shared Secret Request was used, the USERNAME must be a
valid username obtained from a Shared Secret Response within the last
nine minutes. The shared secret for the HMAC is the value of the
PASSWORD attribute obtained from the same Shared Secret Response.
Once formulated, the client sends the Binding Request. Reliability
is accomplished through client retransmissions. Clients SHOULD
retransmit the request starting with an interval of 100ms, doubling
every retransmit until the interval reaches 1.6s. Retransmissions
continue with intervals of 1.6s until a response is received, or a
total of 9 requests have been sent. If no response is received by 1.6
seconds after the last request has been sent, the client SHOULD
consider the transaction to have failed. In other words, requests
would be sent at times 0ms, 100ms, 300ms, 700ms, 1500ms, 3100ms,
4700ms, 6300ms, and 7900ms. At 9500ms, the client considers the
transaction to have failed if no response has been received.
9.4 Processing Binding Responses
The response can either be a Binding Response or Binding Error
Response. Binding Error Responses are always received on the source
address and port the request was sent from. A Binding Response will
be received on the address and port placed in the RESPONSE-ADDRESS
attribute of the request. If none was present, the Binding Responses
will be received on the source address and port the request was sent
from.
If the response is a Binding Error Response, the client checks the
response code from the ERROR-CODE attribute of the response. For a
400 response code, the client SHOULD display the reason phrase to the
user. For a 420 response code, the client SHOULD retry the request,
this time omitting any attributes listed in the UNKNOWN-ATTRIBUTES
attribute of the response. For a 430 response code, the client
SHOULD obtain a new shared secret, and retry the Binding Request with
a new transaction. For 401 and 432 response codes, if the client had
omitted the USERNAME or MESSAGE-INTEGRITY attribute as indicated by
the error, it SHOULD try again with those attributes. For a 431
response code, the client SHOULD alert the user, and MAY try the
request again after obtaining a new username and password. For a 500
response code, the client MAY wait several seconds and then retry the
request. For a 600 response code, the client MUST NOT retry the
request, and SHOULD display the reason phrase to the user. Unknown
attributes between 400 and 499 are treated like a 400, unknown
attributes between 500 and 599 are treated like a 500, and unknown
attributes between 600 and 699 are treated like a 600. Any response
between 100 and 399 MUST result in the cessation of request
retransmissions, but otherwise is discarded.
If a client receives a response with an attribute whose type is
greater than 0x7fff, the attribute MUST be ignored. If the client
receives a response with an attribute whose type is less than or
equal to 0x7fff, request retransmissions MUST cease, but the entire
response is otherwise ignored.
If the response is a Binding Response, the client SHOULD check the
response for a MESSAGE-INTEGRITY attribute. If not present, and the
client placed a MESSAGE-INTEGRITY attribute into the request, it MUST
discard the response. If present, the client computes the HMAC over
the response as described in Section 11.2.8. The key to use depends
on the shared secret mechanism. If the STUN Shared Secret Request
was used, the key MUST be same as used to compute the MESSAGE-
INTEGRITY attribute in the request. If the computed HMAC differs
from the one in the response, the client MUST discard the response,
and SHOULD alert the user about a possible attack. If the computed
HMAC matches the one from the response, processing continues.
Reception of a response (either Binding Error Response or Binding
Response) to a Binding Request will terminate retransmissions of that
request. However, clients MUST continue to listen for responses to a
Binding Request for 10 seconds after the first response. If it
receives any responses in this interval with different message types
(Binding Responses and Binding Error Responses, for example) or
different MAPPED-ADDRESSes, it is an indication of a possible attack.
The client MUST NOT use the MAPPED-ADDRESS from any of the responses
it received (either the first or the additional ones), and SHOULD
alert the user.
Furthermore, if a client receives more than twice as many Binding
Responses as the number of Binding Requests it sent, it MUST NOT use
the MAPPED-ADDRESS from any of those responses, and SHOULD alert the
user about a potential attack.
If the Binding Response is authenticated, and the MAPPED-ADDRESS was
not discarded because of a potential attack, the CLIENT MAY use the
MAPPED-ADDRESS and SOURCE-ADDRESS attributes.
10. Use Cases
The rules of Sections 8 and 9 describe exactly how a client and
server interact to send requests and get responses. However, they do
not dictate how the STUN protocol is used to accomplish useful tasks.
That is at the discretion of the client. Here, we provide some
useful scenarios for applying STUN.
10.1 Discovery Process
In this scenario, a user is running a multimedia application which
needs to determine which of the following scenarios applies to it:
o On the open Internet
o Firewall that blocks UDP
o Firewall that allows UDP out, and responses have to come back to
the source of the request (like a symmetric NAT, but no
translation. We call this a symmetric UDP Firewall)
o Full-cone NAT
o Symmetric NAT
o Restricted cone or restricted port cone NAT
Which of the six scenarios applies can be determined through the flow
chart described in Figure 2. The chart refers only to the sequence
of Binding Requests; Shared Secret Requests will, of course, be
needed to authenticate each Binding Request used in the sequence.
The flow makes use of three tests. In test I, the client sends a
STUN Binding Request to a server, without any flags set in the
CHANGE-REQUEST attribute, and without the RESPONSE-ADDRESS attribute.
This causes the server to send the response back to the address and
port that the request came from. In test II, the client sends a
Binding Request with both the "change IP" and "change port" flags
from the CHANGE-REQUEST attribute set. In test III, the client sends
a Binding Request with only the "change port" flag set.
The client begins by initiating test I. If this test yields no
response, the client knows right away that it is not capable of UDP
connectivity. If the test produces a response, the client examines
the MAPPED-ADDRESS attribute. If this address and port are the same
as the local IP address and port of the socket used to send the
request, the client knows that it is not natted. It executes test
II.
If a response is received, the client knows that it has open access
to the Internet (or, at least, its behind a firewall that behaves
like a full-cone NAT, but without the translation). If no response
is received, the client knows its behind a symmetric UDP firewall.
In the event that the IP address and port of the socket did not match
the MAPPED-ADDRESS attribute in the response to test I, the client
knows that it is behind a NAT. It performs test II. If a response
is received, the client knows that it is behind a full-cone NAT. If
no response is received, it performs test I again, but this time,
does so to the address and port from the CHANGED-ADDRESS attribute