RFC 5389
Session Traversal Utilities for NAT (STUN)
15. 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 18.2. The rest of this section describes the format of the various attributes defined in this specification.15.1. 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.15.2. 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. 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.15.3. USERNAME
The USERNAME attribute is used for message integrity. It identifies the username and password combination used in the message-integrity check. 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].15.4. 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 FINGERPRINT attribute, which appears after MESSAGE-INTEGRITY, agents MUST ignore all other attributes that follow MESSAGE-INTEGRITY.
The key for the HMAC depends on whether long-term or short-term
credentials are in use. For long-term credentials, the key is 16
bytes:
key = MD5(username ":" realm ":" SASLprep(password))
That is, 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 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.
For short-term credentials:
key = SASLprep(password)
where MD5 is defined in RFC 1321 [RFC1321] and SASLprep() is defined
in RFC 4013 [RFC4013].
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.
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.
15.5. 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 8. 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.15.6. 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 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.
15.7. 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.15.8. 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. It MUST be less than 128 characters (which can be as long as 763 bytes).15.9. UNKNOWN-ATTRIBUTES
The UNKNOWN-ATTRIBUTES attribute is present only in an error response when the response code in the ERROR-CODE attribute is 420. 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 8: 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.
15.10. 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]
encoded sequence of less than 128 characters (which can be as long as
763 bytes).
15.11. 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.
16. Security Considerations
16.1. Attacks against the Protocol
16.1.1. 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. 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 14, 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.16.1.2. 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.16.2. 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 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 [MMUSIC-ICE]. Other usages may use other means to prevent these attacks.16.2.1. 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.16.2.2. 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 16.2.1, this attack is only possible when the attacker is on path for packets sent from the STUN server towards this unused IP address.
16.2.3. 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.16.2.4. 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.16.3. 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.17. IAB Considerations
The IAB has studied the problem of Unilateral Self-Address Fixing (UNSAF), which is the general process by which a client attempts to determine its address in another realm on the other side of a NAT
through a collaborative protocol reflection mechanism (RFC3424 [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 mandated that protocols developed for this purpose document a specific set of considerations. Because some STUN usages provide UNSAF functions (such as ICE [MMUSIC-ICE] ), and others do not (such as SIP Outbound [SIP-OUTBOUND]), answers to these considerations need to be addressed by the usages themselves.18. 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".18.1. 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 6. 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.18.2. 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. 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
Comprehension-optional range (0x8000-0xFFFF)
0x8022: SOFTWARE
0x8023: ALTERNATE-SERVER
0x8028: FINGERPRINT
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.
18.3. 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].
The initial values in this registry are given in Section 15.6.
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 7.3.4.18.4. 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.19. Changes since RFC 3489
This specification obsoletes RFC 3489 [RFC3489]. This specification differs from RFC 3489 in the following ways: o Removed the notion that STUN is a complete NAT traversal solution. STUN is now a tool that can be used to produce a NAT traversal solution. As a consequence, changed the name of the protocol to Session Traversal Utilities for NAT. o Introduced the concept of STUN usages, and described what a usage of STUN must document. o Removed the usage of STUN for NAT type detection and binding lifetime discovery. These techniques have proven overly brittle due to wider variations in the types of NAT devices than described in this document. Removed the RESPONSE-ADDRESS, CHANGED-ADDRESS, CHANGE-REQUEST, SOURCE-ADDRESS, and REFLECTED-FROM attributes. o Added a fixed 32-bit magic cookie and reduced length of transaction ID by 32 bits. The magic cookie begins at the same offset as the original transaction ID.
o Added the XOR-MAPPED-ADDRESS attribute, which is included in
Binding responses if the magic cookie is present in the request.
Otherwise, the RFC 3489 behavior is retained (that is, Binding
response includes MAPPED-ADDRESS). See discussion in XOR-MAPPED-
ADDRESS regarding this change.
o Introduced formal structure into the message type header field,
with an explicit pair of bits for indication of request, response,
error response, or indication. Consequently, the message type
field is split into the class (one of the previous four) and
method.
o Explicitly point out that the most significant 2 bits of STUN are
0b00, allowing easy differentiation with RTP packets when used
with ICE.
o Added the FINGERPRINT attribute to provide a method of definitely
detecting the difference between STUN and another protocol when
the two protocols are multiplexed together.
o Added support for IPv6. Made it clear that an IPv4 client could
get a v6 mapped address, and vice versa.
o Added long-term-credential-based authentication.
o Added the SOFTWARE, REALM, NONCE, and ALTERNATE-SERVER attributes.
o Removed the SharedSecret method, and thus the PASSWORD attribute.
This method was almost never implemented and is not needed with
current usages.
o Removed recommendation to continue listening for STUN responses
for 10 seconds in an attempt to recognize an attack.
o Changed transaction timers to be more TCP friendly.
o Removed the STUN example that centered around the separation of
the control and media planes. Instead, provided more information
on using STUN with protocols.
o Defined a generic padding mechanism that changes the
interpretation of the length attribute. This would, in theory,
break backwards compatibility. However, the mechanism in RFC 3489
never worked for the few attributes that weren't aligned naturally
on 32-bit boundaries.
o REALM, SERVER, reason phrases, and NONCE limited to 127
characters. USERNAME to 513 bytes.
o Changed the DNS SRV procedures for TCP and TLS. UDP remains the
same as before.
20. Contributors
Christian Huitema and Joel Weinberger were original co-authors of RFC
3489.
21. Acknowledgements
The authors 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 Rescorla, and Henning
Schulzrinne for IESG and IAB input on this work.
22. References
22.1. Normative References
[ITU.V42.2002] International Telecommunications Union, "Error-
correcting Procedures for DCEs Using Asynchronous-
to-Synchronous Conversion", ITU-T Recommendation
V.42, March 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.
[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. [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission Timer", RFC 2988, November 2000. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names and Passwords", RFC 4013, February 2005.22.2. Informative References
[BEHAVE-NAT] MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery Using STUN", Work in Progress, July 2008. [BEHAVE-TURN] Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)", Work in Progress, July 2008. [KARN87] Karn, P. and C. Partridge, "Improving Round-Trip Time Estimates in Reliable Transport Protocols", SIGCOMM 1987, August 1987. [MMUSIC-ICE] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", Work in Progress, October 2007. [MMUSIC-ICE-TCP] Rosenberg, J., "TCP Candidates with Interactive Connectivity Establishment (ICE)", Work in Progress, July 2008. [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. [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, 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. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [SIP-OUTBOUND] Jennings, C. and R. Mahy, "Managing Client Initiated Connections in the Session Initiation Protocol (SIP)", Work in Progress, June 2008.
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)Authors' Addresses
Jonathan Rosenberg Cisco Edison, NJ US EMail: jdrosen@cisco.com URI: http://www.jdrosen.net Rohan Mahy Unaffiliated EMail: rohan@ekabal.com Philip Matthews Unaffiliated EMail: philip_matthews@magma.ca Dan Wing Cisco 771 Alder Drive San Jose, CA 95035 US EMail: dwing@cisco.com
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