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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 BEHAVE Working Group J. Rosenberg 3 Internet-Draft Cisco 4 Obsoletes: 3489 (if approved) C. Huitema 5 Intended status: Standards Track Microsoft 6 Expires: April 13, 2008 R. Mahy 7 Plantronics 8 P. Matthews 9 Avaya 10 D. Wing 11 Cisco 12 October 11, 2007 14 Session Traversal Utilities for (NAT) (STUN) 15 draft-ietf-behave-rfc3489bis-11 17 Status of this Memo 19 By submitting this Internet-Draft, each author represents that any 20 applicable patent or other IPR claims of which he or she is aware 21 have been or will be disclosed, and any of which he or she becomes 22 aware will be disclosed, in accordance with Section 6 of BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF), its areas, and its working groups. Note that 26 other groups may also distribute working documents as Internet- 27 Drafts. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 The list of current Internet-Drafts can be accessed at 35 http://www.ietf.org/ietf/1id-abstracts.txt. 37 The list of Internet-Draft Shadow Directories can be accessed at 38 http://www.ietf.org/shadow.html. 40 This Internet-Draft will expire on April 13, 2008. 42 Copyright Notice 44 Copyright (C) The IETF Trust (2007). 46 Abstract 48 Session Traversal Utilities for NAT (STUN) is a protocol that serves 49 as a tool for other protocols in dealing with NAT traversal. It can 50 be used by an endpoint to determine the IP address and port allocated 51 to it by a NAT. It can also be used to check connectivity between 52 two endpoints, and as a keep-alive protocol to maintain NAT bindings. 53 STUN works with many existing NATs, and does not require any special 54 behavior from them. 56 STUN is not a NAT traversal solution by itself. Rather, it is a tool 57 to be used in the context of a NAT traversal solution. This is an 58 important change from the previous version of this specification (RFC 59 3489), which presented STUN as a complete solution. 61 This document obsoletes RFC 3489. 63 Table of Contents 65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 66 2. Evolution from RFC 3489 . . . . . . . . . . . . . . . . . . . 4 67 3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5 68 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8 69 5. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 8 70 6. STUN Message Structure . . . . . . . . . . . . . . . . . . . . 9 71 7. Base Protocol Procedures . . . . . . . . . . . . . . . . . . . 12 72 7.1. Forming a Request or an Indication . . . . . . . . . . . 12 73 7.2. Sending the Request or Indication . . . . . . . . . . . . 12 74 7.2.1. Sending over UDP . . . . . . . . . . . . . . . . . . . 13 75 7.2.2. Sending over TCP or TLS-over-TCP . . . . . . . . . . . 13 76 7.3. Receiving a STUN Message . . . . . . . . . . . . . . . . 15 77 7.3.1. Processing a Request . . . . . . . . . . . . . . . . . 16 78 7.3.1.1. Forming a Success or Error Response . . . . . . . 16 79 7.3.1.2. Sending the Success or Error Response . . . . . . 17 80 7.3.2. Processing an Indication . . . . . . . . . . . . . . . 17 81 7.3.3. Processing a Success Response . . . . . . . . . . . . 18 82 7.3.4. Processing an Error Response . . . . . . . . . . . . . 18 83 8. FINGERPRINT Mechanism . . . . . . . . . . . . . . . . . . . . 19 84 9. DNS Discovery of a Server . . . . . . . . . . . . . . . . . . 19 85 10. Authentication and Message-Integrity Mechanisms . . . . . . . 20 86 10.1. Short-Term Credential Mechanism . . . . . . . . . . . . . 21 87 10.1.1. Forming a Request or Indication . . . . . . . . . . . 21 88 10.1.2. Receiving a Request or Indication . . . . . . . . . . 21 89 10.1.3. Receiving a Response . . . . . . . . . . . . . . . . . 22 90 10.2. Long-term Credential Mechanism . . . . . . . . . . . . . 22 91 10.2.1. Forming a Request . . . . . . . . . . . . . . . . . . 23 92 10.2.1.1. First Request . . . . . . . . . . . . . . . . . . 23 93 10.2.1.2. Subsequent Requests . . . . . . . . . . . . . . . 24 94 10.2.2. Receiving a Request . . . . . . . . . . . . . . . . . 24 95 10.2.3. Receiving a Response . . . . . . . . . . . . . . . . . 25 97 11. ALTERNATE-SERVER Mechanism . . . . . . . . . . . . . . . . . . 26 98 12. Backwards Compatibility with RFC 3489 . . . . . . . . . . . . 26 99 12.1. Changes to Client Processing . . . . . . . . . . . . . . 27 100 12.2. Changes to Server Processing . . . . . . . . . . . . . . 27 101 13. STUN Usages . . . . . . . . . . . . . . . . . . . . . . . . . 27 102 14. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 29 103 14.1. MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . . . 30 104 14.2. XOR-MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . 31 105 14.3. USERNAME . . . . . . . . . . . . . . . . . . . . . . . . 32 106 14.4. MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . . 32 107 14.5. FINGERPRINT . . . . . . . . . . . . . . . . . . . . . . . 33 108 14.6. ERROR-CODE . . . . . . . . . . . . . . . . . . . . . . . 33 109 14.7. REALM . . . . . . . . . . . . . . . . . . . . . . . . . . 35 110 14.8. NONCE . . . . . . . . . . . . . . . . . . . . . . . . . . 35 111 14.9. UNKNOWN-ATTRIBUTES . . . . . . . . . . . . . . . . . . . 35 112 14.10. SERVER . . . . . . . . . . . . . . . . . . . . . . . . . 36 113 14.11. ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . . 36 114 15. Security Considerations . . . . . . . . . . . . . . . . . . . 36 115 15.1. Attacks against the Protocol . . . . . . . . . . . . . . 36 116 15.1.1. Outside Attacks . . . . . . . . . . . . . . . . . . . 36 117 15.1.2. Inside Attacks . . . . . . . . . . . . . . . . . . . . 37 118 15.2. Attacks Affecting the Usage . . . . . . . . . . . . . . . 37 119 15.2.1. Attack I: DDoS Against a Target . . . . . . . . . . . 38 120 15.2.2. Attack II: Silencing a Client . . . . . . . . . . . . 38 121 15.2.3. Attack III: Assuming the Identity of a Client . . . . 38 122 15.2.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . . 38 123 15.3. Hash Agility Plan . . . . . . . . . . . . . . . . . . . . 39 124 16. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 39 125 17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39 126 17.1. STUN Methods Registry . . . . . . . . . . . . . . . . . . 39 127 17.2. STUN Attribute Registry . . . . . . . . . . . . . . . . . 40 128 17.3. STUN Error Code Registry . . . . . . . . . . . . . . . . 41 129 18. Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . . 41 130 19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 42 131 20. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43 132 20.1. Normative References . . . . . . . . . . . . . . . . . . 43 133 20.2. Informational References . . . . . . . . . . . . . . . . 43 134 Appendix A. C Snippet to Determine STUN Message Types . . . . . . 45 135 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45 136 Intellectual Property and Copyright Statements . . . . . . . . . . 47 138 1. Introduction 140 The protocol defined in this specification, Session Traversal 141 Utilities for NAT, provides a tool for dealing with NATs. It 142 provides a means for an endpoint to determine the IP address and port 143 allocated by a NAT that corresponds to its private IP address and 144 port. It also provides a way for an endpoint to keep a NAT binding 145 alive. With some extensions, the protocol can be used to do 146 connectivity checks between two endpoints [I-D.ietf-mmusic-ice], or 147 to relay packets between two endpoints [I-D.ietf-behave-turn]. 149 In keeping with its tool nature, this specification defines an 150 extensible packet format, defines operation over several transport 151 protocols, and provides for two forms of authentication. 153 STUN is intended to be used in context of one or more NAT traversal 154 solutions. These solutions are known as STUN usages. Each usage 155 describes how STUN is utilized to achieve the NAT traversal solution. 156 Typically, a usage indicates when STUN messages get sent, which 157 optional attributes to include, what server is used, and what 158 authentication mechanism is to be used. Interactive Connectivity 159 Establishment (ICE) [I-D.ietf-mmusic-ice] is one usage of STUN. SIP 160 Outbound [I-D.ietf-sip-outbound] is another usage of STUN. In some 161 cases, a usage will require extensions to STUN. A STUN extension can 162 be in the form of new methods, attributes, or error response codes. 163 More information on STUN usages can be found in Section 13. 165 2. Evolution from RFC 3489 167 STUN was originally defined in RFC 3489 [RFC3489]. That 168 specification, sometimes referred to as "classic STUN", represented 169 itself as a complete solution to the NAT traversal problem. In that 170 solution, a client would discover whether it was behind a NAT, 171 determine its NAT type, discover its IP address and port on the 172 public side of the outermost NAT, and then utilize that IP address 173 and port within the body of protocols, such as the Session Initiation 174 Protocol (SIP) [RFC3261]. However, experience since the publication 175 of RFC 3489 has found that classic STUN simply does not work 176 sufficiently well to be a deployable solution. The address and port 177 learned through classic STUN are sometimes usable for communications 178 with a peer, and sometimes not. Classic STUN provided no way to 179 discover whether it would, in fact, work or not, and it provided no 180 remedy in cases where it did not. Furthermore, classic STUN's 181 algorithm for classification of NAT types was found to be faulty, as 182 many NATs did not fit cleanly into the types defined there. Classic 183 STUN also had security vulnerabilities which required an extremely 184 complicated mechanism to address, and despite the complexity of the 185 mechanism, were not fully remedied. 187 For these reasons, this specification obsoletes RFC 3489, and instead 188 describes STUN as a tool that is utilized as part of a complete NAT 189 traversal solution. ICE is a complete NAT traversal solution for 190 protocols based on the offer/answer [RFC3264] methodology, such as 191 SIP. SIP Outbound is a complete solution for traversal of SIP 192 signaling, and it uses STUN in a very different way. Though it is 193 possible that a protocol may be able to use STUN by itself (classic 194 STUN) as a traversal solution, such usage is not described here and 195 is strongly discouraged for the reasons described above. 197 The on-the-wire protocol described here is changed only slightly from 198 classic STUN. The protocol now runs over TCP in addition to UDP. 199 Extensibility was added to the protocol in a more structured way. A 200 magic-cookie mechanism for demultiplexing STUN with application 201 protocols was added by stealing 32 bits from the 128 bit transaction 202 ID defined in RFC 3489, allowing the change to be backwards 203 compatible. Mapped addresses are encoded using a new exclusive-or 204 format. There are other, more minor changes. See Section 18 for a 205 more complete listing. 207 Due to the change in scope, STUN has also been renamed from "Simple 208 Traversal of UDP Through NAT" to "Session Traversal Utilities for 209 NAT". The acronym remains STUN, which is all anyone ever remembers 210 anyway. 212 3. Overview of Operation 214 This section is descriptive only. 216 /--------\ 217 // STUN \\ 218 | Agent | 219 \\ (server) // 220 \--------/ 222 +----------------+ Public Internet 223 ................| NAT 2 |....................... 224 +----------------+ 226 +----------------+ Private NET 2 227 ................| NAT 1 |....................... 228 +----------------+ 230 /--------\ 231 // STUN \\ 232 | Agent | 233 \\ (client) // Private NET 1 234 \--------/ 236 Figure 1: One possible STUN Configuration 238 One possible STUN configuration is shown in Figure 1. In this 239 configuration, there are two entities (called STUN agents) that 240 implement the STUN protocol. The lower agent in the figure is 241 connected to private network 1. This network connects to private 242 network 2 through NAT 1. Private network 2 connects to the public 243 Internet through NAT 2. The upper agent in the figure resides on the 244 public Internet. 246 STUN is a client-server protocol. It supports two types of 247 transactions. One is a request/response transaction in which a 248 client sends a request to a server, and the server returns a 249 response. The second is an indication transaction in which a client 250 sends an indication to the server and the server does not respond. 251 Both types of transactions include a transaction ID, which is a 252 randomly selected 96-bit number. For request/response transactions, 253 this transaction ID allows the client to associate the response with 254 the request that generated it; for indications, this simply serves as 255 a debugging aid. 257 All STUN messages start with a fixed header that includes a method, a 258 class, and the transaction ID. The method indicates which of the 259 various requests or indications this is; this specification defines 260 just one method, Binding, but other methods are expected to be 261 defined in other documents. The class indicates whether this is a 262 request, a success response, an error response, or an indication. 263 Following the fixed header comes zero or more attributes, which are 264 type-length-value extensions that convey additional information for 265 the specific message. 267 This document defines a single method called Binding. The Binding 268 method can be used either in request/response transactions or in 269 indication transactions. When used in request/response transactions, 270 the Binding method can be used to determine the particular "binding" 271 a NAT has allocated to a STUN client. When used in either request/ 272 response or in indication transactions, the Binding method can also 273 be used to keep these "bindings" alive. 275 In the Binding request/response transaction, a Binding Request is 276 sent from a STUN client to a STUN server. When the Binding Request 277 arrives at the STUN server, it may have passed through one or more 278 NATs between the STUN client and the STUN server (in Figure 1, there 279 were two such NATs). As the Binding Request message passes through a 280 NAT, the NAT will modify the source transport address (that is, the 281 source IP address and the source port) of the packet. As a result, 282 the source transport address of the request received by the server 283 will be the public IP address and port created by the NAT closest to 284 the server. This is called a reflexive transport address. The STUN 285 server copies that source transport address into an XOR-MAPPED- 286 ADDRESS attribute in the STUN Binding Response and sends the Binding 287 Response back to the the STUN client. As this packet passes back 288 through a NAT, the NAT will modify the destination transport address 289 in the IP header, but the transport address in the XOR-MAPPED-ADDRESS 290 attribute within the body of the STUN response will remain untouched. 291 In this way, the client can learn its reflexive transport address 292 allocated by the outermost NAT with respect to the STUN server. 294 In some usages, STUN must be multiplexed with other protocols (e.g., 295 [I-D.ietf-mmusic-ice], [I-D.ietf-sip-outbound]). In these usages, 296 there must be a way to inspect a packet and determine if it is a STUN 297 packet or not. STUN provides three fields in the STUN header with 298 fixed values that can be used for this purpose. If this is not 299 sufficient, then STUN packets can also contain a FINGERPRINT value 300 which can further be used to distinguish the packets. 302 STUN defines a set of optional procedures that a usage can decide to 303 use, called mechanisms. These mechanisms include DNS discovery, a 304 redirection technique to an alternate server, a fingerprint attribute 305 for demultiplexing, and two authentication and message integrity 306 exchanges. The authentication mechanisms revolve around the use of a 307 username, password, and message-integrity value. Two authentication 308 mechanisms, the long-term credential mechanism and the short-term 309 credential mechanism, are defined in this specification. Each usage 310 specifies the mechanisms allowed with that usage. 312 In the long-term credential mechanism, the client and server share a 313 pre-provisioned username and password and perform a digest challenge/ 314 response exchange inspired by (but differing in details) to the one 315 defined for HTTP [RFC2617]. In the short-term credential mechanism, 316 the client and the server exchange a username and password through 317 some out-of-band method prior to the STUN exchange. For example, in 318 the ICE usage [I-D.ietf-mmusic-ice] the two endpoints use out-of-band 319 signaling to exchange a username and password. These are used to 320 integrity protect and authenticate the request and response. There 321 is no challenge or nonce used. 323 4. Terminology 325 In this document, the key words "MUST", "MUST NOT", "REQUIRED", 326 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 327 and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 328 [RFC2119] and indicate requirement levels for compliant STUN 329 implementations. 331 5. Definitions 333 STUN Agent: An entity that implements the STUN protocol. Agents can 334 act as STUN clients for some transactions and as STUN servers for 335 other transactions. 337 STUN Client: A logical role in the STUN protocol. A STUN client 338 sends STUN requests or STUN indications, and receives STUN 339 responses. The term "STUN client" is also used colloquially to 340 refer to a STUN agent that only acts as a STUN client. 342 STUN Server: A logical role in the STUN protocol. A STUN server 343 receives STUN requests or STUN indications and sends STUN 344 responses. The term "STUN server" is also used colloquially to 345 refer to a STUN agent that only acts as a STUN server. 347 Transport Address: The combination of an IP address and port number 348 (such as a UDP or TCP port number). 350 Reflexive Transport Address: A transport address learned by a client 351 that identifies that client as seen by another host on an IP 352 network, typically a STUN server. When there is an intervening 353 NAT between the client and the other host, the reflexive transport 354 address represents the mapped address allocated to the client on 355 the public side of the NAT. Reflexive transport addresses are 356 learned from the mapped address attribute (MAPPED-ADDRESS or XOR- 357 MAPPED-ADDRESS) in STUN responses. 359 Mapped Address: Same meaning as Reflexive Address. This term is 360 retained only for for historic reasons and due to the naming of 361 the MAPPED-ADDRESS and XOR-MAPPED-ADDRESS attributes. 363 Long Term Credential: A username and associated password that 364 represent a shared secret between client and server. Long term 365 credentials are generally granted to the client when a subscriber 366 enrolls in a service and persist until the subscriber leaves the 367 service or explicitly changes the credential. 369 Long Term Password: The password from a long term credential. 371 Short Term Credential: A temporary username and associated password 372 which represent a shared secret between client and server. Short 373 term credentials are obtained through some kind of protocol 374 mechanism between the client server, preceding the STUN exchange. 375 A short term credential has an explicit temporal scope, which may 376 be based on a specific amount of time (such as 5 minutes) or on an 377 event (such as termination of a SIP dialog). The specific scope 378 of a short term credential is defined by the application usage. 380 Short Term Password: The password component of a short term 381 credential. 383 STUN Indication: A STUN message that does not receive a response 385 Attribute: The STUN term for a Type-Length-Value (TLV) object that 386 can be added to a STUN message. Attributes are divided into two 387 types: comprehension-required and comprehension-optional. STUN 388 agents can safely ignore comprehension-optional attributes they 389 don't understand, but cannot successfully process a message if it 390 contains comprehension-required attributes that are not 391 understood. 393 RTO: Retransmission TimeOut 395 6. STUN Message Structure 397 STUN messages are encoded in binary using network-oriented format 398 (most significant byte or octet first, also commonly known as big- 399 endian). The transmission order is described in detail in Appendix B 400 of RFC791 [RFC0791]. Unless otherwise noted, numeric constants are 401 in decimal (base 10). 403 All STUN messages MUST start with a 20-byte header followed by zero 404 or more Attributes. The STUN header contains a STUN message type, 405 magic cookie, transaction ID, and message length. 407 0 1 2 3 408 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 1 409 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 410 |0 0| STUN Message Type | Message Length | 411 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 412 | Magic Cookie | 413 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 414 | | 415 | Transaction ID (96 bits) | 416 | | 417 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 419 Figure 2: Format of STUN Message Header 421 The most significant two bits of every STUN message MUST be zeroes. 422 This can be used to differentiate STUN packets from other protocols 423 when STUN is multiplexed with other protocols on the same port. 425 The message type defines the message class (request, success 426 response, failure response, or indication) and the message method 427 (the primary function) of the STUN message. Although there are four 428 message classes, there are only two types of transactions in STUN: 429 request/response transactions (which consist of a request message and 430 a response message), and indication transactions (which consists a 431 single indication message). Response classes are split into error 432 and success responses to aid in quickly processing the STUN message. 434 The message type field is decomposed further into the following 435 structure: 437 +--+--+-+-+-+-+-+-+-+-+-+-+-+-+ 438 |M |M |M|M|M|C|M|M|M|C|M|M|M|M| 439 |11|10|9|8|7|1|6|5|4|0|3|2|1|0| 440 +--+--+-+-+-+-+-+-+-+-+-+-+-+-+ 442 Figure 3: Format of STUN Message Type Field 444 Here the bits in the message type field are shown as most-significant 445 (M11) through least-significant (M0). M11 through M0 represent a 12- 446 bit encoding of the method. C1 and C0 represent a 2 bit encoding of 447 the class. A class of 0b00 is a Request, a class of 0b01 is an 448 indication, a class of 0b10 is a success response, and a class of 449 0b11 is an error response. This specification defines a single 450 method, Binding. The method and class are orthogonal, so that four 451 each method, a request, success response, error response and 452 indication are defined for that method. 454 For example, a Binding Request has class=0b00 (request) and 455 method=0b000000000001 (Binding), and is encoded into the first 16 456 bits as 0x0001. A Binding response has class=0b10 (success response) 457 and method=0b000000000001, and is encoded into the first 16 bits as 458 0x0101. 460 Note: This unfortunate encoding is due to assignment of values in 461 [RFC3489] which did not consider encoding Indications, Success, 462 and Errors using bit fields. 464 The magic cookie field MUST contain the fixed value 0x2112A442 in 465 network byte order. In RFC 3489 [RFC3489], this field was part of 466 the transaction ID; placing the magic cookie in this location allows 467 a server to detect if the client will understand certain attributes 468 that were added in this revised specification. In addition, it aids 469 in distinguishing STUN packets from packets of other protocols when 470 STUN is multiplexed with those other protocols on the same port. 472 The transaction ID is a 96 bit identifier, used to uniquely identify 473 STUN transactions. The transaction ID is chosen by the STUN client. 474 It primarily serves to correlate requests with responses, though it 475 also plays a small role in helping to prevent certain types of 476 attacks. As such, the transaction ID MUST be uniformly and randomly 477 chosen from the interval 0 .. 2**96-1. Resends of the same request 478 reuse the same transaction ID, but the client MUST choose a new 479 transaction ID for new transactions unless the new request is bit- 480 wise identical to the previous request and sent from the same 481 transport address to the same IP address. Success and error 482 responses MUST carry the same transaction ID as their corresponding 483 request. When an agent is acting as a STUN server and STUN client on 484 the same port, the transaction IDs in requests sent by the agent have 485 no relationship to the transaction IDs in requests received by the 486 agent. 488 The message length MUST contain the size, in bytes, of the message 489 not including the 20 byte STUN header. Since all STUN attributes are 490 padded to a multiple of four bytes, the last two bits of this field 491 are always zero. This provides another way to distinguish STUN 492 packets from packets of other protocols. 494 Following the STUN fixed portion of the header are zero or more 495 attributes. Each attribute is TLV (type-length-value) encoded. The 496 details of the encoding, and of the attributes themselves is given in 497 Section 14. 499 7. Base Protocol Procedures 501 This section defines the base procedures of the STUN protocol. It 502 describes how messages are formed, how they are sent, and how they 503 are processed when they are received. It also defines the detailed 504 processing of the Binding method. Other sections in this document 505 describe optional procedures that a usage may elect to use in certain 506 situations. Other documents may define other extensions to STUN, by 507 adding new methods, new attributes, or new error response codes. 509 7.1. Forming a Request or an Indication 511 When formulating a request or indication message, the client MUST 512 follow the rules in Section 6 when creating the header. In addition, 513 the message class MUST be either "Request" or "Indication" (as 514 appropriate), and the method must be either Binding or some method 515 defined in another document. 517 The client then adds any attributes specified by the method or the 518 usage. For example, some usages may specify that the client use an 519 authentication method (Section 10) or the FINGERPRINT attribute 520 (Section 8). 522 For the Binding method with no authentication, no attributes are 523 required unless the usage specifies otherwise. 525 All STUN requests (and responses) sent over UDP MUST be less than the 526 path MTU, or 1500 bytes if the MTU is not known. 528 7.2. Sending the Request or Indication 530 The client then sends the request to the server. This document 531 specifies how to send STUN messages over UDP, TCP, or TLS-over-TCP; 532 other transport protocols may be added in the future. The STUN usage 533 must specify which transport protocol is used, and how the client 534 determines the IP address and port of the server. Section 9 535 describes a DNS-based method of determining the IP address and port 536 of a server which a usage may elect to use. STUN may be used with 537 anycast addresses, but only with UDP and in usages where 538 authentication is not used. 540 At any time, a client MAY have multiple outstanding STUN requests 541 with the same STUN server (that is, multiple transactions in 542 progress, with different transaction ids). 544 7.2.1. Sending over UDP 546 When running STUN over UDP it is possible that the STUN message might 547 be dropped by the network. Reliability of STUN request/response 548 transactions is accomplished through retransmissions of the request 549 message by the client application itself. STUN indications are not 550 retransmitted; thus indication transactions over UDP are not 551 reliable. 553 A client SHOULD retransmit a STUN request message starting with an 554 interval of RTO ("Retransmission TimeOut"), doubling after each 555 retransmission. The RTO is an estimate of the round-trip-time, and 556 is computed as described in RFC 2988 [RFC2988], with two exceptions. 557 First, the initial value for RTO SHOULD be configurable (rather than 558 the 3s recommended in RFC 2988) and SHOULD be greater than 100ms. In 559 fixed- line access links, a value of 100ms is RECOMMENDED. Secondly, 560 the value of RTO MUST NOT be rounded up to the nearest second. 561 Rather, a 1ms accuracy MUST be maintained. As with TCP, the usage of 562 Karn's algorithm is RECOMMENDED. When applied to STUN, it means that 563 RTT estimates SHOULD NOT be computed from STUN transactions which 564 result in the retransmission of a request. 566 The value for RTO SHOULD be cached by an client after the completion 567 of the transaction, and used as the starting value for RTO for the 568 next transaction to the same server (based on equality of IP 569 address). The value SHOULD be considered stale and discarded after 570 10 minutes. 572 Retransmissions continue until a response is received, or until a 573 total of 7 requests have been sent. If, after the last request, a 574 duration equal to 16 times the RTO has passed without a response 575 (providing ample time to get a response if only this final request 576 actually succeeds), the client SHOULD consider the transaction to 577 have failed. A STUN transaction over UDP is also considered failed 578 if there has been a transport failure of some sort, such as a fatal 579 ICMP error. For example, assuming an RTO of 100ms, requests would be 580 sent at times 0ms, 100ms, 300ms, 700ms, 1500ms, 3100ms, and 6300ms. 581 If the client has not received a response after 7900ms, the client 582 will consider the transaction to have timed out. 584 7.2.2. Sending over TCP or TLS-over-TCP 586 For TCP and TLS-over-TCP, the client opens a TCP connection to the 587 server. 589 In some usage of STUN, STUN is sent as the only protocol over the TCP 590 connection. In this case, it can be sent without the aid of any 591 additional framing or demultiplexing. In other usages, or with other 592 extensions, it may be multiplexed with other data over a TCP 593 connection. In that case, STUN MUST be run on top of some kind of 594 framing protocol, specified by the usage or extension, which allows 595 for the agent to extract complete STUN messages and complete 596 application layer messages. 598 For TLS-over-TCP, the TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite MUST 599 be supported at a minimum. Implementations MAY also support any 600 other ciphersuite. When it receives the TLS Certificate message, the 601 client SHOULD verify the certificate and inspect the site identified 602 by the certificate. If the certificate is invalid, revoked, or if it 603 does not identify the appropriate party, the client MUST NOT send the 604 STUN message or otherwise proceed with the STUN transaction. The 605 client MUST verify the identity of the server. To do that, it 606 follows the identification procedures defined in Section 3.1 of RFC 607 2818 [RFC2818]. Those procedures assume the client is dereferencing 608 a URI. For purposes of usage with this specification, the client 609 treats the domain name or IP address used in Section 8.1 as the host 610 portion of the URI that has been dereferenced. If DNS was not used, 611 the client MUST be configured with a set of authorized domains whose 612 certificates will be accepted. 614 Reliability of STUN over TCP and TLS-over-TCP is handled by TCP 615 itself, and there are no retransmissions at the STUN protocol level. 616 However, for a request/response transaction, if the client has not 617 received a response 7900ms after it sent the SYN to establish the 618 connection, it considers the transaction to have timed out. This 619 value has been chosen to equalize the TCP and UDP timeouts for the 620 default initial RTO. 622 In addition, if the client is unable to establish the TCP connection, 623 or the TCP connection is reset or fails before a response is 624 received, any request/response transaction in progress is considered 625 to have failed 627 The client MAY send multiple transactions over a single TCP (or TLS- 628 over-TCP) connection, and it MAY send another request before 629 receiving a response to the previous. The client SHOULD keep the 630 connection open until it 632 o has no further STUN requests or indications to send over that 633 connection, and; 635 o has no plans to use any resources (such as a mapped address 636 (MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address 637 [I-D.ietf-behave-turn]) that were learned though STUN requests 638 sent over that connection, and; 640 o if multiplexing other application protocols over that port, has 641 finished using that other application, and; 643 o if using that learned port with a remote peer, has established 644 communications with that remote peer, as is required by some TCP 645 NAT traversal techniques (e.g., [I-D.ietf-mmusic-ice-tcp]). 647 At the server end, the server SHOULD keep the connection open, and 648 let the client close it. If a server becomes overloaded and needs to 649 close connections to free up resources, it SHOULD close an existing 650 connection rather than reject new connection requests. The server 651 SHOULD NOT close a connection if a request was received over that 652 connection for which a response was not sent. A server MUST NOT ever 653 open a connection back towards the client in order to send a 654 response. 656 7.3. Receiving a STUN Message 658 This section specifies the processing of a STUN message. The 659 processing specified here is for STUN messages as defined in this 660 specification; additional rules for backwards compatibility are 661 defined in in Section 12. Those additional procedures are optional, 662 and usages can elect to utilize them. First, a set of processing 663 operations are applied that are independent of the class. This is 664 followed by class-specific processing, described in the subsections 665 which follow. 667 When a STUN agent receives a STUN message, it first checks that the 668 message obeys the rules of Section 6. It checks that the first two 669 bits are 0, that the magic cookie field has the correct value, that 670 the message length is sensible, and that the method value is a 671 supported method. If the message-class is Success Response or Error 672 Response, the agent checks that the transaction ID matches a 673 transaction that is still in progress. If the FINGERPRINT extension 674 is being used, the agent checks that the FINGERPRINT attribute is 675 present and contains the correct value. If any errors are detected, 676 the message is silently discarded. In the case when STUN is being 677 multiplexed with another protocol, an error may indicate that this is 678 not really a STUN message; in this case, the agent should try to 679 parse the message as a different protocol. 681 The STUN agent then does any checks that are required by a 682 authentication mechanism that the usage has specified (see 683 Section 10. 685 Once the authentication checks are done, the STUN agent checks for 686 unknown attributes and known-but-unexpected attributes in the 687 message. Unknown comprehension-optional attributes MUST be ignored 688 by the agent. Known-but-unexpected attributes SHOULD be ignored by 689 the agent. Unknown comprehension-required attributes cause 690 processing that depends on the message-class and is described below. 692 At this point, further processing depends on the message class of the 693 request. 695 7.3.1. Processing a Request 697 If the request contains one or more unknown comprehension-required 698 attributes, the server replies with an error response with an error 699 code of 420 (Unknown Attribute), and includes an UNKNOWN-ATTRIBUTES 700 attribute in the response that lists the unknown comprehension- 701 required attributes. 703 The server then does any additional checking that the method or the 704 specific usage requires. If all the checks succeed, the server 705 formulates a success response as described below. 707 If the request uses UDP transport and is a retransmission of a 708 request for which the server has already generated a success response 709 within the last 10 seconds, the server MUST retransmit the same 710 success response. One way for a server to do this is to remember all 711 transaction IDs received over UDP and their corresponding responses 712 in the last 10 seconds. Another way is to reprocess the request and 713 recompute the response. The latter technique MUST only be applied to 714 requests which are idempotent and result in the same success response 715 for the same request. The Binding method is considered to idempotent 716 in this way (even though certain rare network events could cause the 717 reflexive transport address value to change). Extensions to STUN 718 SHOULD state whether their request types have this property or not. 720 7.3.1.1. Forming a Success or Error Response 722 When forming the response (success or error), the server follows the 723 rules of section 6. The method of the response is the same as that 724 of the request, and the message class is either "Success Response" or 725 "Error Response". 727 For an error response, the server MUST add an ERROR-CODE attribute 728 containing the error code specified in the processing above. The 729 reason phrase is not fixed, but SHOULD be something suitable for the 730 error code. For certain errors, additional attributes are added to 731 the message. These attributes are spelled out in the description 732 where the error code is specified. For example, for an error code of 733 420 (Unknown Attribute), the server MUST include an UNKNOWN- 734 ATTRIBUTES attribute. Certain authentication errors also cause 735 attributes to be added (see Section 10). Extensions may define other 736 errors and/or additional attributes to add in error cases. 738 If the server authenticated the request using an authentication 739 mechanism, then the server SHOULD add the appropriate authentication 740 attributes to the response (see Section 10). 742 The server also adds any attributes required by the specific method 743 or usage. In addition, the server SHOULD add a SERVER attribute to 744 the message. 746 For the Binding method, no additional checking is required unless the 747 usage specifies otherwise. When forming the success response, the 748 server adds a XOR-MAPPED-ADDRESS attribute to the response, where the 749 contents of the attribute are the source transport address of the 750 request message. For UDP, this is the source IP address and source 751 UDP port of the request message. For TCP and TLS-over-TCP, this is 752 the source IP address and source TCP port of the TCP connection as 753 seen by the server. 755 7.3.1.2. Sending the Success or Error Response 757 The response (success or error) is sent over the same transport as 758 the request was received on. If the request was received over UDP, 759 the destination IP address and port of the response is the source IP 760 address and port of the received request message, and the source IP 761 address and port of the response is equal to the destination IP 762 address and port of the received request message. If the request was 763 received over TCP or TLS-over-TCP, the response is sent back on the 764 same TCP connection as the request was received on. 766 7.3.2. Processing an Indication 768 If the indication contains unknown comprehension-required attributes, 769 the indication is discarded and processing ceases. 771 The server then does any additional checking that the method or the 772 specific usage requires. If all the checks succeed, the server then 773 processes the indication. No response is generated for an 774 indication. 776 For the Binding method, no additional checking or processing is 777 required, unless the usage specifies otherwise. The mere receipt of 778 the message by the server has refreshed the "bindings" in the 779 intervening NATs. 781 Since indications are not re-transmitted over UDP (unlike requests), 782 there is no need to handle re-transmissions of indications at the 783 server. 785 7.3.3. Processing a Success Response 787 If the success response contains unknown comprehension-required 788 attributes, the response is discarded and the transaction is 789 considered to have failed. 791 The client then does any additional checking that the method or the 792 specific usage requires. If all the checks succeed, the client then 793 processes the success response. 795 For the Binding method, the client checks that the XOR-MAPPED-ADDRESS 796 attribute is present in the response. The client checks the address 797 family specified. If it is an unsupported address family, the 798 attribute SHOULD be ignored. If it is an unexpected but supported 799 address family (for example, the Binding transaction was sent over 800 IPv4, but the address family specified is IPv6), then the client MAY 801 accept and use the value. 803 7.3.4. Processing an Error Response 805 If the error response contains unknown comprehension-required 806 attributes, or if the error response does not contain an ERROR-CODE 807 attribute, then the transaction is simply considered to have failed. 809 The client then does any processing specified by the authentication 810 mechanism (see Section 10). This may result in a new transaction 811 attempt. 813 The processing at this point depends on the error-code, the method, 814 and the usage; the following are the default rules: 816 o If the error code is 300 through 399, the client SHOULD consider 817 the transaction as failed unless the ALTERNATE-SERVER extension is 818 being used. See Section 11. 820 o If the error code is 400 through 499, the client declares the 821 transaction failed; in the case of 420 (Unknown Attribute), the 822 response should contain a UNKNOWN-ATTRIBUTES attribute that gives 823 additional information. 825 o If the error code is 500 through 599, the client MAY resend the 826 request; clients that do so MUST limit the number of times they do 827 this. 829 Any other error code causes the client to consider the transaction 830 failed. 832 8. FINGERPRINT Mechanism 834 This section describes an optional mechanism for STUN that aids in 835 distinguishing STUN messages from packets of other protocols when the 836 two are multiplexed on the same transport address. This mechanism is 837 optional, and a STUN usage must describe if and when it is used. 839 In some usages, STUN messages are multiplexed on the same transport 840 address as other protocols, such as RTP. In order to apply the 841 processing described in Section 7, STUN messages must first be 842 separated from the application packets. Section 6 describes three 843 fixed fields in the STUN header that can be used for this purpose. 844 However, in some cases, these three fixed fields may not be 845 sufficient. 847 When the FINGERPRINT extension is used, an agent includes the 848 FINGERPRINT attribute in messages it sends to another agent. 849 Section 14.5 describes the placement and value of this attribute. 850 When the agent receives what it believes is a STUN message, then, in 851 addition to other basic checks, the agent also checks that the 852 message contains a FINGERPRINT attribute and that the attribute 853 contains the correct value (see Section 7.3. This additional check 854 helps the agent detect messages of other protocols that might 855 otherwise seem to be STUN messages. 857 9. DNS Discovery of a Server 859 This section describes an optional procedure for STUN that allows a 860 client to use DNS to determine the IP address and port of a server. 861 A STUN usage must describe if and when this extension is used. To 862 use this procedure, the client must have a domain name and a service 863 name; the usage must also describe how the client obtains these. 865 When a client wishes to locate a STUN server in the public Internet 866 that accepts Binding Request/Response transactions, the SRV service 867 name is "stun". STUN usages MAY define additional DNS SRV service 868 names. 870 The domain name is resolved to a transport address using the SRV 871 procedures specified in [RFC2782]. The DNS SRV service name is the 872 service name provided as input to this procedure. The protocol in 873 the SRV lookup is the transport protocol the client will run STUN 874 over: "udp" for UDP, "tcp" for TCP, and "tls" for TLS-over-TCP. If, 875 in the future, additional SRV records are defined for TLS over other 876 transport protocols, those will need to utilize an SRV transport 877 token of the form "tls-foo" for transport protocol "foo". 879 The procedures of RFC 2782 are followed to determine the server to 880 contact. RFC 2782 spells out the details of how a set of SRV records 881 are sorted and then tried. However, RFC2782 only states that the 882 client should "try to connect to the (protocol, address, service)" 883 without giving any details on what happens in the event of failure. 884 When following these procedures, if the STUN transaction times out 885 without receipt of a response, the client SHOULD retry the request to 886 the next server in the list of servers from the DNS SRV response. 887 Such a retry is only possible for request/response transmissions, 888 since indication transactions generate no response or timeout. 890 The default port for STUN requests is 3478, for both TCP and UDP. 891 Administrators SHOULD use this port in their SRV records for UDP and 892 TCP, but MAY use others. There is no default port for STUN over TLS, 893 however a STUN server SHOULD use a port number for TLS different from 894 3478 so that the server can determine whether the first message it 895 will receive after the TCP connection is set up, is a STUN message or 896 a TLS message. 898 If no SRV records were found, the client performs an A or AAAA record 899 lookup of the domain name. The result will be a list of IP 900 addresses, each of which can be contacted at the default port using 901 UDP or TCP, independent of the STUN usage. For usages that require 902 TLS, lack of SRV records is equivalent to a failure of the 903 transaction, since the request or indication MUST NOT be sent unless 904 SRV records provided a transport address specifically for TLS. 906 10. Authentication and Message-Integrity Mechanisms 908 This section defines two mechanisms for STUN that a client and server 909 can use to provide authentication and message-integrity; these two 910 mechanisms are known as the short-term credential mechanism and the 911 long-term credential mechanism. These two mechanisms are optional, 912 and each usage must specify if and when these mechanisms are used. 913 Consequently, both clients and servers will know which mechanism (if 914 any) to follow based on knowledge of which usage applies. For 915 example, a STUN server on the public Internet supporting ICE would 916 have no authentication, whereas the STUN server functionality in an 917 agent supporting connectivity checks would utilize short term 918 credentials. An overview of these two mechanisms is given in 919 Section 3. 921 Each mechanism specifies the additional processing required to use 922 that mechanism, extending the processing specified in Section 7. The 923 additional processing occurs in three different places: when forming 924 a message; when receiving a message immediately after the the basic 925 checks have been performed; and when doing the detailed processing of 926 error responses. 928 10.1. Short-Term Credential Mechanism 930 The short-term credential mechanism assumes that, prior to the STUN 931 transaction, the client and server have used some other protocol to 932 exchange a credential in the form of a username and password. This 933 credential is time-limited. The time-limit is defined by the usage. 934 As an example, in the ICE usage [I-D.ietf-mmusic-ice], the two 935 endpoints use out-of-band signaling to agree on a username and 936 password, and this username and password is applicable for the 937 duration of the media session. 939 This credential is used to form a message integrity check in each 940 request and in many responses. There is no challenge and response as 941 in the long term mechanism; consequently, replay is prevented by 942 virtue of the time-limited nature of the credential. 944 10.1.1. Forming a Request or Indication 946 For a request or indication message, the agent MUST include the 947 USERNAME and MESSAGE-INTEGRITY attributes in the message. The HMAC 948 for the MESSAGE-INTEGRITY attribute is computed as described in 949 Section 14.4. Note that the password is never included in the 950 request or indication. 952 10.1.2. Receiving a Request or Indication 954 After the agent has done the basic processing of a message, the agent 955 performs the checks listed below in order specified: 957 o If the message does not contain both a MESSAGE-INTEGRITY and a 958 USERNAME attribute: 960 * If the message is a request, the server MUST reject the request 961 with an error response. This response MUST use an error code 962 of 400 (Bad Request). 964 * If the message is an indication, the server MUST silently 965 discard the indication. 967 o If the USERNAME does not contain a username value currently valid 968 within the server: 970 * If the message is a request, the server MUST reject the request 971 with an error response. This response MUST use an error code 972 of 401 (Unauthorized). 974 * If the message is an indication, the server MUST silently 975 discard the indication. 977 o Using the password associated with the username, compute the value 978 for the message-integrity as described in Section 14.4. If the 979 resulting value does not match the contents of the MESSAGE- 980 INTEGRITY attribute: 982 * If the message is a request, the server MUST reject the request 983 with an error response. This response MUST use an error code 984 of 401 (Unauthorized). 986 * If the message is an indication, the server MUST silently 987 discard the indication. 989 If these checks pass, the server continues to process the request or 990 indication. Any response generated by the server MUST include the 991 MESSAGE-INTEGRITY attribute, computed using the password utilized to 992 authenticate the request. The response MUST NOT contain the USERNAME 993 attribute. 995 If any of the checks fail, the server MUST NOT include a MESSAGE- 996 INTEGRITY or USERNAME attribute in the error response. This is 997 because, in these failure cases, the server cannot determine the 998 shared secret necessary to compute MESSAGE-INTEGRITY. 1000 10.1.3. Receiving a Response 1002 The client looks for the MESSAGE-INTEGRITY attribute in the response. 1003 If present, the client computes the message integrity over the 1004 response as defined in Section 14.4, using the same password it 1005 utilized for the request. If the resulting value matches the 1006 contents of the MESSAGE-INTEGRITY attribute, the response is 1007 considered authenticated. If the value does not match, or if 1008 MESSAGE-INTEGRITY was absent, the response MUST be discarded, as if 1009 it was never received. This means that retransmits, if applicable, 1010 will continue. 1012 10.2. Long-term Credential Mechanism 1014 The long-term credential mechanism relies on a long term credential, 1015 in the form of a username and password, that are shared between 1016 client and server. The credential is considered long-term since it 1017 is assumed that it is provisioned for a user, and remains in effect 1018 until the user is no longer a subscriber of the system, or is 1019 changed. This is basically a traditional "log-in" username and 1020 password given to users. 1022 Because these usernames and passwords are expected to be valid for 1023 extended periods of time, replay prevention is provided in the form 1024 of a digest challenge. In this mechanism, the client initially sends 1025 a request, without offering any credentials or any integrity checks. 1026 The server rejects this request, providing the user a realm (used to 1027 guide the user or agent in selection of a username and password) and 1028 a nonce. The nonce provides the replay protection. It is a cookie, 1029 selected by the server, and encoded in such a way as to indicate a 1030 duration of validity or client identity from which it is valid. The 1031 client retries the request, this time including its username, the 1032 realm, and echoing the nonce provided by the server. The client also 1033 includes a message-integrity, which provides an HMAC over the entire 1034 request, including the nonce. The server validates the nonce, and 1035 checks the message-integrity. If they match, the request is 1036 authenticated. If the nonce is no longer valid, it is considered 1037 "stale", and the server rejects the request, providing a new nonce. 1039 In subsequent requests to the same server, the client reuses the 1040 nonce, username, realm and password it used previously. In this way, 1041 subsequent requests are not rejected until the nonce becomes invalid 1042 by the server, in which case the rejection provides a new nonce to 1043 the client. 1045 Note that the long-term credential mechanism cannot be used to 1046 protect indications, since indications cannot be challenged. Usages 1047 utilizing indications must either use a short-term credential, or 1048 omit authentication and message integrity for them. 1050 Since the long-term credential mechanism is susceptible to offline 1051 dictionary attacks, deployments SHOULD utilize strong passwords. 1053 10.2.1. Forming a Request 1055 There are two cases when forming a request. In the first case, this 1056 is the first request from the client to the server (as identified by 1057 its IP address and port). In the second case, the client is 1058 submitting a subsequent request once a previous request/response 1059 transaction has completed successfully. Forming a request as a 1060 consequence of a 401 or 438 error response is covered in 1061 Section 10.2.3 and is not considered a "subsequent request" and thus 1062 does not utilize the rules described in Section 10.2.1.2. 1064 10.2.1.1. First Request 1066 If the client has not completed a successful request/response 1067 transaction with the server (as identified by hostname, if the DNS 1068 procedures of Section 9 are used, else IP address if not), it SHOULD 1069 omit the USERNAME, MESSAGE-INTEGRITY, REALM, and NONCE attributes. 1071 In other words, the very first request is sent as if there were no 1072 authentication or message integrity applied. The exception to this 1073 rule are requests sent to another server as a consequence of the 1074 ALTERNATE-SERVER mechanism described in Section 11. Those requests 1075 do include the USERNAME, REALM and NONCE from the original request, 1076 along with a newly computed MESSAGE-INTEGRITY based on them. 1078 10.2.1.2. Subsequent Requests 1080 Once a request/response transaction has completed successfully, the 1081 client will have been been presented a realm and nonce by the server, 1082 and selected a username and password with which it authenticated. 1083 The client SHOULD cache the username, password, realm, and nonce for 1084 subsequent communications with the server. When the client sends a 1085 subsequent request, it SHOULD include the USERNAME, REALM, and NONCE 1086 attributes with these cached values. It SHOULD include a MESSAGE- 1087 INTEGRITY attributed, computed as described in Section 14.4 using the 1088 cached password. 1090 10.2.2. Receiving a Request 1092 After the server has done the basic processing of a request, it 1093 performs the checks listed below in the order specified: 1095 o If the message does not contain a MESSAGE-INTEGRITY attribute, the 1096 server MUST generate an error response with an error code of 401 1097 (Unauthorized). This response MUST include a REALM value. It is 1098 RECOMMENDED that the REALM value be the domain name of the 1099 provider of the STUN server. The response MUST include a NONCE, 1100 selected by the server. The response SHOULD NOT contain a 1101 USERNAME or MESSAGE-INTEGRITY attribute. 1103 o If the message contains a MESSAGE-INTEGRITY attribute, but is 1104 missing the USERNAME, REALM or NONCE attributes, the server MUST 1105 generate an error response with an error code of 400 (Bad 1106 Request). This response SHOULD NOT include a USERNAME, NONCE, 1107 REALM or MESSAGE-INTEGRITY attribute. 1109 o If the NONCE is no longer valid, the server MUST generate an error 1110 response with an error code of 438 (Stale Nonce). This response 1111 MUST include a NONCE and REALM attribute and SHOULD NOT incude the 1112 USERNAME or MESSAGE-INTEGRITY attribute. 1114 o If the username in the USERNAME attribute is not valid, the server 1115 MUST generate an error response with an error code of 401 1116 (Unauthorized). This response MUST include a REALM value. It is 1117 RECOMMENDED that the REALM value be the domain name of the 1118 provider of the STUN server. The response MUST include a NONCE, 1119 selected by the server. The response SHOULD NOT contain a 1120 USERNAME or MESSAGE-INTEGRITY attribute. 1122 o Using the password associated with the username in the USERNAME 1123 attribute, compute the value for the message-integrity as 1124 described in Section 14.4. If the resulting value does not match 1125 the contents of the MESSAGE-INTEGRITY attribute, the server MUST 1126 reject the request with an error response. This response MUST use 1127 an error code of 401 (Unauthorized). It MUST include a REALM and 1128 NONCE attribute and SHOULD NOT include the USERNAME or MESSAGE- 1129 INTEGRITY attribute. 1131 If these checks pass, the server continues to process the request. 1132 Any response generated by the server (excepting the cases described 1133 above) MUST include the MESSAGE-INTEGRITY attribute, computed using 1134 the username and password utilized to authenticate the request. The 1135 REALM, NONCE, and USERNAME attributes SHOULD NOT be included. 1137 10.2.3. Receiving a Response 1139 If the response is an error response, with an error code of 401 1140 (Unauthorized), the client SHOULD retry the request with a new 1141 transaction. This request MUST contain a USERNAME, determined by the 1142 client as the appropriate username for the REALM from the error 1143 response. The request MUST contain the REALM, copied from the error 1144 response. The request MUST contain the NONCE, copied from the error 1145 response. The request MUST contain the MESSAGE-INTEGRITY attribute, 1146 computed using the password associated with the username in the 1147 USERNAME attribute. The client MUST NOT perform this retry if it is 1148 not changing the USERNAME or REALM or its associated password, from 1149 the previous attempt. 1151 If the response is an error response with an error code of 438 (Stale 1152 Nonce), the client MUST retry the request, using the new NONCE 1153 supplied in the 438 (Stale Nonce) response. This retry MUST also 1154 include the USERNAME, REALM and MESSAGE-INTEGRITY. 1156 The client looks for the MESSAGE-INTEGRITY attribute in the response 1157 (either success or failure). If present, the client computes the 1158 message integrity over the response as defined in Section 14.4, using 1159 the same password it utilized for the request. If the resulting 1160 value matches the contents of the MESSAGE-INTEGRITY attribute, the 1161 response is considered authenticated. If the value does not match, 1162 or if MESSAGE-INTEGRITY was absent, the response MUST be discarded, 1163 as if it was never received. This means that retransmits, if 1164 applicable, will continue. 1166 11. ALTERNATE-SERVER Mechanism 1168 This section describes a mechanism in STUN that allows a server to 1169 redirect a client to another server. This extension is optional, and 1170 a usage must define if and when this extension is used. To prevent 1171 denial-of-service attacks, this extension MUST only be used in 1172 situations where the client and server are using an authentication 1173 and message-integrity mechanism. 1175 A server using this extension redirects a client to another server by 1176 replying to a request message with an error response message with an 1177 error code of 300 (Try Alternate). The server MUST include a 1178 ALTERNATE-SERVER attribute in the error response. The error response 1179 message MUST be authenticated, which in practice means the request 1180 message must have passed the authentication checks. 1182 A client using this extension handles a 300 (Try Alternate) error 1183 code as follows. If the error response has passed the authentication 1184 checks, then the client looks for a ALTERNATE-SERVER attribute in the 1185 error response. If one is found, then the client considers the 1186 current transaction as failed, and re-attempts the request with the 1187 server specified in the attribute. The client SHOULD reuse any 1188 authentication credentials from the old request in the new 1189 transaction. 1191 12. Backwards Compatibility with RFC 3489 1193 This section define procedures that allow a degree of backwards 1194 compatible with the original protocol defined in RFC 3489 [RFC3489]. 1195 This mechanism is optional, meant to be utilized only in cases where 1196 a new client can connect to an old server, or vice-a-versa. A usage 1197 must define if and when this procedure is used. 1199 Section 18 lists all the changes between this specification and RFC 1200 3489 [RFC3489]. However, not all of these differences are important, 1201 because "classic STUN" was only used in a few specific ways. For the 1202 purposes of this extension, the important changes are the following. 1203 In RFC 3489: 1205 o UDP was the only supported transport; 1207 o The field that is now the Magic Cookie field was a part of the 1208 transaction id field, and transaction ids were 128 bits long; 1210 o The XOR-MAPPED-ADDRESS attribute did not exist, and the Binding 1211 method used the MAPPED-ADDRESS attribute instead; 1213 o There were two comprehension-required attributes, RESPONSE-ADDRESS 1214 and CHANGE-REQUEST, that have been removed from this 1215 specification; 1217 * These attributes are now part of the NAT Behavior Discovery 1218 usage. 1220 12.1. Changes to Client Processing 1222 A client that wants to interoperate with a [RFC3489] server SHOULD 1223 send a request message that uses the Binding method, contains no 1224 attributes, and uses UDP as the transport protocol to the server. If 1225 successful, the success response received from the server will 1226 contain a MAPPED-ADDRESS attribute rather than an XOR-MAPPED-ADDRESS 1227 attribute; other than this change, the processing of the response is 1228 identical to the procedures described above. 1230 12.2. Changes to Server Processing 1232 A STUN server can detect when a given Binding Request message was 1233 sent from an RFC 3489 [RFC3489] client by the absence of the correct 1234 value in the Magic Cookie field. When the server detects an RFC 3489 1235 client, it SHOULD copy the value seen in the Magic Cookie field in 1236 the Binding Request to the Magic Cookie field in the Binding Response 1237 message, and insert a MAPPED-ADDRESS attribute instead of an XOR- 1238 MAPPED-ADDRESS attribute. 1240 The client might, in rare situations, include either the RESPONSE- 1241 ADDRESS or CHANGE-REQUEST attributes. In these situations, the 1242 server will view these as unknown comprehension-required attributes 1243 and reply with an error response. Since the mechanisms utilizing 1244 those attributes are no longer supported, this behavior is 1245 acceptable. 1247 The RFC 3489 version of STUN lacks both the Magic Cookie and the 1248 FINGERPRINT attribute that allows for a very high probablility of 1249 correctly identifying STUN messages when multiplexed with other 1250 protocols. Therefore, STUN implementations that are backwards 1251 compatible with RFC 3489 SHOULD NOT be used in cases where STUN will 1252 be multiplexed with another protocol. However, that should not be an 1253 issues as such multiplexing was not available in RFC 3489. 1255 13. STUN Usages 1257 STUN by itself is not a solution to the NAT traversal problem. 1258 Rather, STUN defines a tool that can be used inside a larger 1259 solution. The term "STUN Usage" is used for any solution that uses 1260 STUN as a component. 1262 At the time of writing, three STUN usages are defined: Interactive 1263 Connectivity Establishment (ICE) [I-D.ietf-mmusic-ice], Client- 1264 initiated connections for SIP [I-D.ietf-sip-outbound], and NAT 1265 Behavior Discovery [I-D.ietf-behave-nat-behavior-discovery]. Other 1266 STUN usages may be defined in the future. 1268 A STUN usage defines how STUN is actually utilized - when to send 1269 requests, what to do with the responses, and which optional 1270 procedures defined here (or in an extension to STUN) are to be used. 1271 A usage would also define: 1273 o Which STUN methods are used; 1275 o What authentication and message integrity mechanisms are used; 1277 o What mechanisms are used to distinguish STUN messages from other 1278 messages. When STUN is run over TCP, a framing mechanism may be 1279 required; 1281 o How a STUN client determines the IP address and port of the STUN 1282 server; 1284 o Whether backwards compatibility to RFC 3489 is required; 1286 o What optional attributes defined here (such as FINGERPRINT and 1287 ALTERNATE-SERVER) or in other extensions are required. 1289 In addition, any STUN usage must consider the security implications 1290 of using STUN in that usage. A number of attacks against STUN are 1291 known (see the Security Considerations section in this document) and 1292 any usage must consider how these attacks can be thwarted or 1293 mitigated. 1295 Finally, a usage must consider whether its usage of STUN is an 1296 example of the Unilateral Self-Address Fixing approach to NAT 1297 traversal, and if so, address the questions raised in RFC 3424. 1299 14. STUN Attributes 1301 After the STUN header are zero or more attributes. Each attribute 1302 MUST be TLV encoded, with a 16 bit type, 16 bit length, and value. 1303 Each STUN attribute MUST end on a 32 bit boundary. As mentioned 1304 above, all fields in an attribute are transmitted most significant 1305 bit first. 1307 0 1 2 3 1308 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 1309 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1310 | Type | Length | 1311 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1312 | Value (variable) .... 1313 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1315 Figure 4: Format of STUN Attributes 1317 The value in the Length field MUST contain the length of the Value 1318 part of the attribute, prior to padding, measured in bytes. Since 1319 STUN aligns attributes on 32 bit boundaries, attributes whose content 1320 is not a multiple of 4 bytes are padded with 1, 2 or 3 bytes of 1321 padding so that its value contains a multiple of 4 bytes. The 1322 padding bits are ignored, and may be any value. 1324 Any attribute type MAY appear more than once in a STUN message. 1325 Unless specified otherwise, the order of appearance is significant: 1326 only the first occurance needs to be processed by a receiver, and any 1327 duplicates MAY be ignored by a receiver. 1329 To allow future revisions of this specification to add new attributes 1330 if needed, the attribute space is divided into two ranges. 1331 Attributes with type values between 0x0000 and 0x7FFF are 1332 comprehension-required attributes, which means that the STUN agent 1333 cannot successfully process the message unless it understands the 1334 attribute. Attributes with type values between 0x8000 and 0xFFFF are 1335 comprehension-optional attributes, which means that those attributes 1336 can be ignored by the STUN agent if it does not understand them. 1338 The STUN Attribute types defined by this specification are: 1340 Comprehension-required range (0x0000-0x7FFF): 1341 0x0000: (Reserved) 1342 0x0001: MAPPED-ADDRESS 1343 0x0006: USERNAME 1344 0x0007: (Reserved; was PASSWORD) 1345 0x0008: MESSAGE-INTEGRITY 1346 0x0009: ERROR-CODE 1347 0x000A: UNKNOWN-ATTRIBUTES 1348 0x0014: REALM 1349 0x0015: NONCE 1350 0x0020: XOR-MAPPED-ADDRESS 1352 Comprehension-optional range (0x8000-0xFFFF) 1353 0x8022: SERVER 1354 0x8023: ALTERNATE-SERVER 1355 0x8028: FINGERPRINT 1357 The rest of this section describes the format of the various 1358 attributes defined in this specification. 1360 14.1. MAPPED-ADDRESS 1362 The MAPPED-ADDRESS attribute indicates a reflexive transport address 1363 of the client. It consists of an eight bit address family, and a 1364 sixteen bit port, followed by a fixed length value representing the 1365 IP address. If the address family is IPv4, the address MUST be 32 1366 bits. If the address family is IPv6, the address MUST be 128 bits. 1367 All fields must be in network byte order. 1369 The format of the MAPPED-ADDRESS attribute is: 1371 0 1 2 3 1372 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 1373 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1374 |0 0 0 0 0 0 0 0| Family | Port | 1375 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1376 | | 1377 | Address (32 bits or 128 bits) | 1378 | | 1379 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1381 Figure 6: Format of MAPPED-ADDRESS attribute 1383 The address family can take on the following values: 1385 0x01:IPv4 1386 0x02:IPv6 1388 The first 8 bits of the MAPPED-ADDRESS MUST be set to 0 and MUST be 1389 ignored by receivers. These bits are present for aligning parameters 1390 on natural 32 bit boundaries. 1392 This attribute is used only by servers for achieving backwards 1393 compatibility with RFC 3489 [RFC3489] clients. 1395 14.2. XOR-MAPPED-ADDRESS 1397 The XOR-MAPPED-ADDRESS attribute is identical to the MAPPED-ADDRESS 1398 attribute, except that the reflexive transport address is obfuscated 1399 through the XOR function. 1401 The format of the XOR-MAPPED-ADDRESS is: 1403 0 1 2 3 1404 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 1405 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1406 |x x x x x x x x| Family | X-Port | 1407 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1408 | X-Address (Variable) 1409 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1411 Figure 8: Format of XOR-MAPPED-ADDRESS Attribute 1413 The Family represents the IP address family, and is encoded 1414 identically to the Family in MAPPED-ADDRESS. 1416 X-Port is computed by taking the mapped port in host byte order, 1417 XOR'ing it with the most significant 16 bits of the magic cookie, and 1418 then the converting the result to network byte order. If the IP 1419 address family is IPv4, X-Address is computed by taking the mapped IP 1420 address in host byte order, XOR'ing it with the magic cookie, and 1421 converting the result to network byte order. If the IP address 1422 family is IPv6, X-Address is computed by taking the mapped IP address 1423 in host byte order, XOR'ing it with the magic cookie and the 96-bit 1424 transaction ID, and converting the result to network byte order. 1426 The rules for encoding and processing the first 8 bits of the 1427 attribute's value, the rules for handling multiple occurrences of the 1428 attribute, and the rules for processing addresses families are the 1429 same as for MAPPED-ADDRESS. 1431 NOTE: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their 1432 encoding of the transport address. The former encodes the transport 1433 address by exclusive-or'ing it with the magic cookie. The latter 1434 encodes it directly in binary. RFC 3489 originally specified only 1435 MAPPED-ADDRESS. However, deployment experience found that some NATs 1436 rewrite the 32-bit binary payloads containing the NAT's public IP 1437 address, such as STUN's MAPPED-ADDRESS attribute, in the well-meaning 1438 but misguided attempt at providing a generic ALG function. Such 1439 behavior interferes with the operation of STUN and also causes 1440 failure of STUN's message integrity checking. 1442 14.3. USERNAME 1444 The USERNAME attribute is used for message integrity. It identifies 1445 the username and password combination used in the message integrity 1446 check. 1448 The value of USERNAME is a variable length value. It MUST contain a 1449 UTF-8 encoded sequence of less than 513 bytes. 1451 14.4. MESSAGE-INTEGRITY 1453 The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [RFC2104] of 1454 the STUN message. The MESSAGE-INTEGRITY attribute can be present in 1455 any STUN message type. Since it uses the SHA1 hash, the HMAC will be 1456 20 bytes. The text used as input to HMAC is the STUN message, 1457 including the header, up to and including the attribute preceding the 1458 MESSAGE-INTEGRITY attribute. With the exception of the FINGERPRINT 1459 attribute, which appears after MESSAGE-INTEGRITY, agents MUST ignore 1460 all other attributes that follow MESSAGE-INTEGRITY. 1462 The key for the HMAC depends on whether long term or short term 1463 credentials are in use. For long term credentials: 1465 key = MD5(username ":" realm ":" password) 1467 For short term credentials: 1469 key = password 1471 The structure of the key when used with long term credentials 1472 facilitates deployment in systems that also utilize SIP. Typically, 1473 SIP systems utilizing SIP's digest authentication mechanism do not 1474 actually store the password in the database. Rather, they store a 1475 value called H(A1), which is equal to the key defined above. 1477 Based on the rules above, the hash includes the length field from the 1478 STUN message header. This length indicates the length of the entire 1479 message, including the MESSAGE-INTEGRITY attribute itself. 1480 Consequently, the MESSAGE-INTEGRITY attribute MUST be inserted into 1481 the message (with dummy content) prior to the computation of the 1482 integrity check. Once the computation is performed, the value of the 1483 attribute can be filled in. This ensures the length has the correct 1484 value when the hash is performed. Similarly, when validating the 1485 MESSAGE-INTEGRITY, the length field should be adjusted to point to 1486 the end of the MESSAGE-INTEGRITY attribute prior to calculating the 1487 HMAC. Such adjustment is necessary when attributes, such as 1488 FINGERPRINT, appear after MESSAGE-INTEGRITY. 1490 14.5. FINGERPRINT 1492 The FINGERPRINT attribute may be present in all STUN messages. The 1493 value of the attribute is computed as the CRC-32 of the STUN message 1494 up to (but excluding) the FINGERPRINT attribute itself, xor-d with 1495 the 32 bit value 0x5354554e (the XOR helps in cases where an 1496 application packet is also using CRC-32 in it). The 32 bit CRC is 1497 the one defined in ITU V.42 [ITU.V42.1994], which has a generator 1498 polynomial of x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1. 1499 When present, the FINGERPRINT attribute MUST be the last attribute in 1500 the message, and thus will appear after MESSAGE-INTEGRITY. 1502 The FINGERPRINT attribute can aid in distinguishing STUN packets from 1503 packets of other protocols. See Section 8. 1505 As with MESSAGE-INTEGRITY, the CRC used in the FINGERPRINT attribute 1506 covers the length field from the STUN message header. Therefore, 1507 this value must be correct, and include the CRC attribute as part of 1508 the message length, prior to computation of the CRC. When using the 1509 FINGERPRINT attribute in a message, the attribute is first placed 1510 into the message with a dummy value, then the CRC is computed, and 1511 then the value of the attribute is updated. If the MESSAGE-INTEGRITY 1512 attribute is also present, then it must be present with the correct 1513 message-integrity value before the CRC is computed, since the CRC is 1514 done over the value of the MESSAGE-INTEGRITY attribute as well. 1516 14.6. ERROR-CODE 1518 The ERROR-CODE attribute is used in Error Response messages. It 1519 contains a numeric error code value in the range of 300 to 699 plus a 1520 textual reason phrase encoded in UTF-8, and is consistent in its code 1521 assignments and semantics with SIP [RFC3261] and HTTP [RFC2616]. The 1522 reason phrase is meant for user consumption, and can be anything 1523 appropriate for the error code. Recommended reason phrases for the 1524 defined error codes are presented below. The reason phrase MUST be a 1525 UTF-8 encoded sequence of less than 128 characters (which can be as 1526 long as 763 bytes). 1528 To facilitate processing, the class of the error code (the hundreds 1529 digit) is encoded separately from the rest of the code. 1531 0 1 2 3 1532 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 1533 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1534 | Reserved, should be 0 |Class| Number | 1535 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1536 | Reason Phrase (variable) .. 1537 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1539 The Reserved bits SHOULD be 0, and are for alignment on 32-bit 1540 boundaries. Receivers MUST ignore these bits. The Class represents 1541 the hundreds digit of the error code. The value MUST be between 3 1542 and 6. The number represents the error code modulo 100, and its 1543 value MUST be between 0 and 99. 1545 The following error codes, along with their recommended reason 1546 phrases (in brackets) are defined: 1548 300 Try Alternate: The client should contact an alternate server for 1549 this request. This error response MUST only be sent if the 1550 request included a USERNAME attribute and a valid MESSAGE- 1551 INTEGRITY attribute; otherwise it MUST NOT be sent and error 1552 code 400 (Bad Request) is suggested. This error response MUST 1553 be protected with the MESSAGE-INTEGRITY attribute, and receivers 1554 MUST validate the MESSAGE-INTEGRITY of this response before 1555 redirecting themselves to an alternate server. 1557 Note: failure to generate and validate message-integrity 1558 for a 300 response allows an on-path attacker to falsify a 1559 300 response thus causing subsequent STUN messages to be 1560 sent to a victim. 1562 400 Bad Request: The request was malformed. The client SHOULD NOT 1563 retry the request without modification from the previous 1564 attempt. The server may not be able to generate a valid 1565 MESSAGE-INTEGRITY for this error, so the client MUST NOT expect 1566 a valid MESSAGE-INTEGRITY attribute on this response. 1568 401 Unauthorized: The request did not contain the expected MESSAGE- 1569 INTEGRITY attribute. The server MAY include the MESSAGE- 1570 INTEGRITY attribute in its error response. 1572 420 Unknown Attribute: The server received STUN packet containing a 1573 comprehension-required attribute which it did not understand. 1574 The server MUST put this unknown attribute in the UNKNOWN- 1575 ATTRIBUTE attribute of its error response. 1577 438 Stale Nonce: The NONCE used by the client was no longer valid. 1578 The client should retry, using the NONCE provided in the 1579 response. 1581 500 Server Error: The server has suffered a temporary error. The 1582 client should try again. 1584 14.7. REALM 1586 The REALM attribute may be present in requests and responses. It 1587 contains text which meets the grammar for "realm-value" as described 1588 in RFC 3261 [RFC3261] but without the double quotes and their 1589 surrounding whitespace. That is, it is an unquoted realm-value. It 1590 MUST be a UTF-8 encoded sequence of less than 128 characters (which 1591 can be as long as 763 bytes). 1593 Presence of the REALM attribute in a request indicates that long-term 1594 credentials are being used for authentication. Presence in certain 1595 error responses indicates that the server wishes the client to use a 1596 long-term credential for authentication. 1598 14.8. NONCE 1600 The NONCE attribute may be present in requests and responses. It 1601 contains a sequence of qdtext or quoted-pair, which are defined in 1602 RFC 3261 [RFC3261]. See RFC 2617 [RFC2617], Section 4.3, for 1603 guidance on selection of nonce values in a server. It MUST be less 1604 than 128 characters (which can be as long as 763 bytes). 1606 14.9. UNKNOWN-ATTRIBUTES 1608 The UNKNOWN-ATTRIBUTES attribute is present only in an error response 1609 when the response code in the ERROR-CODE attribute is 420. 1611 The attribute contains a list of 16 bit values, each of which 1612 represents an attribute type that was not understood by the server. 1614 0 1 2 3 1615 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 1616 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1617 | Attribute 1 Type | Attribute 2 Type | 1618 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1619 | Attribute 3 Type | Attribute 4 Type ... 1620 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1622 Figure 12: Format of UNKNOWN-ATTRIBUTES attribute 1624 Note: In [RFC3489], this field was padded to 32 by duplicating the 1625 last attribute. In this version of the specification, the normal 1626 padding rules for attributes are used instead. 1628 14.10. SERVER 1630 The server attribute contains a textual description of the software 1631 being used by the server, including manufacturer and version number. 1632 The attribute has no impact on operation of the protocol, and serves 1633 only as a tool for diagnostic and debugging purposes. The value of 1634 SERVER is variable length. It MUST be a UTF-8 encoded sequence of 1635 less than 128 characters (which can be as long as 763 bytes). 1637 14.11. ALTERNATE-SERVER 1639 The alternate server represents an alternate transport address 1640 identifying a different STUN server which the STUN client should try. 1642 It is encoded in the same way as MAPPED-ADDRESS, and thus refers to a 1643 single server by IP address. The IP address family MUST be identical 1644 to that of the source IP address of the request. 1646 This attribute MUST only appear in an error response that contains a 1647 MESSAGE-INTEGRITY attribute. This prevents it from being used in 1648 denial-of-service attacks. 1650 15. Security Considerations 1652 15.1. Attacks against the Protocol 1654 15.1.1. Outside Attacks 1656 An attacker can try to modify STUN messages in transit, in order to 1657 cause a failure in STUN operation. These attacks are detected for 1658 both requests and responses through the message integrity mechanism, 1659 using either a short term or long term credential. Of course, once 1660 detected, the manipulated packets will be dropped, causing the STUN 1661 transaction to effectively fail. This attack is possible only by an 1662 on-path attacker. 1664 An attacker that can observe, but not modify STUN messages in-transit 1665 (for example, an attacker present on a shared access medium, such as 1666 Wi-Fi), can see a STUN request, and then immediately send a STUN 1667 response, typically an error response, in order to disrupt STUN 1668 processing. This attack is also prevented for messages that utilize 1669 MESSAGE-INTEGRITY. However, some error responses, those related to 1670 authentication in particular, cannot be protected by MESSAGE- 1671 INTEGRITY. When STUN itself is run over a secure transport protocol 1672 (e.g., TLS), these attacks are completely mitigated. 1674 15.1.2. Inside Attacks 1676 A rogue client may try to launch a DoS attack against a server by 1677 sending it a large number of STUN requests. Fortunately, STUN 1678 requests can be processed statelessly by a server, making such 1679 attacks hard to launch. 1681 A rogue client may use a STUN server as a reflector, sending it 1682 requests with a falsified source IP address and port. In such a 1683 case, the response would be delivered to that source IP and port. 1684 There is no amplification of the number of packets with this attack 1685 (the STUN server sends one packet for each packet sent by the 1686 client), though there is a small increase in the amount of data, 1687 since STUN responses are typically larger than requests. This attack 1688 is mitigated by ingress source address filtering. 1690 15.2. Attacks Affecting the Usage 1692 This section lists attacks that might be launched against a usage of 1693 STUN. Each STUN usage must consider whether these attacks are 1694 applicable to it, and if so, discuss counter-measures. 1696 Most of the attacks in this section revolve around an attacker 1697 modifying the reflexive address learned by a STUN client through a 1698 Binding Request/Binding Response transaction. Since the usage of the 1699 reflexive address is a function of the usage, the applicability and 1700 remediation of these attacks is usage-specific. In common 1701 situations, modification of the reflexive address by an on-path 1702 attacker is easy to do. Consider, for example, the common situation 1703 where STUN is run directly over UDP. In this case, an on-path 1704 attacker can modify the source IP address of the Binding Request 1705 before it arrives at the STUN server. The STUN server will then 1706 return this IP address in the XOR-MAPPED-ADDRESS attribute to the 1707 client, and send the response back to that (falsified) IP address and 1708 port. If the attacker can also intercept this response, it can 1709 direct it back towards the client. Protecting against this attack by 1710 using a message-integrity check is impossible, since a message- 1711 integrity value cannot cover the source IP address, since the 1712 intervening NAT must be able to modify this value. Instead, one 1713 solution to preventing the attacks listed below is for the client to 1714 verify the reflexive address learned, as is done in ICE 1715 [I-D.ietf-mmusic-ice]. Other usages may use other means to prevent 1716 these attacks. 1718 15.2.1. Attack I: DDoS Against a Target 1720 In this attack, the attacker provides one or more clients with the 1721 same faked reflexive address that points to the intended target. 1722 This will trick the STUN clients into thinking that their reflexive 1723 addresses are equal to that of the target. If the clients hand out 1724 that reflexive address in order to receive traffic on it (for 1725 example, in SIP messages), the traffic will instead be sent to the 1726 target. This attack can provide substantial amplification, 1727 especially when used with clients that are using STUN to enable 1728 multimedia applications. However, it can only be launched against 1729 targets for which packets from the STUN server to the target pass 1730 through the attacker, limiting the cases in which it is possible 1732 15.2.2. Attack II: Silencing a Client 1734 In this attack, the attacker provides a STUN client with a faked 1735 reflexive address. The reflexive address it provides is a transport 1736 address that routes to nowhere. As a result, the client won't 1737 receive any of the packets it expects to receive when it hands out 1738 the reflexive address. This exploitation is not very interesting for 1739 the attacker. It impacts a single client, which is frequently not 1740 the desired target. Moreover, any attacker that can mount the attack 1741 could also deny service to the client by other means, such as 1742 preventing the client from receiving any response from the STUN 1743 server, or even a DHCP server. As with the attack in Section 15.2.1, 1744 this attack is only possible when the attacker is on path for packets 1745 sent from the STUN server towards this unused IP address. 1747 15.2.3. Attack III: Assuming the Identity of a Client 1749 This attack is similar to attack II. However, the faked reflexive 1750 address points to the attacker itself. This allows the attacker to 1751 receive traffic which was destined for the client. 1753 15.2.4. Attack IV: Eavesdropping 1755 In this attack, the attacker forces the client to use a reflexive 1756 address that routes to itself. It then forwards any packets it 1757 receives to the client. This attack would allow the attacker to 1758 observe all packets sent to the client. However, in order to launch 1759 the attack, the attacker must have already been able to observe 1760 packets from the client to the STUN server. In most cases (such as 1761 when the attack is launched from an access network), this means that 1762 the attacker could already observe packets sent to the client. This 1763 attack is, as a result, only useful for observing traffic by 1764 attackers on the path from the client to the STUN server, but not 1765 generally on the path of packets being routed towards the client. 1767 15.3. Hash Agility Plan 1769 This specification uses HMAC-SHA-1 for computation of the message 1770 integrity. If, at a later time, HMAC-SHA-1 is found to be 1771 compromised, the following is the remedy that will be applied. 1773 We will define a STUN extension which introduces a new message 1774 integrity attribute, computed using a new hash. Clients would be 1775 required to include both the new and old message integrity attributes 1776 in their requests or indications. A new server will utilize the new 1777 message integrity attribute, and an old one, the old. After a 1778 transition period where mixed implementations are in deployment, the 1779 old message-integrity attribute will be deprecated by another 1780 specification, and clients will cease including it in requests. 1782 16. IAB Considerations 1784 The IAB has studied the problem of "Unilateral Self Address Fixing" 1785 (UNSAF), which is the general process by which a client attempts to 1786 determine its address in another realm on the other side of a NAT 1787 through a collaborative protocol reflection mechanism (RFC3424 1788 [RFC3424]). STUN can be used to perform this function using a 1789 Binding Request/Response transaction if one agent is behind a NAT and 1790 the other is on the public side of the NAT. 1792 The IAB has mandated that protocols developed for this purpose 1793 document a specific set of considerations. Because some STUN usages 1794 provide UNSAF functions (such as ICE [I-D.ietf-mmusic-ice] ), and 1795 others do not (such as SIP Outbound [I-D.ietf-sip-outbound]), answers 1796 to these considerations need to be addressed by the usages 1797 themselves. 1799 17. IANA Considerations 1801 IANA is hereby requested to create three new registries: a STUN 1802 methods registry, a STUN Attributes registry, and a STUN Error Codes 1803 registry. 1805 17.1. STUN Methods Registry 1807 A STUN method is a hex number in the range 0x000 - 0x3FF. The 1808 encoding of STUN method into a STUN message is described in 1809 Section 6. 1811 The initial STUN methods are: 1813 0x000: (Reserved) 1814 0x001: Binding 1815 0x002: (Reserved; was SharedSecret) 1817 STUN methods in the range 0x000 - 0x1FF are assigned by IETF 1818 Consensus [RFC2434]. STUN methods in the range 0x200 - 0x3FF are 1819 assigned on a First Come First Served basis [RFC2434] 1821 17.2. STUN Attribute Registry 1823 A STUN Attribute type is a hex number in the range 0x0000 - 0xFFFF. 1824 STUN attribute types in the range 0x0000 - 0x7FFF are considered 1825 comprehension-required; STUN attribute types in the range 0x8000 - 1826 0xFFFF are considered comprehension-optional. A STUN agent handles 1827 unknown comprehension-required and comprehension-optional attributes 1828 differently. 1830 The initial STUN Attributes types are: 1832 Comprehension-required range (0x0000-0x7FFF): 1833 0x0000: (Reserved) 1834 0x0001: MAPPED-ADDRESS 1835 0x0006: USERNAME 1836 0x0007: (Reserved; was PASSWORD) 1837 0x0008: MESSAGE-INTEGRITY 1838 0x0009: ERROR-CODE 1839 0x000A: UNKNOWN-ATTRIBUTES 1840 0x0014: REALM 1841 0x0015: NONCE 1842 0x0020: XOR-MAPPED-ADDRESS 1844 Comprehension-optional range (0x8000-0xFFFF) 1845 0x8022: SERVER 1846 0x8023: ALTERNATE-SERVER 1847 0x8028: FINGERPRINT 1849 STUN Attribute types in the first half of the comprehension-required 1850 range (0x0000 - 0x3FFF) and in the first half of the comprehension- 1851 optional range (0x8000 - 0xBFFF) are assigned by IETF Consensus 1852 [RFC2434]. STUN Attribute types in the second half of the 1853 comprehension-required range (0x4000 - 0x7FFF) and in the second half 1854 of the comprehension-optional range (0xC000 - 0xFFFF) are assigned on 1855 a First Come First Served basis [RFC2434]. 1857 17.3. STUN Error Code Registry 1859 A STUN Error code is a number in the range 0 - 699. STUN error codes 1860 are accompanied by a textual reason phrase in UTF-8 which is intended 1861 only for human consumption and can be anything appropriate; this 1862 document proposes only suggested values. 1864 STUN error codes are consistent in codepoint assignments and 1865 semantics with SIP [RFC3261] and HTTP [RFC2616]. 1867 The initial values in this registry are given in Section 14.6. 1869 New STUN error codes are assigned on a Specification-Required basis 1870 [RFC2434]. The specification must carefully consider how clients 1871 that do not understand this error code will process it before 1872 granting the request. See the rules in Section 7.3.4. 1874 18. Changes Since RFC 3489 1876 This specification obsoletes RFC3489 [RFC3489]. This specification 1877 differs from RFC3489 in the following ways: 1879 o Removed the notion that STUN is a complete NAT traversal solution. 1880 STUN is now a tool that can be used to produce a NAT traversal 1881 solution. As a consequence, changed the name of the protocol to 1882 Session Traversal Utilities for NAT. 1884 o Introduced the concept of STUN usages, and described what a usage 1885 of STUN must document. 1887 o Removed the usage of STUN for NAT type detection and binding 1888 lifetime discovery. These techniques have proven overly brittle 1889 due to wider variations in the types of NAT devices than described 1890 in this document. Removed the RESPONSE-ADDRESS, CHANGED-ADDRESS, 1891 CHANGE-REQUEST, SOURCE-ADDRESS, and REFLECTED-FROM attributes. 1893 o Added a fixed 32-bit magic cookie and reduced length of 1894 transaction ID by 32 bits. The magic cookie begins at the same 1895 offset as the original transaction ID. 1897 o Added the XOR-MAPPED-ADDRESS attribute, which is included in 1898 Binding Responses if the magic cookie is present in the request. 1899 Otherwise the RFC3489 behavior is retained (that is, Binding 1900 Response includes MAPPED-ADDRESS). See discussion in XOR-MAPPED- 1901 ADDRESS regarding this change. 1903 o Introduced formal structure into the Message Type header field, 1904 with an explicit pair of bits for indication of request, response, 1905 error response or indication. Consequently, the message type 1906 field is split into the class (one of the previous four) and 1907 method. 1909 o Explicitly point out that the most significant two bits of STUN 1910 are 0b00, allowing easy differentiation with RTP packets when used 1911 with ICE. 1913 o Added the FINGERPRINT attribute to provide a method of definitely 1914 detecting the difference between STUN and another protocol when 1915 the two protocols are multiplexed together. 1917 o Added support for IPv6. Made it clear that an IPv4 client could 1918 get a v6 mapped address, and vice-a-versa. 1920 o Added long-term credential-based authentication. 1922 o Added the SERVER, REALM, NONCE, and ALTERNATE-SERVER attributes. 1924 o Removed the SharedSecret method, and thus the PASSWORD attribute. 1925 This method was almost never implemented and is not needed with 1926 current usages. 1928 o Removed recommendation to continue listening for STUN Responses 1929 for 10 seconds in an attempt to recognize an attack. 1931 o Changed transaction timers to be more TCP friendly. 1933 o Removed the STUN example that centered around the separation of 1934 the control and media planes. Instead, provided more information 1935 on using STUN with protocols. 1937 o Defined a generic padding mechanism that changes the 1938 interpretation of the length attribute. This would, in theory, 1939 break backwards compatibility. However, the mechanism in RFC 3489 1940 never worked for the few attributes that weren't aligned naturally 1941 on 32 bit boundaries. 1943 o REALM, SERVER, reason phrases and NONCE limited to 127 characters. 1944 USERNAME to 513 bytes. 1946 19. Acknowledgements 1948 The authors would like to thank Cedric Aoun, Pete Cordell, Cullen 1949 Jennings, Bob Penfield, Xavier Marjou, Bruce Lowekamp and Chris 1950 Sullivan for their comments, and Baruch Sterman and Alan Hawrylyshen 1951 for initial implementations. Thanks for Leslie Daigle, Allison 1952 Mankin, Eric Rescorla, and Henning Schulzrinne for IESG and IAB input 1953 on this work. 1955 20. References 1957 20.1. Normative References 1959 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1960 Requirement Levels", BCP 14, RFC 2119, March 1997. 1962 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 1963 September 1981. 1965 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 1966 specifying the location of services (DNS SRV)", RFC 2782, 1967 February 2000. 1969 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. 1971 [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., 1972 Leach, P., Luotonen, A., and L. Stewart, "HTTP 1973 Authentication: Basic and Digest Access Authentication", 1974 RFC 2617, June 1999. 1976 [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission 1977 Timer", RFC 2988, November 2000. 1979 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 1980 Hashing for Message Authentication", RFC 2104, 1981 February 1997. 1983 [ITU.V42.1994] 1984 International Telecommunications Union, "Error-correcting 1985 Procedures for DCEs Using Asynchronous-to-Synchronous 1986 Conversion", ITU-T Recommendation V.42, 1994. 1988 20.2. Informational References 1990 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1991 A., Peterson, J., Sparks, R., Handley, M., and E. 1992 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1993 June 2002. 1995 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1996 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1997 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 1999 [I-D.ietf-mmusic-ice] 2000 Rosenberg, J., "Interactive Connectivity Establishment 2001 (ICE): A Protocol for Network Address Translator (NAT) 2002 Traversal for Offer/Answer Protocols", 2003 draft-ietf-mmusic-ice-17 (work in progress), July 2007. 2005 [RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, 2006 "STUN - Simple Traversal of User Datagram Protocol (UDP) 2007 Through Network Address Translators (NATs)", RFC 3489, 2008 March 2003. 2010 [I-D.ietf-behave-turn] 2011 Rosenberg, J., "Traversal Using Relays around NAT (TURN): 2012 Relay Extensions to Session Traversal Utilities for NAT 2013 (STUN)", draft-ietf-behave-turn-04 (work in progress), 2014 July 2007. 2016 [I-D.ietf-sip-outbound] 2017 Jennings, C. and R. Mahy, "Managing Client Initiated 2018 Connections in the Session Initiation Protocol (SIP)", 2019 draft-ietf-sip-outbound-10 (work in progress), July 2007. 2021 [I-D.ietf-behave-nat-behavior-discovery] 2022 MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery 2023 Using STUN", draft-ietf-behave-nat-behavior-discovery-01 2024 (work in progress), July 2007. 2026 [I-D.ietf-mmusic-ice-tcp] 2027 Rosenberg, J., "TCP Candidates with Interactive 2028 Connectivity Establishment (ICE", 2029 draft-ietf-mmusic-ice-tcp-04 (work in progress), 2030 July 2007. 2032 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 2033 with Session Description Protocol (SDP)", RFC 3264, 2034 June 2002. 2036 [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral 2037 Self-Address Fixing (UNSAF) Across Network Address 2038 Translation", RFC 3424, November 2002. 2040 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an 2041 IANA Considerations Section in RFCs", BCP 26, RFC 2434, 2042 October 1998. 2044 Appendix A. C Snippet to Determine STUN Message Types 2046 Given an 16-bit STUN message type value in host byte order in 2047 msg_type parameter, below are C macros to determine the STUN message 2048 types: 2050 #define IS_REQUEST(msg_type) (((msg_type) & 0x0110) == 0x0000) 2051 #define IS_INDICATION(msg_type) (((msg_type) & 0x0110) == 0x0010) 2052 #define IS_SUCCESS_RESP(msg_type) (((msg_type) & 0x0110) == 0x0100) 2053 #define IS_ERR_RESP(msg_type) (((msg_type) & 0x0110) == 0x0110) 2055 Authors' Addresses 2057 Jonathan Rosenberg 2058 Cisco 2059 Edison, NJ 2060 US 2062 Email: jdrosen@cisco.com 2063 URI: http://www.jdrosen.net 2065 Christian Huitema 2066 Microsoft 2067 One Microsoft Way 2068 Redmond, WA 98052 2069 US 2071 Email: huitema@microsoft.com 2073 Rohan Mahy 2074 Plantronics 2075 345 Encinal Street 2076 Santa Cruz, CA 95060 2077 US 2079 Email: rohan@ekabal.com 2080 Philip Matthews 2081 Avaya 2082 1135 Innovation Drive 2083 Ottawa, Ontario K2K 3G7 2084 Canada 2086 Phone: +1 613 592 4343 x224 2087 Fax: 2088 Email: philip_matthews@magma.ca 2089 URI: 2091 Dan Wing 2092 Cisco 2093 771 Alder Drive 2094 San Jose, CA 95035 2095 US 2097 Email: dwing@cisco.com 2099 Full Copyright Statement 2101 Copyright (C) The IETF Trust (2007). 2103 This document is subject to the rights, licenses and restrictions 2104 contained in BCP 78, and except as set forth therein, the authors 2105 retain all their rights. 2107 This document and the information contained herein are provided on an 2108 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 2109 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 2110 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 2111 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 2112 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 2113 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 2115 Intellectual Property 2117 The IETF takes no position regarding the validity or scope of any 2118 Intellectual Property Rights or other rights that might be claimed to 2119 pertain to the implementation or use of the technology described in 2120 this document or the extent to which any license under such rights 2121 might or might not be available; nor does it represent that it has 2122 made any independent effort to identify any such rights. 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