TRAM M. Petit-Huguenin
Internet-Draft Impedance Mismatch
Obsoletes: 5389 (if approved) G. Salgueiro
Intended status: Standards Track J. Rosenberg
Expires: September 10, 2015 D. Wing
Cisco
R. Mahy
Plantronics
P. Matthews
Avaya
March 09, 2015
Session Traversal Utilities for NAT (STUN)
draft-ietf-tram-stunbis-02
Abstract
Session Traversal Utilities for NAT (STUN) is a protocol that serves
as a tool for other protocols in dealing with Network Address
Translator (NAT) traversal. It can be used by an endpoint to
determine the IP address and port allocated to it by a NAT. It can
also be used to check connectivity between two endpoints, and as a
keep-alive protocol to maintain NAT bindings. STUN works with many
existing NATs, and does not require any special behavior from them.
STUN is not a NAT traversal solution by itself. Rather, it is a tool
to be used in the context of a NAT traversal solution.
This document obsoletes RFC 5389.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 10, 2015.
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Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. STUN Message Structure . . . . . . . . . . . . . . . . . . . 9
6. Base Protocol Procedures . . . . . . . . . . . . . . . . . . 11
6.1. Forming a Request or an Indication . . . . . . . . . . . 11
6.2. Sending the Request or Indication . . . . . . . . . . . . 12
6.2.1. Sending over UDP or DTLS-over-UDP . . . . . . . . . . 13
6.2.2. Sending over TCP or TLS-over-TCP . . . . . . . . . . 14
6.2.3. Sending over SCTP-over-UDP or SCTP-over-DTLS-over-UDP 15
6.2.4. Sending over TLS-over-TCP or DTLS-over-UDP or SCTP-
over-DTLS-over-UDP . . . . . . . . . . . . . . . . . 17
6.3. Receiving a STUN Message . . . . . . . . . . . . . . . . 18
6.3.1. Processing a Request . . . . . . . . . . . . . . . . 18
6.3.1.1. Forming a Success or Error Response . . . . . . . 19
6.3.1.2. Sending the Success or Error Response . . . . . . 20
6.3.2. Processing an Indication . . . . . . . . . . . . . . 20
6.3.3. Processing a Success Response . . . . . . . . . . . . 21
6.3.4. Processing an Error Response . . . . . . . . . . . . 21
7. FINGERPRINT Mechanism . . . . . . . . . . . . . . . . . . . . 22
8. DNS Discovery of a Server . . . . . . . . . . . . . . . . . . 22
8.1. STUN URI Scheme Semantics . . . . . . . . . . . . . . . . 23
9. Authentication and Message-Integrity Mechanisms . . . . . . . 24
9.1. Short-Term Credential Mechanism . . . . . . . . . . . . . 24
9.1.1. HMAC Key . . . . . . . . . . . . . . . . . . . . . . 24
9.1.2. Forming a Request or Indication . . . . . . . . . . . 25
9.1.3. Receiving a Request or Indication . . . . . . . . . . 25
9.1.4. Receiving a Response . . . . . . . . . . . . . . . . 26
9.1.5. Sending Subsequent Requests . . . . . . . . . . . . . 26
9.2. Long-Term Credential Mechanism . . . . . . . . . . . . . 26
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9.2.1. HMAC Key . . . . . . . . . . . . . . . . . . . . . . 27
9.2.2. Forming a Request . . . . . . . . . . . . . . . . . . 28
9.2.2.1. First Request . . . . . . . . . . . . . . . . . . 28
9.2.2.2. Subsequent Requests . . . . . . . . . . . . . . . 28
9.2.3. Receiving a Request . . . . . . . . . . . . . . . . . 29
9.2.4. Receiving a Response . . . . . . . . . . . . . . . . 30
10. ALTERNATE-SERVER Mechanism . . . . . . . . . . . . . . . . . 31
11. Backwards Compatibility with RFC 5389 . . . . . . . . . . . . 32
12. Basic Server Behavior . . . . . . . . . . . . . . . . . . . . 32
13. STUN Usages . . . . . . . . . . . . . . . . . . . . . . . . . 33
14. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 34
14.1. MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . . . 35
14.2. XOR-MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . 35
14.3. USERNAME . . . . . . . . . . . . . . . . . . . . . . . . 36
14.4. MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . 37
14.5. MESSAGE-INTEGRITY2 . . . . . . . . . . . . . . . . . . . 37
14.6. FINGERPRINT . . . . . . . . . . . . . . . . . . . . . . 38
14.7. ERROR-CODE . . . . . . . . . . . . . . . . . . . . . . . 39
14.8. REALM . . . . . . . . . . . . . . . . . . . . . . . . . 40
14.9. NONCE . . . . . . . . . . . . . . . . . . . . . . . . . 40
14.10. PASSWORD-ALGORITHMS . . . . . . . . . . . . . . . . . . 41
14.11. PASSWORD-ALGORITHM . . . . . . . . . . . . . . . . . . . 41
14.12. UNKNOWN-ATTRIBUTES . . . . . . . . . . . . . . . . . . . 42
14.13. SOFTWARE . . . . . . . . . . . . . . . . . . . . . . . . 42
14.14. ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . . 43
14.15. ALTERNATE-DOMAIN . . . . . . . . . . . . . . . . . . . . 43
15. Security Considerations . . . . . . . . . . . . . . . . . . . 43
15.1. Attacks against the Protocol . . . . . . . . . . . . . . 43
15.1.1. Outside Attacks . . . . . . . . . . . . . . . . . . 43
15.1.2. Inside Attacks . . . . . . . . . . . . . . . . . . . 44
15.2. Attacks Affecting the Usage . . . . . . . . . . . . . . 44
15.2.1. Attack I: Distributed DoS (DDoS) against a Target . 45
15.2.2. Attack II: Silencing a Client . . . . . . . . . . . 45
15.2.3. Attack III: Assuming the Identity of a Client . . . 46
15.2.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . 46
15.3. Hash Agility Plan . . . . . . . . . . . . . . . . . . . 46
16. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 46
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
17.1. STUN Methods Registry . . . . . . . . . . . . . . . . . 47
17.2. STUN Attribute Registry . . . . . . . . . . . . . . . . 47
17.3. STUN Error Code Registry . . . . . . . . . . . . . . . . 48
17.4. Password Algorithm Registry . . . . . . . . . . . . . . 49
17.4.1. Password Algorithms . . . . . . . . . . . . . . . . 49
17.4.1.1. Salted SHA256 . . . . . . . . . . . . . . . . . 49
17.5. STUN UDP and TCP Port Numbers . . . . . . . . . . . . . 49
18. Changes since RFC 5389 . . . . . . . . . . . . . . . . . . . 50
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
19.1. Normative References . . . . . . . . . . . . . . . . . . 50
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19.2. Informational References . . . . . . . . . . . . . . . . 52
Appendix A. C Snippet to Determine STUN Message Types . . . . . 53
Appendix B. Release notes . . . . . . . . . . . . . . . . . . . 54
B.1. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 54
B.2. Modifications between draft-ietf-tram-stunbis-02 and
draft-ietf-tram-stunbis-01 . . . . . . . . . . . . . . . 54
B.3. Modifications between draft-ietf-tram-stunbis-01 and
draft-ietf-tram-stunbis-00 . . . . . . . . . . . . . . . 55
B.4. Modifications between draft-salgueiro-tram-stunbis-02 and
draft-ietf-tram-stunbis-00 . . . . . . . . . . . . . . . 55
B.5. Modifications between draft-salgueiro-tram-stunbis-02 and
draft-salgueiro-tram-stunbis-01 . . . . . . . . . . . . . 56
B.6. Modifications between draft-salgueiro-tram-stunbis-01 and
draft-salgueiro-tram-stunbis-00 . . . . . . . . . . . . . 56
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 56
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 57
1. Introduction
The protocol defined in this specification, Session Traversal
Utilities for NAT, provides a tool for dealing with NATs. It
provides a means for an endpoint to determine the IP address and port
allocated by a NAT that corresponds to its private IP address and
port. It also provides a way for an endpoint to keep a NAT binding
alive. With some extensions, the protocol can be used to do
connectivity checks between two endpoints [RFC5245], or to relay
packets between two endpoints [RFC5766].
In keeping with its tool nature, this specification defines an
extensible packet format, defines operation over several transport
protocols, and provides for two forms of authentication.
STUN is intended to be used in context of one or more NAT traversal
solutions. These solutions are known as STUN usages. Each usage
describes how STUN is utilized to achieve the NAT traversal solution.
Typically, a usage indicates when STUN messages get sent, which
optional attributes to include, what server is used, and what
authentication mechanism is to be used. Interactive Connectivity
Establishment (ICE) [RFC5245] is one usage of STUN. SIP Outbound
[RFC5626] is another usage of STUN. In some cases, a usage will
require extensions to STUN. A STUN extension can be in the form of
new methods, attributes, or error response codes. More information
on STUN usages can be found in Section 13.
Implementations and deployments of a STUN usage should follow the
recommendations in [I-D.ietf-uta-tls-bcp].
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2. Overview of Operation
This section is descriptive only.
/-----\
// STUN \\
| Server |
\\ //
\-----/
+--------------+ Public Internet
................| NAT 2 |.......................
+--------------+
+--------------+ Private NET 2
................| NAT 1 |.......................
+--------------+
/-----\
// STUN \\
| Client |
\\ // Private NET 1
\-----/
Figure 1: One Possible STUN Configuration
One possible STUN configuration is shown in Figure 1. In this
configuration, there are two entities (called STUN agents) that
implement the STUN protocol. The lower agent in the figure is the
client, and 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 upper agent in the figure is
the server, and resides on the public Internet.
STUN is a client-server protocol. It supports two types of
transactions. One is a request/response transaction in which a
client sends a request to a server, and the server returns a
response. The second is an indication transaction in which either
agent -- client or server -- sends an indication that generates no
response. Both types of transactions include a transaction ID, which
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is a randomly selected 96-bit number. For request/response
transactions, this transaction ID allows the client to associate the
response with the request that generated it; for indications, the
transaction ID serves as a debugging aid.
All STUN messages start with a fixed header that includes a method, a
class, and the transaction ID. The method indicates which of the
various requests or indications this is; this specification defines
just one method, Binding, but other methods are expected to be
defined in other documents. The class indicates whether this is a
request, a success response, an error response, or an indication.
Following the fixed header comes zero or more attributes, which are
Type-Length-Value extensions that convey additional information for
the specific message.
This document defines a single method called Binding. The Binding
method can be used either in request/response transactions or in
indication transactions. When used in request/response transactions,
the Binding method can be used to determine the particular "binding"
a NAT has allocated to a STUN client. When used in either request/
response or in indication transactions, the Binding method can also
be used to keep these "bindings" alive.
In the Binding request/response transaction, a Binding request is
sent from a STUN client to a STUN server. When the Binding request
arrives at the STUN server, it may have passed through one or more
NATs between the STUN client and the STUN server (in Figure 1, there
were two such NATs). As the Binding request message passes through a
NAT, the NAT will modify the source transport address (that is, the
source IP address and the source port) of the packet. As a result,
the source transport address of the request received by the server
will be the public IP address and port created by the NAT closest to
the server. This is called a reflexive transport address. The STUN
server copies that source transport address into an XOR-MAPPED-
ADDRESS attribute in the STUN Binding response and sends the Binding
response back to the STUN client. As this packet passes back through
a NAT, the NAT will modify the destination transport address in the
IP header, but the transport address in the XOR-MAPPED-ADDRESS
attribute within the body of the STUN response will remain untouched.
In this way, the client can learn its reflexive transport address
allocated by the outermost NAT with respect to the STUN server.
In some usages, STUN must be multiplexed with other protocols (e.g.,
[RFC5245], [RFC5626]). In these usages, there must be a way to
inspect a packet and determine if it is a STUN packet or not. STUN
provides three fields in the STUN header with fixed values that can
be used for this purpose. If this is not sufficient, then STUN
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packets can also contain a FINGERPRINT value, which can further be
used to distinguish the packets.
STUN defines a set of optional procedures that a usage can decide to
use, called mechanisms. These mechanisms include DNS discovery, a
redirection technique to an alternate server, a fingerprint attribute
for demultiplexing, and two authentication and message-integrity
exchanges. The authentication mechanisms revolve around the use of a
username, password, and message-integrity value. Two authentication
mechanisms, the long-term credential mechanism and the short-term
credential mechanism, are defined in this specification. Each usage
specifies the mechanisms allowed with that usage.
In the long-term credential mechanism, the client and server share a
pre-provisioned username and password and perform a digest challenge/
response exchange inspired by (but differing in details) to the one
defined for HTTP [RFC2617]. In the short-term credential mechanism,
the client and the server exchange a username and password through
some out-of-band method prior to the STUN exchange. For example, in
the ICE usage [RFC5245] the two endpoints use out-of-band signaling
to exchange a username and password. These are used to integrity
protect and authenticate the request and response. There is no
challenge or nonce used.
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
[RFC2119] and indicate requirement levels for compliant STUN
implementations.
4. Definitions
STUN Agent: A STUN agent is an entity that implements the STUN
protocol. The entity can be either a STUN client or a STUN
server.
STUN Client: A STUN client is an entity that sends STUN requests and
receives STUN responses. A STUN client can also send indications.
In this specification, the terms STUN client and client are
synonymous.
STUN Server: A STUN server is an entity that receives STUN requests
and sends STUN responses. A STUN server can also send
indications. In this specification, the terms STUN server and
server are synonymous.
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Transport Address: The combination of an IP address and port number
(such as a UDP or TCP port number).
Reflexive Transport Address: A transport address learned by a client
that identifies that client as seen by another host on an IP
network, typically a STUN server. When there is an intervening
NAT between the client and the other host, the reflexive transport
address represents the mapped address allocated to the client on
the public side of the NAT. Reflexive transport addresses are
learned from the mapped address attribute (MAPPED-ADDRESS or XOR-
MAPPED-ADDRESS) in STUN responses.
Mapped Address: Same meaning as reflexive address. This term is
retained only for historic reasons and due to the naming of the
MAPPED-ADDRESS and XOR-MAPPED-ADDRESS attributes.
Long-Term Credential: A username and associated password that
represent a shared secret between client and server. Long-term
credentials are generally granted to the client when a subscriber
enrolls in a service and persist until the subscriber leaves the
service or explicitly changes the credential.
Long-Term Password: The password from a long-term credential.
Short-Term Credential: A temporary username and associated password
that represent a shared secret between client and server. Short-
term credentials are obtained through some kind of protocol
mechanism between the client and server, preceding the STUN
exchange. A short-term credential has an explicit temporal scope,
which may be based on a specific amount of time (such as 5
minutes) or on an event (such as termination of a SIP dialog).
The specific scope of a short-term credential is defined by the
application usage.
Short-Term Password: The password component of a short-term
credential.
STUN Indication: A STUN message that does not receive a response.
Attribute: The STUN term for a Type-Length-Value (TLV) object that
can be added to a STUN message. Attributes are divided into two
types: comprehension-required and comprehension-optional. STUN
agents can safely ignore comprehension-optional attributes they
don't understand, but cannot successfully process a message if it
contains comprehension-required attributes that are not
understood.
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RTO: Retransmission TimeOut, which defines the initial period of
time between transmission of a request and the first retransmit of
that request.
5. STUN Message Structure
STUN messages are encoded in binary using network-oriented format
(most significant byte or octet first, also commonly known as big-
endian). The transmission order is described in detail in Appendix B
of RFC791 [RFC0791]. Unless otherwise noted, numeric constants are
in decimal (base 10).
All STUN messages MUST start with a 20-byte header followed by zero
or more Attributes. The STUN header contains a STUN message type,
magic cookie, transaction ID, and message length.
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 (96 bits) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format of STUN Message Header
The most significant 2 bits of every STUN message MUST be zeroes.
This can be used to differentiate STUN packets from other protocols
when STUN is multiplexed with other protocols on the same port.
The message type defines the message class (request, success
response, failure response, or indication) and the message method
(the primary function) of the STUN message. Although there are four
message classes, there are only two types of transactions in STUN:
request/response transactions (which consist of a request message and
a response message) and indication transactions (which consist of a
single indication message). Response classes are split into error
and success responses to aid in quickly processing the STUN message.
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The message type field is decomposed further into the following
structure:
0 1
2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
|M |M |M|M|M|C|M|M|M|C|M|M|M|M|
|11|10|9|8|7|1|6|5|4|0|3|2|1|0|
+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Format of STUN Message Type Field
Here the bits in the message type field are shown as most significant
(M11) through least significant (M0). M11 through M0 represent a
12-bit encoding of the method. C1 and C0 represent a 2-bit encoding
of the class. A class of 0b00 is a request, a class of 0b01 is an
indication, a class of 0b10 is a success response, and a class of
0b11 is an error response. This specification defines a single
method, Binding. The method and class are orthogonal, so that for
each method, a request, success response, error response, and
indication are possible for that method. Extensions defining new
methods MUST indicate which classes are permitted for that method.
For example, a Binding request has class=0b00 (request) and
method=0b000000000001 (Binding) and is encoded into the first 16 bits
as 0x0001. A Binding response has class=0b10 (success response) and
method=0b000000000001, and is encoded into the first 16 bits as
0x0101.
Note: This unfortunate encoding is due to assignment of values in
[RFC3489] that did not consider encoding Indications, Success, and
Errors using bit fields.
The magic cookie field MUST contain the fixed value 0x2112A442 in
network byte order. In RFC 3489 [RFC3489], this field was part of
the transaction ID; placing the magic cookie in this location allows
a server to detect if the client will understand certain attributes
that were added in this revised specification. In addition, it aids
in distinguishing STUN packets from packets of other protocols when
STUN is multiplexed with those other protocols on the same port.
The transaction ID is a 96-bit identifier, used to uniquely identify
STUN transactions. For request/response transactions, the
transaction ID is chosen by the STUN client for the request and
echoed by the server in the response. For indications, it is chosen
by the agent sending the indication. It primarily serves to
correlate requests with responses, though it also plays a small role
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in helping to prevent certain types of attacks. The server also uses
the transaction ID as a key to identify each transaction uniquely
across all clients. As such, the transaction ID MUST be uniformly
and randomly chosen from the interval 0 .. 2**96-1, and SHOULD be
cryptographically random. Resends of the same request reuse the same
transaction ID, but the client MUST choose a new transaction ID for
new transactions unless the new request is bit-wise identical to the
previous request and sent from the same transport address to the same
IP address. Success and error responses MUST carry the same
transaction ID as their corresponding request. When an agent is
acting as a STUN server and STUN client on the same port, the
transaction IDs in requests sent by the agent have no relationship to
the transaction IDs in requests received by the agent.
The message length MUST contain the size, in bytes, of the message
not including the 20-byte STUN header. Since all STUN attributes are
padded to a multiple of 4 bytes, the last 2 bits of this field are
always zero. This provides another way to distinguish STUN packets
from packets of other protocols.
Following the STUN fixed portion of the header are zero or more
attributes. Each attribute is TLV (Type-Length-Value) encoded. The
details of the encoding, and of the attributes themselves are given
in Section 14.
6. Base Protocol Procedures
This section defines the base procedures of the STUN protocol. It
describes how messages are formed, how they are sent, and how they
are processed when they are received. It also defines the detailed
processing of the Binding method. Other sections in this document
describe optional procedures that a usage may elect to use in certain
situations. Other documents may define other extensions to STUN, by
adding new methods, new attributes, or new error response codes.
6.1. Forming a Request or an Indication
When formulating a request or indication message, the agent MUST
follow the rules in Section 5 when creating the header. In addition,
the message class MUST be either "Request" or "Indication" (as
appropriate), and the method must be either Binding or some method
defined in another document.
The agent then adds any attributes specified by the method or the
usage. For example, some usages may specify that the agent use an
authentication method (Section 9) or the FINGERPRINT attribute
(Section 7).
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If the agent is sending a request, it SHOULD add a SOFTWARE attribute
to the request. Agents MAY include a SOFTWARE attribute in
indications, depending on the method. Extensions to STUN should
discuss whether SOFTWARE is useful in new indications.
For the Binding method with no authentication, no attributes are
required unless the usage specifies otherwise.
All STUN messages sent over UDP or DTLS-over-UDP [RFC6347] SHOULD be
less than the path MTU, if known.
If the path MTU is unknown for UDP, messages SHOULD be the smaller of
576 bytes and the first-hop MTU for IPv4 [RFC1122] and 1280 bytes for
IPv6 [RFC2460]. This value corresponds to the overall size of the IP
packet. Consequently, for IPv4, the actual STUN message would need
to be less than 548 bytes (576 minus 20-byte IP header, minus 8-byte
UDP header, assuming no IP options are used).
If the path MTU is unknown for DTLS-over-UDP, the rules described in
the previous paragraph need to be adjusted to take into account the
size of the (13-byte) DTLS Record header, the MAC size, and the
padding size.
If a STUN client needs to send requests that are larger than the MTU,
or if an application envisions that a response would be larger than
the MTU, then it MUST use SCTP-over-UDP or SCTP-over-DTLS-over-UDP as
a transport, unless the purpose of sending an oversized packet is to
probe for MTU characteristics (see [RFC5780]).
Adding an ORIGIN attribute to a request, as specified in
[I-D.ietf-tram-stun-origin], may increase the size of a request
beyond the MTU such that it consequently triggers the use of SCTP-
over-UDP or SCTP-over-DTLS-over-UDP as a transport.
6.2. Sending the Request or Indication
The agent then sends the request or indication. This document
specifies how to send STUN messages over UDP, TCP, TLS-over-TCP,
DTLS-over-UDP, SCTP-over-UDP, or SCTP-over-DTLS-over-UDP; other
transport protocols may be added in the future. The STUN usage must
specify which transport protocol is used, and how the agent
determines the IP address and port of the recipient. Section 8
describes a DNS-based method of determining the IP address and port
of a server that a usage may elect to use. STUN may be used with
anycast addresses, but only with UDP and in usages where
authentication is not used.
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At any time, a client MAY have multiple outstanding STUN requests
with the same STUN server (that is, multiple transactions in
progress, with different transaction IDs). Absent other limits to
the rate of new transactions (such as those specified by ICE for
connectivity checks or when STUN is run over TCP), a client SHOULD
space new parallel transactions to a server by RTO and SHOULD limit
itself to ten outstanding transactions to the same server.
Parallel transactions are defined as transactions that can be sent
independently of each other. Serial transactions, on the other hand,
are a series of transactions that are each dependent on the
completion of the previous transaction (e.g., the second transaction
of a long term authentication). In contrast to parallel
transactions, a client need not space new serial transactions to a
server by RTO.
6.2.1. Sending over UDP or DTLS-over-UDP
When running STUN over UDP or STUN over DTLS-over-UDP [RFC7350], it
is possible that the STUN message might be dropped by the network.
Reliability of STUN request/response transactions is accomplished
through retransmissions of the request message by the client
application itself. STUN indications are not retransmitted; thus,
indication transactions over UDP or DTLS-over-UDP are not reliable.
A client SHOULD retransmit a STUN request message starting with an
interval of RTO ("Retransmission TimeOut"), doubling after each
retransmission. The RTO is an estimate of the round-trip time (RTT),
and is computed as described in RFC 6298 [RFC6298], with two
exceptions. First, the initial value for RTO SHOULD be greater than
500 ms. The exception cases for this "SHOULD" are when other
mechanisms are used to derive congestion thresholds (such as the ones
defined in ICE for fixed rate streams), or when STUN is used in non-
Internet environments with known network capacities. In fixed-line
access links, a value of 500 ms is RECOMMENDED. Second, the value of
RTO SHOULD NOT be rounded up to the nearest second. Rather, a 1 ms
accuracy SHOULD be maintained. As with TCP, the usage of Karn's
algorithm is RECOMMENDED [KARN87]. When applied to STUN, it means
that RTT estimates SHOULD NOT be computed from STUN transactions that
result in the retransmission of a request.
The value for RTO SHOULD be cached by a client after the completion
of the transaction, and used as the starting value for RTO for the
next transaction to the same server (based on equality of IP
address). The value SHOULD be considered stale and discarded after
10 minutes without any transactions to the same server.
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Retransmissions continue until a response is received, or until a
total of Rc requests have been sent. Rc SHOULD be configurable and
SHOULD have a default of 7. If, after the last request, a duration
equal to Rm times the RTO has passed without a response (providing
ample time to get a response if only this final request actually
succeeds), the client SHOULD consider the transaction to have failed.
Rm SHOULD be configurable and SHOULD have a default of 16. A STUN
transaction over UDP or DTLS-over-UDP is also considered failed if
there has been a hard ICMP error [RFC1122]. For example, assuming an
RTO of 500ms, requests would be sent at times 0 ms, 500 ms, 1500 ms,
3500 ms, 7500 ms, 15500 ms, and 31500 ms. If the client has not
received a response after 39500 ms, the client will consider the
transaction to have timed out.
6.2.2. Sending over TCP or TLS-over-TCP
For TCP and TLS-over-TCP [RFC5246], the client opens a TCP connection
to the server.
In some usages of STUN, STUN is sent as the only protocol over the
TCP connection. In this case, it can be sent without the aid of any
additional framing or demultiplexing. In other usages, or with other
extensions, it may be multiplexed with other data over a TCP
connection. In that case, STUN MUST be run on top of some kind of
framing protocol, specified by the usage or extension, which allows
for the agent to extract complete STUN messages and complete
application layer messages. The STUN service running on the well-
known port or ports discovered through the DNS procedures in
Section 8 is for STUN alone, and not for STUN multiplexed with other
data. Consequently, no framing protocols are used in connections to
those servers. When additional framing is utilized, the usage will
specify how the client knows to apply it and what port to connect to.
For example, in the case of ICE connectivity checks, this information
is learned through out-of-band negotiation between client and server.
Reliability of STUN over TCP and TLS-over-TCP is handled by TCP
itself, and there are no retransmissions at the STUN protocol level.
However, for a request/response transaction, if the client has not
received a response by Ti seconds after it sent the SYN to establish
the connection, it considers the transaction to have timed out. Ti
SHOULD be configurable and SHOULD have a default of 39.5s. This
value has been chosen to equalize the TCP and UDP timeouts for the
default initial RTO.
In addition, if the client is unable to establish the TCP connection,
or the TCP connection is reset or fails before a response is
received, any request/response transaction in progress is considered
to have failed.
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The client MAY send multiple transactions over a single TCP (or TLS-
over-TCP) connection, and it MAY send another request before
receiving a response to the previous. The client SHOULD keep the
connection open until it:
o has no further STUN requests or indications to send over that
connection, and
o has no plans to use any resources (such as a mapped address
(MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address
[RFC5766]) that were learned though STUN requests sent over that
connection, and
o if multiplexing other application protocols over that port, has
finished using that other application, and
o if using that learned port with a remote peer, has established
communications with that remote peer, as is required by some TCP
NAT traversal techniques (e.g., [RFC6544]).
At the server end, the server SHOULD keep the connection open, and
let the client close it, unless the server has determined that the
connection has timed out (for example, due to the client
disconnecting from the network). Bindings learned by the client will
remain valid in intervening NATs only while the connection remains
open. Only the client knows how long it needs the binding. The
server SHOULD NOT close a connection if a request was received over
that connection for which a response was not sent. A server MUST NOT
ever open a connection back towards the client in order to send a
response. Servers SHOULD follow best practices regarding connection
management in cases of overload.
6.2.3. Sending over SCTP-over-UDP or SCTP-over-DTLS-over-UDP
For SCTP-over-UDP [RFC6951] and SCTP-over-DTLS-over-UDP
[I-D.ietf-tsvwg-sctp-dtls-encaps], the client opens a Stream Control
Transmission Protocol (SCTP) connection to the server.
For some STUN usages, STUN is sent over SCTP as the only protocol
over the UDP association. In this case, it can be sent without the
aid of any additional demultiplexing. In other usages, or with other
extensions, it may be multiplexed with other data over a UDP
association. In that case, the SCTP source and destination ports
MUST be chosen so the two most significant bits are 0b11.
Reliability of STUN over SCTP-over-UDP and STUN over SCTP-over-DTLS-
over-UDP is handled by SCTP itself and there are no retransmissions
at the STUN protocol level. However, for a request/response
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transaction, if the client has not received a response by Ti seconds
after it sent the INIT to establish the connection, it considers the
transaction to have timed out. Ti SHOULD be configurable and SHOULD
have a default of 39.5s. This value has been chosen to equalize the
SCTP-over-UDP, TCP, and UDP timeouts for the default initial RTO.
In addition, if the client is unable to establish the SCTP
connection, or the SCTP connection is reset or fails before a
response is received, any request/response transaction in progress is
considered to have failed.
The client MAY send multiple transactions over a single SCTP (or
SCTP-over-DTLS) connection and it MAY send another request before
receiving a response to the previous. Each transaction MUST use a
different SCTP stream ID. The client SHOULD keep the connection open
until it:
o has no further STUN requests or indications to send over that
connection, and
o has no plans to use any resources (such as a mapped address
(MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address
[RFC5766]) that were learned through STUN requests sent over that
connection, and
o has finished using all corresponding applications if multiplexing
other application protocols over that port
When using SCTP-over-UDP, the SCTP source port and destination port
MUST be selected so the two most significant bits are set to "1".
This permits multiplexing of STUN-over-UDP, STUN-over-SCTP-over-UDP,
DTLS, and RTP/RTCP on the same socket.
STUN indications MAY be sent unreliably by using the SCTP extension
in [RFC3758], augmented with the policies of
[I-D.ietf-tsvwg-sctp-prpolicies]. Each STUN usage MUST specify the
conditions under which STUN indications are sent reliably or not, and
MUST specify the policy for allocating an SCTP stream ID. The NAT
Discovery usage described in this document does not use STUN
indications.
At the server end, the server SHOULD keep the connection open and let
the client close it unless the server has determined that the
connection has timed out (for example, due to the client
disconnecting from the network). Bindings learned by the client will
remain valid in intervening NATs only while the connection remains
open. Only the client knows how long it needs the binding. The
server SHOULD NOT close a connection if a request was received over
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that connection for which a response was not sent. A server MUST NOT
ever open a connection back towards the client in order to send a
response. Servers SHOULD follow best practices regarding connection
management in cases of overload.
6.2.4. Sending over TLS-over-TCP or DTLS-over-UDP or SCTP-over-DTLS-
over-UDP
When STUN is run by itself over TLS-over-TCP or DTLS-over-UDP or
SCTP-over-DTLS-over-UDP, the TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 cipher suites MUST be
implemented and other cipher suites MAY be implemented. Perfect
Forward Secrecy (PFS) cipher suites MUST be preferred over non-PFS
cipher suites. Cipher suites with known weaknesses, such as those
based on (single) DES and RC4, MUST NOT be used. Implementations
MUST disable TLS-level compression.
When it receives the TLS Certificate message, the client SHOULD
verify the certificate and inspect the site identified by the
certificate. If the certificate is invalid or revoked, or if it does
not identify the appropriate party, the client MUST NOT send the STUN
message or otherwise proceed with the STUN transaction. The client
MUST verify the identity of the server using the following procedure.
STUN clients that are using the mechanism in Section 8, and that have
established that all DNS Resource Records from the Source Domain to
the Host Name are secure according to DNSsec [RFC4033] (i.e., that
the AD bit is set in all the DNS responses) MUST lookup a TLSA
Resource Record [RFC6698] for the protocol, port and Host Name
selected. If the TLSA Resource Record is secure then the STUN client
MUST use it to validate the certificate presented by the STUN server.
If there is no TLSA Resource Record or if the Resource Record is not
secure, then the client MUST fallback to the validation process
defined in Section 3.1 of RFC 2818 [RFC2818].
Alternatively, a client MAY be configured with a set of domains or IP
addresses that are trusted. If a certificate is received that
identifies one of those trusted domains or IP addresses, the client
considers the identity of the server to be verified.
When STUN is multiplexed with other protocols over a TLS-over-TCP
connection or a DTLS-over-UDP or a SCTP-over-DTLS-over-UDP
association, the mandatory ciphersuites and TLS handling procedures
operate as defined by those protocols.
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6.3. Receiving a STUN Message
This section specifies the processing of a STUN message. The
processing specified here is for STUN messages as defined in this
specification; additional rules for backwards compatibility are
defined in Section 11. Those additional procedures are optional, and
usages can elect to utilize them. First, a set of processing
operations is applied that is independent of the class. This is
followed by class-specific processing, described in the subsections
that follow.
When a STUN agent receives a STUN message, it first checks that the
message obeys the rules of Section 5. It checks that the first two
bits are 0, that the magic cookie field has the correct value, that
the message length is sensible, and that the method value is a
supported method. It checks that the message class is allowed for
the particular method. If the message class is "Success Response" or
"Error Response", the agent checks that the transaction ID matches a
transaction that is still in progress. If the FINGERPRINT extension
is being used, the agent checks that the FINGERPRINT attribute is
present and contains the correct value. If any errors are detected,
the message is silently discarded. In the case when STUN is being
multiplexed with another protocol, an error may indicate that this is
not really a STUN message; in this case, the agent should try to
parse the message as a different protocol.
The STUN agent then does any checks that are required by a
authentication mechanism that the usage has specified (see
Section 9).
Once the authentication checks are done, the STUN agent checks for
unknown attributes and known-but-unexpected attributes in the
message. Unknown comprehension-optional attributes MUST be ignored
by the agent. Known-but-unexpected attributes SHOULD be ignored by
the agent. Unknown comprehension-required attributes cause
processing that depends on the message class and is described below.
At this point, further processing depends on the message class of the
request.
6.3.1. Processing a Request
If the request contains one or more unknown comprehension-required
attributes, the server replies with an error response with an error
code of 420 (Unknown Attribute), and includes an UNKNOWN-ATTRIBUTES
attribute in the response that lists the unknown comprehension-
required attributes.
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The server then does any additional checking that the method or the
specific usage requires. If all the checks succeed, the server
formulates a success response as described below.
When run over UDP or DTLS-over-UDP or SCTP-over-UDP or SCTP-over-
DTLS-over-UDP, a request received by the server could be the first
request of a transaction, or a retransmission. The server MUST
respond to retransmissions such that the following property is
preserved: if the client receives the response to the retransmission
and not the response that was sent to the original request, the
overall state on the client and server is identical to the case where
only the response to the original retransmission is received, or
where both responses are received (in which case the client will use
the first). The easiest way to meet this requirement is for the
server to remember all transaction IDs received over UDP or DTLS-
over-UDP and their corresponding responses in the last 40 seconds.
However, this requires the server to hold state, and will be
inappropriate for any requests which are not authenticated. Another
way is to reprocess the request and recompute the response. The
latter technique MUST only be applied to requests that are idempotent
(a request is considered idempotent when the same request can be
safely repeated without impacting the overall state of the system)
and result in the same success response for the same request. The
Binding method is considered to be idempotent. Note that there are
certain rare network events that could cause the reflexive transport
address value to change, resulting in a different mapped address in
different success responses. Extensions to STUN MUST discuss the
implications of request retransmissions on servers that do not store
transaction state.
6.3.1.1. Forming a Success or Error Response
When forming the response (success or error), the server follows the
rules of Section 6. The method of the response is the same as that
of the request, and the message class is either "Success Response" or
"Error Response".
For an error response, the server MUST add an ERROR-CODE attribute
containing the error code specified in the processing above. The
reason phrase is not fixed, but SHOULD be something suitable for the
error code. For certain errors, additional attributes are added to
the message. These attributes are spelled out in the description
where the error code is specified. For example, for an error code of
420 (Unknown Attribute), the server MUST include an UNKNOWN-
ATTRIBUTES attribute. Certain authentication errors also cause
attributes to be added (see Section 9). Extensions may define other
errors and/or additional attributes to add in error cases.
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If the server authenticated the request using an authentication
mechanism, then the server SHOULD add the appropriate authentication
attributes to the response (see Section 9).
The server also adds any attributes required by the specific method
or usage. In addition, the server SHOULD add a SOFTWARE attribute to
the message.
For the Binding method, no additional checking is required unless the
usage specifies otherwise. When forming the success response, the
server adds a XOR-MAPPED-ADDRESS attribute to the response, where the
contents of the attribute are the source transport address of the
request message. For UDP and DTLS-over-UDP, this is the source IP
address and source UDP port of the request message. For TCP and TLS-
over-TCP, this is the source IP address and source TCP port of the
TCP connection as seen by the server.
6.3.1.2. Sending the Success or Error Response
The response (success or error) is sent over the same transport as
the request was received on. If the request was received over UDP or
DTLS-over-UDP, the destination IP address and port of the response
are the source IP address and port of the received request message,
and the source IP address and port of the response are equal to the
destination IP address and port of the received request message. If
the request was received over TCP or TLS-over-TCP, the response is
sent back on the same TCP connection as the request was received on.
6.3.2. Processing an Indication
If the indication contains unknown comprehension-required attributes,
the indication is discarded and processing ceases.
The agent then does any additional checking that the method or the
specific usage requires. If all the checks succeed, the agent then
processes the indication. No response is generated for an
indication.
For the Binding method, no additional checking or processing is
required, unless the usage specifies otherwise. The mere receipt of
the message by the agent has refreshed the "bindings" in the
intervening NATs.
Since indications are not re-transmitted over UDP or DTLS-over-UDP
(unlike requests), there is no need to handle re-transmissions of
indications at the sending agent.
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6.3.3. Processing a Success Response
If the success response contains unknown comprehension-required
attributes, the response is discarded and the transaction is
considered to have failed.
The client then does any additional checking that the method or the
specific usage requires. If all the checks succeed, the client then
processes the success response.
For the Binding method, the client checks that the XOR-MAPPED-ADDRESS
attribute is present in the response. The client checks the address
family specified. If it is an unsupported address family, the
attribute SHOULD be ignored. If it is an unexpected but supported
address family (for example, the Binding transaction was sent over
IPv4, but the address family specified is IPv6), then the client MAY
accept and use the value.
6.3.4. Processing an Error Response
If the error response contains unknown comprehension-required
attributes, or if the error response does not contain an ERROR-CODE
attribute, then the transaction is simply considered to have failed.
The client then does any processing specified by the authentication
mechanism (see Section 9). This may result in a new transaction
attempt.
The processing at this point depends on the error code, the method,
and the usage; the following are the default rules:
o If the error code is 300 through 399, the client SHOULD consider
the transaction as failed unless the ALTERNATE-SERVER extension is
being used. See Section 10.
o If the error code is 400 through 499, the client declares the
transaction failed; in the case of 420 (Unknown Attribute), the
response should contain a UNKNOWN-ATTRIBUTES attribute that gives
additional information.
o If the error code is 500 through 599, the client MAY resend the
request; clients that do so MUST limit the number of times they do
this.
Any other error code causes the client to consider the transaction
failed.
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7. FINGERPRINT Mechanism
This section describes an optional mechanism for STUN that aids in
distinguishing STUN messages from packets of other protocols when the
two are multiplexed on the same transport address. This mechanism is
optional, and a STUN usage must describe if and when it is used. The
FINGERPRINT mechanism is not backwards compatible with RFC3489, and
cannot be used in environments where such compatibility is required.
In some usages, STUN messages are multiplexed on the same transport
address as other protocols, such as the Real Time Transport Protocol
(RTP). In order to apply the processing described in Section 6, STUN
messages must first be separated from the application packets.
Section 5 describes three fixed fields in the STUN header that can be
used for this purpose. However, in some cases, these three fixed
fields may not be sufficient.
When the FINGERPRINT extension is used, an agent includes the
FINGERPRINT attribute in messages it sends to another agent.
Section 14.6 describes the placement and value of this attribute.
When the agent receives what it believes is a STUN message, then, in
addition to other basic checks, the agent also checks that the
message contains a FINGERPRINT attribute and that the attribute
contains the correct value. Section 6.3 describes when in the
overall processing of a STUN message the FINGERPRINT check is
performed. This additional check helps the agent detect messages of
other protocols that might otherwise seem to be STUN messages.
8. DNS Discovery of a Server
This section describes an optional procedure for STUN that allows a
client to use DNS to determine the IP address and port of a server.
A STUN usage must describe if and when this extension is used. To
use this procedure, the client must know a STUN URI [RFC7064]; the
usage must also describe how the client obtains this URI. Hard-
coding a STUN URI into software is NOT RECOMMENDED in case the domain
name is lost or needs to change for legal or other reasons.
When a client wishes to locate a STUN server on the public Internet
that accepts Binding request/response transactions, the STUN URI
scheme is "stun". When it wishes to locate a STUN server that
accepts Binding request/response transactions over a TLS, or DTLS, or
SCTP-over-DTLS session, the URI scheme is "stuns".
The syntax of the "stun" and "stuns" URIs are defined in Section 3.1
of [RFC7064]. STUN usages MAY define additional URI schemes.
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8.1. STUN URI Scheme Semantics
If the <host> part contains an IP address, then this IP address is
used directly to contact the server. A "stuns" URI containing an IP
address MUST be rejected, unless the domain name is provided by the
same mechanism that provided the STUN URI, and that domain name can
be passed to the verification code.
If the URI does not contain an IP address, the domain name contained
in the <host> part is resolved to a transport address using the SRV
procedures specified in [RFC2782]. The DNS SRV service name is the
content of the <host> part. The protocol in the SRV lookup is the
transport protocol the client will run STUN over: "udp" for UDP,
"tcp" for TCP, and "sctp-udp" for SCTP-over-UDP.
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
is sorted and then tried. However, RFC 2782 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.
When following these procedures, if the STUN transaction times out
without receipt of a response, the client SHOULD retry the request to
the next server in the ordered defined by RFC 2782. Such a retry is
only possible for request/response transmissions, since indication
transactions generate no response or timeout.
The default port for STUN requests is 3478, for both TCP and UDP.
The default port for STUN over TLS and STUN over DTLS requests is
5349. The default port for STUN over SCTP-over-UDP requests is XXXX.
The default port for STUN over SCTP-over-DTLS-over-UDP requests is
XXXX. Servers can run STUN over DTLS on the same port as STUN over
UDP if the server software supports determining whether the initial
message is a DTLS or STUN message. Servers can run STUN over TLS on
the same port as STUN over TCP if the server software supports
determining whether the initial message is a TLS or STUN message.
Administrators of STUN servers SHOULD use these ports in their SRV
records for UDP and TCP. In all cases, the port in DNS MUST reflect
the one on which the server is listening.
If no SRV records were found, the client performs an A or AAAA record
lookup of the domain name. The result will be a list of IP
addresses, each of which can be contacted at the default port using
UDP or TCP, independent of the STUN usage. For usages that require
TLS, the client connects to one of the IP addresses using the default
STUN over TLS port. For usages that require DTLS, the client
connects to one of the IP addresses using the default STUN over DTLS
port.
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9. Authentication and Message-Integrity Mechanisms
This section defines two mechanisms for STUN that a client and server
can use to provide authentication and message integrity; these two
mechanisms are known as the short-term credential mechanism and the
long-term credential mechanism. These two mechanisms are optional,
and each usage must specify if and when these mechanisms are used.
Consequently, both clients and servers will know which mechanism (if
any) to follow based on knowledge of which usage applies. For
example, a STUN server on the public Internet supporting ICE would
have no authentication, whereas the STUN server functionality in an
agent supporting connectivity checks would utilize short-term
credentials. An overview of these two mechanisms is given in
Section 2.
Each mechanism specifies the additional processing required to use
that mechanism, extending the processing specified in Section 6. The
additional processing occurs in three different places: when forming
a message, when receiving a message immediately after the basic
checks have been performed, and when doing the detailed processing of
error responses.
9.1. Short-Term Credential Mechanism
The short-term credential mechanism assumes that, prior to the STUN
transaction, the client and server have used some other protocol to
exchange a credential in the form of a username and password. This
credential is time-limited. The time limit is defined by the usage.
As an example, in the ICE usage [RFC5245], the two endpoints use out-
of-band signaling to agree on a username and password, and this
username and password are applicable for the duration of the media
session.
This credential is used to form a message-integrity check in each
request and in many responses. There is no challenge and response as
in the long-term mechanism; consequently, replay is prevented by
virtue of the time-limited nature of the credential.
9.1.1. HMAC Key
For short-term credentials the HMAC key is defined as follow:
key = SASLprep(password)
where SASLprep() is defined in RFC 4013 [RFC4013].
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9.1.2. Forming a Request or Indication
For a request or indication message, the agent MUST include the
USERNAME, MESSAGE-INTEGRITY2, and MESSAGE-INTEGRITY attributes in the
message. The HMAC for the MESSAGE-INTEGRITY attribute is computed as
described in Section 14.4 and the HMAC for the MESSAGE-INTEGRITY2
attributes is computed as described in Section 14.5. Note that the
password is never included in the request or indication.
9.1.3. Receiving a Request or Indication
After the agent has done the basic processing of a message, the agent
performs the checks listed below in order specified:
o If the message does not contain 1) a MESSAGE-INTEGRITY or a
MESSAGE-INTEGRITY2 attribute and 2) a USERNAME attribute:
* If the message is a request, the server MUST reject the request
with an error response. This response MUST use an error code
of 400 (Bad Request).
* If the message is an indication, the agent MUST silently
discard the indication.
o If the USERNAME does not contain a username value currently valid
within the server:
* If the message is a request, the server MUST reject the request
with an error response. This response MUST use an error code
of 401 (Unauthorized).
* If the message is an indication, the agent MUST silently
discard the indication.
o If the MESSAGE-INTEGRITY2 attribute is present compute the value
for the message integrity as described in Section 14.5, using the
password associated with the username. If the MESSAGE-INTEGRITY2
attribute is not present, and using the same password, compute the
value for the message integrity as described in Section 14.4. If
the resulting value does not match the contents of the MESSAGE-
INTEGRITY2 attribute or the MESSAGE-INTEGRITY attribute:
* If the message is a request, the server MUST reject the request
with an error response. This response MUST use an error code
of 401 (Unauthorized).
* If the message is an indication, the agent MUST silently
discard the indication.
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If these checks pass, the agent continues to process the request or
indication. Any response generated by a server to a request that
contains a MESSAGE-INTEGRITY2 attribute MUST include the MESSAGE-
INTEGRITY2 attribute, computed using the password utilized to
authenticate the request. Any response generated by a server to a
request that contains only a MESSAGE-INTEGRITY attribute MUST include
the MESSAGE-INTEGRITY attribute, computed using the password utilized
to authenticate the request. The response MUST NOT contain the
USERNAME attribute.
If any of the checks fail, a server MUST NOT include a MESSAGE-
INTEGRITY2, MESSAGE-INTEGRITY, or USERNAME attribute in the error
response. This is because, in these failure cases, the server cannot
determine the shared secret necessary to compute the MESSAGE-
INTEGRITY2 or MESSAGE-INTEGRITY attributes.
9.1.4. Receiving a Response
The client looks for the MESSAGE-INTEGRITY2 or the MESSAGE-INTEGRITY
attribute in the response. If present, the client computes the
message integrity over the response as defined in Section 14.4 or
Section 14.5, using the same password it utilized for the request.
If the resulting value matches the contents of the MESSAGE-INTEGRITY
or MESSAGE-INTEGRITY2 attribute, the response is considered
authenticated. If the value does not match, or if both MESSAGE-
INTEGRITY and MESSAGE-INTEGRITY2 were absent, the response MUST be
discarded, as if it was never received. This means that retransmits,
if applicable, will continue.
9.1.5. Sending Subsequent Requests
A client sending subsequent requests to the same server a MAY choose
to send only the MESSAGE-INTEGRITY2 or the MESSAGE-INTEGRITY
attribute depending upon the attribute that was received in the
response to the initial request.
9.2. Long-Term Credential Mechanism
The long-term credential mechanism relies on a long-term credential,
in the form of a username and password that are shared between client
and server. The credential is considered long-term since it is
assumed that it is provisioned for a user, and remains in effect
until the user is no longer a subscriber of the system, or is
changed. This is basically a traditional "log-in" username and
password given to users.
Because these usernames and passwords are expected to be valid for
extended periods of time, replay prevention is provided in the form
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of a digest challenge. In this mechanism, the client initially sends
a request, without offering any credentials or any integrity checks.
The server rejects this request, providing the user a realm (used to
guide the user or agent in selection of a username and password) and
a nonce. The nonce provides the replay protection. It is a cookie,
selected by the server, and encoded in such a way as to indicate a
duration of validity or client identity from which it is valid. The
client retries the request, this time including its username and the
realm, and echoing the nonce provided by the server. The client also
includes a message-integrity, which provides an HMAC over the entire
request, including the nonce. The server validates the nonce and
checks the message integrity. If they match, the request is
authenticated. If the nonce is no longer valid, it is considered
"stale", and the server rejects the request, providing a new nonce.
In subsequent requests to the same server, the client reuses the
nonce, username, realm, and password it used previously. In this
way, subsequent requests are not rejected until the nonce becomes
invalid by the server, in which case the rejection provides a new
nonce to the client.
Note that the long-term credential mechanism cannot be used to
protect indications, since indications cannot be challenged. Usages
utilizing indications must either use a short-term credential or omit
authentication and message integrity for them.
Since the long-term credential mechanism is susceptible to offline
dictionary attacks, deployments SHOULD utilize passwords that are
difficult to guess. In cases where the credentials are not entered
by the user, but are rather placed on a client device during device
provisioning, the password SHOULD have at least 128 bits of
randomness. In cases where the credentials are entered by the user,
they should follow best current practices around password structure.
9.2.1. HMAC Key
For long-term credentials that do not use a different algorithm, as
specified by the PASSWORD-ALGORITHM attribute, the key is 16 bytes:
key = MD5(username ":" realm ":" SASLprep(password))
Where MD5 is defined in RFC 1321 [RFC1321] and SASLprep() is defined
in RFC 4013 [RFC4013].
The 16-byte key is formed by taking the MD5 hash of the result of
concatenating the following five fields: (1) the username, with any
quotes and trailing nulls removed, as taken from the USERNAME
attribute (in which case SASLprep has already been applied); (2) a
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single colon; (3) the realm, with any quotes and trailing nulls
removed; (4) a single colon; and (5) the password, with any trailing
nulls removed and after processing using SASLprep. For example, if
the username was 'user', the realm was 'realm', and the password was
'pass', then the 16-byte HMAC key would be the result of performing
an MD5 hash on the string 'user:realm:pass', the resulting hash being
0x8493fbc53ba582fb4c044c456bdc40eb.
The structure of the key when used with long-term credentials
facilitates deployment in systems that also utilize SIP. Typically,
SIP systems utilizing SIP's digest authentication mechanism do not
actually store the password in the database. Rather, they store a
value called H(A1), which is equal to the key defined above.
When a PASSWORD-ALGORITHM is used, the key length and algorithm to
use are described in Section 17.4.1.
9.2.2. Forming a Request
There are two cases when forming a request. In the first case, this
is the first request from the client to the server (as identified by
its IP address and port). In the second case, the client is
submitting a subsequent request once a previous request/response
transaction has completed successfully. Forming a request as a
consequence of a 401 or 438 error response is covered in
Section 9.2.4 and is not considered a "subsequent request" and thus
does not utilize the rules described in Section 9.2.2.2.
9.2.2.1. First Request
If the client has not completed a successful request/response
transaction with the server (as identified by hostname, if the DNS
procedures of Section 8 are used, else IP address if not), it SHOULD
omit the USERNAME, MESSAGE-INTEGRITY, MESSAGE-INTEGRITY2, REALM,
NONCE, PASSWORD-ALGORITHMS, and PASSWORD-ALGORITHM attributes. In
other words, the very first request is sent as if there were no
authentication or message integrity applied.
9.2.2.2. Subsequent Requests
Once a request/response transaction has completed successfully, the
client will have been presented a realm and nonce by the server, and
selected a username and password with which it authenticated. The
client SHOULD cache the username, password, realm, and nonce for
subsequent communications with the server. When the client sends a
subsequent request, it SHOULD include the USERNAME, REALM, NONCE, and
PASSWORD-ALGORITHM attributes with these cached values. It SHOULD
include a MESSAGE-INTEGRITY attribute or a MESSAGE-INTEGRITY2
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attribute, computed as described in Section 14.4 and Section 14.5
using the cached password. The choice between the two attributes
depends on the attribute received in the response to the first
request.
9.2.3. Receiving a Request
After the server has done the basic processing of a request, it
performs the checks listed below in the order specified:
o If the message does not contain a MESSAGE-INTEGRITY or MESSAGE-
INTEGRITY2 attribute, the server MUST generate an error response
with an error code of 401 (Unauthorized). This response MUST
include a REALM value. It is RECOMMENDED that the REALM value be
the domain name of the provider of the STUN server. The response
MUST include a NONCE, selected by the server. The server MUST
ensure that the same NONCE cannot be selected for clients that use
different IP addresses and/or different ports. The server MAY
support alternate password algorithms, in which case it can list
them in preferential order in a PASSWORD-ALGORITHMS attribute. If
the server adds a PASSWORD-ALGORITHMS attribute it MUST prepend
the NONCE attribute value with the character string "obMatJos2".
The response SHOULD NOT contain a USERNAME, MESSAGE-INTEGRITY or
MESSAGE-INTEGRITY2 attribute.
o If the message contains a MESSAGE-INTEGRITY or a MESSAGE-
INTEGRITY2 attribute, but is missing the USERNAME, REALM, or NONCE
attribute, the server MUST generate an error response with an
error code of 400 (Bad Request). This response SHOULD NOT include
a USERNAME, NONCE, REALM, MESSAGE-INTEGRITY or MESSAGE-INTEGRITY2
attribute.
o If the NONCE attribute starts with the value "obMatJos2" but the
PASSWORD-ALGORITHMS attribute is not present or is not identical
to the PASSWORD-ALGORITHMS attribute sent in the response, the
server MUST generate an error response with an error code of 400
(Bad Request). This response SHOULD NOT include a USERNAME,
NONCE, REALM, MESSAGE-INTEGRITY, or MESSAGE-INTEGRITY2 attribute.
o If the NONCE is no longer valid, the server MUST generate an error
response with an error code of 438 (Stale Nonce). This response
MUST include NONCE and REALM attributes and SHOULD NOT include the
USERNAME, MESSAGE-INTEGRITY, or MESSAGE-INTEGRITY2 attribute.
Servers can invalidate nonces in order to provide additional
security. See Section 4.3 of [RFC2617] for guidelines.
o If the username in the USERNAME attribute is not valid, the server
MUST generate an error response with an error code of 401
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(Unauthorized). This response MUST include a REALM value. It is
RECOMMENDED that the REALM value be the domain name of the
provider of the STUN server. The response MUST include a NONCE,
selected by the server. The response SHOULD NOT contain a
USERNAME, MESSAGE-INTEGRITY or MESSAGE-INTEGRITY2 attribute.
o If the MESSAGE-INTEGRITY2 attribute is present compute the value
for the message integrity as described in Section 14.5, using the
password associated with the username. Else, using the same
password, compute the value for the message integrity as described
in Section 14.4. If the resulting value does not match the
contents of the MESSAGE-INTEGRITY attribute or the MESSAGE-
INTEGRITY2 attribute, the server MUST reject the request with an
error response. This response MUST use an error code of 401
(Unauthorized). It MUST include REALM and NONCE attributes and
SHOULD NOT include the USERNAME, MESSAGE-INTEGRITY, or MESSAGE-
INTEGRITY2 attribute.
If these checks pass, the server continues to process the request.
Any response generated by the server (excepting the cases described
above) MUST include both the MESSAGE-INTEGRITY and MESSAGE-INTEGRITY2
attributes, computed using the username and password utilized to
authenticate the request. The REALM, NONCE, and USERNAME attributes
SHOULD NOT be included.
9.2.4. Receiving a Response
If the response is an error response with an error code of 401
(Unauthorized), the client MUST test if the NONCE attribute value
starts with the character string "obMatJos2". If the test succeeds
and no PASSWORD-ALGORITHMS attribute is present, then the client MUST
NOT retry the request with a new transaction.
If the response is an error response with an error code of 401
(Unauthorized), the client SHOULD retry the request with a new
transaction. This request MUST contain a USERNAME, determined by the
client as the appropriate username for the REALM from the error
response. The request MUST contain the REALM, copied from the error
response. The request MUST contain the NONCE, copied from the error
response. If the response contains a PASSWORD-ALGORITHMS attribute,
the request MUST contain the PASSWORD-ALGORITHMS attribute with the
same content. If the response contains a PASSWORD-ALGORITHMS
attribute, and this attribute contains at least one algorithm that is
supported by the client then the request MUST contain a PASSWORD-
ALGORITHM attribute with the first algorithm supported on the list.
if the response contains a MESSAGE-INTEGRITY2 attribute then the
request MUST contain a MESSAGE-INTEGRITY2 attribute, computed using
the password associated with the username in the USERNAME attribute.
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Else the request MUST contain the MESSAGE-INTEGRITY attribute,
computed using the password associated with the username in the
USERNAME attribute. The client MUST NOT perform this retry if it is
not changing the USERNAME or REALM or its associated password, from
the previous attempt.
If the response is an error response with an error code of 438 (Stale
Nonce), the client MUST retry the request, using the new NONCE
attribute supplied in the 438 (Stale Nonce) response. This retry
MUST also include the USERNAME, REALM and either the MESSAGE-
INTEGRITY or MESSAGE-INTEGRITY2 attributes.
The client looks for the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY2
attribute in the response (either success or failure). If present,
the client computes the message integrity over the response as
defined in Section 14.4 or Section 14.5, using the same password it
utilized for the request. If the resulting value matches the
contents of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY2 attribute,
the response is considered authenticated. If the value does not
match, or if both MESSAGE-INTEGRITY and MESSAGE-INTEGRITY2 were
absent, the response MUST be discarded, as if it was never received.
This means that retransmits, if applicable, will continue.
10. ALTERNATE-SERVER Mechanism
This section describes a mechanism in STUN that allows a server to
redirect a client to another server. This extension is optional, and
a usage must define if and when this extension is used.
A server using this extension redirects a client to another server by
replying to a request message with an error response message with an
error code of 300 (Try Alternate). The server MUST include an
ALTERNATE-SERVER attribute in the error response. The error response
message MAY be authenticated; however, there are uses cases for
ALTERNATE-SERVER where authentication of the response is not possible
or practical. If the transaction uses TLS or DTLS and if the
transaction is authenticated by a MESSAGE-INTEGRITY2 attribute and if
the server wants to redirect to a server that uses a different
certificate, then it MUST include an ALTERNATE-DOMAIN attribute
containing the subjectAltName of that certificate.
A client using this extension handles a 300 (Try Alternate) error
code as follows. The client looks for an ALTERNATE-SERVER attribute
in the error response. If one is found, then the client considers
the current transaction as failed, and reattempts the request with
the server specified in the attribute, using the same transport
protocol used for the previous request. That request, if
authenticated, MUST utilize the same credentials that the client
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would have used in the request to the server that performed the
redirection. If the transport protocol uses TLS or DTLS, then the
client looks for an ALTERNATE-DOMAIN attribute. If the attribute is
found, the domain MUST be used to validate the cartificate. If the
attribute is not found, the same domain that was used for the
original request MUST be used to validate the certificate. If the
client has been redirected to a server on which it has already tried
this request within the last five minutes, it MUST ignore the
redirection and consider the transaction to have failed. This
prevents infinite ping-ponging between servers in case of redirection
loops.
11. Backwards Compatibility with RFC 5389
In addition to the backward compatibility already described in
Section 12 of [RFC5389], DTLS MUST NOT be used with RFC 3489 STUN
[RFC3489] (also referred to as "classic STUN"). Any STUN request or
indication without the magic cookie (see Section 6 of [RFC5389]) over
DTLS MUST always result in an error.
12. Basic Server Behavior
This section defines the behavior of a basic, stand-alone STUN
server. A basic STUN server provides clients with server reflexive
transport addresses by receiving and replying to STUN Binding
requests.
The STUN server MUST support the Binding method. It SHOULD NOT
utilize the short-term or long-term credential mechanism. This is
because the work involved in authenticating the request is more than
the work in simply processing it. It SHOULD NOT utilize the
ALTERNATE-SERVER mechanism for the same reason. It MUST support UDP
and TCP. It MAY support STUN over TCP/TLS or STUN over UDP/DTLS;
however, DTLS and TLS provide minimal security benefits in this basic
mode of operation. It MAY utilize the FINGERPRINT mechanism but MUST
NOT require it. Since the stand-alone server only runs STUN,
FINGERPRINT provides no benefit. Requiring it would break
compatibility with RFC 3489, and such compatibility is desirable in a
stand-alone server. Stand-alone STUN servers SHOULD support
backwards compatibility with [RFC3489] clients, as described in
Section 11.
It is RECOMMENDED that administrators of STUN servers provide DNS
entries for those servers as described in Section 8.
A basic STUN server is not a solution for NAT traversal by itself.
However, it can be utilized as part of a solution through STUN
usages. This is discussed further in Section 13.
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13. STUN Usages
STUN by itself is not a solution to the NAT traversal problem.
Rather, STUN defines a tool that can be used inside a larger
solution. The term "STUN usage" is used for any solution that uses
STUN as a component.
At the time of writing, three STUN usages are defined: Interactive
Connectivity Establishment (ICE) [RFC5245], Client-initiated
connections for SIP [RFC5626], and NAT Behavior Discovery [RFC5780].
Other STUN usages may be defined in the future.
A STUN usage defines how STUN is actually utilized -- when to send
requests, what to do with the responses, and which optional
procedures defined here (or in an extension to STUN) are to be used.
A usage would also define:
o Which STUN methods are used.
o What transports are used. If DTLS-over-UDP is used then
implementing the denial-of-service countermeasure described in
Section 4.2.1 of [RFC6347] is mandatory.
o What authentication and message-integrity mechanisms are used.
o The considerations around manual vs. automatic key derivation for
the integrity mechanism, as discussed in [RFC4107].
o What mechanisms are used to distinguish STUN messages from other
messages. When STUN is run over TCP, a framing mechanism may be
required.
o How a STUN client determines the IP address and port of the STUN
server.
o Whether backwards compatibility to RFC 3489 is required.
o What optional attributes defined here (such as FINGERPRINT and
ALTERNATE-SERVER) or in other extensions are required.
In addition, any STUN usage must consider the security implications
of using STUN in that usage. A number of attacks against STUN are
known (see the Security Considerations section in this document), and
any usage must consider how these attacks can be thwarted or
mitigated.
Finally, a usage must consider whether its usage of STUN is an
example of the Unilateral Self-Address Fixing approach to NAT
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traversal, and if so, address the questions raised in RFC 3424
[RFC3424].
14. STUN Attributes
After the STUN header are zero or more attributes. Each attribute
MUST be TLV encoded, with a 16-bit type, 16-bit length, and value.
Each STUN attribute MUST end on a 32-bit boundary. As mentioned
above, all fields in an attribute are transmitted most significant
bit first.
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 (variable) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Format of STUN Attributes
The value in the length field MUST contain the length of the Value
part of the attribute, prior to padding, measured in bytes. Since
STUN aligns attributes on 32-bit boundaries, attributes whose content
is not a multiple of 4 bytes are padded with 1, 2, or 3 bytes of
padding so that its value contains a multiple of 4 bytes. The
padding bits are ignored, and may be any value.
Any attribute type MAY appear more than once in a STUN message.
Unless specified otherwise, the order of appearance is significant:
only the first occurrence needs to be processed by a receiver, and
any duplicates MAY be ignored by a receiver.
To allow future revisions of this specification to add new attributes
if needed, the attribute space is divided into two ranges.
Attributes with type values between 0x0000 and 0x7FFF are
comprehension-required attributes, which means that the STUN agent
cannot successfully process the message unless it understands the
attribute. Attributes with type values between 0x8000 and 0xFFFF are
comprehension-optional attributes, which means that those attributes
can be ignored by the STUN agent if it does not understand them.
The set of STUN attribute types is maintained by IANA. The initial
set defined by this specification is found in Section 17.2.
The rest of this section describes the format of the various
attributes defined in this specification.
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14.1. MAPPED-ADDRESS
The MAPPED-ADDRESS attribute indicates a reflexive transport address
of the client. It consists of an 8-bit address family and a 16-bit
port, followed by a fixed-length value representing the IP address.
If the address family is IPv4, the address MUST be 32 bits. If the
address family is IPv6, the address MUST be 128 bits. All fields
must be in network byte order.
The format of the MAPPED-ADDRESS attribute is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0| Family | Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Address (32 bits or 128 bits) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Format of MAPPED-ADDRESS Attribute
The address family can take on the following values:
0x01:IPv4
0x02:IPv6
The first 8 bits of the MAPPED-ADDRESS MUST be set to 0 and MUST be
ignored by receivers. These bits are present for aligning parameters
on natural 32-bit boundaries.
This attribute is used only by servers for achieving backwards
compatibility with RFC 3489 [RFC3489] clients.
14.2. XOR-MAPPED-ADDRESS
The XOR-MAPPED-ADDRESS attribute is identical to the MAPPED-ADDRESS
attribute, except that the reflexive transport address is obfuscated
through the XOR function.
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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 6: 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 computed by taking the mapped port in host byte order,
XOR'ing it with the most significant 16 bits of the magic cookie, and
then the converting the result to network byte order. If the IP
address family is IPv4, X-Address is computed by taking the mapped IP
address in host byte order, XOR'ing it with the magic cookie, and
converting the result to network byte order. If the IP address
family is IPv6, X-Address is computed by taking the mapped IP address
in host byte order, XOR'ing it with the concatenation of the magic
cookie and the 96-bit transaction ID, and converting the result to
network byte order.
The rules for encoding and processing the first 8 bits of the
attribute's value, the rules for handling multiple occurrences of the
attribute, and the rules for processing address families are the same
as for MAPPED-ADDRESS.
Note: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their
encoding of the transport address. The former encodes the transport
address by exclusive-or'ing it with the magic cookie. The latter
encodes it directly in binary. RFC 3489 originally specified only
MAPPED-ADDRESS. However, deployment experience found that some NATs
rewrite the 32-bit binary payloads containing the NAT's public IP
address, such as STUN's MAPPED-ADDRESS attribute, in the well-meaning
but misguided attempt at providing a generic ALG function. Such
behavior interferes with the operation of STUN and also causes
failure of STUN's message-integrity checking.
14.3. USERNAME
The USERNAME attribute is used for message integrity. It identifies
the username and password combination used in the message-integrity
check.
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The value of USERNAME is a variable-length value. It MUST contain a
UTF-8 [RFC3629] encoded sequence of less than 513 bytes, and MUST
have been processed using SASLprep [RFC4013].
14.4. MESSAGE-INTEGRITY
The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [RFC2104] of
the STUN message. The MESSAGE-INTEGRITY attribute can be present in
any STUN message type. Since it uses the SHA1 hash, the HMAC will be
20 bytes. The text used as input to HMAC is the STUN message,
including the header, up to and including the attribute preceding the
MESSAGE-INTEGRITY attribute. With the exception of the MESSAGE-
INTEGRITY2 and FINGERPRINT attributes, which appear after MESSAGE-
INTEGRITY, agents MUST ignore all other attributes that follow
MESSAGE-INTEGRITY.
The key for the HMAC depends on which credential mechanism is in use.
Section 9.1.1 defines the key for the short-term credential mechanism
and Section 9.2.1 defines the key for the long-term credential
mechanism. Other credential mechanisms MUST define the key that is
used for the HMAC.
Based on the rules above, the hash used to construct MESSAGE-
INTEGRITY includes the length field from the STUN message header.
Prior to performing the hash, the MESSAGE-INTEGRITY attribute MUST be
inserted into the message (with dummy content). The length MUST then
be set to point to the length of the message up to, and including,
the MESSAGE-INTEGRITY attribute itself, but excluding any attributes
after it. Once the computation is performed, the value of the
MESSAGE-INTEGRITY attribute can be filled in, and the value of the
length in the STUN header can be set to its correct value -- the
length of the entire message. Similarly, when validating the
MESSAGE-INTEGRITY, the length field should be adjusted to point to
the end of the MESSAGE-INTEGRITY attribute prior to calculating the
HMAC. Such adjustment is necessary when attributes, such as
FINGERPRINT, appear after MESSAGE-INTEGRITY.
14.5. MESSAGE-INTEGRITY2
The MESSAGE-INTEGRITY2 attribute contains an HMAC-SHA-256 [RFC2104]
of the STUN message. The MESSAGE-INTEGRITY2 attribute can be present
in any STUN message type. Since it uses the SHA-256 hash, the HMAC
will be 32 bytes. The text used as input to HMAC is the STUN
message, including the header, up to and including the attribute
preceding the MESSAGE-INTEGRITY2 attribute. With the exception of
the FINGERPRINT attribute, which appears after MESSAGE-INTEGRITY2,
agents MUST ignore all other attributes that follow MESSAGE-
INTEGRITY2.
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The key for the HMAC depends on which credential mechanism is in use.
Section 9.1.1 defines the key for the short-term credential mechanism
and Section 9.2.1 defines the key for the long-term credential
mechanism. Other credential mechanism MUST define the key that is
used for the HMAC.
Based on the rules above, the hash used to construct MESSAGE-
INTEGRITY2 includes the length field from the STUN message header.
Prior to performing the hash, the MESSAGE-INTEGRITY2 attribute MUST
be inserted into the message (with dummy content). The length MUST
then be set to point to the length of the message up to, and
including, the MESSAGE-INTEGRITY2 attribute itself, but excluding any
attributes after it. Once the computation is performed, the value of
the MESSAGE-INTEGRITY2 attribute can be filled in, and the value of
the length in the STUN header can be set to its correct value -- the
length of the entire message. Similarly, when validating the
MESSAGE-INTEGRITY2, the length field should be adjusted to point to
the end of the MESSAGE-INTEGRITY2 attribute prior to calculating the
HMAC. Such adjustment is necessary when attributes, such as
FINGERPRINT, appear after MESSAGE-INTEGRITY2.
14.6. FINGERPRINT
The FINGERPRINT attribute MAY be present in all STUN messages. The
value of the attribute is computed as the CRC-32 of the STUN message
up to (but excluding) the FINGERPRINT attribute itself, XOR'ed with
the 32-bit value 0x5354554e (the XOR helps in cases where an
application packet is also using CRC-32 in it). The 32-bit CRC is
the one defined in ITU V.42 [ITU.V42.2002], which has a generator
polynomial of x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1.
When present, the FINGERPRINT attribute MUST be the last attribute in
the message, and thus will appear after MESSAGE-INTEGRITY.
The FINGERPRINT attribute can aid in distinguishing STUN packets from
packets of other protocols. See Section 7.
As with MESSAGE-INTEGRITY, the CRC used in the FINGERPRINT attribute
covers the length field from the STUN message header. Therefore,
this value must be correct and include the CRC attribute as part of
the message length, prior to computation of the CRC. When using the
FINGERPRINT attribute in a message, the attribute is first placed
into the message with a dummy value, then the CRC is computed, and
then the value of the attribute is updated. If the MESSAGE-INTEGRITY
attribute is also present, then it must be present with the correct
message-integrity value before the CRC is computed, since the CRC is
done over the value of the MESSAGE-INTEGRITY attribute as well.
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14.7. ERROR-CODE
The ERROR-CODE attribute is used in error response messages. It
contains a numeric error code value in the range of 300 to 699 plus a
textual reason phrase encoded in UTF-8 [RFC3629], and is consistent
in its code assignments and semantics with SIP [RFC3261] and HTTP
[RFC2616]. The reason phrase is meant for user consumption, and can
be anything appropriate for the error code. Recommended reason
phrases for the defined error codes are included in the IANA registry
for error codes. The reason phrase MUST be a UTF-8 [RFC3629] encoded
sequence of less than 128 characters (which can be as long as 763
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved, should be 0 |Class| Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason Phrase (variable) ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: ERROR-CODE Attribute
To facilitate processing, the class of the error code (the hundreds
digit) is encoded separately from the rest of the code, as shown in
Figure 7.
The Reserved bits SHOULD be 0, and are for alignment on 32-bit
boundaries. Receivers MUST ignore these bits. The Class represents
the hundreds digit of the error code. The value MUST be between 3
and 6. The Number represents the error code modulo 100, and its
value MUST be between 0 and 99.
The following error codes, along with their recommended reason
phrases, are defined:
300 Try Alternate: The client should contact an alternate server for
this request. This error response MUST only be sent if the
request included a USERNAME attribute and a valid MESSAGE-
INTEGRITY attribute; otherwise, it MUST NOT be sent and error
code 400 (Bad Request) is suggested. This error response MUST
be protected with the MESSAGE-INTEGRITY attribute, and receivers
MUST validate the MESSAGE-INTEGRITY of this response before
redirecting themselves to an alternate server.
Note: Failure to generate and validate message integrity for
a 300 response allows an on-path attacker to falsify a 300
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response thus causing subsequent STUN messages to be sent to
a victim.
400 Bad Request: The request was malformed. The client SHOULD NOT
retry the request without modification from the previous
attempt. The server may not be able to generate a valid
MESSAGE-INTEGRITY for this error, so the client MUST NOT expect
a valid MESSAGE-INTEGRITY attribute on this response.
401 Unauthorized: The request did not contain the correct
credentials to proceed. The client should retry the request
with proper credentials.
420 Unknown Attribute: The server received a STUN packet containing
a comprehension-required attribute that it did not understand.
The server MUST put this unknown attribute in the UNKNOWN-
ATTRIBUTE attribute of its error response.
438 Stale Nonce: The NONCE used by the client was no longer valid.
The client should retry, using the NONCE provided in the
response.
500 Server Error: The server has suffered a temporary error. The
client should try again.
14.8. REALM
The REALM attribute may be present in requests and responses. It
contains text that meets the grammar for "realm-value" as described
in RFC 3261 [RFC3261] but without the double quotes and their
surrounding whitespace. That is, it is an unquoted realm-value (and
is therefore a sequence of qdtext or quoted-pair). It MUST be a
UTF-8 [RFC3629] encoded sequence of less than 128 characters (which
can be as long as 763 bytes), and MUST have been processed using
SASLprep [RFC4013].
Presence of the REALM attribute in a request indicates that long-term
credentials are being used for authentication. Presence in certain
error responses indicates that the server wishes the client to use a
long-term credential for authentication.
14.9. NONCE
The NONCE attribute may be present in requests and responses. It
contains a sequence of qdtext or quoted-pair, which are defined in
RFC 3261 [RFC3261]. Note that this means that the NONCE attribute
will not contain actual quote characters. See RFC 2617 [RFC2617],
Section 4.3, for guidance on selection of nonce values in a server.
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It MUST be less than 128 characters (which can be as long as 763
bytes).
14.10. PASSWORD-ALGORITHMS
The PASSWORD-ALGORITHMS attribute is present only in responses. It
contains the list of algorithms that the server can use to derive the
long-term password.
The set of known algorithms is maintained by IANA. The initial set
defined by this specification is found in Section 17.4.
The attribute contains a list of algorithm numbers and variable
length parameters. The algorithm number is a 16-bit value as defined
in Section 17.4. The parameters start with the actual length of the
parameters as a 16-bit value, followed by the parameters that are
specific to each algorithm. The parameters are padded to a 32-bit
boundary, in the same manner as an attribute.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 1 | Algorithm 1 Parameters Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 1 Parameters (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 2 | Algorithm 2 Parameters Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 2 Parameter (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...
Figure 8: Format of PASSWORD-ALGORITHMS Attribute
14.11. PASSWORD-ALGORITHM
The PASSWORD-ALGORITHM attribute is present only in requests. It
contains the algorithms that the server must use to derive the long-
term password.
The set of known algorithms is maintained by IANA. The initial set
defined by this specification is found in Section 17.4.
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The attribute contains an algorithm number and variable length
parameters. The algorithm number is a 16-bit value as defined in
Section 17.4. The parameters starts with the actual length of the
parameters as a 16-bit value, followed by the parameters that are
specific to the algorithm. The parameters are padded to a 32-bit
boundary, in the same manner as an attribute.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm | Algorithm Parameters Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm Parameters (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Format of PASSWORD-ALGORITHM Attribute
14.12. UNKNOWN-ATTRIBUTES
The UNKNOWN-ATTRIBUTES attribute is present only in an 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.
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 10: Format of UNKNOWN-ATTRIBUTES Attribute
Note: In [RFC3489], this field was padded to 32 by duplicating the
last attribute. In this version of the specification, the normal
padding rules for attributes are used instead.
14.13. SOFTWARE
The SOFTWARE attribute contains a textual description of the software
being used by the agent sending the message. It is used by clients
and servers. Its value SHOULD include manufacturer and version
number. The attribute has no impact on operation of the protocol,
and serves only as a tool for diagnostic and debugging purposes. The
value of SOFTWARE is variable length. It MUST be a UTF-8 [RFC3629]
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encoded sequence of less than 128 characters (which can be as long as
763 bytes).
14.14. ALTERNATE-SERVER
The alternate server represents an alternate transport address
identifying a different STUN server that the STUN client should try.
It is encoded in the same way as MAPPED-ADDRESS, and thus refers to a
single server by IP address. The IP address family MUST be identical
to that of the source IP address of the request.
14.15. ALTERNATE-DOMAIN
The alternate domain represents the domain name that is used to
verify the IP address in the ALTERNATE-SERVER attribute when the
transport protocol uses TLS or DTLS.
The value of ALTERNATE-DOMAIN is variable length. It MUST be a UTF-8
[RFC3629] encoded sequence of less than 128 characters (which can be
as long as 763 bytes).
15. Security Considerations
15.1. Attacks against the Protocol
15.1.1. Outside Attacks
An attacker can try to modify STUN messages in transit, in order to
cause a failure in STUN operation. These attacks are detected for
both requests and responses through the message-integrity mechanism,
using either a short-term or long-term credential. Of course, once
detected, the manipulated packets will be dropped, causing the STUN
transaction to effectively fail. This attack is possible only by an
on-path attacker.
An attacker that can observe, but not modify, STUN messages in-
transit (for example, an attacker present on a shared access medium,
such as Wi-Fi), can see a STUN request, and then immediately send a
STUN response, typically an error response, in order to disrupt STUN
processing. This attack is also prevented for messages that utilize
MESSAGE-INTEGRITY. However, some error responses, those related to
authentication in particular, cannot be protected by MESSAGE-
INTEGRITY. When STUN itself is run over a secure transport protocol
(e.g., TLS), these attacks are completely mitigated.
Depending on the STUN usage, these attacks may be of minimal
consequence and thus do not require message integrity to mitigate.
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For example, when STUN is used to a basic STUN server to discover a
server reflexive candidate for usage with ICE, authentication and
message integrity are not required since these attacks are detected
during the connectivity check phase. The connectivity checks
themselves, however, require protection for proper operation of ICE
overall. As described in Section 13, STUN usages describe when
authentication and message integrity are needed.
Since STUN uses the HMAC of a shared secret for authentication and
integrity protection, it is subject to offline dictionary attacks.
When authentication is utilized, it SHOULD be with a strong password
that is not readily subject to offline dictionary attacks.
Protection of the channel itself, using TLS, mitigates these attacks.
However, STUN is most often run over UDP and in those cases, strong
passwords are the only way to protect against these attacks.
15.1.2. Inside Attacks
A rogue client may try to launch a DoS attack against a server by
sending it a large number of STUN requests. Fortunately, STUN
requests can be processed statelessly by a server, making such
attacks hard to launch.
A rogue client may use a STUN server as a reflector, sending it
requests with a falsified source IP address and port. In such a
case, the response would be delivered to that source IP and port.
There is no amplification of the number of packets with this attack
(the STUN server sends one packet for each packet sent by the
client), though there is a small increase in the amount of data,
since STUN responses are typically larger than requests. This attack
is mitigated by ingress source address filtering.
Revealing the specific software version of the agent through the
SOFTWARE attribute might allow them to become more vulnerable to
attacks against software that is known to contain security holes.
Implementers SHOULD make usage of the SOFTWARE attribute a
configurable option.
15.2. Attacks Affecting the Usage
This section lists attacks that might be launched against a usage of
STUN. Each STUN usage must consider whether these attacks are
applicable to it, and if so, discuss counter-measures.
Most of the attacks in this section revolve around an attacker
modifying the reflexive address learned by a STUN client through a
Binding request/response transaction. Since the usage of the
reflexive address is a function of the usage, the applicability and
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remediation of these attacks are usage-specific. In common
situations, modification of the reflexive address by an on-path
attacker is easy to do. Consider, for example, the common situation
where STUN is run directly over UDP. In this case, an on-path
attacker can modify the source IP address of the Binding request
before it arrives at the STUN server. The STUN server will then
return this IP address in the XOR-MAPPED-ADDRESS attribute to the
client, and send the response back to that (falsified) IP address and
port. If the attacker can also intercept this response, it can
direct it back towards the client. Protecting against this attack by
using a message-integrity check is impossible, since a message-
integrity value cannot cover the source IP address, since the
intervening NAT must be able to modify this value. Instead, one
solution to preventing the attacks listed below is for the client to
verify the reflexive address learned, as is done in ICE [RFC5245].
Other usages may use other means to prevent these attacks.
15.2.1. Attack I: Distributed DoS (DDoS) against a Target
In this attack, the attacker provides one or more clients with the
same faked reflexive address that points to the intended target.
This will trick the STUN clients into thinking that their reflexive
addresses are equal to that of the target. If the clients hand out
that reflexive address in order to receive traffic on it (for
example, in SIP messages), the traffic will instead be sent to the
target. This attack can provide substantial amplification,
especially when used with clients that are using STUN to enable
multimedia applications. However, it can only be launched against
targets for which packets from the STUN server to the target pass
through the attacker, limiting the cases in which it is possible.
15.2.2. Attack II: Silencing a Client
In this attack, the attacker provides a STUN client with a faked
reflexive address. The reflexive address it provides is a transport
address that routes to nowhere. As a result, the client won't
receive any of the packets it expects to receive when it hands out
the reflexive address. This exploitation is not very interesting for
the attacker. It impacts a single client, which is frequently not
the desired target. Moreover, any attacker that can mount the attack
could also deny service to the client by other means, such as
preventing the client from receiving any response from the STUN
server, or even a DHCP server. As with the attack in Section 15.2.1,
this attack is only possible when the attacker is on path for packets
sent from the STUN server towards this unused IP address.
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15.2.3. Attack III: Assuming the Identity of a Client
This attack is similar to attack II. However, the faked reflexive
address points to the attacker itself. This allows the attacker to
receive traffic that was destined for the client.
15.2.4. Attack IV: Eavesdropping
In this attack, the attacker forces the client to use a reflexive
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.
15.3. Hash Agility Plan
This specification uses HMAC-SHA-1 for computation of the message
integrity. If, at a later time, HMAC-SHA-1 is found to be
compromised, the following is the remedy that will be applied.
We will define a STUN extension that introduces a new message-
integrity attribute, computed using a new hash. Clients would be
required to include both the new and old message-integrity attributes
in their requests or indications. A new server will utilize the new
message-integrity attribute, and an old one, the old. After a
transition period where mixed implementations are in deployment, the
old message-integrity attribute will be deprecated by another
specification, and clients will cease including it in requests.
It is also important to note that the HMAC is done using a key that
is itself computed using an MD5 of the user's password. The choice
of the MD5 hash was made because of the existence of legacy databases
that store passwords in that form. If future work finds that an HMAC
of an MD5 input is not secure, and a different hash is needed, it can
also be changed using this plan. However, this would require
administrators to repopulate their databases.
16. IAB Considerations
The IAB has studied the problem of Unilateral Self-Address Fixing
(UNSAF), which is the general process by which a client attempts to
determine its address in another realm on the other side of a NAT
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through a collaborative protocol reflection mechanism (RFC3424
[RFC3424]). STUN can be used to perform this function using a
Binding request/response transaction if one agent is behind a NAT and
the other is on the public side of the NAT.
The IAB has suggested that protocols developed for this purpose
document a specific set of considerations. Because some STUN usages
provide UNSAF functions (such as ICE [RFC5245] ), and others do not
(such as SIP Outbound [RFC5626]), answers to these considerations
need to be addressed by the usages themselves.
17. IANA Considerations
IANA has created three new registries: a "STUN Methods Registry", a
"STUN Attributes Registry", and a "STUN Error Codes Registry". IANA
has also changed the name of the assigned IANA port for STUN from
"nat-stun-port" to "stun".
17.1. STUN Methods Registry
A STUN method is a hex number in the range 0x000 - 0xFFF. The
encoding of STUN method into a STUN message is described in
Section 5.
The initial STUN methods are:
0x000: (Reserved)
0x001: Binding
0x002: (Reserved; was SharedSecret)
STUN methods in the range 0x000 - 0x7FF are assigned by IETF Review
[RFC5226]. STUN methods in the range 0x800 - 0xFFF are assigned by
Designated Expert [RFC5226]. The responsibility of the expert is to
verify that the selected codepoint(s) are not in use and that the
request is not for an abnormally large number of codepoints.
Technical review of the extension itself is outside the scope of the
designated expert responsibility.
17.2. STUN Attribute Registry
A STUN Attribute type is a hex number in the range 0x0000 - 0xFFFF.
STUN attribute types in the range 0x0000 - 0x7FFF are considered
comprehension-required; STUN attribute types in the range 0x8000 -
0xFFFF are considered comprehension-optional. A STUN agent handles
unknown comprehension-required and comprehension-optional attributes
differently.
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The initial STUN Attributes types are:
Comprehension-required range (0x0000-0x7FFF):
0x0000: (Reserved)
0x0001: MAPPED-ADDRESS
0x0002: (Reserved; was RESPONSE-ADDRESS)
0x0003: (Reserved; was CHANGE-ADDRESS)
0x0004: (Reserved; was SOURCE-ADDRESS)
0x0005: (Reserved; was CHANGED-ADDRESS)
0x0006: USERNAME
0x0007: (Reserved; was PASSWORD)
0x0008: MESSAGE-INTEGRITY
0x0009: ERROR-CODE
0x000A: UNKNOWN-ATTRIBUTES
0x000B: (Reserved; was REFLECTED-FROM)
0x0014: REALM
0x0015: NONCE
0x0020: XOR-MAPPED-ADDRESS
0xXXXX: PASSWORD-ALGORITHM
Comprehension-optional range (0x8000-0xFFFF)
0x8022: SOFTWARE
0x8023: ALTERNATE-SERVER
0x8028: FINGERPRINT
0xXXXX: PASSSORD-ALGORITHMS
0xXXXX: ALTERNATE-DOMAIN
STUN Attribute types in the first half of the comprehension-required
range (0x0000 - 0x3FFF) and in the first half of the comprehension-
optional range (0x8000 - 0xBFFF) are assigned by IETF Review
[RFC5226]. STUN Attribute types in the second half of the
comprehension-required range (0x4000 - 0x7FFF) and in the second half
of the comprehension-optional range (0xC000 - 0xFFFF) are assigned by
Designated Expert [RFC5226]. The responsibility of the expert is to
verify that the selected codepoint(s) are not in use, and that the
request is not for an abnormally large number of codepoints.
Technical review of the extension itself is outside the scope of the
designated expert responsibility.
17.3. STUN Error Code Registry
A STUN error code is a number in the range 0 - 699. STUN error codes
are accompanied by a textual reason phrase in UTF-8 [RFC3629] that is
intended only for human consumption and can be anything appropriate;
this document proposes only suggested values.
STUN error codes are consistent in codepoint assignments and
semantics with SIP [RFC3261] and HTTP [RFC2616].
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The initial values in this registry are given in Section 14.7.
New STUN error codes are assigned based on IETF Review [RFC5226].
The specification must carefully consider how clients that do not
understand this error code will process it before granting the
request. See the rules in Section 6.3.4.
17.4. Password Algorithm Registry
A Password Algorithm is a hex number in the range 0x0000 - 0xFFFF.
The initial Password Algorithms are:
0x0001: Salted SHA256
Password Algorithms in the first half of the range (0x0000 - 0x7FFF)
are assigned by IETF Review [RFC5226]. Password Algorithms in the
second half of the range (0x8000 - 0xFFFF) are assigned by Designated
Expert [RFC5226].
17.4.1. Password Algorithms
The initial list of password algorithms is taken from
[I-D.veltri-sip-alt-auth].
17.4.1.1. Salted SHA256
The key length is 32 bytes and the parameters contains the salt.
key = SHA256(username ":" realm ":" SASLprep(password) ":" salt)
17.5. STUN UDP and TCP Port Numbers
IANA has previously assigned port 3478 for STUN. This port appears
in the IANA registry under the moniker "nat-stun-port". In order to
align the DNS SRV procedures with the registered protocol service,
IANA is requested to change the name of protocol assigned to port
3478 from "nat-stun-port" to "stun", and the textual name from
"Simple Traversal of UDP Through NAT (STUN)" to "Session Traversal
Utilities for NAT", so that the IANA port registry would read:
stun 3478/tcp Session Traversal Utilities for NAT (STUN) port
stun 3478/udp Session Traversal Utilities for NAT (STUN) port
In addition, IANA has assigned port number 5349 for the "stuns"
service, defined over TCP and UDP. The UDP port is not currently
defined; however, it is reserved for future use.
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18. Changes since RFC 5389
This specification obsoletes RFC 5389 [RFC5389]. This specification
differs from RFC 5389 in the following ways:
o
19. References
19.1. Normative References
[I-D.ietf-tsvwg-sctp-dtls-encaps]
Jesup, R., Loreto, S., Stewart, R., and M. Tuexen, "DTLS
Encapsulation of SCTP Packets for RTCWEB", draft-ietf-
tsvwg-sctp-dtls-encaps-00 (work in progress), February
2013.
[I-D.ietf-tsvwg-sctp-prpolicies]
Tuexen, M., Seggelmann, R., Stewart, R., and S. Loreto,
"Additional Policies for the Partial Reliability Extension
of the Stream Control Transmission Protocol", draft-ietf-
tsvwg-sctp-prpolicies-05 (work in progress), November
2014.
[ITU.V42.2002]
International Telecommunications Union, "Error-correcting
Procedures for DCEs Using Asynchronous-to-Synchronous
Conversion", ITU-T Recommendation V.42, 2002.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
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[RFC2617] 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.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758, May 2004.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, February 2005.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298, June
2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951, May 2013.
[RFC7064] Nandakumar, S., Salgueiro, G., Jones, P., and M. Petit-
Huguenin, "URI Scheme for the Session Traversal Utilities
for NAT (STUN) Protocol", RFC 7064, November 2013.
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[RFC7350] Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
Layer Security (DTLS) as Transport for Session Traversal
Utilities for NAT (STUN)", RFC 7350, August 2014.
19.2. Informational References
[I-D.ietf-tram-stun-origin]
Johnston, A., Uberti, J., Yoakum, J., and K. Singh, "An
Origin Attribute for the STUN Protocol", draft-ietf-tram-
stun-origin-05 (work in progress), February 2015.
[I-D.ietf-uta-tls-bcp]
Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of TLS and DTLS", draft-
ietf-uta-tls-bcp-11 (work in progress), February 2015.
[I-D.veltri-sip-alt-auth]
Veltri, L., Salsano, S., and A. Polidoro, "HTTP digest
authentication using alternate password storage schemes",
draft-veltri-sip-alt-auth-00 (work in progress), April
2008.
[KARN87] Karn, P. and C. Partridge, "Improving Round-Trip Time
Estimates in Reliable Transport Protocols", SIGCOMM 1987,
August 1987.
[RFC2616] 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.
[RFC3261] 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.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[RFC3489] 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.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, June 2005.
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[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5626] Jennings, C., Mahy, R., and F. Audet, "Managing Client-
Initiated Connections in the Session Initiation Protocol
(SIP)", RFC 5626, October 2009.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC5780] MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
Using Session Traversal Utilities for NAT (STUN)", RFC
5780, May 2010.
[RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
"TCP Candidates with Interactive Connectivity
Establishment (ICE)", RFC 6544, March 2012.
Appendix A. C Snippet to Determine STUN Message Types
Given a 16-bit STUN message type value in host byte order in msg_type
parameter, below are C macros to determine the STUN message types:
#define IS_REQUEST(msg_type) (((msg_type) & 0x0110) == 0x0000)
#define IS_INDICATION(msg_type) (((msg_type) & 0x0110) == 0x0010)
#define IS_SUCCESS_RESP(msg_type) (((msg_type) & 0x0110) == 0x0100)
#define IS_ERR_RESP(msg_type) (((msg_type) & 0x0110) == 0x0110)
A function to convert method and class into a message type:
int type(int method, int cls) {
return (method & 0x0F80) << 9 | (method & 0x0070) << 5
| (method & 0x000F) | (cls & 0x0002) << 8
| (cls & 0x0001) << 4;
}
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A function to extract the method from the message type:
int method(int type) {
return (type & 0x3E00) >> 2 | (type & 0x00E0) >> 1
| (type & 0x000F);
}
A function to extract the class from the message type:
int cls(int type) {
return (type & 0x0100) >> 7 | (type & 0x0010) >> 4;
}
Appendix B. Release notes
This section must be removed before publication as an RFC.
B.1. Open Issues
1. Integrate RFC 5769 (stun vectors) as examples
B.2. Modifications between draft-ietf-tram-stunbis-02 and draft-ietf-
tram-stunbis-01
o Prevent the server to allocate the same NONCE to clients with
different IP address and/or different port. This prevent sharing
the nonce between TURN allocations in TURN.
o Add reference to draft-ietf-uta-tls-bcp
o Add a new attribute ALTERNATE-DOMAIN to verify the certificate of
the ALTERNATE-SERVER after a 300 over (D)TLS.
o The RTP delay between transactions applies only to parallel
transactions, not to serial transactions. That prevents a 3RTT
delay between the first transaction and the second transaction
with long term authentication.
o Add text saying ORIGIN can increase a request size beyond the MTU
and so require an SCTPoUDP transport.
o Add a new attribute ALTERNATE-DOMAIN to verify the certificate of
the ALTERNATE-SERVER after a 300 over (D)TLS.
o The RTP delay between transactions applies only to parallel
transactions, not to serial transactions. That prevents a 3RTT
delay between the first transaction and the second transaction
with long term authentication.
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o Add text saying ORIGIN can increase a request size beyond the MTU
and so require an SCTPoUDP transport.
o Move the Acknowledgments and Contributor sections to the end of
the document, in accordance with RFC 7322 section 4.
B.3. Modifications between draft-ietf-tram-stunbis-01 and draft-ietf-
tram-stunbis-00
o Add negotiation mechanism for new password algorithms.
o Describe the MESSAGE-INTEGRITY/MESSAGE-INTEGRITY2 protocol.
o Add support for SCTP to solve the fragmentation problem.
o Merge RFC 7350:
* Split the "Sending over..." sections in 3.
* Add DTLS-over-UDP as transport.
* Update the cipher suites and cipher/compression restrictions.
* A stuns uri with an IP address is rejected.
* Replace most of the RFC 3489 compatibility by a reference to
the section in RFC 5389.
* Update the STUN Usages list with transport applicability.
o Merge RFC 7064:
* DNS discovery is done from the URI.
* Reorganized the text about default ports.
o Add more C snippets.
o Make clear that the cached RTO is discarded only if there is no
new transations for 10 minutes.
B.4. Modifications between draft-salgueiro-tram-stunbis-02 and draft-
ietf-tram-stunbis-00
o Draft adopted as WG item.
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B.5. Modifications between draft-salgueiro-tram-stunbis-02 and draft-
salgueiro-tram-stunbis-01
o Add definition of MESSAGE-INTEGRITY2.
o Update text and reference from RFC 2988 to RFC 6298.
o s/The IAB has mandated/The IAB has suggested/ (Errata #3737).
o Fix the figure for the UNKNOWN-ATTRIBUTES (Errata #2972).
o Fix section number and make clear that the original domain name is
used for the server certificate verification. This is consistent
with what RFC 5922 (section 4) is doing. (Errata #2010)
o Remove text transitioning from RFC 3489.
o Add definition of MESSAGE-INTEGRITY2.
o Update text and reference from RFC 2988 to RFC 6298.
o s/The IAB has mandated/The IAB has suggested/ (Errata #3737).
o Fix the figure for the UNKNOWN-ATTRIBUTES (Errata #2972).
o Fix section number and make clear that the original domain name is
used for the server certificate verification. This is consistent
with what RFC 5922 (section 4) is doing. (Errata #2010)
B.6. Modifications between draft-salgueiro-tram-stunbis-01 and draft-
salgueiro-tram-stunbis-00
o Restore the RFC 5389 text.
o Add list of open issues.
Acknowledgements
Thanks to Michael Tuexen, Tirumaleswar Reddy, Oleg Moskalenko, Simon
Perreault, and Benjamin Schwartz for the comments, suggestions, and
questions that helped improve this document.
The authors of RFC 5389 would like to thank Cedric Aoun, Pete
Cordell, Cullen Jennings, Bob Penfield, Xavier Marjou, Magnus
Westerlund, Miguel Garcia, Bruce Lowekamp, and Chris Sullivan for
their comments, and Baruch Sterman and Alan Hawrylyshen for initial
implementations. Thanks for Leslie Daigle, Allison Mankin, Eric
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Rescorla, and Henning Schulzrinne for IESG and IAB input on this
work.
Contributors
Christian Huitema and Joel Weinberger were original co-authors of RFC
3489.
Authors' Addresses
Marc Petit-Huguenin
Impedance Mismatch
Email: marc@petit-huguenin.org
Gonzalo Salgueiro
Cisco
7200-12 Kit Creek Road
Research Triangle Park, NC 27709
US
Email: gsalguei@cisco.com
Jonathan Rosenberg
Cisco
Edison, NJ
US
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
Dan Wing
Cisco
771 Alder Drive
San Jose, CA 95035
US
Email: dwing@cisco.com
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Rohan Mahy
Plantronics
345 Encinal Street
Santa Cruz, CA 95060
US
Email: rohan@ekabal.com
Philip Matthews
Avaya
1135 Innovation Drive
Ottawa, Ontario K2K 3G7
Canada
Phone: +1 613 592 4343 x224
Email: philip_matthews@magma.ca
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