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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: March 16, 2008 R. Mahy 7 Plantronics 8 P. Matthews 9 Avaya 10 D. Wing 11 Cisco 12 September 13, 2007 14 Session Traversal Utilities for (NAT) (STUN) 15 draft-ietf-behave-rfc3489bis-10 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 March 16, 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 . . . . . . . . . . . . . . . . . . 24 92 10.2.1.1. First Request . . . . . . . . . . . . . . . . . . 24 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 . . . . . . . . . . . . . . . . . . . . . . . . . 28 102 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 . . . . . . . . . . . 37 120 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 . . . . . . . . . . . . . . . . . . . . 38 124 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 . . . . . . . . . . . . . . . . 40 129 18. Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . . 41 130 19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 42 131 20. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42 132 20.1. Normative References . . . . . . . . . . . . . . . . . . 42 133 20.2. Informational References . . . . . . . . . . . . . . . . 43 134 Appendix A. C Snippet to Determine STUN Message Types . . . . . . 44 135 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, the 575 client SHOULD consider the transaction to have failed. A STUN 576 transaction over UDP is also considered failed if there has been a 577 transport failure of some sort, such as a fatal ICMP error. For 578 example, assuming an RTO of 100ms, requests would be sent at times 579 0ms, 100ms, 300ms, 700ms, 1500ms, 3100ms, and 6300ms. If the client 580 has not received a response after 7900ms, the client will consider 581 the transaction to have timed out. 583 7.2.2. Sending over TCP or TLS-over-TCP 585 For TCP and TLS-over-TCP, the client opens a TCP connection to the 586 server. 588 In some usage of STUN, STUN is sent as the only protocol over the TCP 589 connection. In this case, it can be sent without the aid of any 590 additional framing or demultiplexing. In other usages, or with other 591 extensions, it may be multiplexed with other data over a TCP 592 connection. In that case, STUN MUST be run on top of some kind of 593 framing protocol, specified by the usage or extension, which allows 594 for the agent to extract complete STUN messages and complete 595 application layer messages. 597 For TLS-over-TCP, the TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite MUST 598 be supported at a minimum. Implementations MAY also support any 599 other ciphersuite. When it receives the TLS Certificate message, the 600 client SHOULD verify the certificate and inspect the site identified 601 by the certificate. If the certificate is invalid, revoked, or if it 602 does not identify the appropriate party, the client MUST NOT send the 603 STUN message or otherwise proceed with the STUN transaction. The 604 client MUST verify the identity of the server. To do that, it 605 follows the identification procedures defined in Section 3.1 of RFC 606 2818 [RFC2818]. Those procedures assume the client is dereferencing 607 a URI. For purposes of usage with this specification, the client 608 treats the domain name or IP address used in Section 8.1 as the host 609 portion of the URI that has been dereferenced. If DNS was not used, 610 the client MUST be configured with a set of authorized domains whose 611 certificates will be accepted. 613 Reliability of STUN over TCP and TLS-over-TCP is handled by TCP 614 itself, and there are no retransmissions at the STUN protocol level. 615 However, for a request/response transaction, if the client has not 616 received a response 7900ms after it sent the SYN to establish the 617 connection, it considers the transaction to have timed out. This 618 value has been chosen to equalize the TCP and UDP timeouts for the 619 default initial RTO. 621 In addition, if the client is unable to establish the TCP connection, 622 or the TCP connection is reset or fails before a response is 623 received, any request/response transaction in progress is considered 624 to have failed 626 The client MAY send multiple transactions over a single TCP (or TLS- 627 over-TCP) connection, and it MAY send another request before 628 receiving a response to the previous. The client SHOULD keep the 629 connection open until it 631 o has no further STUN requests or indications to send over that 632 connection, and; 634 o has no plans to use any resources (such as a mapped address 635 (MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address 636 [I-D.ietf-behave-turn]) that were learned though STUN requests 637 sent over that connection, and; 639 o if multiplexing other application protocols over that port, has 640 finished using that other application, and; 642 o if using that learned port with a remote peer, has established 643 communications with that remote peer, as is required by some TCP 644 NAT traversal techniques (e.g., [I-D.ietf-mmusic-ice-tcp]). 646 At the server end, the server SHOULD keep the connection open, and 647 let the client close it. If a server becomes overloaded and needs to 648 close connections to free up resources, it SHOULD close an existing 649 connection rather than reject new connection requests. The server 650 SHOULD NOT close a connection if a request was received over that 651 connection for which a response was not sent. A server MUST NOT ever 652 open a connection back towards the client in order to send a 653 response. 655 7.3. Receiving a STUN Message 657 This section specifies the processing of a STUN message. The 658 processing specified here is for STUN messages as defined in this 659 specification; additional rules for backwards compatibility are 660 defined in in Section 12. Those additional procedures are optional, 661 and usages can elect to utilize them. First, a set of processing 662 operations are applied that are independent of the class. This is 663 followed by class-specific processing, described in the subsections 664 which follow. 666 When a STUN agent receives a STUN message, it first checks that the 667 message obeys the rules of Section 6. It checks that the first two 668 bits are 0, that the magic cookie field has the correct value, that 669 the message length is sensible, and that the method value is a 670 supported method. If the message-class is Success Response or Error 671 Response, the agent checks that the transaction ID matches a 672 transaction that is still in progress. If the FINGERPRINT extension 673 is being used, the agent checks that the FINGERPRINT attribute is 674 present and contains the correct value. If any errors are detected, 675 the message is silently discarded. In the case when STUN is being 676 multiplexed with another protocol, an error may indicate that this is 677 not really a STUN message; in this case, the agent should try to 678 parse the message as a different protocol. 680 The STUN agent then does any checks that are required by a 681 authentication mechanism that the usage has specified (see 682 Section 10. 684 Once the authentication checks are done, the STUN agent checks for 685 unknown attributes and known-but-unexpected attributes in the 686 message. Unknown comprehension-optional attributes MUST be ignored 687 by the agent. Known-but-unexpected attributes SHOULD be ignored by 688 the agent. Unknown comprehension-required attributes cause 689 processing that depends on the message-class and is described below. 691 At this point, further processing depends on the message class of the 692 request. 694 7.3.1. Processing a Request 696 If the request contains one or more unknown comprehension-required 697 attributes, the server replies with an error response with an error 698 code of 420 (Unknown Attribute), and includes an UNKNOWN-ATTRIBUTES 699 attribute in the response that lists the unknown comprehension- 700 required attributes. 702 The server then does any additional checking that the method or the 703 specific usage requires. If all the checks succeed, the server 704 formulates a success response as described below. 706 If the request uses UDP transport and is a retransmission of a 707 request for which the server has already generated a success response 708 within the last 10 seconds, the server MUST retransmit the same 709 success response. One way for a server to do this is to remember all 710 transaction IDs received over UDP and their corresponding responses 711 in the last 10 seconds. Another way is to reprocess the request and 712 recompute the response. The latter technique MUST only be applied to 713 requests which are idempotent and result in the same success response 714 for the same request. The Binding method is considered to idempotent 715 in this way (even though certain rare network events could cause the 716 reflexive transport address value to change). Extensions to STUN 717 SHOULD state whether their request types have this property or not. 719 7.3.1.1. Forming a Success or Error Response 721 When forming the response (success or error), the server follows the 722 rules of section 6. The method of the response is the same as that 723 of the request, and the message class is either "Success Response" or 724 "Error Response". 726 For an error response, the server MUST add an ERROR-CODE attribute 727 containing the error code specified in the processing above. The 728 reason phrase is not fixed, but SHOULD be something suitable for the 729 error code. For certain errors, additional attributes are added to 730 the message. These attributes are spelled out in the description 731 where the error code is specified. For example, for an error code of 732 420 (Unknown Attribute), the server MUST include an UNKNOWN- 733 ATTRIBUTES attribute. Certain authentication errors also cause 734 attributes to be added (see Section 10). Extensions may define other 735 errors and/or additional attributes to add in error cases. 737 If the server authenticated the request using an authentication 738 mechanism, then the server SHOULD add the appropriate authentication 739 attributes to the response (see Section 10). 741 The server also adds any attributes required by the specific method 742 or usage. In addition, the server SHOULD add a SERVER attribute to 743 the message. 745 For the Binding method, no additional checking is required unless the 746 usage specifies otherwise. When forming the success response, the 747 server adds a XOR-MAPPED-ADDRESS attribute to the response, where the 748 contents of the attribute are the source transport address of the 749 request message. For UDP, this is the source IP address and source 750 UDP port of the request message. For TCP and TLS-over-TCP, this is 751 the source IP address and source TCP port of the TCP connection as 752 seen by the server. 754 7.3.1.2. Sending the Success or Error Response 756 The response (success or error) is sent over the same transport as 757 the request was received on. If the request was received over UDP, 758 the destination IP address and port of the response is the source IP 759 address and port of the received request message, and the source IP 760 address and port of the response is equal to the destination IP 761 address and port of the received request message. If the request was 762 received over TCP or TLS-over-TCP, the response is sent back on the 763 same TCP connection as the request was received on. 765 7.3.2. Processing an Indication 767 If the indication contains unknown comprehension-required attributes, 768 the indication is discarded and processing ceases. 770 The server then does any additional checking that the method or the 771 specific usage requires. If all the checks succeed, the server then 772 processes the indication. No response is generated for an 773 indication. 775 For the Binding method, no additional checking or processing is 776 required, unless the usage specifies otherwise. The mere receipt of 777 the message by the server has refreshed the "bindings" in the 778 intervening NATs. 780 Since indications are not re-transmitted over UDP (unlike requests), 781 there is no need to handle re-transmissions of indications at the 782 server. 784 7.3.3. Processing a Success Response 786 If the success response contains unknown comprehension-required 787 attributes, the response is discarded and the transaction is 788 considered to have failed. 790 The client then does any additional checking that the method or the 791 specific usage requires. If all the checks succeed, the client then 792 processes the success response. 794 For the Binding method, the client checks that the XOR-MAPPED-ADDRESS 795 attribute is present in the response. The client checks the address 796 family specified. If it is an unsupported address family, the 797 attribute SHOULD be ignored. If it is an unexpected but supported 798 address family (for example, the Binding transaction was sent over 799 IPv4, but the address family specified is IPv6), then the client MAY 800 accept and use the value. 802 7.3.4. Processing an Error Response 804 If the error response contains unknown comprehension-required 805 attributes, or if the error response does not contain an ERROR-CODE 806 attribute, then the transaction is simply considered to have failed. 808 The client then does any processing specified by the authentication 809 mechanism (see Section 10). This may result in a new transaction 810 attempt. 812 The processing at this point depends on the error-code, the method, 813 and the usage; the following are the default rules: 815 o If the error code is 300 through 399, the client SHOULD consider 816 the transaction as failed unless the ALTERNATE-SERVER extension is 817 being used. See Section 11. 819 o If the error code is 400 through 499, the client declares the 820 transaction failed; in the case of 420 (Unknown Attribute), the 821 response should contain a UNKNOWN-ATTRIBUTES attribute that gives 822 additional information. 824 o If the error code is 500 through 599, the client MAY resend the 825 request; clients that do so MUST limit the number of times they do 826 this. 828 Any other error code causes the client to consider the transaction 829 failed. 831 8. FINGERPRINT Mechanism 833 This section describes an optional mechanism for STUN that aids in 834 distinguishing STUN messages from packets of other protocols when the 835 two are multiplexed on the same transport address. This mechanism is 836 optional, and a STUN usage must describe if and when it is used. 838 In some usages, STUN messages are multiplexed on the same transport 839 address as other protocols, such as RTP. In order to apply the 840 processing described in Section 7, STUN messages must first be 841 separated from the application packets. Section 6 describes three 842 fixed fields in the STUN header that can be used for this purpose. 843 However, in some cases, these three fixed fields may not be 844 sufficient. 846 When the FINGERPRINT extension is used, an agent includes the 847 FINGERPRINT attribute in messages it sends to another agent. 848 Section 14.5 describes the placement and value of this attribute. 849 When the agent receives what it believes is a STUN message, then, in 850 addition to other basic checks, the agent also checks that the 851 message contains a FINGERPRINT attribute and that the attribute 852 contains the correct value (see Section 7.3. This additional check 853 helps the agent detect messages of other protocols that might 854 otherwise seem to be STUN messages. 856 9. DNS Discovery of a Server 858 This section describes an optional procedure for STUN that allows a 859 client to use DNS to determine the IP address and port of a server. 860 A STUN usage must describe if and when this extension is used. To 861 use this procedure, the client must have a domain name and a service 862 name; the usage must also describe how the client obtains these. 864 When a client wishes to locate a STUN server in the public Internet 865 that accepts Binding Request/Response transactions, the SRV service 866 name is "stun". STUN usages MAY define additional DNS SRV service 867 names. 869 The domain name is resolved to a transport address using the SRV 870 procedures specified in [RFC2782]. The DNS SRV service name is the 871 service name provided as input to this procedure. The protocol in 872 the SRV lookup is the transport protocol the client will run STUN 873 over: "udp" for UDP, "tcp" for TCP, and "tls" for TLS-over-TCP. If, 874 in the future, additional SRV records are defined for TLS over other 875 transport protocols, those will need to utilize an SRV transport 876 token of the form "tls-foo" for transport protocol "foo". 878 The procedures of RFC 2782 are followed to determine the server to 879 contact. RFC 2782 spells out the details of how a set of SRV records 880 are sorted and then tried. However, RFC2782 only states that the 881 client should "try to connect to the (protocol, address, service)" 882 without giving any details on what happens in the event of failure. 883 When following these procedures, if the STUN transaction times out 884 without receipt of a response, the client SHOULD retry the request to 885 the next server in the list of servers from the DNS SRV response. 886 Such a retry is only possible for request/response transmissions, 887 since indication transactions generate no response or timeout. 889 The default port for STUN requests is 3478, for both TCP and UDP. 890 Administrators SHOULD use this port in their SRV records for UDP and 891 TCP, but MAY use others. There is no default port for STUN over TLS, 892 however a STUN server SHOULD use a port number for TLS different from 893 3478 so that the server can determine whether the first message it 894 will receive after the TCP connection is set up, is a STUN message or 895 a TLS message. 897 If no SRV records were found, the client performs an A or AAAA record 898 lookup of the domain name. The result will be a list of IP 899 addresses, each of which can be contacted at the default port using 900 UDP or TCP, independent of the STUN usage. For usages that require 901 TLS, lack of SRV records is equivalent to a failure of the 902 transaction, since the request or indication MUST NOT be sent unless 903 SRV records provided a transport address specifically for TLS. 905 10. Authentication and Message-Integrity Mechanisms 907 This section defines two mechanisms for STUN that a client and server 908 can use to provide authentication and message-integrity; these two 909 mechanisms are known as the short-term credential mechanism and the 910 long-term credential mechanism. These two mechanisms are optional, 911 and each usage must specify if and when these mechanisms are used. 912 Consequently, both clients and servers will know which mechanism (if 913 any) to follow based on knowledge of which usage applies. For 914 example, a STUN server on the public Internet supporting ICE would 915 have no authentication, whereas the STUN server functionality in an 916 agent supporting connectivity checks would utilize short term 917 credentials. An overview of these two mechanisms is given in 918 Section 3. 920 Each mechanism specifies the additional processing required to use 921 that mechanism, extending the processing specified in Section 7. The 922 additional processing occurs in three different places: when forming 923 a message; when receiving a message immediately after the the basic 924 checks have been performed; and when doing the detailed processing of 925 error responses. 927 10.1. Short-Term Credential Mechanism 929 The short-term credential mechanism assumes that, prior to the STUN 930 transaction, the client and server have used some other protocol to 931 exchange a credential in the form of a username and password. This 932 credential is time-limited. The time-limit is defined by the usage. 933 As an example, in the ICE usage [I-D.ietf-mmusic-ice], the two 934 endpoints use out-of-band signaling to agree on a username and 935 password, and this username and password is applicable for the 936 duration of the media session. 938 This credential is used to form a message integrity check in each 939 request and in many responses. There is no challenge and response as 940 in the long term mechanism; consequently, replay is prevented by 941 virtue of the time-limited nature of the credential. 943 10.1.1. Forming a Request or Indication 945 For a request or indication message, the agent MUST include the 946 USERNAME and MESSAGE-INTEGRITY attributes in the message. The HMAC 947 for the MESSAGE-INTEGRITY attribute is computed as described in 948 Section 14.4. The key for the HMAC is the password. Note that the 949 password is never included in the request or indication. 951 10.1.2. Receiving a Request or Indication 953 After the agent has done the basic processing of a message, the agent 954 performs the checks listed below in order specified: 956 o If the message does not contain both a MESSAGE-INTEGRITY and a 957 USERNAME attribute: 959 * If the message is a request, the server MUST reject the request 960 with an error response. This response MUST use an error code 961 of 400 (Bad Request). 963 * If the message is an indication, the server MUST silently 964 discard the indication. 966 o If the USERNAME does not contain a username value currently valid 967 within the server: 969 * If the message is a request, the server MUST reject the request 970 with an error response. This response MUST use an error code 971 of 401 (Unauthorized). 973 * If the message is an indication, the server MUST silently 974 discard the indication. 976 o Using the password associated with the username, compute the value 977 for the message-integrity as described in Section 14.4. If the 978 resulting value does not match the contents of the MESSAGE- 979 INTEGRITY attribute: 981 * If the message is a request, the server MUST reject the request 982 with an error response. This response MUST use an error code 983 of 401 (Unauthorized). 985 * If the message is an indication, the server MUST silently 986 discard the indication. 988 If these checks pass, the server continues to process the request or 989 indication. Any response generated by the server MUST include the 990 MESSAGE-INTEGRITY attribute, computed using the password utilized to 991 authenticate the request. The response MUST NOT contain the USERNAME 992 attribute. 994 If any of the checks fail, the server MUST NOT include a MESSAGE- 995 INTEGRITY or USERNAME attribute in the error response. This is 996 because, in these failure cases, the server cannot determine the 997 shared secret necessary to compute MESSAGE-INTEGRITY. 999 10.1.3. Receiving a Response 1001 The client looks for the MESSAGE-INTEGRITY attribute in the response. 1002 If present, the client computes the message integrity over the 1003 response as defined in Section 14.4, using the same password it 1004 utilized for the request. If the resulting value matches the 1005 contents of the MESSAGE-INTEGRITY attribute, the response is 1006 considered authenticated. If the value does not match, or if 1007 MESSAGE-INTEGRITY was absent, the response MUST be discarded, as if 1008 it was never received. This means that retransmits, if applicable, 1009 will continue. 1011 10.2. Long-term Credential Mechanism 1013 The long-term credential mechanism relies on a long term credential, 1014 in the form of a username and password, that are shared between 1015 client and server. The credential is considered long-term since it 1016 is assumed that it is provisioned for a user, and remains in effect 1017 until the user is no longer a subscriber of the system, or is 1018 changed. This is basically a traditional "log-in" username and 1019 password given to users. 1021 Because these usernames and passwords are expected to be valid for 1022 extended periods of time, replay prevention is provided in the form 1023 of a digest challenge. In this mechanism, the client initially sends 1024 a request, without offering any credentials or any integrity checks. 1025 The server rejects this request, providing the user a realm (used to 1026 guide the user or agent in selection of a username and password) and 1027 a nonce. The nonce provides the replay protection. It is a cookie, 1028 selected by the server, and encoded in such a way as to indicate a 1029 duration of validity or client identity from which it is valid. The 1030 client retries the request, this time including its username, the 1031 realm, and echoing the nonce provided by the server. The client also 1032 includes a message-integrity, which provides an HMAC over the entire 1033 request, including the nonce. The server validates the nonce, and 1034 checks the message-integrity. If they match, the request is 1035 authenticated. If the nonce is no longer valid, it is considered 1036 "stale", and the server rejects the request, providing a new nonce. 1038 In subsequent requests to the same server, the client reuses the 1039 nonce, username, realm and password it used previously. In this way, 1040 subsequent requests are not rejected until the nonce becomes invalid 1041 by the server, in which case the rejection provides a new nonce to 1042 the client. 1044 Note that the long-term credential mechanism cannot be used to 1045 protect indications, since indications cannot be challenged. Usages 1046 utilizing indications must either use a short-term credential, or 1047 omit authentication and message integrity for them. 1049 Since the long-term credential mechanism is susceptible to offline 1050 dictionary attacks, deployments SHOULD utilize strong passwords. 1052 For STUN servers used in conjunction with SIP servers, it is 1053 desirable to use the same credentials for authentication to the SIP 1054 server and STUN server. Typically, SIP systems utilizing SIP's 1055 digest authentication mechanism do not actually store the password in 1056 the database. Rather, they store a value called H(A1), which is 1057 computed as: 1059 H(A1) = MD5(username ":" realm ":" password) 1061 If a system wishes to utilize this credential, the STUN password 1062 would be computed by taking the user-entered username and password, 1063 and using H(A1) as the STUN password. It is RECOMMENDED that clients 1064 utilize this construction for the STUN password. 1066 10.2.1. Forming a Request 1068 There are two cases when forming a request. In the first case, this 1069 is the first request from the client to the server (as identified by 1070 its IP address and port). In the second case, the client is 1071 submitting a subsequent request once a previous request/response 1072 transaction has completed successfully. Forming a request as a 1073 consequence of a 401 or 438 error response is covered in 1074 Section 10.2.3 and is not considered a "subsequent request" and thus 1075 does not utilize the rules described in Section 10.2.1.2. 1077 10.2.1.1. First Request 1079 If the client has not completed a successful request/response 1080 transaction with the server (as identified by hostname, if the DNS 1081 procedures of Section 9 are used, else IP address if not), it SHOULD 1082 omit the USERNAME, MESSAGE-INTEGRITY, REALM, and NONCE attributes. 1083 In other words, the very first request is sent as if there were no 1084 authentication or message integrity applied. The exception to this 1085 rule are requests sent to another server as a consequence of the 1086 ALTERNATE-SERVER mechanism described in Section 11. Those requests 1087 do include the USERNAME, REALM and NONCE from the original request, 1088 along with a newly computed MESSAGE-INTEGRITY based on them. 1090 10.2.1.2. Subsequent Requests 1092 Once a request/response transaction has completed successfully, the 1093 client will have been been presented a realm and nonce by the server, 1094 and selected a username and password with which it authenticated. 1095 The client SHOULD cache the username, password, realm, and nonce for 1096 subsequent communications with the server. When the client sends a 1097 subsequent request, it SHOULD include the USERNAME, REALM, and NONCE 1098 attributes with these cached values. It SHOULD include a MESSAGE- 1099 INTEGRITY attributed, computed as described in Section 14.4 using the 1100 cached password as the key. 1102 10.2.2. Receiving a Request 1104 After the server has done the basic processing of a request, it 1105 performs the checks listed below in the order specified: 1107 o If the message does not contain a MESSAGE-INTEGRITY attribute, the 1108 server MUST generate an error response with an error code of 401 1109 (Unauthorized). This response MUST include a REALM value. It is 1110 RECOMMENDED that the REALM value be the domain name of the 1111 provider of the STUN server. The response MUST include a NONCE, 1112 selected by the server. The response SHOULD NOT contain a 1113 USERNAME or MESSAGE-INTEGRITY attribute. 1115 o If the message contains a MESSAGE-INTEGRITY attribute, but is 1116 missing the USERNAME, REALM or NONCE attributes, the server MUST 1117 generate an error response with an error code of 400 (Bad 1118 Request). This response SHOULD NOT include a USERNAME, NONCE, 1119 REALM or MESSAGE-INTEGRITY attribute. 1121 o If the NONCE is no longer valid, the server MUST generate an error 1122 response with an error code of 438 (Stale Nonce). This response 1123 MUST include a NONCE and REALM attribute and SHOULD NOT incude the 1124 USERNAME or MESSAGE-INTEGRITY attribute. 1126 o If the username in the USERNAME attribute is not valid, the server 1127 MUST generate an error response with an error code of 401 1128 (Unauthorized). This response MUST include a REALM value. It is 1129 RECOMMENDED that the REALM value be the domain name of the 1130 provider of the STUN server. The response MUST include a NONCE, 1131 selected by the server. The response SHOULD NOT contain a 1132 USERNAME or MESSAGE-INTEGRITY attribute. 1134 o Using the password associated with the username in the USERNAME 1135 attribute, compute the value for the message-integrity as 1136 described in Section 14.4. If the resulting value does not match 1137 the contents of the MESSAGE-INTEGRITY attribute, the server MUST 1138 reject the request with an error response. This response MUST use 1139 an error code of 401 (Unauthorized). It MUST include a REALM and 1140 NONCE attribute and SHOULD NOT include the USERNAME or MESSAGE- 1141 INTEGRITY attribute. 1143 If these checks pass, the server continues to process the request. 1144 Any response generated by the server (excepting the cases described 1145 above) MUST include the MESSAGE-INTEGRITY attribute, computed using 1146 the username and password utilized to authenticate the request. The 1147 REALM, NONCE, and USERNAME attributes SHOULD NOT be included. 1149 10.2.3. Receiving a Response 1151 If the response is an error response, with an error code of 401 1152 (Unauthorized), the client SHOULD retry the request with a new 1153 transaction. This request MUST contain a USERNAME, determined by the 1154 client as the appropriate username for the REALM from the error 1155 response. The request MUST contain the REALM, copied from the error 1156 response. The request MUST contain the NONCE, copied from the error 1157 response. The request MUST contain the MESSAGE-INTEGRITY attribute, 1158 computed using the password associated with the username in the 1159 USERNAME attribute. The client MUST NOT perform this retry if it is 1160 not changing the USERNAME or REALM or its associated password, from 1161 the previous attempt. 1163 If the response is an error response with an error code of 438 (Stale 1164 Nonce), the client MUST retry the request, using the new NONCE 1165 supplied in the 438 (Stale Nonce) response. This retry MUST also 1166 include the USERNAME, REALM and MESSAGE-INTEGRITY. 1168 The client looks for the MESSAGE-INTEGRITY attribute in the response 1169 (either success or failure). If present, the client computes the 1170 message integrity over the response as defined in Section 14.4, using 1171 the same password it utilized for the request. If the resulting 1172 value matches the contents of the MESSAGE-INTEGRITY attribute, the 1173 response is considered authenticated. If the value does not match, 1174 or if MESSAGE-INTEGRITY was absent, the response MUST be discarded, 1175 as if it was never received. This means that retransmits, if 1176 applicable, will continue. 1178 11. ALTERNATE-SERVER Mechanism 1180 This section describes a mechanism in STUN that allows a server to 1181 redirect a client to another server. This extension is optional, and 1182 a usage must define if and when this extension is used. To prevent 1183 denial-of-service attacks, this extension MUST only be used in 1184 situations where the client and server are using an authentication 1185 and message-integrity mechanism. 1187 A server using this extension redirects a client to another server by 1188 replying to a request message with an error response message with an 1189 error code of 300 (Try Alternate). The server MUST include a 1190 ALTERNATE-SERVER attribute in the error response. The error response 1191 message MUST be authenticated, which in practice means the request 1192 message must have passed the authentication checks. 1194 A client using this extension handles a 300 (Try Alternate) error 1195 code as follows. If the error response has passed the authentication 1196 checks, then the client looks for a ALTERNATE-SERVER attribute in the 1197 error response. If one is found, then the client considers the 1198 current transaction as failed, and re-attempts the request with the 1199 server specified in the attribute. The client SHOULD reuse any 1200 authentication credentials from the old request in the new 1201 transaction. 1203 12. Backwards Compatibility with RFC 3489 1205 This section define procedures that allow a degree of backwards 1206 compatible with the original protocol defined in RFC 3489 [RFC3489]. 1207 This mechanism is optional, meant to be utilized only in cases where 1208 a new client can connect to an old server, or vice-a-versa. A usage 1209 must define if and when this procedure is used. 1211 Section 18 lists all the changes between this specification and RFC 1212 3489 [RFC3489]. However, not all of these differences are important, 1213 because "classic STUN" was only used in a few specific ways. For the 1214 purposes of this extension, the important changes are the following. 1215 In RFC 3489: 1217 o UDP was the only supported transport; 1219 o The field that is now the Magic Cookie field was a part of the 1220 transaction id field, and transaction ids were 128 bits long; 1222 o The XOR-MAPPED-ADDRESS attribute did not exist, and the Binding 1223 method used the MAPPED-ADDRESS attribute instead; 1225 o There were two comprehension-required attributes, RESPONSE-ADDRESS 1226 and CHANGE-REQUEST, that have been removed from this 1227 specification; 1229 * These attributes are now part of the NAT Behavior Discovery 1230 usage. 1232 12.1. Changes to Client Processing 1234 A client that wants to interoperate with a [RFC3489] server SHOULD 1235 send a request message that uses the Binding method, contains no 1236 attributes, and uses UDP as the transport protocol to the server. If 1237 successful, the success response received from the server will 1238 contain a MAPPED-ADDRESS attribute rather than an XOR-MAPPED-ADDRESS 1239 attribute; other than this change, the processing of the response is 1240 identical to the procedures described above. 1242 12.2. Changes to Server Processing 1244 A STUN server can detect when a given Binding Request message was 1245 sent from an RFC 3489 [RFC3489] client by the absence of the correct 1246 value in the Magic Cookie field. When the server detects an RFC 3489 1247 client, it SHOULD copy the value seen in the Magic Cookie field in 1248 the Binding Request to the Magic Cookie field in the Binding Response 1249 message, and insert a MAPPED-ADDRESS attribute instead of an XOR- 1250 MAPPED-ADDRESS attribute. 1252 The client might, in rare situations, include either the RESPONSE- 1253 ADDRESS or CHANGE-REQUEST attributes. In these situations, the 1254 server will view these as unknown comprehension-required attributes 1255 and reply with an error response. Since the mechanisms utilizing 1256 those attributes are no longer supported, this behavior is 1257 acceptable. 1259 13. STUN Usages 1261 STUN by itself is not a solution to the NAT traversal problem. 1262 Rather, STUN defines a tool that can be used inside a larger 1263 solution. The term "STUN Usage" is used for any solution that uses 1264 STUN as a component. 1266 At the time of writing, three STUN usages are defined: Interactive 1267 Connectivity Establishment (ICE) [I-D.ietf-mmusic-ice], Client- 1268 initiated connections for SIP [I-D.ietf-sip-outbound], and NAT 1269 Behavior Discovery [I-D.ietf-behave-nat-behavior-discovery]. Other 1270 STUN usages may be defined in the future. 1272 A STUN usage defines how STUN is actually utilized - when to send 1273 requests, what to do with the responses, and which optional 1274 procedures defined here (or in an extension to STUN) are to be used. 1275 A usage would also define: 1277 o Which STUN methods are used; 1279 o What authentication and message integrity mechanisms are used; 1281 o What mechanisms are used to distinguish STUN messages from other 1282 messages. When STUN is run over TCP, a framing mechanism may be 1283 required; 1285 o How a STUN client determines the IP address and port of the STUN 1286 server; 1288 o Whether backwards compatibility to RFC 3489 is required; 1290 o What optional attributes defined here (such as FINGERPRINT and 1291 ALTERNATE-SERVER) or in other extensions are required. 1293 In addition, any STUN usage must consider the security implications 1294 of using STUN in that usage. A number of attacks against STUN are 1295 known (see the Security Considerations section in this document) and 1296 any usage must consider how these attacks can be thwarted or 1297 mitigated. 1299 Finally, a usage must consider whether its usage of STUN is an 1300 example of the Unilateral Self-Address Fixing approach to NAT 1301 traversal, and if so, address the questions raised in RFC 3424. 1303 14. STUN Attributes 1305 After the STUN header are zero or more attributes. Each attribute 1306 MUST be TLV encoded, with a 16 bit type, 16 bit length, and value. 1307 Each STUN attribute MUST end on a 32 bit boundary. As mentioned 1308 above, all fields in an attribute are transmitted most significant 1309 bit first. 1311 0 1 2 3 1312 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 1313 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1314 | Type | Length | 1315 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1316 | Value (variable) .... 1317 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1319 Figure 5: Format of STUN Attributes 1321 The value in the Length field MUST contain the length of the Value 1322 part of the attribute, prior to padding, measured in bytes. Since 1323 STUN aligns attributes on 32 bit boundaries, attributes whose content 1324 is not a multiple of 4 bytes are padded with 1, 2 or 3 bytes of 1325 padding so that its value contains a multiple of 4 bytes. The 1326 padding bits are ignored, and may be any value. 1328 Any attribute type MAY appear more than once in a STUN message. 1329 Unless specified otherwise, the order of appearance is significant: 1330 only the first occurance needs to be processed by a receiver, and any 1331 duplicates MAY be ignored by a receiver. 1333 To allow future revisions of this specification to add new attributes 1334 if needed, the attribute space is divided into two ranges. 1335 Attributes with type values between 0x0000 and 0x7FFF are 1336 comprehension-required attributes, which means that the STUN agent 1337 cannot successfully process the message unless it understands the 1338 attribute. Attributes with type values between 0x8000 and 0xFFFF are 1339 comprehension-optional attributes, which means that those attributes 1340 can be ignored by the STUN agent if it does not understand them. 1342 The STUN Attribute types defined by this specification are: 1344 Comprehension-required range (0x0000-0x7FFF): 1345 0x0000: (Reserved) 1346 0x0001: MAPPED-ADDRESS 1347 0x0006: USERNAME 1348 0x0007: (Reserved; was PASSWORD) 1349 0x0008: MESSAGE-INTEGRITY 1350 0x0009: ERROR-CODE 1351 0x000A: UNKNOWN-ATTRIBUTES 1352 0x0014: REALM 1353 0x0015: NONCE 1354 0x0020: XOR-MAPPED-ADDRESS 1356 Comprehension-optional range (0x8000-0xFFFF) 1357 0x8022: SERVER 1358 0x8023: ALTERNATE-SERVER 1359 0x8028: FINGERPRINT 1361 The rest of this section describes the format of the various 1362 attributes defined in this specification. 1364 14.1. MAPPED-ADDRESS 1366 The MAPPED-ADDRESS attribute indicates a reflexive transport address 1367 of the client. It consists of an eight bit address family, and a 1368 sixteen bit port, followed by a fixed length value representing the 1369 IP address. If the address family is IPv4, the address MUST be 32 1370 bits. If the address family is IPv6, the address MUST be 128 bits. 1371 All fields must be in network byte order. 1373 The format of the MAPPED-ADDRESS attribute is: 1375 0 1 2 3 1376 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 1377 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1378 |0 0 0 0 0 0 0 0| Family | Port | 1379 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1380 | | 1381 | Address (32 bits or 128 bits) | 1382 | | 1383 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1385 Figure 7: Format of MAPPED-ADDRESS attribute 1387 The address family can take on the following values: 1389 0x01:IPv4 1390 0x02:IPv6 1392 The first 8 bits of the MAPPED-ADDRESS MUST be set to 0 and MUST be 1393 ignored by receivers. These bits are present for aligning parameters 1394 on natural 32 bit boundaries. 1396 This attribute is used only by servers for achieving backwards 1397 compatibility with RFC 3489 [RFC3489] clients. 1399 14.2. XOR-MAPPED-ADDRESS 1401 The XOR-MAPPED-ADDRESS attribute is identical to the MAPPED-ADDRESS 1402 attribute, except that the reflexive transport address is obfuscated 1403 through the XOR function. 1405 The format of the XOR-MAPPED-ADDRESS is: 1407 0 1 2 3 1408 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 1409 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1410 |x x x x x x x x| Family | X-Port | 1411 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1412 | X-Address (Variable) 1413 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1415 Figure 9: Format of XOR-MAPPED-ADDRESS Attribute 1417 The Family represents the IP address family, and is encoded 1418 identically to the Family in MAPPED-ADDRESS. 1420 X-Port is computed by taking the mapped port in host byte order, 1421 XOR'ing it with the most significant 16 bits of the magic cookie, and 1422 then the converting the result to network byte order. If the IP 1423 address family is IPv4, X-Address is computed by taking the mapped IP 1424 address in host byte order, XOR'ing it with the magic cookie, and 1425 converting the result to network byte order. If the IP address 1426 family is IPv6, X-Address is computed by taking the mapped IP address 1427 in host byte order, XOR'ing it with the magic cookie and the 96-bit 1428 transaction ID, and converting the result to network byte order. 1430 The rules for encoding and processing the first 8 bits of the 1431 attribute's value, the rules for handling multiple occurrences of the 1432 attribute, and the rules for processing addresses families are the 1433 same as for MAPPED-ADDRESS. 1435 NOTE: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their 1436 encoding of the transport address. The former encodes the transport 1437 address by exclusive-or'ing it with the magic cookie. The latter 1438 encodes it directly in binary. RFC 3489 originally specified only 1439 MAPPED-ADDRESS. However, deployment experience found that some NATs 1440 rewrite the 32-bit binary payloads containing the NAT's public IP 1441 address, such as STUN's MAPPED-ADDRESS attribute, in the well-meaning 1442 but misguided attempt at providing a generic ALG function. Such 1443 behavior interferes with the operation of STUN and also causes 1444 failure of STUN's message integrity checking. 1446 14.3. USERNAME 1448 The USERNAME attribute is used for message integrity. It identifies 1449 the username and password combination used in the message integrity 1450 check. 1452 The value of USERNAME is a variable length value. It MUST contain a 1453 UTF-8 encoded sequence of less than 513 bytes. 1455 14.4. MESSAGE-INTEGRITY 1457 The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [RFC2104] of 1458 the STUN message. The MESSAGE-INTEGRITY attribute can be present in 1459 any STUN message type. Since it uses the SHA1 hash, the HMAC will be 1460 20 bytes. The text used as input to HMAC is the STUN message, 1461 including the header, up to and including the attribute preceding the 1462 MESSAGE-INTEGRITY attribute. With the exception of the FINGERPRINT 1463 attribute, which appears after MESSAGE-INTEGRITY, agents MUST ignore 1464 all other attributes that follow MESSAGE-INTEGRITY. 1466 The key used as input to HMAC is the password. 1468 Based on the rules above, the hash includes the length field from the 1469 STUN message header. This length indicates the length of the entire 1470 message, including the MESSAGE-INTEGRITY attribute itself. 1471 Consequently, the MESSAGE-INTEGRITY attribute MUST be inserted into 1472 the message (with dummy content) prior to the computation of the 1473 integrity check. Once the computation is performed, the value of the 1474 attribute can be filled in. This ensures the length has the correct 1475 value when the hash is performed. Similarly, when validating the 1476 MESSAGE-INTEGRITY, the length field should be adjusted to point to 1477 the end of the MESSAGE-INTEGRITY attribute prior to calculating the 1478 HMAC. Such adjustment is necessary when attributes, such as 1479 FINGERPRINT, appear after MESSAGE-INTEGRITY. 1481 14.5. FINGERPRINT 1483 The FINGERPRINT attribute may be present in all STUN messages. The 1484 value of the attribute is computed as the CRC-32 of the STUN message 1485 up to (but excluding) the FINGERPRINT attribute itself, xor-d with 1486 the 32 bit value 0x5354554e (the XOR helps in cases where an 1487 application packet is also using CRC-32 in it). The 32 bit CRC is 1488 the one defined in ITU V.42 [ITU.V42.1994], which has a generator 1489 polynomial of x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1. 1490 When present, the FINGERPRINT attribute MUST be the last attribute in 1491 the message, and thus will appear after MESSAGE-INTEGRITY. 1493 The FINGERPRINT attribute can aid in distinguishing STUN packets from 1494 packets of other protocols. See Section 8. 1496 As with MESSAGE-INTEGRITY, the CRC used in the FINGERPRINT attribute 1497 covers the length field from the STUN message header. Therefore, 1498 this value must be correct, and include the CRC attribute as part of 1499 the message length, prior to computation of the CRC. When using the 1500 FINGERPRINT attribute in a message, the attribute is first placed 1501 into the message with a dummy value, then the CRC is computed, and 1502 then the value of the attribute is updated. If the MESSAGE-INTEGRITY 1503 attribute is also present, then it must be present with the correct 1504 message-integrity value before the CRC is computed, since the CRC is 1505 done over the value of the MESSAGE-INTEGRITY attribute as well. 1507 14.6. ERROR-CODE 1509 The ERROR-CODE attribute is used in Error Response messages. It 1510 contains a numeric error code value in the range of 300 to 699 plus a 1511 textual reason phrase encoded in UTF-8, and is consistent in its code 1512 assignments and semantics with SIP [RFC3261] and HTTP [RFC2616]. The 1513 reason phrase is meant for user consumption, and can be anything 1514 appropriate for the error code. Recommended reason phrases for the 1515 defined error codes are presented below. The reason phrase MUST be a 1516 UTF-8 encoded sequence of less than 128 characters (which can be as 1517 long as 763 bytes). 1519 To facilitate processing, the class of the error code (the hundreds 1520 digit) is encoded separately from the rest of the code. 1522 0 1 2 3 1523 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 1524 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1525 | Reserved, should be 0 |Class| Number | 1526 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1527 | Reason Phrase (variable) .. 1528 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1530 The Reserved bits SHOULD be 0, and are for alignment on 32-bit 1531 boundaries. Receivers MUST ignore these bits. The Class represents 1532 the hundreds digit of the error code. The value MUST be between 3 1533 and 6. The number represents the error code modulo 100, and its 1534 value MUST be between 0 and 99. 1536 The following error codes, along with their recommended reason 1537 phrases (in brackets) are defined: 1539 300 Try Alternate: The client should contact an alternate server for 1540 this request. This error response MUST only be sent if the 1541 request included a USERNAME attribute and a valid MESSAGE- 1542 INTEGRITY attribute; otherwise it MUST NOT be sent and error 1543 code 400 (Bad Request) is suggested. This error response MUST 1544 be protected with the MESSAGE-INTEGRITY attribute, and receivers 1545 MUST validate the MESSAGE-INTEGRITY of this response before 1546 redirecting themselves to an alternate server. 1548 Note: failure to generate and validate message-integrity 1549 for a 300 response allows an on-path attacker to falsify a 1550 300 response thus causing subsequent STUN messages to be 1551 sent to a victim. 1553 400 Bad Request: The request was malformed. The client SHOULD NOT 1554 retry the request without modification from the previous 1555 attempt. The server may not be able to generate a valid 1556 MESSAGE-INTEGRITY for this error, so the client MUST NOT expect 1557 a valid MESSAGE-INTEGRITY attribute on this response. 1559 401 Unauthorized: The request did not contain the expected MESSAGE- 1560 INTEGRITY attribute. The server MAY include the MESSAGE- 1561 INTEGRITY attribute in its error response. 1563 420 Unknown Attribute: The server received STUN packet containing a 1564 comprehension-required attribute which it did not understand. 1565 The server MUST put this unknown attribute in the UNKNOWN- 1566 ATTRIBUTE attribute of its error response. 1568 438 Stale Nonce: The NONCE used by the client was no longer valid. 1569 The client should retry, using the NONCE provided in the 1570 response. 1572 500 Server Error: The server has suffered a temporary error. The 1573 client should try again. 1575 14.7. REALM 1577 The REALM attribute may be present in requests and responses. It 1578 contains text which meets the grammar for "realm-value" as described 1579 in RFC 3261 [RFC3261] but without the double quotes and their 1580 surrounding whitespace. That is, it is an unquoted realm-value. It 1581 MUST be a UTF-8 encoded sequence of less than 128 characters (which 1582 can be as long as 763 bytes). 1584 Presence of the REALM attribute in a request indicates that long-term 1585 credentials are being used for authentication. Presence in certain 1586 error responses indicates that the server wishes the client to use a 1587 long-term credential for authentication. 1589 14.8. NONCE 1591 The NONCE attribute may be present in requests and responses. It 1592 contains a sequence of qdtext or quoted-pair, which are defined in 1593 RFC 3261 [RFC3261]. See RFC 2617 [RFC2617], Section 4.3, for 1594 guidance on selection of nonce values in a server. It MUST be less 1595 than 128 characters (which can be as long as 763 bytes). 1597 14.9. UNKNOWN-ATTRIBUTES 1599 The UNKNOWN-ATTRIBUTES attribute is present only in an error response 1600 when the response code in the ERROR-CODE attribute is 420. 1602 The attribute contains a list of 16 bit values, each of which 1603 represents an attribute type that was not understood by the server. 1605 0 1 2 3 1606 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 1607 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1608 | Attribute 1 Type | Attribute 2 Type | 1609 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1610 | Attribute 3 Type | Attribute 4 Type ... 1611 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1613 Figure 11: Format of UNKNOWN-ATTRIBUTES attribute 1615 Note: In [RFC3489], this field was padded to 32 by duplicating the 1616 last attribute. In this version of the specification, the normal 1617 padding rules for attributes are used instead. 1619 14.10. SERVER 1621 The server attribute contains a textual description of the software 1622 being used by the server, including manufacturer and version number. 1623 The attribute has no impact on operation of the protocol, and serves 1624 only as a tool for diagnostic and debugging purposes. The value of 1625 SERVER is variable length. It MUST be a UTF-8 encoded sequence of 1626 less than 128 characters (which can be as long as 763 bytes). 1628 14.11. ALTERNATE-SERVER 1630 The alternate server represents an alternate transport address 1631 identifying a different STUN server which the STUN client should try. 1633 It is encoded in the same way as MAPPED-ADDRESS, and thus refers to a 1634 single server by IP address. The IP address family MUST be identical 1635 to that of the source IP address of the request. 1637 This attribute MUST only appear in an error response that contains a 1638 MESSAGE-INTEGRITY attribute. This prevents it from being used in 1639 denial-of-service attacks. 1641 15. Security Considerations 1643 15.1. Attacks against the Protocol 1645 15.1.1. Outside Attacks 1647 An attacker can try to modify STUN messages in transit, in order to 1648 cause a failure in STUN operation. These attacks are detected for 1649 both requests and responses through the message integrity mechanism, 1650 using either a short term or long term credential. Of course, once 1651 detected, the manipulated packets will be dropped, causing the STUN 1652 transaction to effectively fail. This attack is possible only by an 1653 on-path attacker. 1655 An attacker that can observe, but not modify STUN messages in-transit 1656 (for example, an attacker present on a shared access medium, such as 1657 Wi-Fi), can see a STUN request, and then immediately send a STUN 1658 response, typically an error response, in order to disrupt STUN 1659 processing. This attack is also prevented for messages that utilize 1660 MESSAGE-INTEGRITY. However, some error responses, those related to 1661 authentication in particular, cannot be protected by MESSAGE- 1662 INTEGRITY. When STUN itself is run over a secure transport protocol 1663 (e.g., TLS), these attacks are completely mitigated. 1665 15.1.2. Inside Attacks 1667 A rogue client may try to launch a DoS attack against a server by 1668 sending it a large number of STUN requests. Fortunately, STUN 1669 requests can be processed statelessly by a server, making such 1670 attacks hard to launch. 1672 A rogue client may use a STUN server as a reflector, sending it 1673 requests with a falsified source IP address and port. In such a 1674 case, the response would be delivered to that source IP and port. 1675 There is no amplification with this attack, and it is mitigated by 1676 ingress source address filtering. 1678 15.2. Attacks Affecting the Usage 1680 This section lists attacks that might be launched against a usage of 1681 STUN. Each STUN usage must consider whether these attacks are 1682 applicable to it, and if so, discuss counter-measures. 1684 Most of the attacks in this section revolve around an attacker 1685 modifying the reflexive address learned by a STUN client through a 1686 Binding Request/Binding Response transaction. Since the usage of the 1687 reflexive address is a function of the usage, the applicability and 1688 remediation of these attacks is usage-specific. In common 1689 situations, modification of the reflexive address by an on-path 1690 attacker is easy to do. Consider, for example, the common situation 1691 where STUN is run directly over UDP. In this case, an on-path 1692 attacker can modify the source IP address of the Binding Request 1693 before it arrives at the STUN server. The STUN server will then 1694 return this IP address in the XOR-MAPPED-ADDRESS attribute to the 1695 client. Protecting against this attack by using a message-integrity 1696 check is impossible, since a message-integrity value cannot cover the 1697 source IP address, since the intervening NAT must be able to modify 1698 this value. Instead, one solution to preventing the attacks listed 1699 below is for the client to verify the reflexive address learned, as 1700 is done in ICE [I-D.ietf-mmusic-ice]. Other usages may use other 1701 means to prevent these attacks. 1703 15.2.1. Attack I: DDoS Against a Target 1705 In this attack, the attacker provides one or more clients with the 1706 same faked reflexive address that points to the intended target. 1707 This will trick the STUN clients into thinking that their reflexive 1708 addresses are equal to that of the target. If the clients hand out 1709 that reflexive address in order to receive traffic on it (for 1710 example, in SIP messages), the traffic will instead be sent to the 1711 target. This attack can provide substantial amplification, 1712 especially when used with clients that are using STUN to enable 1713 multimedia applications. 1715 15.2.2. Attack II: Silencing a Client 1717 In this attack, the attacker provides a STUN client with a faked 1718 reflexive address. The reflexive address it provides is a transport 1719 address that routes to nowhere. As a result, the client won't 1720 receive any of the packets it expects to receive when it hands out 1721 the reflexive address. This exploitation is not very interesting for 1722 the attacker. It impacts a single client, which is frequently not 1723 the desired target. Moreover, any attacker that can mount the attack 1724 could also deny service to the client by other means, such as 1725 preventing the client from receiving any response from the STUN 1726 server, or even a DHCP server. 1728 15.2.3. Attack III: Assuming the Identity of a Client 1730 This attack is similar to attack II. However, the faked reflexive 1731 address points to the attacker itself. This allows the attacker to 1732 receive traffic which was destined for the client. 1734 15.2.4. Attack IV: Eavesdropping 1736 In this attack, the attacker forces the client to use a reflexive 1737 address that routes to itself. It then forwards any packets it 1738 receives to the client. This attack would allow the attacker to 1739 observe all packets sent to the client. However, in order to launch 1740 the attack, the attacker must have already been able to observe 1741 packets from the client to the STUN server. In most cases (such as 1742 when the attack is launched from an access network), this means that 1743 the attacker could already observe packets sent to the client. This 1744 attack is, as a result, only useful for observing traffic by 1745 attackers on the path from the client to the STUN server, but not 1746 generally on the path of packets being routed towards the client. 1748 15.3. Hash Agility Plan 1750 This specification uses SHA-1 for computation of the message 1751 integrity. If, at a later time, SHA-1 is found to be compromised, 1752 the following is the remedy that will be applied. 1754 We will define a STUN extension which introduces a new message 1755 integrity attribute, computed using a new hash. Clients would be 1756 required to include both the new and old message integrity attributes 1757 in their requests or indications. A new server will utilize the new 1758 message integrity attribute, and an old one, the old. After a 1759 transition period where mixed implementations are in deployment, the 1760 old message-integrity attribute will be deprecated by another 1761 specification, and clients will cease including it in requests. 1763 16. IAB Considerations 1765 The IAB has studied the problem of "Unilateral Self Address Fixing" 1766 (UNSAF), which is the general process by which a client attempts to 1767 determine its address in another realm on the other side of a NAT 1768 through a collaborative protocol reflection mechanism (RFC3424 1769 [RFC3424]). STUN can be used to perform this function using a 1770 Binding Request/Response transaction if one agent is behind a NAT and 1771 the other is on the public side of the NAT. 1773 The IAB has mandated that protocols developed for this purpose 1774 document a specific set of considerations. Because some STUN usages 1775 provide UNSAF functions (such as ICE [I-D.ietf-mmusic-ice] ), and 1776 others do not (such as SIP Outbound [I-D.ietf-sip-outbound]), answers 1777 to these considerations need to be addressed by the usages 1778 themselves. 1780 17. IANA Considerations 1782 IANA is hereby requested to create three new registries: a STUN 1783 methods registry, a STUN Attributes registry, and a STUN Error Codes 1784 registry. 1786 17.1. STUN Methods Registry 1788 A STUN method is a hex number in the range 0x000 - 0x3FF. The 1789 encoding of STUN method into a STUN message is described in 1790 Section 6. 1792 The initial STUN methods are: 1794 0x000: (Reserved) 1795 0x001: Binding 1796 0x002: (Reserved; was SharedSecret) 1798 STUN methods in the range 0x000 - 0x1FF are assigned by IETF 1799 Consensus [RFC2434]. STUN methods in the range 0x200 - 0x3FF are 1800 assigned on a First Come First Served basis [RFC2434] 1802 17.2. STUN Attribute Registry 1804 A STUN Attribute type is a hex number in the range 0x0000 - 0xFFFF. 1805 STUN attribute types in the range 0x0000 - 0x7FFF are considered 1806 comprehension-required; STUN attribute types in the range 0x8000 - 1807 0xFFFF are considered comprehension-optional. A STUN agent handles 1808 unknown comprehension-required and comprehension-optional attributes 1809 differently. 1811 The initial STUN Attributes types are: 1813 Comprehension-required range (0x0000-0x7FFF): 1814 0x0000: (Reserved) 1815 0x0001: MAPPED-ADDRESS 1816 0x0006: USERNAME 1817 0x0007: (Reserved; was PASSWORD) 1818 0x0008: MESSAGE-INTEGRITY 1819 0x0009: ERROR-CODE 1820 0x000A: UNKNOWN-ATTRIBUTES 1821 0x0014: REALM 1822 0x0015: NONCE 1823 0x0020: XOR-MAPPED-ADDRESS 1825 Comprehension-optional range (0x8000-0xFFFF) 1826 0x8022: SERVER 1827 0x8023: ALTERNATE-SERVER 1828 0x8028: FINGERPRINT 1830 STUN Attribute types in the first half of the comprehension-required 1831 range (0x0000 - 0x3FFF) and in the first half of the comprehension- 1832 optional range (0x8000 - 0xBFFF) are assigned by IETF Consensus 1833 [RFC2434]. STUN Attribute types in the second half of the 1834 comprehension-required range (0x4000 - 0x7FFF) and in the second half 1835 of the comprehension-optional range (0xC000 - 0xFFFF) are assigned on 1836 a First Come First Served basis [RFC2434]. 1838 17.3. STUN Error Code Registry 1840 A STUN Error code is a number in the range 0 - 699. STUN error codes 1841 are accompanied by a textual reason phrase in UTF-8 which is intended 1842 only for human consumption and can be anything appropriate; this 1843 document proposes only suggested values. 1845 STUN error codes are consistent in codepoint assignments and 1846 semantics with SIP [RFC3261] and HTTP [RFC2616]. 1848 The initial values in this registry are given in Section 14.6. 1850 New STUN error codes are assigned on a Specification-Required basis 1851 [RFC2434]. The specification must carefully consider how clients 1852 that do not understand this error code will process it before 1853 granting the request. See the rules in Section 7.3.4. 1855 18. Changes Since RFC 3489 1857 This specification obsoletes RFC3489 [RFC3489]. This specification 1858 differs from RFC3489 in the following ways: 1860 o Removed the notion that STUN is a complete NAT traversal solution. 1861 STUN is now a tool that can be used to produce a NAT traversal 1862 solution. As a consequence, changed the name of the protocol to 1863 Session Traversal Utilities for NAT. 1865 o Introduced the concept of STUN usages, and described what a usage 1866 of STUN must document. 1868 o Removed the usage of STUN for NAT type detection and binding 1869 lifetime discovery. These techniques have proven overly brittle 1870 due to wider variations in the types of NAT devices than described 1871 in this document. Removed the RESPONSE-ADDRESS, CHANGED-ADDRESS, 1872 CHANGE-REQUEST, SOURCE-ADDRESS, and REFLECTED-FROM attributes. 1874 o Added a fixed 32-bit magic cookie and reduced length of 1875 transaction ID by 32 bits. The magic cookie begins at the same 1876 offset as the original transaction ID. 1878 o Added the XOR-MAPPED-ADDRESS attribute, which is included in 1879 Binding Responses if the magic cookie is present in the request. 1880 Otherwise the RFC3489 behavior is retained (that is, Binding 1881 Response includes MAPPED-ADDRESS). See discussion in XOR-MAPPED- 1882 ADDRESS regarding this change. 1884 o Introduced formal structure into the Message Type header field, 1885 with an explicit pair of bits for indication of request, response, 1886 error response or indication. Consequently, the message type 1887 field is split into the class (one of the previous four) and 1888 method. 1890 o Explicitly point out that the most significant two bits of STUN 1891 are 0b00, allowing easy differentiation with RTP packets when used 1892 with ICE. 1894 o Added the FINGERPRINT attribute to provide a method of definitely 1895 detecting the difference between STUN and another protocol when 1896 the two protocols are multiplexed together. 1898 o Added support for IPv6. Made it clear that an IPv4 client could 1899 get a v6 mapped address, and vice-a-versa. 1901 o Added long-term credential-based authentication. 1903 o Added the SERVER, REALM, NONCE, and ALTERNATE-SERVER attributes. 1905 o Removed the SharedSecret method, and thus the PASSWORD attribute. 1906 This method was almost never implemented and is not needed with 1907 current usages. 1909 o Removed recommendation to continue listening for STUN Responses 1910 for 10 seconds in an attempt to recognize an attack. 1912 o Changed transaction timers to be more TCP friendly. 1914 o Removed the STUN example that centered around the separation of 1915 the control and media planes. Instead, provided more information 1916 on using STUN with protocols. 1918 o Defined a generic padding mechanism that changes the 1919 interpretation of the length attribute. This would, in theory, 1920 break backwards compatibility. However, the mechanism in RFC 3489 1921 never worked for the few attributes that weren't aligned naturally 1922 on 32 bit boundaries. 1924 o REALM, SERVER, reason phrases and NONCE limited to 127 characters. 1925 USERNAME to 513 bytes. 1927 19. Acknowledgements 1929 The authors would like to thank Cedric Aoun, Pete Cordell, Cullen 1930 Jennings, Bob Penfield, Xavier Marjou, Bruce Lowekamp and Chris 1931 Sullivan for their comments, and Baruch Sterman and Alan Hawrylyshen 1932 for initial implementations. Thanks for Leslie Daigle, Allison 1933 Mankin, Eric Rescorla, and Henning Schulzrinne for IESG and IAB input 1934 on this work. 1936 20. References 1938 20.1. Normative References 1940 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1941 Requirement Levels", BCP 14, RFC 2119, March 1997. 1943 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 1944 September 1981. 1946 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 1947 specifying the location of services (DNS SRV)", RFC 2782, 1948 February 2000. 1950 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. 1952 [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., 1953 Leach, P., Luotonen, A., and L. Stewart, "HTTP 1954 Authentication: Basic and Digest Access Authentication", 1955 RFC 2617, June 1999. 1957 [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission 1958 Timer", RFC 2988, November 2000. 1960 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 1961 Hashing for Message Authentication", RFC 2104, 1962 February 1997. 1964 [ITU.V42.1994] 1965 International Telecommunications Union, "Error-correcting 1966 Procedures for DCEs Using Asynchronous-to-Synchronous 1967 Conversion", ITU-T Recommendation V.42, 1994. 1969 20.2. Informational References 1971 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1972 A., Peterson, J., Sparks, R., Handley, M., and E. 1973 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1974 June 2002. 1976 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1977 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1978 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 1980 [I-D.ietf-mmusic-ice] 1981 Rosenberg, J., "Interactive Connectivity Establishment 1982 (ICE): A Protocol for Network Address Translator (NAT) 1983 Traversal for Offer/Answer Protocols", 1984 draft-ietf-mmusic-ice-17 (work in progress), July 2007. 1986 [RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, 1987 "STUN - Simple Traversal of User Datagram Protocol (UDP) 1988 Through Network Address Translators (NATs)", RFC 3489, 1989 March 2003. 1991 [I-D.ietf-behave-turn] 1992 Rosenberg, J., "Traversal Using Relays around NAT (TURN): 1993 Relay Extensions to Session Traversal Utilities for NAT 1994 (STUN)", draft-ietf-behave-turn-04 (work in progress), 1995 July 2007. 1997 [I-D.ietf-sip-outbound] 1998 Jennings, C. and R. Mahy, "Managing Client Initiated 1999 Connections in the Session Initiation Protocol (SIP)", 2000 draft-ietf-sip-outbound-10 (work in progress), July 2007. 2002 [I-D.ietf-behave-nat-behavior-discovery] 2003 MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery 2004 Using STUN", draft-ietf-behave-nat-behavior-discovery-01 2005 (work in progress), July 2007. 2007 [I-D.ietf-mmusic-ice-tcp] 2008 Rosenberg, J., "TCP Candidates with Interactive 2009 Connectivity Establishment (ICE", 2010 draft-ietf-mmusic-ice-tcp-04 (work in progress), 2011 July 2007. 2013 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 2014 with Session Description Protocol (SDP)", RFC 3264, 2015 June 2002. 2017 [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral 2018 Self-Address Fixing (UNSAF) Across Network Address 2019 Translation", RFC 3424, November 2002. 2021 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an 2022 IANA Considerations Section in RFCs", BCP 26, RFC 2434, 2023 October 1998. 2025 Appendix A. C Snippet to Determine STUN Message Types 2027 Given an 16-bit STUN message type value in host byte order in 2028 msg_type parameter, below are C macros to determine the STUN message 2029 types: 2031 #define IS_REQUEST(msg_type) (((msg_type) & 0x0110) == 0x0000) 2032 #define IS_INDICATION(msg_type) (((msg_type) & 0x0110) == 0x0010) 2033 #define IS_SUCCESS_RESP(msg_type) (((msg_type) & 0x0110) == 0x0100) 2034 #define IS_ERR_RESP(msg_type) (((msg_type) & 0x0110) == 0x0110) 2036 Authors' Addresses 2038 Jonathan Rosenberg 2039 Cisco 2040 Edison, NJ 2041 US 2043 Email: jdrosen@cisco.com 2044 URI: http://www.jdrosen.net 2046 Christian Huitema 2047 Microsoft 2048 One Microsoft Way 2049 Redmond, WA 98052 2050 US 2052 Email: huitema@microsoft.com 2054 Rohan Mahy 2055 Plantronics 2056 345 Encinal Street 2057 Santa Cruz, CA 95060 2058 US 2060 Email: rohan@ekabal.com 2062 Philip Matthews 2063 Avaya 2064 1135 Innovation Drive 2065 Ottawa, Ontario K2K 3G7 2066 Canada 2068 Phone: +1 613 592 4343 x224 2069 Fax: 2070 Email: philip_matthews@magma.ca 2071 URI: 2073 Dan Wing 2074 Cisco 2075 771 Alder Drive 2076 San Jose, CA 95035 2077 US 2079 Email: dwing@cisco.com 2081 Full Copyright Statement 2083 Copyright (C) The IETF Trust (2007). 2085 This document is subject to the rights, licenses and restrictions 2086 contained in BCP 78, and except as set forth therein, the authors 2087 retain all their rights. 2089 This document and the information contained herein are provided on an 2090 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 2091 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 2092 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 2093 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 2094 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 2095 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 2097 Intellectual Property 2099 The IETF takes no position regarding the validity or scope of any 2100 Intellectual Property Rights or other rights that might be claimed to 2101 pertain to the implementation or use of the technology described in 2102 this document or the extent to which any license under such rights 2103 might or might not be available; nor does it represent that it has 2104 made any independent effort to identify any such rights. 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