RFC 8489
Session Traversal Utilities for NAT (STUN)
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. Note that agents MUST ignore all attributes that follow MESSAGE- INTEGRITY, with the exception of the MESSAGE-INTEGRITY-SHA256 and FINGERPRINT attributes. Similarly, agents MUST ignore all attributes that follow the MESSAGE-INTEGRITY-SHA256 attribute if the MESSAGE- INTEGRITY attribute is not present, with the exception of the FINGERPRINT attribute.
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 [RFC8445], 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 limited by virtue of the time-limited nature of the credential.9.1.1. HMAC Key
For short-term credentials, the Hash-Based Message Authentication Code (HMAC) key is defined as follow: key = OpaqueString(password) where the OpaqueString profile is defined in [RFC8265]. The encoding used is UTF-8 [RFC3629].9.1.2. Forming a Request or Indication
For a request or indication message, the agent MUST include the USERNAME, MESSAGE-INTEGRITY-SHA256, and MESSAGE-INTEGRITY attributes in the message unless the agent knows from an external mechanism which message integrity algorithm is supported by both agents. In this case, either MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 MUST be included in addition to USERNAME. The HMAC for the MESSAGE- INTEGRITY attribute is computed as described in Section 14.5, and the HMAC for the MESSAGE-INTEGRITY-SHA256 attributes is computed as described in Section 14.6. 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 the order specified: o If the message does not contain 1) a MESSAGE-INTEGRITY or a MESSAGE-INTEGRITY-SHA256 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 (Unauthenticated).
* If the message is an indication, the agent MUST silently
discard the indication.
o If the MESSAGE-INTEGRITY-SHA256 attribute is present, compute the
value for the message integrity as described in Section 14.6,
using the password associated with the username. If the MESSAGE-
INTEGRITY-SHA256 attribute is not present, then use the same
password to compute the value for the message integrity as
described in Section 14.5. If the resulting value does not match
the contents of the corresponding attribute (MESSAGE-INTEGRITY-
SHA256 or MESSAGE-INTEGRITY):
* 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 (Unauthenticated).
* If the message is an indication, the agent MUST silently
discard the indication.
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-INTEGRITY-SHA256 attribute MUST include the
MESSAGE-INTEGRITY-SHA256 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. This means that only
one of these attributes can appear in a response. The response MUST
NOT contain the USERNAME attribute.
If any of the checks fail, a server MUST NOT include a MESSAGE- INTEGRITY-SHA256, 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- INTEGRITY-SHA256 or MESSAGE-INTEGRITY attributes.9.1.4. Receiving a Response
The client looks for the MESSAGE-INTEGRITY or the MESSAGE-INTEGRITY- SHA256 attribute in the response. If present and if the client only sent one of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attributes in the request (because of the external indication in Section 9.1.2 or because this is a subsequent request as defined in Section 9.1.5), the algorithm in the response has to match; otherwise, the response MUST be discarded. The client then computes the message integrity over the response as defined in Section 14.5 for the MESSAGE-INTEGRITY attribute or Section 14.6 for the MESSAGE-INTEGRITY-SHA256 attribute, using the same password it utilized for the request. If the resulting value matches the contents of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY- SHA256 attribute, respectively, the response is considered authenticated. If the value does not match, or if both MESSAGE- INTEGRITY and MESSAGE-INTEGRITY-SHA256 are absent, the processing depends on whether the request was sent over a reliable or an unreliable transport. If the request was sent over an unreliable transport, the response MUST be discarded, as if it had never been received. This means that retransmits, if applicable, will continue. If all the responses received are discarded, then instead of signaling a timeout after ending the transaction, the layer MUST signal that the integrity protection was violated. If the request was sent over a reliable transport, the response MUST be discarded, and the layer MUST immediately end the transaction and signal that the integrity protection was violated.9.1.5. Sending Subsequent Requests
A client sending subsequent requests to the same server MUST send only the MESSAGE-INTEGRITY-SHA256 or the MESSAGE-INTEGRITY attribute that matches the attribute that was received in the response to the initial request. Here, "same server" means same IP address and port number, not just the same URI or SRV lookup result.
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 until it 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 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 a limited 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. Only the server needs to know about the internal structure of the cookie. 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 one of the message-integrity attributes defined in this document, 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. To indicate that it supports this specification, a server MUST prepend the NONCE attribute value with the character string composed of "obMatJos2" concatenated with the (4-character) base64 [RFC4648] encoding of the 24-bit STUN Security Features as defined in Section 18.1. The 24-bit Security Feature set is encoded as 3 bytes, with bit 0 as the most significant bit of the first byte and bit 23 as the least significant bit of the third byte. If no security features are used, then a byte array with all 24 bits set to zero
MUST be encoded instead. For the remainder of this document, the term "nonce cookie" will refer to the complete 13-character string prepended to the NONCE attribute value. 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. Bid-Down Attack Prevention
This document introduces two new security features that provide the ability to choose the algorithm used for password protection as well as the ability to use an anonymous username. Both of these capabilities are optional in order to remain backwards compatible with previous versions of the STUN protocol. These new capabilities are subject to bid-down attacks whereby an attacker in the message path can remove these capabilities and force weaker security properties. To prevent these kinds of attacks from going undetected, the nonce is enhanced with additional information. The value of the "nonce cookie" will vary based on the specific STUN Security Feature bits selected. When this document makes reference to the "nonce cookie" in a section discussing a specific STUN Security Feature it is understood that the corresponding STUN Security Feature bit in the "nonce cookie" is set to 1. For example, when the PASSWORD-ALGORITHMS security feature (defined in Section 9.2.4) is used, the corresponding "Password algorithms" bit (defined in Section 18.1) is set to 1 in the "nonce cookie".9.2.2. 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 ":" OpaqueString(realm) ":" OpaqueString(password)) Where MD5 is defined in [RFC1321] and [RFC6151], and the OpaqueString profile is defined in [RFC8265]. The encoding used is UTF-8 [RFC3629].
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 OpaqueString has already been applied); (2) a single colon; (3) the realm, with any quotes and trailing nulls removed and after processing using OpaqueString; (4) a single colon; and (5) the password, with any trailing nulls removed and after processing using OpaqueString. For example, if the username is 'user', the realm is 'realm', and the password is '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 [RFC3261]. 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. For example, this mechanism can be used with the authentication extensions defined in [RFC5090]. When a PASSWORD-ALGORITHM is used, the key length and algorithm to use are described in Section 18.5.1.9.2.3. Forming a Request
The first request from the client to the server (as identified by hostname if the DNS procedures of Section 8 are used and by IP address if not) is handled according to the rules in Section 9.2.3.1. When the client initiates a subsequent request once a previous request/response transaction has completed successfully, it follows the rules in Section 9.2.3.2. Forming a request as a consequence of a 401 (Unauthenticated) or 438 (Stale Nonce) error response is covered in Section 9.2.5 and is not considered a "subsequent request" and thus does not utilize the rules described in Section 9.2.3.2. Each of these types of requests have a different mandatory attributes.9.2.3.1. First Request
If the client has not completed a successful request/response transaction with the server, it MUST omit the USERNAME, USERHASH, MESSAGE-INTEGRITY, MESSAGE-INTEGRITY-SHA256, REALM, NONCE, PASSWORD- ALGORITHMS, and PASSWORD-ALGORITHM attributes. In other words, the first request is sent as if there were no authentication or message integrity applied.
9.2.3.2. Subsequent Requests
Once a request/response transaction has completed, 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 MUST include either the USERNAME or USERHASH, REALM, NONCE, and PASSWORD-ALGORITHM attributes with these cached values. It MUST include a MESSAGE-INTEGRITY attribute or a MESSAGE-INTEGRITY- SHA256 attribute, computed as described in Sections 14.5 and 14.6 using the cached password. The choice between the two attributes depends on the attribute received in the response to the first request.9.2.4. Receiving a Request
After the server has done the basic processing of a request, it performs the checks listed below in the order specified. Note that it is RECOMMENDED that the REALM value be the domain name of the provider of the STUN server: o If the message does not contain a MESSAGE-INTEGRITY or MESSAGE- INTEGRITY-SHA256 attribute, the server MUST generate an error response with an error code of 401 (Unauthenticated). This response MUST include a REALM value. The response MUST include a NONCE, selected by the server. The server MUST NOT choose the same NONCE for two requests unless they have the same source IP address and port. 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 set the STUN Security Feature "Password algorithms" bit to 1. The server MAY support anonymous username, in which case it MUST set the STUN Security Feature "Username anonymity" bit set to 1. The response SHOULD NOT contain a USERNAME, USERHASH, MESSAGE-INTEGRITY, or MESSAGE- INTEGRITY-SHA256 attribute. Note: Reusing a NONCE for different source IP addresses or ports was not explicitly forbidden in [RFC5389]. o If the message contains a MESSAGE-INTEGRITY or a MESSAGE- INTEGRITY-SHA256 attribute, but is missing either the USERNAME or USERHASH, 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, USERHASH, NONCE, or REALM
attribute. The response cannot contain a MESSAGE-INTEGRITY or
MESSAGE-INTEGRITY-SHA256 attribute, as the attributes required to
generate them are missing.
o If the NONCE attribute starts with the "nonce cookie" with the
STUN Security Feature "Password algorithms" bit set to 1, the
server performs these checks in the order specified:
* If the request contains neither the PASSWORD-ALGORITHMS nor the
PASSWORD-ALGORITHM algorithm, then the request is processed as
though PASSWORD-ALGORITHM were MD5.
* Otherwise, unless (1) PASSWORD-ALGORITHM and PASSWORD-
ALGORITHMS are both present, (2) PASSWORD-ALGORITHMS matches
the value sent in the response that sent this NONCE, and (3)
PASSWORD-ALGORITHM matches one of the entries in PASSWORD-
ALGORITHMS, the server MUST generate an error response with an
error code of 400 (Bad Request).
o If the value of the USERNAME or USERHASH attribute is not valid,
the server MUST generate an error response with an error code of
401 (Unauthenticated). This response MUST include a REALM value.
The response MUST include a NONCE, selected by the server. The
response MUST include a PASSWORD-ALGORITHMS attribute. The
response SHOULD NOT contain a USERNAME or USERHASH attribute. The
response MAY include a MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute, using the previous key to calculate it.
o If the MESSAGE-INTEGRITY-SHA256 attribute is present, compute the
value for the message integrity as described in Section 14.6,
using the password associated with the username. Otherwise, using
the same password, compute the value for the MESSAGE-INTEGRITY
attribute as described in Section 14.5. If the resulting value
does not match the contents of the MESSAGE-INTEGRITY attribute or
the MESSAGE-INTEGRITY-SHA256 attribute, the server MUST reject the
request with an error response. This response MUST use an error
code of 401 (Unauthenticated). It MUST include the REALM and
NONCE attributes and SHOULD NOT include the USERNAME, USERHASH,
MESSAGE-INTEGRITY, or MESSAGE-INTEGRITY-SHA256 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, REALM, and PASSWORD-ALGORITHMS attributes and
SHOULD NOT include the USERNAME and USERHASH attributes. The
NONCE attribute value MUST be valid. The response MAY include a
MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute, using the
previous NONCE to calculate it. Servers can revoke nonces in
order to provide additional security. See Section 5.4 of
[RFC7616] for guidelines.
If these checks pass, the server continues to process the request.
Any response generated by the server MUST include the MESSAGE-
INTEGRITY-SHA256 attribute, computed using the username and password
utilized to authenticate the request, unless the request was
processed as though PASSWORD-ALGORITHM was MD5 (because the request
contained neither PASSWORD-ALGORITHMS nor PASSWORD-ALGORITHM). In
that case, the MESSAGE-INTEGRITY attribute MUST be used instead of
the MESSAGE-INTEGRITY-SHA256 attribute, and the REALM, NONCE,
USERNAME, and USERHASH attributes SHOULD NOT be included.
9.2.5. Receiving a Response
If the response is an error response with an error code of 401
(Unauthenticated) or 438 (Stale Nonce), the client MUST test if the
NONCE attribute value starts with the "nonce cookie". If so and the
"nonce cookie" has the STUN Security Feature "Password algorithms"
bit set to 1 but 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
(Unauthenticated), the client SHOULD retry the request with a new
transaction. This request MUST contain a USERNAME or a USERHASH,
determined by the client as the appropriate username for the REALM
from the error response. If the "nonce cookie" is present and has
the STUN Security Feature "Username anonymity" bit set to 1, then the
USERHASH attribute MUST be used; else, the USERNAME attribute MUST be
used. 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 PASSWORD-ALGORITHMS attribute, and this
attribute does not contain any algorithm that is supported by the
client, then the client MUST NOT retry the request with a new
transaction. The client MUST NOT perform this retry if it is not
changing the USERNAME, USERHASH, 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 either the USERNAME or USERHASH, the REALM, and
either the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute.
For all other responses, if the NONCE attribute starts with the
"nonce cookie" with the STUN Security Feature "Password algorithms"
bit set to 1 but PASSWORD-ALGORITHMS is not present, the response
MUST be ignored.
If the response is an error response with an error code of 400 (Bad
Request) and does not contain either the MESSAGE-INTEGRITY or
MESSAGE-INTEGRITY-SHA256 attribute, then the response MUST be
discarded, as if it were never received. This means that
retransmits, if applicable, will continue.
Note: In this case, the 400 response will never reach the
application, resulting in a timeout.
The client looks for the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute in the response (either success or failure). If
present, the client computes the message integrity over the response
as defined in Sections 14.5 or 14.6, using the same password it
utilized for the request. If the resulting value matches the
contents of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
attribute, the response is considered authenticated. If the value
does not match, or if both MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-
SHA256 are absent, the processing depends on the request being sent
over a reliable or an unreliable transport.
If the request was sent over an unreliable transport, the response
MUST be discarded, as if it had never been received. This means that
retransmits, if applicable, will continue. If all the responses
received are discarded, then instead of signaling a timeout after
ending the transaction, the layer MUST signal that the integrity
protection was violated.
If the request was sent over a reliable transport, the response MUST
be discarded, and the layer MUST immediately end the transaction and
signal that the integrity protection was violated.
If the response contains a PASSWORD-ALGORITHMS attribute, all the
subsequent requests MUST be authenticated using MESSAGE-INTEGRITY-
SHA256 only.
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. The ALTERNATE-SERVER attribute carries an IP address. 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 at least one ALTERNATE-SERVER attribute in the error response, which MUST contain an IP address of the same address family as the source IP address of the request message. The server SHOULD include an additional ALTERNATE-SERVER attribute, after the mandatory one, that contains an IP address of the address family other than the source IP address of the request message. The error response message MAY be authenticated; however, there are use cases for ALTERNATE-SERVER where authentication of the response is not possible or practical. If the transaction uses TLS or DTLS, if the transaction is authenticated by a MESSAGE-INTEGRITY-SHA256 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 name inside the subjectAltName of that certificate. This series of conditions on the MESSAGE-INTEGRITY-SHA256 attribute indicates that the transaction is authenticated and that the client implements this specification and therefore can process the ALTERNATE-DOMAIN attribute. 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 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 certificate using the recommendations in [RFC6125]. The certificate MUST contain an identifier of type DNS-ID or CN-ID (eventually with wildcards) but not of type SRV-ID or URI- ID. 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 to which it has already sent 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 3489
In addition to the backward compatibility already described in Section 12 of [RFC5389], DTLS MUST NOT be used with [RFC3489] (referred to as "classic STUN"). Any STUN request or indication without the magic cookie (see Section 6 of [RFC5389]) over DTLS MUST be considered invalid: all requests MUST generate a 500 (Server Error) error response, and indications MUST be ignored.12. Basic Server Behavior
This section defines the behavior of a basic, stand-alone STUN server. Historically, "classic STUN" [RFC3489] only defined the behavior of a server that was providing clients with server reflexive transport addresses by receiving and replying to STUN Binding requests. [RFC5389] redefined the protocol as an extensible framework, and the server functionality became the sole STUN Usage defined in that document. This STUN Usage is also known as "Basic STUN Server". 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 does not require a keep-alive mechanism because a TCP or TLS-over-TCP connection is closed after the end of the Binding transaction. 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 clients using [RFC3489], as described in Section 11. It is RECOMMENDED that administrators of STUN servers provide DNS entries for those servers as described in Section 8. If both A and AAAA resource records are returned, then the client can simultaneously send STUN Binding requests to the IPv4 and IPv6 addresses (as specified in [RFC8305]), as the Binding request is idempotent. Note that the MAPPED-ADDRESS or XOR-MAPPED-ADDRESS attributes that are returned will not necessarily match the address family of the server address used.
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 13.13. 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. 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 also defines: 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 or TLS-over-TCP, a framing mechanism may be required. o How a STUN client determines the IP address and port of the STUN server. o How simultaneous use of IPv4 and IPv6 addresses (Happy Eyeballs [RFC8305]) works with non-idempotent transactions when both address families are found for 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. o If MESSAGE-INTEGRITY-SHA256 truncation is permitted, and the limits permitted for truncation. o The keep-alive mechanism if STUN is run over TCP or TLS-over-TCP.
o If anycast addresses can be used for the server in case 1) TCP or
TLS-over-TCP or 2) authentication is used.
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
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 MUST be set to zero on sending and MUST be ignored by
the receiver.
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 18.3. The rest of this section describes the format of the various attributes defined in this specification.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 [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. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0| Family | X-Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | X-Address (Variable) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: Format of XOR-MAPPED-ADDRESS Attribute The Family field represents the IP address family and is encoded identically to the Family field in MAPPED-ADDRESS. X-Port is computed by XOR'ing the mapped port with the most significant 16 bits of the magic cookie. If the IP address family is IPv4, X-Address is computed by XOR'ing the mapped IP address with the magic cookie. If the IP address family is IPv6, X-Address is computed by XOR'ing the mapped IP address with the concatenation of the magic cookie and the 96-bit transaction ID. In all cases, the XOR operation works on its inputs in network byte order (that is, the order they will be encoded in the message). 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 XOR'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 to provide a generic Application Layer Gateway (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. The value of USERNAME is a variable-length value containing the authentication username. It MUST contain a UTF-8-encoded [RFC3629] sequence of fewer than 509 bytes and MUST have been processed using the OpaqueString profile [RFC8265]. A compliant implementation MUST be able to parse a UTF-8-encoded sequence of 763 or fewer octets to be compatible with [RFC5389]. Note: [RFC5389] mistakenly referenced the definition of UTF-8 in [RFC2279]. [RFC2279] assumed up to 6 octets per characters encoded. [RFC2279] was replaced by [RFC3629], which allows only 4 octets per character encoded, consistent with changes made in Unicode 2.0 and ISO/IEC 10646. Note: This specification uses the OpaqueString profile instead of the UsernameCasePreserved profile for username string processing in order to improve compatibility with deployed password stores. Many password databases used for HTTP and SIP Digest authentication store the MD5 hash of username:realm:password instead of storing a plain text password. In [RFC3489], STUN authentication was designed to be compatible with these existing databases to the extent possible, which like SIP and HTTP performed no pre-processing of usernames and passwords other than prohibiting non-space ASCII control characters. The next revision of the STUN specification, [RFC5389], used the SASLprep [RFC4013] stringprep [RFC3454] profile to pre-process usernames and passwords. SASLprep uses Unicode Normalization Form KC (Compatibility Decomposition, followed by Canonical Composition) [UAX15] and prohibits various control, space, and non-text, deprecated, or inappropriate codepoints. The PRECIS framework [RFC8264] obsoletes stringprep. PRECIS handling of usernames and passwords [RFC8265] uses Unicode Normalization Form C (Canonical Decomposition, followed by Canonical Composition). While there are specific cases where different username strings under HTTP Digest could be mapped to a single STUN username processed with OpaqueString, these cases are extremely unlikely and easy to detect and correct. With a UsernameCasePreserved profile, it would be more likely that valid usernames under HTTP Digest would not match their processed forms (specifically usernames containing bidirectional text and compatibility forms). Operators are free to further restrict the allowed codepoints in usernames to avoid problematic characters.
14.4. USERHASH
The USERHASH attribute is used as a replacement for the USERNAME attribute when username anonymity is supported. The value of USERHASH has a fixed length of 32 bytes. The username MUST have been processed using the OpaqueString profile [RFC8265], and the realm MUST have been processed using the OpaqueString profile [RFC8265] before hashing. The following is the operation that the client will perform to hash the username: userhash = SHA-256(OpaqueString(username) ":" OpaqueString(realm))14.5. 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 SHA-1 hash, the HMAC will be 20 bytes. 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 9.2.2 defines the key for the long-term credential mechanism. Other credential mechanisms MUST define the key that is used for the HMAC. The text used as input to HMAC is the STUN message, up to and including the attribute preceding the MESSAGE-INTEGRITY attribute. The Length field of the STUN message header is adjusted to point to the end of the MESSAGE-INTEGRITY attribute. The value of the MESSAGE-INTEGRITY attribute is set to a dummy value. Once the computation is performed, the value of the MESSAGE-INTEGRITY attribute is filled in, and the value of the length in the STUN header is set to its correct value -- the length of the entire message. Similarly, when validating the MESSAGE-INTEGRITY, the Length field in the STUN header must be adjusted to point to the end of the MESSAGE-INTEGRITY attribute prior to calculating the HMAC over the STUN message, up to and including the attribute preceding the MESSAGE-INTEGRITY attribute. Such adjustment is necessary when attributes, such as FINGERPRINT and MESSAGE-INTEGRITY-SHA256, appear after MESSAGE-INTEGRITY. See also [RFC5769] for examples of such calculations.
14.6. MESSAGE-INTEGRITY-SHA256
The MESSAGE-INTEGRITY-SHA256 attribute contains an HMAC-SHA256 [RFC2104] of the STUN message. The MESSAGE-INTEGRITY-SHA256 attribute can be present in any STUN message type. The MESSAGE- INTEGRITY-SHA256 attribute contains an initial portion of the HMAC- SHA-256 [RFC2104] of the STUN message. The value will be at most 32 bytes, but it MUST be at least 16 bytes and MUST be a multiple of 4 bytes. The value must be the full 32 bytes unless the STUN Usage explicitly specifies that truncation is allowed. STUN Usages may specify a minimum length longer than 16 bytes. 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 9.2.2 defines the key for the long-term credential mechanism. Other credential mechanism MUST define the key that is used for the HMAC. The text used as input to HMAC is the STUN message, up to and including the attribute preceding the MESSAGE-INTEGRITY-SHA256 attribute. The Length field of the STUN message header is adjusted to point to the end of the MESSAGE-INTEGRITY-SHA256 attribute. The value of the MESSAGE-INTEGRITY-SHA256 attribute is set to a dummy value. Once the computation is performed, the value of the MESSAGE- INTEGRITY-SHA256 attribute is filled in, and the value of the length in the STUN header is set to its correct value -- the length of the entire message. Similarly, when validating the MESSAGE-INTEGRITY- SHA256, the Length field in the STUN header must be adjusted to point to the end of the MESSAGE-INTEGRITY-SHA256 attribute prior to calculating the HMAC over the STUN message, up to and including the attribute preceding the MESSAGE-INTEGRITY-SHA256 attribute. Such adjustment is necessary when attributes, such as FINGERPRINT, appear after MESSAGE-INTEGRITY-SHA256. See also Appendix B.1 for examples of such calculations.14.7. 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 operation ensures that the FINGERPRINT test will not report a false positive on a packet containing a CRC-32 generated by an application protocol.) The 32-bit CRC is the one defined in ITU V.42 [ITU.V42.2002], which
has a generator polynomial of x^32 + x^26 + x^23 + x^22 + x^16 + x^12 + x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1. See the sample code for the CRC-32 in Section 8 of [RFC1952]. When present, the FINGERPRINT attribute MUST be the last attribute in the message and thus will appear after MESSAGE-INTEGRITY and MESSAGE- INTEGRITY-SHA256. The FINGERPRINT attribute can aid in distinguishing STUN packets from packets of other protocols. See Section 7. As with MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256, the CRC used in the FINGERPRINT attribute covers the Length field from the STUN message header. Therefore, prior to computation of the CRC, this value must be correct and include the CRC attribute as part of the message length. 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 the value of the attribute is updated. If the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 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 and MESSAGE-INTEGRITY-SHA256 attributes as well.14.8. 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]; it is also consistent in its code assignments and semantics with SIP [RFC3261] and HTTP [RFC7231]. The reason phrase is meant for diagnostic purposes 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-encoded [RFC3629] sequence of fewer than 128 characters (which can be as long as 509 bytes when encoding them or 763 bytes when decoding them). 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: Format of 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 binary encoding of 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 either a USERNAME or USERHASH attribute and a
valid MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 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 or MESSAGE-INTEGRITY-SHA256 attribute, and
receivers MUST validate the MESSAGE-INTEGRITY or MESSAGE-
INTEGRITY-SHA256 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 or MESSAGE-INTEGRITY-SHA256 for this error, so
the client MUST NOT expect a valid MESSAGE-INTEGRITY or MESSAGE-
INTEGRITY-SHA256 attribute on this response.
401 Unauthenticated: 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.
14.9. REALM
The REALM attribute may be present in requests and responses. It
contains text that meets the grammar for "realm-value" as described
in [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-encoded
[RFC3629] sequence of fewer than 128 characters (which can be as long
as 509 bytes when encoding them and as long as 763 bytes when
decoding them) and MUST have been processed using the OpaqueString
profile [RFC8265].
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 in that realm for authentication.
14.10. NONCE
The NONCE attribute may be present in requests and responses. It
contains a sequence of qdtext or quoted-pair, which are defined in
[RFC3261]. Note that this means that the NONCE attribute will not
contain the actual surrounding quote characters. The NONCE attribute
MUST be fewer than 128 characters (which can be as long as 509 bytes
when encoding them and a long as 763 bytes when decoding them). See
Section 5.4 of [RFC7616] for guidance on selection of nonce values in
a server.
14.11. PASSWORD-ALGORITHMS
The PASSWORD-ALGORITHMS attribute may be present in requests and
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 18.5.
The attribute contains a list of algorithm numbers and variable
length parameters. The algorithm number is a 16-bit value as defined
in Section 18.5. The parameters start with the length (prior to
padding) 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 Parameters (variable) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... Figure 8: Format of PASSWORD-ALGORITHMS Attribute14.12. PASSWORD-ALGORITHM
The PASSWORD-ALGORITHM attribute is present only in requests. It contains the algorithm that the server must use to derive a key from the long-term password. The set of known algorithms is maintained by IANA. The initial set defined by this specification is found in Section 18.5. The attribute contains an algorithm number and variable length parameters. The algorithm number is a 16-bit value as defined in Section 18.5. The parameters starts with the length (prior to padding) 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. Similarly, the padding bits MUST be set to zero on sending and MUST be ignored by the receiver. 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 Attribute14.13. UNKNOWN-ATTRIBUTES
The UNKNOWN-ATTRIBUTES attribute is present only in an error response when the response code in the ERROR-CODE attribute is 420 (Unknown Attribute).
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.14.14. 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-encoded [RFC3629] sequence of fewer than 128 characters (which can be as long as 509 when encoding them and as long as 763 bytes when decoding them).14.15. 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.14.16. 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 valid DNS name [RFC1123] (including A-labels [RFC5890]) of 255 or fewer ASCII characters.
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