BEHAVE J. Rosenberg
Internet-Draft Cisco Systems
Expires: August 5, 2006 C. Huitema
Microsoft
R. Mahy
Plantronics
D. Wing
Cisco Systems
February 2006
Simple Traversal of UDP Through Network Address Translators (NAT) (STUN)
draft-ietf-behave-rfc3489bis-03
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Copyright (C) The Internet Society (2006).
Abstract
Simple Traversal of UDP Through NATs (STUN) is a lightweight protocol
that provides the ability for applications to determine the public IP
addresses and ports allocated to them by the NAT and to keep NAT
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bindings open. These addresses and ports can be placed into protocol
payloads where a client needs to provide a publically routable IP
address. STUN works with many existing NATs, and does not require
any special behavior from them. As a result, it allows a wide
variety of applications to work through existing NAT infrastructure.
Table of Contents
1. Applicability Statement . . . . . . . . . . . . . . . . . . . 5
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7
6. STUN Message Structure . . . . . . . . . . . . . . . . . . . . 9
7. STUN Transactions . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Request Transaction Reliability . . . . . . . . . . . . . 11
8. General Client Behavior . . . . . . . . . . . . . . . . . . . 12
8.1. Request Message Types . . . . . . . . . . . . . . . . . . 12
8.1.1. Discovery . . . . . . . . . . . . . . . . . . . . . . 12
8.1.2. Obtaining a Shared Secret . . . . . . . . . . . . . . 13
8.1.3. Formulating the Request Message . . . . . . . . . . . 14
8.1.4. Processing Responses . . . . . . . . . . . . . . . . . 14
8.1.5. Using the Mapped Address . . . . . . . . . . . . . . . 15
8.2. Indication Message Types . . . . . . . . . . . . . . . . 17
8.2.1. Formulating the Indication Message . . . . . . . . . . 17
9. General Server Behavior . . . . . . . . . . . . . . . . . . . 17
9.1. Request Message Types . . . . . . . . . . . . . . . . . . 17
9.1.1. Receive Request Message . . . . . . . . . . . . . . . 17
9.1.2. Constructing the Response . . . . . . . . . . . . . . 19
9.1.3. Sending the Response . . . . . . . . . . . . . . . . . 19
9.2. Indication Message Types . . . . . . . . . . . . . . . . 19
10. Short-Term Passwords . . . . . . . . . . . . . . . . . . . . . 19
11. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 20
11.1. MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . . . 21
11.2. RESPONSE-ADDRESS . . . . . . . . . . . . . . . . . . . . 21
11.3. CHANGED-ADDRESS . . . . . . . . . . . . . . . . . . . . . 22
11.4. CHANGE-REQUEST . . . . . . . . . . . . . . . . . . . . . 22
11.5. SOURCE-ADDRESS . . . . . . . . . . . . . . . . . . . . . 23
11.6. USERNAME . . . . . . . . . . . . . . . . . . . . . . . . 23
11.7. PASSWORD . . . . . . . . . . . . . . . . . . . . . . . . 23
11.8. MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . . 23
11.9. ERROR-CODE . . . . . . . . . . . . . . . . . . . . . . . 24
11.10. REFLECTED-FROM . . . . . . . . . . . . . . . . . . . . . 26
11.11. ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . . 26
11.12. REALM . . . . . . . . . . . . . . . . . . . . . . . . . . 26
11.13. NONCE . . . . . . . . . . . . . . . . . . . . . . . . . . 26
11.14. UNKNOWN-ATTRIBUTES . . . . . . . . . . . . . . . . . . . 27
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11.15. XOR-MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . 27
11.16. SERVER . . . . . . . . . . . . . . . . . . . . . . . . . 28
11.17. ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . . 28
11.18. BINDING-LIFETIME . . . . . . . . . . . . . . . . . . . . 29
12. STUN Usages . . . . . . . . . . . . . . . . . . . . . . . . . 29
12.1. Defined STUN Usages . . . . . . . . . . . . . . . . . . . 29
12.2. Binding Discovery . . . . . . . . . . . . . . . . . . . . 29
12.2.1. Applicability . . . . . . . . . . . . . . . . . . . . 29
12.2.2. Client Discovery of Server . . . . . . . . . . . . . . 30
12.2.3. Server Determination of Usage . . . . . . . . . . . . 30
12.2.4. New Requests or Indications . . . . . . . . . . . . . 30
12.2.5. New Attributes . . . . . . . . . . . . . . . . . . . . 30
12.2.6. New Error Response Codes . . . . . . . . . . . . . . . 30
12.2.7. Client Procedures . . . . . . . . . . . . . . . . . . 30
12.2.8. Server Procedures . . . . . . . . . . . . . . . . . . 30
12.2.9. Security Considerations for Binding Discovery . . . . 30
12.3. Connectivity Check . . . . . . . . . . . . . . . . . . . 31
12.3.1. Applicability . . . . . . . . . . . . . . . . . . . . 31
12.3.2. Client Discovery of Server . . . . . . . . . . . . . . 31
12.3.3. Server Determination of Usage . . . . . . . . . . . . 31
12.3.4. New Requests or Indications . . . . . . . . . . . . . 31
12.3.5. New Attributes . . . . . . . . . . . . . . . . . . . . 31
12.3.6. New Error Response Codes . . . . . . . . . . . . . . . 31
12.3.7. Client Procedures . . . . . . . . . . . . . . . . . . 31
12.3.8. Server Procedures . . . . . . . . . . . . . . . . . . 32
12.3.9. Security Considerations for Connectivity Check . . . . 32
12.4. NAT Keepalives . . . . . . . . . . . . . . . . . . . . . 32
12.4.1. Applicability . . . . . . . . . . . . . . . . . . . . 32
12.4.2. Client Discovery of Server . . . . . . . . . . . . . . 32
12.4.3. Server Determination of Usage . . . . . . . . . . . . 32
12.4.4. New Requests or Indications . . . . . . . . . . . . . 33
12.4.5. New Attributes . . . . . . . . . . . . . . . . . . . . 33
12.4.6. New Error Response Codes . . . . . . . . . . . . . . . 33
12.4.7. Client Procedures . . . . . . . . . . . . . . . . . . 33
12.4.8. Server Procedures . . . . . . . . . . . . . . . . . . 33
12.4.9. Security Considerations for NAT Keepalives . . . . . . 33
12.5. Short-Term Password . . . . . . . . . . . . . . . . . . . 33
12.5.1. Applicability . . . . . . . . . . . . . . . . . . . . 33
12.5.2. Client Discovery of Server . . . . . . . . . . . . . . 34
12.5.3. Server Determination of Usage . . . . . . . . . . . . 34
12.5.4. New Requests or Indications . . . . . . . . . . . . . 34
12.5.5. New Attributes . . . . . . . . . . . . . . . . . . . . 35
12.5.6. New Error Response Codes . . . . . . . . . . . . . . . 35
12.5.7. Client Procedures . . . . . . . . . . . . . . . . . . 35
12.5.8. Server Procedures . . . . . . . . . . . . . . . . . . 35
12.5.9. Security Considerations for Short-Term Password . . . 35
13. Security Considerations . . . . . . . . . . . . . . . . . . . 36
13.1. Attacks on STUN . . . . . . . . . . . . . . . . . . . . . 36
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13.1.1. Attack I: DDoS Against a Target . . . . . . . . . . . 36
13.1.2. Attack II: Silencing a Client . . . . . . . . . . . . 36
13.1.3. Attack III: Assuming the Identity of a Client . . . . 37
13.1.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . . 37
13.2. Launching the Attacks . . . . . . . . . . . . . . . . . . 37
13.2.1. Approach I: Compromise a Legitimate STUN Server . . . 38
13.2.2. Approach II: DNS Attacks . . . . . . . . . . . . . . . 38
13.2.3. Approach III: Rogue Router or NAT . . . . . . . . . . 38
13.2.4. Approach IV: Man in the Middle . . . . . . . . . . . . 39
13.2.5. Approach V: Response Injection Plus DoS . . . . . . . 39
13.2.6. Approach VI: Duplication . . . . . . . . . . . . . . . 39
13.3. Countermeasures . . . . . . . . . . . . . . . . . . . . . 40
13.4. Residual Threats . . . . . . . . . . . . . . . . . . . . 42
14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 42
14.1. Problem Definition . . . . . . . . . . . . . . . . . . . 42
14.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 43
14.3. Brittleness Introduced by STUN . . . . . . . . . . . . . 43
14.4. Requirements for a Long Term Solution . . . . . . . . . . 45
14.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 46
14.6. In Closing . . . . . . . . . . . . . . . . . . . . . . . 46
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
15.1. STUN Message Type Registry . . . . . . . . . . . . . . . 47
15.2. STUN Attribute Registry . . . . . . . . . . . . . . . . . 47
16. Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . . 48
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 49
18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 49
18.1. Normative References . . . . . . . . . . . . . . . . . . 49
18.2. Informational References . . . . . . . . . . . . . . . . 50
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 52
Intellectual Property and Copyright Statements . . . . . . . . . . 53
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1. Applicability Statement
This protocol is not a cure-all for the problems associated with NAT.
It does not enable incoming TCP connections through NAT. It allows
incoming UDP packets through NAT, but only through a subset of
existing NAT types. In particular, STUN does not enable incoming UDP
packets through "symmetric NATs", which is
a NAT 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.
This type of NAT is common in large enterprises. STUN does not work
when it is used to obtain an address to communicate with a peer which
happens to be behind the same NAT. STUN does not work when the STUN
server is not in a common shared address realm.
In order to work with such a NAT, a media relay such as TURN [3] is
required. All other types of NATs work without a media relay.
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 [17] 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.
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Many existing proprietary protocols, such as those for online games
(such as the games described in RFC3027 [18]) 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 of UDP Through NAT
(STUN), provides a toolkit of functions. These functions allow
entities behind a NAT to learn the address bindings allocated by the
NAT, to keep those bindings open, and communicate with other STUN-
aware to validate connecivity. 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.
STUN Server
A STUN Server (also just referred to as a server) is an entity
that receives STUN requests, and sends STUN responses.
Transport Address
The combination of an IP address and (UDP or TCP) port.
Reflexive Transport Address
A transport address learned by a client which 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 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.
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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.
5. Overview of Operation
This section is descriptive only. Normative behavior is described in
Section 8 and Section .
/----\
// 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. Two types of messages are
available -- request/response in which client sends a request to a
server, and the server returns a response; and indications which can
be initiated by the client or by the server and which do not elicit a
response. There are two types of requests defined in this
specification - Binding Requests, sent over UDP, and Shared Secret
Requests, sent over TLS [6] over TCP. Shared Secret Requests ask the
server to return a temporary username and password. This username
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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 --
this is the 'mapped address'. There are attributes for providing
message integrity and authentication.
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 [14]) 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 [4], and it is generally assumed
that the client is configured with the domain to use to find the STUN
server. Generally, this will be the domain of the provider of the
service the application is using (such a provider is incented to
deploy STUN servers in order to allow its customers to use its
application through NAT). Of course, a client can determine the
address or domain name of a STUN server through other means. A STUN
server can even be embedded within an end system.
The STUN Binding Request is used to discover the public IP address
and port mappings generated by the NAT. Binding Requests are sent to
the STUN server using UDP. When a Binding Request arrives at the
STUN server, it may have passed through one or more NATs between the
STUN client and the STUN server. As a result, the source address of
the request received by the server will be the mapped address created
by the NAT closest to the server. The STUN server copies that source
IP address and port into a STUN Binding Response, and sends it back
to the source IP address and port of the STUN request. Every type of
NAT will route that response so that it arrives 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 knows is behind one or more NATs. If the STUN server
is publicly routable the IP address and port in the STUN Binding
Response are also publicly routable, and can be used by any host on
the public Internet to send packets to the application that sent the
STUN request. An application need only listen on the IP address and
port from which the STUN request was sent. Packets sent by a host on
the public Internet to the public address and port learned by STUN
will be received by the application, so long as conditions permit.
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The conditions in which these packets will not be received by the
client are described in Section 1.
It should be noted that the configuration in Figure 1 is not the only
permissible configuration. The STUN server can be located anywhere,
including within another client. The only requirement is that the
STUN server is reachable by the client, and if the client is trying
to obtain a publicly routable address, that the server reside on the
public Internet.
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, transaction ID,
and length. The length indicates the total length of the STUN
payload, not including the 20-byte header.
There are two categories of STUN message types: Requests and
Indications.
Upon receiving a STUN request, a STUN server will send a STUN success
response or a STUN error response. All STUN success responses MUST
have a type whose value is 0x100 higher than their associated
request, and all STUN error responses MUST have a type whose value is
0x110 higher than their associated request. Any newly defined STUN
message types MUST use message type values 0x100 and 0x110 higher for
their success and error responses, respectively. STUN Requests are
sent reliably (Section 7.1). 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 are not sent
reliably do not have an associated success response message type or
associated error response message type. Indication messages can be
sent by the STUN client to the server, or from the STUN server to the
client. 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 0b00. This,
combined with the magic cookie, aids in differentiating STUN packets
from other protocols when STUN is multiplexed with other protocols on
the same port.
The STUN message types Binding Request, Response, and Error Response
are defined in Section 8 and Section 9.1. The Shared Secret Request,
Response, and Error Response are described in Section 12.5. Their
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 [13] this field was part of the
transaction ID. This fixed value affords easy identification of a
STUN message when STUN is multiplexed with other protocols on the
same port, as is done for example in [12] and [15]. 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 and error responses.
The transaction ID is a 96 bit identifier. STUN transactions are
identified by their unique 96-bit transaction ID. This transaction
ID is chosen by the STUN client and MUST be unique for each new STUN
transaction by that STUN client. Any two requests that are not bit-
wise identical, and not sent to the same server from the same IP
address and port, MUST have a different transaction ID. The
transaction ID MUST be uniformly and randomly distributed between 0
and 2**96 - 1. The large range is needed because the transaction ID
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serves as a form of randomization, helping to prevent replays of
previously signed responses from the server.
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:
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 3: Format of STUN Attributes
The attribute types defined in this specification are in Section 11 .
7. STUN 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.
7.1. Request Transaction Reliability
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 is accomplished through client
retransmissions. Clients SHOULD retransmit the request starting with
an interval of 100ms, doubling every retransmit until the interval
reaches 1.6 seconds. Retransmissions continue with intervals of 1.6
seconds 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.
When running STUN over TCP, TCP is responsible for ensuring delivery.
The STUN application SHOULD NOT retransmit STUN requests when running
over TCP.
For STUN requests, failure occurs if there is a transport failure of
some sort (generally, due to fatal ICMP errors in UDP or connection
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failures in TCP) or if retransmissions of the same STUN Request
doesn't elicit a Response. If a failure occurs and the SRV query
indicated other STUN servers are available, 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 Indication message types are not sent reliably.
8. General Client Behavior
There are two classes of client behavior -- one for the request
message types and another for the indication message types.
8.1. Request Message Types
This section applies to client behavior for the Request message types
-- Binding Request and Shared Secret Request. For Request message
types, the client must discover the STUN server's address and port,
obtain a shared secret, formulate the Request, transmit the request
reliability, process the Binding Response, and use the information in
the Response.
8.1.1. 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 an IP address and
port using the SRV procedures specified in RFC2782 [4]. The
mechanism for configuring the STUN client with the domain name to
look up is not in scope of this document.
The DNS SRV service name is "stun". The protocol is "udp" for
sending Binding Requests, or "tcp" for sending Shared Secret
Requests. The procedures of RFC 2782 are followed to determine the
server to contact. RFC 2782 spells out the details of how a set of
SRV records are sorted and then tried. However, 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 8.1.3.
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 or
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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.
8.1.2. 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 (for example Kerberos [16] or ICE [12]). The shared secret
MUST have at least 128 bits of randomness.
When a client is needs to send a Request or an Indication, it can do
one of three things:
1. send the message without MESSAGE-INTEGRITY, if permitted by the
STUN usage.
2. use a short term credential, as determined by the STUN usage. In
this case, the STUN Request or STUN Indication would contain the
USERNAME and MESSAGE-INTEGRITY attributes. The message would not
contain the NONCE attribute. The key for MESSAGE-INTEGRITY is
the password.
3. use long term credential, as determined by STUN usage. In this
case, the STUN request contains the USERNAME, REALM, and MESSAGE-
INTEGRITY attributes. The request does not contain the NONCE
attribute. The key for MESSAGE-INTEGRITY is MD5(unq(USERNAME-
value) ":" unq(REALM-value) ":" password).
Based on the STUN usage, the server does one of four things:
1. The server processes the request and generates a response. If
the request included the MESSAGE-INTEGRITY attribute, the server
would also include MESSAGE-INTEGRITY in its response.
2. The server generates an error response indicating that MESSAGE-
INTEGRITY with short-term or with long-term credentials are
required (error 401). To indicate that short-term credentials
are required, the REALM attribute MUST NOT be present in the
error response. To indicate short-term credentials are required,
the REALM attribute MOST be present in the error response.
3. The server generates an error response indicating that a NONCE
attribute is required (error 435) or that the supplied NONCE
attribute's value is stale (error 437).
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4. The server generates an error response indicating that the short-
term credentials are no longer valid (error 430). The client
will have to obtain new short-term credentials appropriate to its
STUN usage.
In all of the above error responses, the NONCE attribute MAY
optionally be included in the error response, in which case the
client MUST include that NONCE in the subsequent STUN transaction.
The NONCE value is not stored by the STUN client; it is only valid
for the subsequent STUN transaction and that transactions
retransmissions.
STUN messages generated in order to obtain the shared secret are
formulated like other messages by following Section 8.1.3.
8.1.3. Formulating the Request Message
The client follows the syntax rules defined in Section 6 and the
transmission rules of Section 7. The message type of the MUST be a
request type; "Binding Request" or "Shared Secret Request" are the
two defined by this document.
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.
Once formulated, the client sends the Binding Request. Reliability
is accomplished through client retransmissions, following the
procedure in Section 7.1.
The client MAY send multiple requests on the connection, and it may
pipeline requests (that is, it can have multiple requests outstanding
at the same time). When using TCP the client SHOULD close the
connection as soon as it has received the STUN Response.
8.1.4. Processing Responses
All responses, 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 11.8. 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
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attribute in the request.
If the computed HMAC matches the one from the response, processing
continues. The response can either be a Binding Response or Binding
Error Response.
If the response is an 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 one-time username and password, and retry the
Allocate 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. A new one-time username and password is needed in that
case. 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 300 response code, the client SHOULD attempt a new
transaction to the server indicated in the ALTERNATE-SERVER
attribute. For a 500 response code, the client MAY wait several
seconds and then retry the request with a new username and password.
For a 600 response code, client MUST NOT retry the request and SHOULD
display the reason phrase to the user. 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.
Binding Responses containing unknown optional attributes (greater
than 0x7FFF) MUST be ignored by the STUN client. Binding Responses
containing unknown mandatory attributions (less than or equal to
0x7FFF) MUST be discarded and considered immediately as a failed
transaction.
It is also possible for an IPv4 host to receive a XOR-MAPPED-ADDRESS
or MAPPED-ADDRESS containing an IPv6 address, or for an IPv6 host to
receive a XOR-MAPPED-ADDRESS or MAPPED-ADDRESS containing an IPv4
address. Clients MUST be prepared for this case.
8.1.5. Using the Mapped Address
This section applies to the Binding Response message type. The
Binding Response message type always includes either the MAPPED-
ADDRESS attribute or the XOR-MAPPED-ADDRESS attribute, depending on
the presence of the magic cookie in the corresponding Binding
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Request.
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 which require a client to
insert an IP address and port into a message, to which subsequent
messages will be delivered by other entities in a network. Normally,
the client would insert the IP address and port from a local
interface into the message. However, if the client is behind a NAT,
this local interface will be a private address. Clients within other
address realms will not be able to send messages to that address.
An example of a such an application is SIP, which requires a client
to include IP address and port information in several places,
including the Session Description Protocol (SDP [19]) body carried by
SIP. The IP address and port present in the SDP is used for receipt
of media.
To use STUN as a technique for traversal of SIP and other protocols,
when the client wishes to send a protocol message, it figures out the
places in the protocol data unit where it is supposed to insert its
own IP address along with a port. Instead of directly using a port
allocated from a local interface, the client allocates a port from
the local interface, and from that port, initiates the STUN
procedures described above. The mapped address in the Binding
Response (XOR-MAPPED-ADDRESS or MAPPED- ADDRESS) provides the client
with an alternative IP address and port which it can then include in
the protocol payload. This IP address and port may be within a
different address family than the local interfaces used by the
client. This is not an error condition. In such a case, the client
would use the learned IP address and port as if the client was a host
with an interface within that address family.
In the case of SIP, to populate the SDP appropriately, a client would
generate two STUN Binding Request messages at the time a call is
initiated or answered. One is used to obtain the IP address and port
for RTP, and the other, for the Real Time Control Protocol
(RTCP)[14]. The client might also need to use STUN to obtain IP
addresses and ports for usage in other parts of the SIP message. The
detailed usage of STUN to facilitate SIP NAT traversal is outside the
scope of this specification.
As discussed above, the addresses learned by STUN may not be usable
with all entities with whom a client might wish to communicate. The
way in which this problem is handled depends on the application
protocol. The ideal solution is for a protocol to allow a client to
include a multiplicity of addresses and ports in the PDU. One of
those can be the address and port determined from STUN, and the
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others can include addresses and ports learned from other techniques.
The application protocol would then provide a means for dynamically
detecting which one works. An example of such an an approach is
Interactive Connectivity Establishment (ICE [12]).
8.2. Indication Message Types
This section applies to client behavior for the Indication message
types.
8.2.1. Formulating the Indication Message
The client follows the syntax rules defined in Section 6 and the
transmission rules of Section 7. The message type MUST be one of the
Indication message types; none are defined by this document.
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.
Once formulated, the client sends the Indication message. Indication
message types are not sent reliably, do not elicit a response from
the server, and are not retransmitted.
The client MAY send multiple indications on the connection, and it
may pipeline indication messages. When using TCP the client SHOULD
close the TCP connection as soon as it has transmitted the indication
message.
9. General Server Behavior
9.1. Request Message Types
The server behavior for receiving request message types is described
in this section.
9.1.1. Receive Request Message
A STUN server MUST be prepared to receive Request and Indication
messages on the IP address and UDP or TCP port that will be
discovered by the STUN client when the STUN client follows its
discovery procedures described in Section 8.1.1. 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. The usages describe how the STUN server determines
the usage.
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The server checks the request for a MESSAGE-INTEGRITY attribute. If
not present, the server generates an error response with an ERROR-
CODE attribute and a response code of 401. That error response MUST
include a NONCE attribute, containing a nonce that the server wishes
the client to reflect back in a subsequent request (and therefore
include in the message integrity computation). The error response
MUST include a REALM attribute, containing a realm from which the
username and password are scoped [8].
If the MESSAGE-INTEGRITY attribute was present, 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. That error response MUST also include a NONCE and a REALM
attribute.
If the REALM attribute was present, 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. That error response MUST include a NONCE attribute and a REALM
attribute.
If the NONCE 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. It MUST
include a NONCE attribute and a REALM attribute.
If the USERNAME attribute was present, the server computes the HMAC
over the request as described in Section 11.8. The key is computed
as MD5(unq(USERNAME-value) ":" unq(REALM-value) ":" password), where
the password is the password associated with the username and realm
provided in the request. If the server does not have a record for
that username within that realm, the server generates an error
response. That error response MUST include an ERROR-CODE attribute
with a response code of 436. That error response MUST include a
NONCE attribute and a REALM attribute.
This format for the key was chosen so as to enable a common
authentication database for SIP and STUN, as it is expected that
credentials are usually stored in their hashed forms.
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. This
response MUST include a NONCE attribute and a REALM attribute.
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If the computed HMAC doesn't differ from the one in the request, but
the nonce is stale, the server MUST generate an error response. That
error response MUST include an ERROR-CODE attribute with response
code 430. That error response MUST include a NONCE attribute and a
REALM attribute.
The server MUST check for any mantadory attributes in the request
(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. 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 server then copies the
Transaction ID from the Request to the Response. If the STUN
response is a success response, the STUN server adds 0x100 to the
Message Type; if a failure response the STUN server adds 0x110 to the
Message Type.
Depending in the Request message type and the message attributes of
the request, the response is constructed; see Figure 4.
9.1.3. Sending the Response
All Response messages are sent to the IP address and port the
associated Binding Request came from, and sent from the IP address
and port the Binding Request was sent to.
9.2. Indication Message Types
Indication messages cause the server to change its state. Indication
message types to not cause the server to send a response message.
Indication message types are defined in other documents, for example
in [3].
10. Short-Term Passwords
Short-term passwords are useful to provide authentication and
integrity protection to STUN Request and STUN Response messages.
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
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long-term password, the issuance of a short-term password is useful
to prevent dictionary attacks.
Short-term passwords can be used multiple times for as long as a
usage allows the same short-term password to be used. The duration
of validity is determined by usage.
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.
In order to align attributes on word boundaries, the length of the
all message attributes values MUST be 0 or a multiple of 4 bytes.
Extensions to this specification MUST also follow this requirement.
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
______________________________________________
MAPPED-ADDRESS - C -
USERNAME O - -
PASSWORD - - -
MESSAGE-INTEGRITY O O O
ERROR-CODE - - M
ALTERNATE-SERVER - - C
REALM C C C
NONCE C - C
UNKNOWN-ATTRIBUTES - - C
XOR-MAPPED-ADDRESS - M -
XOR-ONLY O - -
SERVER - O O
BINDING-LIFETIME - O -
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Figure 4: Mandatory Attributes and Message Types
11.1. MAPPED-ADDRESS
The MAPPED-ADDRESS attribute indicates the mapped IP address and
port. It consists of an eight bit address family, and a sixteen bit
port, followed by a fixed length value representing the IP address.
If the address family is IPv4, the address is 32 bits. If the
address family is IPv6, the address is 128 bits.
For backwards compatibility with RFC3489-compliant STUN clients, if
the magic cookie was not present in the associated Binding Request,
this attribute MUST be present in the associated response.
Discussion: Some NATs rewrite the 32-bit binary payloads
containing the NAT's public IP address, such as STUN's MAPPED-
ADDRESS attribute. Such behavior interferes with the operation of
STUN and also causes failure of STUN's message integrity checking.
Presence of the magic cookie in the STUN Request indicates the
client is compatible with this specification and is capable of
processing XOR-MAPPED-ADDRESS.
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 5: 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
Binding Request arrived from.
The first 8 bits of the MAPPED-ADDRESS are ignored for the purposes
of aligning parameters on natural boundaries.
11.2. RESPONSE-ADDRESS
The RESPONSE-ADDRESS attribute indicates where the response to a
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Binding Request should be sent. Its syntax is identical to MAPPED-
ADDRESS.
This attribute is not used by any STUN usages defined in this
document except for backwards compatibility with RFC3489 clients when
using the Binding Discovery usage (Section 12.2). Section 12.2.8
describes when this attribute must be included in a binding response.
Issue: should this attribute be made specific to Binding
Discovery or moved to another document entirely.
11.3. CHANGED-ADDRESS
The CHANGED-ADDRESS attribute indicates the IP address and port where
responses would have been sent from if the "change IP" and "change
port" flags had been set in the CHANGE-REQUEST attribute of the
Binding Request. Its syntax is identical to MAPPED-ADDRESS.
This attribute is not used by any STUN usages defined in this
document except for backwards compatibility with RFC3489 clients when
using the Binding Discovery usage (Section 12.2). Section 12.2.8
describes when this attribute must be included in a binding response.
Issue: should this attribute be made specific to Binding
Discovery or moved to another document entirely.
11.4. CHANGE-REQUEST
The CHANGE-REQUEST attribute is used by the client to request that
the server use a different address and/or port when sending the
response. The attribute is 32 bits long, although only two bits (A
and B) are used:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 A B 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The meaning of the flags are:
A: This is the "change IP" flag. If true, it requests the server to
send the Binding Response with a different IP address than the one
the Binding Request was received on.
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B: This is the "change port" flag. If true, it requests the server
to send the Binding Response with a different port than the one
the Binding Request was received on.
This attribute is not used by any STUN usages defined in this
document.
Issue: should this attribute be made specific to Binding
Discovery or moved to another document entirely.
11.5. SOURCE-ADDRESS
The SOURCE-ADDRESS attribute is present in Binding Responses. It
indicates the source IP address and port that the server is sending
the response from. Its syntax is identical to that of MAPPED-
ADDRESS.
This attribute is not used by any STUN usages defined in this
document except for backwards compatibility with RFC3489 clients when
using the Binding Discovery usage (Section 12.2). Section 12.2.8
describes when this attribute must be included in a binding response.
Issue: should this attribute be made specific to Binding
Discovery or moved to another document entirely.
11.6. USERNAME
The USERNAME attribute is used for message integrity. It identifies
the shared secret used in the message integrity check. The USERNAME
is always present in a Shared Secret Response, along with the
PASSWORD. When message integrity is used with Binding Request
messages, the USERNAME attribute MUST be included.
The value of USERNAME is a variable length opaque value.
11.7. 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
in the MESSAGE-INTEGRITY attribute of a subsequent STUN transaction.
11.8. MESSAGE-INTEGRITY
The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [9] of the STUN
message. The MESSAGE-INTEGRITY attribute can be present in any STUN
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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 the last attribute in any STUN message. However, STUN
clients MUST be able to successfully parse and process STUN messages
which have additional attributes after the MESSAGE-INTEGRITY
attribute. STUN clients that are compliant with this specification
SHOULD ignore attributes that are after the MESSAGE-INTEGRITY
attribute.
The key used as input to HMAC depends on the STUN usage and the
shared secret mechanism.
11.9. ERROR-CODE
The ERROR-CODE attribute is present in the Binding Error Response and
Shared Secret Error Response. It is a numeric value in the range of
100 to 699 plus a textual reason phrase encoded in UTF-8, and is
consistent in its code assignments and semantics with SIP [10] and
HTTP [11]. The reason phrase is meant for user consumption, and can
be anything appropriate for the response code. The length of the
reason phrase MUST be a multiple of 4 (measured in bytes),
accomplished by added spaces to the end of the text, if necessary.
Recommended reason phrases for the defined response codes are
presented below.
To facilitate processing, the class of the error code (the hundreds
digit) is encoded separately from the rest of the code.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |Class| Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason Phrase (variable) ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The class represents the hundreds digit of the response code. The
value MUST be between 1 and 6. The number represents the response
code modulo 100, and its value MUST be between 0 and 99.
The following response codes, along with their recommended reason
phrases (in brackets) are defined at this time:
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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 Binding Request did not contain a MESSAGE-
INTEGRITY attribute.
420 (Unknown Attribute): The server did not understand a mandatory
attribute in the request.
430 (Stale Credentials): The Binding Request did contain a MESSAGE-
INTEGRITY attribute, but it used a shared secret that has
expired. The client should obtain a new shared secret and try
again.
431 (Integrity Check Failure): The Binding Request contained a
MESSAGE-INTEGRITY attribute, but the HMAC failed verification.
This could be a sign of a potential attack, or client
implementation error.
432 (Missing Username): The Binding Request contained a MESSAGE-
INTEGRITY attribute, but not a USERNAME attribute. Both
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 in the given REALM.
437 (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.
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600 (Global Failure): The server is refusing to fulfill the
request. The client should not retry.
Issue: Do 300/500/600 mean that other STUN servers returned in
the same SRV lookup should be retried / not retried? With same
SRV Priority?
11.10. REFLECTED-FROM
The REFLECTED-FROM attribute is present only in Binding Responses,
when the Binding Request contained a RESPONSE-ADDRESS attribute. The
attribute contains the identity (in terms of IP address) of the
source where the request came from. Its purpose is to provide
traceability, so that a STUN server cannot be used as a reflector for
denial-of-service attacks. Its syntax is identical to the MAPPED-
ADDRESS attribute.
This attribute is not used by any STUN usages defined in this
document.
Issue: should this attribute be made specific to Binding
Discovery or moved to another document entirely.
11.11. ALTERNATE-SERVER
The alternate server represents an alternate IP address and port for
a different TURN server to try. It is encoded in the same way as
MAPPED-ADDRESS.
11.12. REALM
The REALM attribute is present in Requests and Responses. It
contains text which meets the grammar for "realm" as described in
RFC3261 [10], and will thus contain a quoted string (including the
quotes).
Presence of the REALM attribute indicates that long-term credentials
are used for the values of the USERNAME, PASSWORD, and MESSAGE-
INTEGRITY attributes.
11.13. 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
RFC3261 [10].
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11.14. UNKNOWN-ATTRIBUTES
The UNKNOWN-ATTRIBUTES attribute is present only in a Binding Error
Response or Shared Secret Error Response when the response code in
the ERROR-CODE attribute is 420.
The attribute contains a list of 16 bit values, each of which
represents an attribute type that was not understood by the server.
If the number of unknown attributes is an odd number, one of the
attributes MUST be repeated in the list, so that the total length of
the list is a multiple of 4 bytes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute 1 Type | Attribute 2 Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute 3 Type | Attribute 4 Type ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Format of UNKNOWN-ATTRIBUTES attribute
11.15. XOR-MAPPED-ADDRESS
The XOR-MAPPED-ADDRESS attribute is only present in Binding
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. See the
discussion in Section 11.1.
This attribute MUST always be present in a Binding Response.
Note: Version -02 of this Internet Draft used 0x8020 for this
attribute, which was in the Optional range of attributes. This
attribute has been moved back to 0x0020 as a Mandatory attribute.
[This paragraph should be removed prior to publication as an RFC.]
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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 mapped IP address exclusive or'd with the magic 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.
Issue: The motivation for XORing the IP address is clear. Is
there a motivation for XORing the port?
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.
11.16. 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 length of
the server attribute MUST be a multiple of 4 (measured in bytes),
accomplished by added spaces to the end of the text, if necessary.
The value of SERVER is variable length.
11.17. ALTERNATE-SERVER
The alternate server represents an alternate IP address and port for
a different STUN server to try. It is encoded in the same way as
MAPPED-ADDRESS.
This attribute is MUST only appear in an Error Response. This
attribute MUST only appear when using the TURN usage.
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11.18. BINDING-LIFETIME
The binding lifetime indicates the number of seconds the NAT binding
will be valid. This attribute MUST only be present in Response
messages. This attribute MUST NOT be present unless the STUN server
is aware of the minimum binding lifetime of all NATs on the path
between the STUN client and the STUN server.
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 usages may differ in how STUN servers
are discovered, the message types, and the message attributes that
are supported.
This specification defines the STUN usages for binding discovery
(Section 12.2), connectivity check (Section 12.3), NAT keepalives
(Section 12.4) and short-term password (Section 12.5).
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. Defined STUN Usages
12.2. Binding Discovery
The previous version of this specification, RFC3489 [13], described
only this binding discovery usage.
12.2.1. Applicability
Binding discovery is useful to learn reflexive addresses from servers
on the network. That is, it is used to determine your dynamically-
bound 'public' IP address and UDP port that is assigned by a NAT
between a STUN client and a STUN server. This usage is used with ICE
[12].
When short-term passwords are used with binding discovery, the
username and password are valid for subsequent transactions for nine
(9) minutes.
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12.2.2. Client Discovery of Server
The general client discovery of server behavior is sufficient for
this usage.
12.2.3. Server Determination of Usage
The general binding server behavior is sufficient for this usage.
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.
12.2.6. New Error Response Codes
This usage does not define any new error response codes.
12.2.7. Client Procedures
This usage does not define any new client procedures.
12.2.8. Server Procedures
In this usage, the short-term password is valid for 30 seconds after
its initial assignment.
For backwards compatibility with RFC3489-compliant STUN servers, if
the STUN server receives a Binding request without the magic cookie,
the STUN server MUST include the following attributes in the Binding
response; otherwise these attribute MUST NOT be included:
MAPPED-ADDRESS
SOURCE-ADDRESS
Likewise if the STUN server receives a Binding Request containing the
CHANGE-REQUEST attribute without the magic cookie, the STUN server
MUST include the CHANGED-ADDRESS attribute in its Binding Response.
12.2.9. Security Considerations for Binding Discovery
Issue: Currently, the security considerations applies to all the
various usages. Split it up to talk about each one? Create
subsections talking about each usage?
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12.3. Connectivity Check
12.3.1. Applicability
This STUN usage primarily provides a connectivity check to a peer
discovered through rendezvous protocols and additionally allows
learning reflexive address discovery to the peer.
The username and password exchanged in the rendezvous protocol is
valid for the duration of the connection being checked.
12.3.2. Client Discovery of Server
The client does not follow the general procedure in Section 8.1.1.
Instead, the client discovers the STUN server's IP address and port
through a rendezvous protocol such as Session Description Protocol
(SDP [19]). An example of such a discovery technique is ICE [12].
12.3.3. Server Determination of Usage
The server is aware of this usage because it signalsed this port
through the rendezvous protocol.
When operating in this usage, the STUN server is listening on an
ephemeral port rather than the IANA-assigned STUN port. The server
is typically multiplexing two protocols on this port, one protocol is
STUN and the other protocol is the peer-to-peer protocol using that
same port. When used with ICE, the two protocols multiplexed on the
same port are STUN and RTP [14].
12.3.4. New Requests or Indications
This usage does not define any new message types.
12.3.5. New Attributes
This usage does not define any new message attributes.
12.3.6. New Error Response Codes
This usage does not define any new error response codes.
12.3.7. Client Procedures
This usage does not define any new client procedures.
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12.3.8. Server Procedures
In this usage, the short-term password is valid as long as the UDP
port is listening for STUN packets. For example when used with ICE,
the short-term password would be valid as long as the RTP session
(which multiplexes STUN and RTP) is active.
12.3.9. Security Considerations for Connectivity Check
The username and password, which are used for STUN's message
integrity, are exchanged in the rendezvous protocol. Failure to
encrypt and integrity protect the rendezvous protocol is equivalent
in risk to using STUN without message integrity.
12.4. NAT Keepalives
12.4.1. Applicability
This usage is useful in two cases: keeping a NAT binding open in a
client connection to a server and detecting server failure and NAT
reboots.
The username and password used for STUN integrity can be used for 24
hours.
Issue: do we need message integrity for keepalives when doing
STUN and SIP on the same port? Do we need message integrity for
keepalives when doing STUN and RTP on the same port (recvonly,
inactive)
If yes, do we continue using same STUN username/password forever
(days?)
12.4.2. Client Discovery of Server
In this usage, the STUN server and the application protocol are using
the same fixed port. While the multiplexing of two applications on
the same port is similar to the connectivity check (Section 12.3)
usage, this usage is differs as the server's port is fixed and the
server's port isn't communicated using a rendezvous protocol.
12.4.3. 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.
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12.4.4. New Requests or Indications
This usage does not define any new message types.
12.4.5. New Attributes
This usage does not define any new message attributes.
12.4.6. New Error Response Codes
This usage does not define any new error response codes.
12.4.7. 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 should send a new registration message indicating the new
mapped address.
12.4.8. Server Procedures
In this usage no authentication is used so there is no duration of
the short-term password.
12.4.9. Security Considerations for NAT Keepalives
Issue: Currently, the security considerations applies to all the
various usages. Split it up to talk about each one? Create
subsections talking about each usage?
12.5. Short-Term Password
In order to ensure interoperability, this usage describes a TLS-based
mechanism to obtain a short-term username and short-term password.
12.5.1. 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 hosts that have the same SRV Priority value as
discovered via Section 12.5.2. The username and password obtained
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with this usage are used as the USERNAME and as the HMAC for the
MESSAGE-ID in a subsequent STUN message, respectively.
The duration of validity of the username and password obtained via
this usage depends on the usage of the subsequent STUN messages that
are protected with that username and password.
12.5.2. Client Discovery of Server
The client follows the procedures in Section 8.1.1, except the SRV
protocol is TCP rather than UDP and the service name "stun-tls".
For example a client would look up "_stun-tls._tcp.example.com" in
DNS.
12.5.3. Server Determination of Usage
The server advertises this port in the DNS as capable of receiving
TLS-protected STUN messages for this usage. The server MAY also
advertise this same port in DNS for other TCP usages if the server is
capable of multiplexing those different usages. For example, the
server could advertise
12.5.4. 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 N/A 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 - M -
PASSWORD - M -
ERROR-CODE - - M
UNKNOWN-ATTRIBUTES - - C
SERVER - O O
REALM C - C
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Note: As this usage requires running over TLS, MESSAGE-INTEGRITY
isn't necessary.
12.5.5. New Attributes
No new attributes are defined by this usage.
12.5.6. New Error Response Codes
This usage does not define any new error response codes.
12.5.7. Client Procedures
The client opens up the connection to that address and port, and
immediately begins TLS negotiation[6]. The client MUST verify the
identity of the server. To do that, it follows the identification
procedures defined in Section 3.1 of RFC2818 [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.
If the response was a Shared Secret Error Response, the client checks
the response code in the ERROR-CODE attribute. 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.
12.5.8. Server Procedures
After a client has established a TLS session, the server should
expect a STUN message containing a Shared Secret Request. The server
will generates a response, which can either be a Shared Secret
Response or a Shared Secret Error Response.
12.5.9. Security Considerations for Short-Term Password
Issue: Currently, the security considerations applies to all the
various usages. Split it up to talk about each one? Create
subsections talking about each usage?
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13. Security Considerations
Issue: This section has not been revised to properly consider the
attacks on each of STUN's different usages. This needs to be done.
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. STUN servers create state through
the Shared Secret Request mechanism. To prevent being swamped with
traffic, a STUN server SHOULD limit the number of simultaneous TLS
connections it will hold open by dropping an existing connection when
a new connection request arrives (based on an Least Recently Used
(LRU) policy, for example). Similarly, if the server is storing
short-term passwords it SHOULD limit the number of shared secrets it
will store. 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
XOR-MAPPED-ADDRESS or MAPPED-ADDRESS. In the sections below, we
refer to either the XOR-MAPPED-ADDRESS or MAPPED-ADDRESS as just the
mapped address (note the lower case). The attacks that can be
launched using such a technique include:
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
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 an IP address that routes to nowhere. As a result, the
client won't receive any of the packets it expects to receive when it
hands out the mapped address. This exploitation is not very
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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
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 IP address and UDP ports 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:
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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.
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 address of the packet to be that of the
desired mapped address. This address cannot be arbitrary. If the
attacker is on the public Internet (that is, there are no NATs
between it and the STUN server), and the attacker doesn't modify the
STUN request, the address has to have the property that packets sent
from the STUN server to that address would route through the
compromised router. This is because the STUN server will send the
responses back to the source address of the request. With a modified
source address, the only way they can reach the client is if the
compromised router directs them there. If the attacker is on the
public Internet, but they can modify the STUN request, they can
insert a RESPONSE-ADDRESS attribute into the request, containing the
actual source address of the STUN request. This will cause the
server to send the response to the client, independent of the source
address the STUN server sees. This gives the attacker the ability to
forge an arbitrary source address when it forwards the STUN request.
Todo: RESPONSE-ADDRESS has been removed from this version of the
specification. Reword or remove above paragraph accordingly.
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
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force the server to generate arbitrary mapped addresses in responses.
They will only be able force the STUN server to generate mapped
addresses which route to the private network. This is because the
NAT between the attacker and the STUN server will rewrite the source
address of the STUN request, mapping it to a public address that
routes to the private network. Because of this, the attacker can
only force the server to generate faked mapped addresses that route
to the private network. Unfortunately, it is possible that a low
quality NAT would be willing to map an allocated public address to
another public address (as opposed to an internal private address),
in which case the attacker could forge the source address in a STUN
request to be an arbitrary public address. This kind of behavior
from NATs does appear to be rare.
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
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
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attacker listens on the network for a STUN request. When it sees it,
it generates its own STUN request towards the server. This STUN
request is identical to the one it saw, but with a spoofed source IP
address. The spoofed address is equal to the one that the attacker
desires to have placed in the mapped address of the STUN response.
In fact, the attacker generates a flood of such packets. The STUN
server will receive the one original request, plus a flood of
duplicate fake ones. It generates responses to all of them. If the
flood is sufficiently large for the responses to congest routers or
some other equipment, there is a reasonable probability that the one
real response is lost (along with many of the faked ones), but the
net result is that only the faked responses are received by the STUN
client. These responses are all identical and all contain the mapped
address that the attacker wanted the client to use.
The flood of duplicate packets is not needed (that is, only one faked
request is sent), so long as the faked response beats the real
response back to the client, and the client uses the first response,
and ignores the second (even though it's different).
Note that, in this approach, launching a DoS attack against the STUN
server or the IP network, to prevent the valid response from being
sent or received, is problematic. The attacker needs the STUN server
to be available to handle its own request. Due to the periodic
retransmissions of the request from the client, this leaves a very
tiny window of opportunity. The attacker must start the DoS attack
immediately after the actual request from the client, causing the
correct response to be discarded, and then cease the DoS attack in
order to send its own request, all before the next retransmission
from the client. Due to the close spacing of the retransmits (100ms
to a few seconds), this is very difficult to do.
Besides DoS attacks, there may be other ways to prevent the actual
request from the client from reaching the server. Layer 2
manipulations, for example, might be able to accomplish it.
Fortunately, 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.
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 [7]. This is particularly
important for the NATs themselves. As Section 13.2.3 explains, NATs
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which do not perform this check can be used as "reflectors" in DDoS
attacks. Most NATs do perform this check as a default mode of
operation. We strongly advise people that purchase NATs to ensure
that this capability is present and enabled.
Secondly, it is RECOMMENDED that STUN servers be run on hosts
dedicated to STUN, with all UDP and TCP ports disabled except for the
STUN ports. This is to prevent viruses and Trojan horses from
infecting STUN servers, in order to prevent their compromise. This
helps mitigate Approach I (Section 13.2.1).
Thirdly, to prevent the DNS attack of Section 13.2.2, Section 8.1.2
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 [20].
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 further prevented by using the message
integrity mechanism provided in STUN, described in Section 11.8.
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. All three approaches are most
potent when the attacker can modify the request, inserting a
RESPONSE-ADDRESS that routes to the client. Fortunately, such
modifications are preventable using the message integrity techniques
described in Section 11.8. However, these three approaches are still
functional when the attacker modifies nothing but the source address
of the STUN request. Sadly, this is the one thing that cannot be
protected through cryptographic means, as this is the change that
STUN itself is seeking to detect and report. It is therefore an
inherent weakness in NAT, and not fixable in STUN.
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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
Todo: The diagnostic usages have been removed from this document,
which reduces the brittleness of STUN. This section should be
updated accordingly.
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 [21]).
STUN is an example of a protocol that performs this type of function.
The IAB has mandated that any protocols developed for this purpose
document a specific set of considerations. This section meets those
requirements.
14.1. Problem Definition
From RFC3424 [21], 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 a means for a
client to obtain an address on the public Internet from a non-
symmetric NAT, for the express purpose of receiving incoming UDP
traffic from another host, targeted to that address. STUN does not
address traversal of NATs using TCP, either incoming or outgoing, and
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does not address outgoing UDP communications.
14.2. Exit Strategy
From RFC3424 [21], 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
[12]), which allow a client to determine whether addresses learned
from STUN are needed, or whether other addresses, such as the one on
the local interface, will work when communicating with another host.
With such a detection technique, as a client finds that the addresses
provided by STUN are never used, STUN queries can cease to be made,
thus allowing them to phase out.
STUN can also help facilitate the introduction of other NAT traversal
techniques such as MIDCOM [22]. As midcom-capable NATs are deployed,
applications will, instead of using STUN (which also resides at the
application layer), first allocate an address binding using midcom.
However, it is a well-known limitation of MIDCOM that it only works
when the agent knows the middleboxes through which its traffic will
flow. Once bindings have been allocated from those middleboxes, a
STUN detection procedure can validate that there are no additional
middleboxes on the path from the public Internet to the client. If
this is the case, the application can continue operation using the
address bindings allocated from MIDCOM. If it is not the case, STUN
provides a mechanism for self-address fixing through the remaining
MIDCOM-unaware middleboxes. Thus, STUN provides a way to help
transition to full MIDCOM-aware networks.
14.3. Brittleness Introduced by STUN
From RFC3424 [21], any UNSAF proposal must provide:
Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.
STUN introduces brittleness into the system in several ways:
o The binding acquisition usage is dependant on NAT's behavior when
forwarding UDP packets from arbitrary hosts on the public side of
the NAT. Application specific processing will generally be
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needed. For symmetric NATs, the binding acquisition will not
yield a usable address. The tight dependency on the specific type
of NAT makes the protocol brittle.
o STUN assumes that the server exists on the public Internet. If
the server is located in another private address realm, the user
may or may not be able to use its discovered address to
communicate with other users. There is no way to detect such a
condition.
o The bindings allocated from the NAT need to be continuously
refreshed. Since the timeouts for these bindings is very
implementation specific, the refresh interval cannot easily be
determined. When the binding is not being actively used to
receive traffic, but to wait for an incoming message, the binding
refresh will needlessly consume network bandwidth.
o The use of the STUN server as an additional network element
introduces another point of potential security attack. These
attacks are largely prevented by the security measures provided by
STUN, but not entirely.
o The use of the STUN server as an additional network element
introduces another point of failure. If the client cannot locate
a STUN server, or if the server should be unavailable due to
failure, the application cannot function.
o The use of STUN to discover address bindings will result in an
increase in latency for applications. For example, a Voice over
IP application will see an increase of call setup delays equal to
at least one RTT to the STUN server.
o STUN imposes some restrictions on the network topologies for
proper operation. If client A obtains an address from STUN server
X, and sends it to client B, B may not be able to send to A using
that IP address. The address will not work if any of the
following is true:
* The STUN server is not in an address realm that is a common
ancestor (topologically) of both clients A and B. For example,
consider client A and B, both of which have residential NAT
devices. Both devices connect them to their cable operators,
but both clients have different providers. Each provider has a
NAT in front of their entire network, connecting it to the
public Internet. If the STUN server used by A is in A's cable
operator's network, an address obtained by it will not be
usable by B. The STUN server must be in the network which is a
common ancestor to both - in this case, the public Internet.
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* The STUN server is in an address realm that is a common
ancestor to both clients, but both clients are behind the same
NAT connecting to that address realm. For example, if the two
clients in the previous example had the same cable operator,
that cable operator had a single NAT connecting their network
to the public Internet, and the STUN server was on the public
Internet, the address obtained by A would not be usable by B.
That is because some NATs will not accept an internal packet
sent to a public IP address which is mapped back to an internal
address. To deal with this, additional protocol mechanisms or
configuration parameters need to be introduced which detect
this case.
o Most significantly, STUN introduces potential security threats
which cannot be eliminated. This specification describes
heuristics that can be used to mitigate the problem, but it is
provably unsolvable given what STUN is trying to accomplish.
These security problems are described fully in Section 13.
14.4. Requirements for a Long Term Solution
From RFC3424 [21], any UNSAF proposal must provide:
Identify requirements for longer term, sound technical solutions
-- contribute to the process of finding the right longer term
solution.
Our experience with STUN has led to the following requirements for a
long term solution to the NAT problem:
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.
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. This is only true for first-party controls; third-
party controls are best handled using the MIDCOM framework.
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
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binding to reach the address where the control packets are coming
from.
o Simplicity is Paramount. The control protocol will need to be
implement in very simple clients. The servers will need to
support extremely high loads. The protocol will need to be
extremely robust, being the precursor to a host of application
protocols. As such, simplicity is key.
14.5. Issues with Existing NAPT Boxes
From RFC3424 [21], any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports.
Several of the practical issues with STUN involve future proofing -
breaking the protocol when new NAT types get deployed. Fortunately,
this is not an issue at the current time, since most of the deployed
NATs are of the types assumed by STUN. The primary usage STUN has
found is in the area of VoIP, to facilitate allocation of addresses
for receiving RTP [14] traffic. In that application, the periodic
keepalives are usually (but not always) provided by the RTP traffic
itself. However, several practical problems arise for RTP. First,
in the absence of [23], RTP assumes that RTCP traffic is on a port
one higher than the RTP traffic. This pairing property cannot be
guaranteed through NATs that are not directly controllable. As a
result, RTCP traffic may not be properly received. [23] mitigates
this by allowing the client to signal a different port for RTCP but
there will be interoperability problems for some time.
For VoIP, silence suppression can cause a gap in the transmission of
RTP packets. If that silence period exceeds the NAT binding timeout,
this could result in the loss of a NAT binding in the middle of a
call. This can be mitigated by sending occasional packets to keep
the binding alive. However, the result is additional brittleness.
14.6. In Closing
The problems with STUN are not design flaws in STUN. The problems in
STUN have to do with the lack of standardized behaviors and controls
in NATs. The result of this lack of standardization has been a
proliferation of devices whose behavior is highly unpredictable,
extremely variable, and uncontrollable. STUN does the best it can in
such a hostile environment. Ultimately, the solution is to make the
environment less hostile, and to introduce controls and standardized
behaviors into NAT. However, until such time as that happens, STUN
provides a good short term solution given the terrible conditions
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under which it is forced to operate.
15. IANA Considerations
IANA is hereby requsted to create two new registries STUN Message
Types and STUN Attributes. IANA must assign the following values to
both registeries before publication of this document as an RFC. New
values for both STUN Message Type and STUN Attributes are assigned
through the IETF consensus process via RFCs approved by the IESG.
15.1. STUN Message Type Registry
For STUN Message Types that are request message types, they MUST be
registered including associated Response message types and Error
Response message types, and those responses must have values that are
0x100 and 0x110 higher than their respective Request values.
For STUN Message Types that are Indication message types, no
associated restriction applies. As the message type field is only 14
bits the range of valid values is 0x001 through 0x3FFF.
The initial STUN Message Types are:
0x0001 : Binding Request
0x0101 : Binding Response
0x0111 : Binding Error Response
0x0002 : Shared Secret Request
0x0102 : Shared Secret Response
0x0112 : Shared Secret Error Response
0x0002 : Shared Secret Request
0x0102 : Shared Secret Responsed
0x0112 : Shared Secret Error Response
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.
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The initial STUN Attributes are:
0x0001: MAPPED-ADDRESS
0x0002: RESPONSE-ADDRESS
0x0003: CHANGE-REQUEST
0x0004: SOURCE-ADDRESS
0X0005: CHANGED-ADDRESS
0x0006: USERNAME
0x0007: PASSWORD
0x0008: MESSAGE-INTEGRITY
0x0009: ERROR-CODE
0x000A: UNKNOWN-ATTRIBUTES
0x000B: REFLECTED-FROM
0x000E: ALTERNATE-SERVER
0x0014: REALM
0x0015: NONCE
0x0020: XOR-MAPPED-ADDRESS
0x8022: SERVER
0x8023: ALTERNATE-SERVER
0x8024: BINDING-LIFETIME
16. Changes Since RFC 3489
This specification updates RFC3489 [13]. 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 Removed the STUN example that centered around the separation of
the control and media planes. Instead, provided more information
on using STUN with protocols.
o Added 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.
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o Explicitly point out that the most significant two bits of STUN
are 0b00, allowing easy differentiation with RTP packets when used
with ICE.
o Added support for IPv6. Made it clear that an IPv4 client could
get a v6 mapped address, and vice-a-versa.
o Added the SERVER, 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.
17. Acknowledgements
The authors would like to thank Cedric Aoun, Pete Cordell, Cullen
Jennings, Bob Penfield and Chris Sullivan for their comments, and
Baruch Sterman and Alan Hawrylyshen for initial implementations.
Thanks for Leslie Daigle, Allison Mankin, Eric Rescorla, and Henning
Schulzrinne for IESG and IAB input on this work.
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] Rosenberg, J., "Traversal Using Relay NAT (TURN)",
draft-rosenberg-midcom-turn-08 (work in progress),
September 2005.
[4] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[5] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[6] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[7] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating
Denial of Service Attacks which employ IP Source Address
Spoofing", BCP 38, RFC 2827, May 2000.
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[8] 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.
18.2. Informational References
[9] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[10] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[11] 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.
[12] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for
Offer/Answer Protocols", draft-ietf-mmusic-ice-06 (work in
progress), October 2005.
[13] 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.
[14] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[15] Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-01 (work in progress), October 2005.
[16] Kohl, J. and B. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.
[17] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
[18] Holdrege, M. and P. Srisuresh, "Protocol Complications with the
IP Network Address Translator", RFC 3027, January 2001.
[19] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[20] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
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RFC 3711, March 2004.
[21] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
[22] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A.
Rayhan, "Middlebox communication architecture and framework",
RFC 3303, August 2002.
[23] Huitema, C., "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October 2003.
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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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