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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Unused Reference: 'RFC4472' is defined on line 899, but no explicit reference was found in the text == Unused Reference: 'RFC8415' is defined on line 959, but no explicit reference was found in the text ** Obsolete normative reference: RFC 4941 (Obsoleted by RFC 8981) == Outdated reference: A later version (-15) exists of draft-ietf-mboned-ieee802-mcast-problems-11 Summary: 2 errors (**), 0 flaws (~~), 5 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IPv6 Maintenance (6man) Working Group F. Gont 3 Internet-Draft SI6 Networks / UTN-FRH 4 Obsoletes: rfc4941 (if approved) S. Krishnan 5 Intended status: Standards Track Ericsson Research 6 Expires: August 12, 2020 T. Narten 7 IBM Corporation 8 R. Draves 9 Microsoft Research 10 February 9, 2020 12 Privacy Extensions for Stateless Address Autoconfiguration in IPv6 13 draft-ietf-6man-rfc4941bis-06 15 Abstract 17 Nodes use IPv6 stateless address autoconfiguration to generate 18 addresses using a combination of locally available information and 19 information advertised by routers. Addresses are formed by combining 20 network prefixes with an interface identifier. This document 21 describes an extension that causes nodes to generate global scope 22 addresses with randomized interface identifiers that change over 23 time. Changing global scope addresses over time makes it more 24 difficult for eavesdroppers and other information collectors to 25 identify when different addresses used in different transactions 26 correspond to the same node. This document obsoletes RFC4941. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at https://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on August 12, 2020. 45 Copyright Notice 47 Copyright (c) 2020 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (https://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 63 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 64 1.2. Problem Statement . . . . . . . . . . . . . . . . . . . . 4 65 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 66 2.1. Extended Use of the Same Identifier . . . . . . . . . . . 4 67 2.2. Possible Approaches . . . . . . . . . . . . . . . . . . . 6 68 3. Protocol Description . . . . . . . . . . . . . . . . . . . . 6 69 3.1. Design Guidelines . . . . . . . . . . . . . . . . . . . . 6 70 3.2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 7 71 3.3. Generation of Randomized Interface Identifiers . . . . . 8 72 3.3.1. Simple Randomized Interface Identifiers . . . . . . . 8 73 3.3.2. Hash-based Generation of Randomized Interface 74 Identifiers . . . . . . . . . . . . . . . . . . . . . 9 75 3.4. Generating Temporary Addresses . . . . . . . . . . . . . 10 76 3.5. Expiration of Temporary Addresses . . . . . . . . . . . . 12 77 3.6. Regeneration of Temporary Addresses . . . . . . . . . . . 12 78 3.7. Implementation Considerations . . . . . . . . . . . . . . 14 79 3.8. Defined Constants . . . . . . . . . . . . . . . . . . . . 14 80 4. Implications of Changing Interface Identifiers . . . . . . . 15 81 5. Significant Changes from RFC4941 . . . . . . . . . . . . . . 16 82 6. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 16 83 7. Security Considerations . . . . . . . . . . . . . . . . . . . 16 84 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 85 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 86 9.1. Normative References . . . . . . . . . . . . . . . . . . 17 87 9.2. Informative References . . . . . . . . . . . . . . . . . 19 88 Appendix A. Changes from RFC4941 [to be removed by the RFC- 89 Editor before publication . . . . . . . . . . . . . 21 90 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 92 1. Introduction 94 Stateless address autoconfiguration (SLAAC) [RFC4862] defines how an 95 IPv6 node generates addresses without the need for a Dynamic Host 96 Configuration Protocol for IPv6 (DHCPv6) server. The security and 97 privacy implications of such addresses have been discussed in great 98 detail in [RFC7721],[RFC7217], and RFC7707. This document specifies 99 an extension for SLAAC to generate temporary addresses, such that the 100 aforementioned issues are mitigated. This is a revision of RFC4941, 101 and formally obsoletes RFC4941. Section 5 describes the changes from 102 [RFC4941]. 104 The default address selection for IPv6 has been specified in 105 [RFC6724]. The determination as to whether to use stable versus 106 temporary addresses can in some cases only be made by an application. 107 For example, some applications may always want to use temporary 108 addresses, while others may want to use them only in some 109 circumstances or not at all. An API such as that specified in 110 [RFC5014] can enable individual applications to indicate a preference 111 for the use of temporary addresses. 113 Section 2 provides background information on the issue. Section 3 114 describes a procedure for generating temporary interface identifiers 115 and global scope addresses. Section 4 discusses implications of 116 changing interface identifiers. Section 5 describes the changes from 117 [RFC4941]. 119 1.1. Terminology 121 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 122 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 123 "OPTIONAL" in this document are to be interpreted as described in BCP 124 14 [RFC2119] [RFC8174] when, and only when, they appear in all 125 capitals, as shown here. 127 The terms "public address", "stable address", "temporary address", 128 "constant IID", "stable IID", and "temporary IID" are to be 129 interpreted as specified in [RFC7721]. 131 The term "global scope addresses" is used in this document to 132 collectively refer to "Global unicast addresses" as defined in 133 [RFC4291] and "Unique local addresses" as defined in [RFC4193], and 134 not to "globally reachable" as defined in [RFC8190]. 136 1.2. Problem Statement 138 Addresses generated using stateless address autoconfiguration 139 [RFC4862] contain an embedded interface identifier, which may remain 140 stable over time. Anytime a fixed identifier is used in multiple 141 contexts, it becomes possible to correlate seemingly unrelated 142 activity using this identifier. 144 The correlation can be performed by 146 o An attacker who is in the path between the node in question and 147 the peer(s) to which it is communicating, and who can view the 148 IPv6 addresses present in the datagrams. 150 o An attacker who can access the communication logs of the peers 151 with which the node has communicated. 153 Since the identifier is embedded within the IPv6 address, it cannot 154 be hidden. This document proposes a solution to this issue by 155 generating interface identifiers that vary over time. 157 Note that an attacker, who is on path, may be able to perform 158 significant correlation on unencrypted packets based on 160 o The payload contents of the packets on the wire 162 o The characteristics of the packets such as packet size and timing 164 Use of temporary addresses will not prevent such payload-based 165 correlation, which can only be addressed by widespread deployment of 166 encryption as advocated in [RFC7624]. Nor will it prevent an on-link 167 observer (e.g. the node's default router) to track all the node's 168 addresses. 170 2. Background 172 This section discusses the problem in more detail, and provides 173 context for evaluating the significance of the concerns in specific 174 environments and makes comparisons with existing practices. 176 2.1. Extended Use of the Same Identifier 178 The use of a non-changing interface identifier to form addresses is a 179 specific instance of the more general case where a constant 180 identifier is reused over an extended period of time and in multiple 181 independent activities. Any time the same identifier is used in 182 multiple contexts, it becomes possible for that identifier to be used 183 to correlate seemingly unrelated activity. For example, a network 184 sniffer placed strategically on a link across which all traffic to/ 185 from a particular host crosses could keep track of which destinations 186 a node communicated with and at what times. Such information can in 187 some cases be used to infer things, such as what hours an employee 188 was active, when someone is at home, etc. Although it might appear 189 that changing an address regularly in such environments would be 190 desirable to lessen privacy concerns, it should be noted that the 191 network prefix portion of an address also serves as a constant 192 identifier. All nodes at, say, a home, would have the same network 193 prefix, which identifies the topological location of those nodes. 194 This has implications for privacy, though not at the same granularity 195 as the concern that this document addresses. Specifically, all nodes 196 within a home could be grouped together for the purposes of 197 collecting information. If the network contains a very small number 198 of nodes, say, just one, changing just the interface identifier will 199 not enhance privacy, since the prefix serves as a constant 200 identifier. 202 One of the requirements for correlating seemingly unrelated 203 activities is the use (and reuse) of an identifier that is 204 recognizable over time within different contexts. IP addresses 205 provide one obvious example, but there are more. Many nodes also 206 have DNS names associated with their addresses, in which case the DNS 207 name serves as a similar identifier. Although the DNS name 208 associated with an address is more work to obtain (it may require a 209 DNS query), the information is often readily available. In such 210 cases, changing the address on a machine over time would do little to 211 address the concerns raised in this document, unless the DNS name is 212 changed as well (see Section 4). 214 Web browsers and servers typically exchange "cookies" with each other 215 [RFC6265]. Cookies allow web servers to correlate a current activity 216 with a previous activity. One common usage is to send back targeted 217 advertising to a user by using the cookie supplied by the browser to 218 identify what earlier queries had been made (e.g., for what type of 219 information). Based on the earlier queries, advertisements can be 220 targeted to match the (assumed) interests of the end-user. 222 The use of a constant identifier within an address is of special 223 concern because addresses are a fundamental requirement of 224 communication and cannot easily be hidden from eavesdroppers and 225 other parties. Even when higher layers encrypt their payloads, 226 addresses in packet headers appear in the clear. Consequently, if a 227 mobile host (e.g., laptop) accessed the network from several 228 different locations, an eavesdropper might be able to track the 229 movement of that mobile host from place to place, even if the upper 230 layer payloads were encrypted. 232 The security and privacy implications of IPv6 addresses are discussed 233 in detail in [RFC7721], [RFC7707], and [RFC7217]. 235 Using temporary addresses alone is not sufficient to prevent all 236 forms of tracking. It is however clear that temporary addresses are 237 useful to improve user privacy. 239 2.2. Possible Approaches 241 One approach, compatible with the stateless address autoconfiguration 242 architecture, would be to change the interface identifier portion of 243 an address over time. Changing the interface identifier can make it 244 more difficult to look at the IP addresses in independent 245 transactions and identify which ones actually correspond to the same 246 node, both in the case where the routing prefix portion of an address 247 changes and when it does not. 249 Many machines function as both clients and servers. In such cases, 250 the machine would need a DNS name for its use as a server. Whether 251 the address stays fixed or changes has little privacy implication 252 since the DNS name remains constant and serves as a constant 253 identifier. When acting as a client (e.g., initiating 254 communication), however, such a machine may want to vary the 255 addresses it uses. In such environments, one may need multiple 256 addresses: a stable address registered in the DNS, that is used to 257 accept incoming connection requests from other machines, and a 258 temporary address used to shield the identity of the client when it 259 initiates communication. 261 On the other hand, a machine that functions only as a client may want 262 to employ only temporary addresses for public communication. 264 To make it difficult to make educated guesses as to whether two 265 different interface identifiers belong to the same node, the 266 algorithm for generating alternate identifiers must include input 267 that has an unpredictable component from the perspective of the 268 outside entities that are collecting information. 270 3. Protocol Description 272 The following subsections define the procedures for the generation of 273 IPv6 temporary addresses. 275 3.1. Design Guidelines 277 Temporary addresses observe the following properties: 279 1. Temporary addresses are typically employed for initiating 280 outgoing sessions. 282 2. Temporary addresses are used for a short period of time 283 (typically hours to days) and are subsequently deprecated. 284 Deprecated addresses can continue to be used for established 285 connections, but are not used to initiate new connections. 287 3. New temporary addresses are generated periodically to replace 288 temporary addresses that expire. 290 4. Temporary addresses must have a limited lifetime (limited "valid 291 lifetime" and "preferred lifetime" from [RFC4862]), that should 292 be statistically different for different addresses. The lifetime 293 of an address should be further reduced when privacy-meaningful 294 events (such as a node attaching to a different network, or the 295 regeneration of a new randomized MAC address) takes place. 297 5. By default, one address is generated for each prefix advertised 298 by stateless address autoconfiguration. The resulting Interface 299 Identifiers must be statistically different when addresses are 300 configured for different prefixes. That is, when temporary 301 addresses are generated for different autoconfiguration prefixes 302 for the same network interface, the resulting Interface 303 Identifiers must be statistically different. This means that, 304 given two addresses that employ different prefixes, it must be 305 difficult for an outside entity to tell whether the addresses 306 correspond to the same network interface or even whether they 307 have been generated by the same host. 309 6. It must be difficult for an outside entity to predict the 310 Interface Identifiers that will be employed for temporary 311 addresses, even with knowledge of the algorithm/method employed 312 to generate them and/or knowledge of the Interface Identifiers 313 previously employed for other temporary addresses. These 314 Interface Identifiers must be semantically opaque [RFC7136] and 315 must not follow any specific patterns. 317 3.2. Assumptions 319 The following algorithm assumes that for a given temporary address, 320 an implementation can determine the prefix from which it was 321 generated. When a temporary address is deprecated, a new temporary 322 address is generated. The specific valid and preferred lifetimes for 323 the new address are dependent on the corresponding lifetime values 324 set for the prefix from which it was generated. 326 Finally, this document assumes that when a node initiates outgoing 327 communication, temporary addresses can be given preference over 328 stable addresses (if available), when the device is configured to do 329 so. [RFC6724] mandates implementations to provide a mechanism, which 330 allows an application to configure its preference for temporary 331 addresses over stable addresses. It also allows for an 332 implementation to prefer temporary addresses by default, so that the 333 connections initiated by the node can use temporary addresses without 334 requiring application-specific enablement. This document also 335 assumes that an API will exist that allows individual applications to 336 indicate whether they prefer to use temporary or stable addresses and 337 override the system defaults (see e.g. [RFC5014]). 339 3.3. Generation of Randomized Interface Identifiers 341 The following subsections specify example algorithms for generating 342 temporary interface identifiers that follow the guidelines in 343 Section 3.1 of this document. The algorithm specified in 344 Section 3.3.1 benefits from a Pseudo-Random Number Generator (PRNG) 345 available on the system. The algorithm specified in Section 3.3.2 346 allows for code reuse by nodes that implement [RFC7217]. 348 3.3.1. Simple Randomized Interface Identifiers 350 One approach is to select a pseudorandom number of the appropriate 351 length. A node employing this algorithm should generate IIDs as 352 follows: 354 1. Obtain a random number (see [RFC4086] for randomness requirements 355 for security). 357 2. The Interface Identifier is obtained by taking as many bits from 358 the random number obtained in the previous step as necessary. 359 Note: there are no special bits in an Interface Identifier 360 [RFC7136]. 362 We note that [RFC4291] requires that the Interface IDs of all 363 unicast addresses (except those that start with the binary 364 value 000) be 64 bits long. However, the method discussed in 365 this document could be employed for generating Interface IDs 366 of any arbitrary length, albeit at the expense of reduced 367 entropy (when employing Interface IDs smaller than 64 bits). 368 The privacy implications of the IID length are discussed in 369 [RFC7421]. 371 3. The resulting Interface Identifier SHOULD be compared against the 372 reserved IPv6 Interface Identifiers [RFC5453] [IANA-RESERVED-IID] 373 and against those Interface Identifiers already employed in an 374 address of the same network interface and the same network 375 prefix. In the event that an unacceptable identifier has been 376 generated, a new interface identifier should be generated, by 377 repeating the algorithm from the first step. 379 3.3.2. Hash-based Generation of Randomized Interface Identifiers 381 The algorithm in [RFC7217] can be augmented for the generation of 382 temporary addresses. The benefit of this would be that a node could 383 employ a single algorithm for generating stable and temporary 384 addresses, by employing appropriate parameters. 386 Nodes would employ the following algorithm for generating the 387 temporary IID: 389 1. Compute a random identifier with the expression: 391 RID = F(Prefix, Net_Iface, Network_ID, Time, DAD_Counter, 392 secret_key) 394 Where: 396 RID: 397 Random Identifier 399 F(): 400 A pseudorandom function (PRF) that MUST NOT be computable from 401 the outside (without knowledge of the secret key). F() MUST 402 also be difficult to reverse, such that it resists attempts to 403 obtain the secret_key, even when given samples of the output 404 of F() and knowledge or control of the other input parameters. 405 F() SHOULD produce an output of at least 64 bits. F() could 406 be implemented as a cryptographic hash of the concatenation of 407 each of the function parameters. SHA-256 [FIPS-SHS] is one 408 possible option for F(). Note: MD5 [RFC1321] is considered 409 unacceptable for F() [RFC6151]. 411 Prefix: 412 The prefix to be used for SLAAC, as learned from an ICMPv6 413 Router Advertisement message. 415 Net_Iface: 416 The MAC address corresponding to the underlying network 417 interface card, in the case the link uses IEEE802 link-layer 418 identifiers. Employing the MAC address for this parameter 419 (over the other suggested options in RFC7217) means that the 420 re-generation of a randomized MAC address will result in a 421 different temporary address. 423 Network_ID: 424 Some network-specific data that identifies the subnet to which 425 this interface is attached -- for example, the IEEE 802.11 426 Service Set Identifier (SSID) corresponding to the network to 427 which this interface is associated. Additionally, Simple DNA 428 [RFC6059] describes ideas that could be leveraged to generate 429 a Network_ID parameter. This parameter is SHOULD be employed 430 if some form of "Network_ID" is available. 432 Time: 433 An implementation-dependent representation of time. One 434 possible example is the representation in UNIX-like systems 435 [OPEN-GROUP], that measure time in terms of the number of 436 seconds elapsed since the Epoch (00:00:00 Coordinated 437 Universal Time (UTC), 1 January 1970). The addition of the 438 "Time" argument results in (statistically) different interface 439 identifiers over time. 441 DAD_Counter: 442 A counter that is employed to resolve Duplicate Address 443 Detection (DAD) conflicts. 445 secret_key: 446 A secret key that is not known by the attacker. The secret 447 key SHOULD be of at least 128 bits. It MUST be initialized to 448 a pseudo-random number (see [RFC4086] for randomness 449 requirements for security) when the operating system is 450 "bootstrapped". 452 2. The Interface Identifier is finally obtained by taking as many 453 bits from the RID value (computed in the previous step) as 454 necessary, starting from the least significant bit. The 455 resulting Interface Identifier SHOULD be compared against the 456 reserved IPv6 Interface Identifiers [RFC5453] [IANA-RESERVED-IID] 457 and against those Interface Identifiers already employed in an 458 address of the same network interface and the same network 459 prefix. In the event that an unacceptable identifier has been 460 generated, the value DAD_Counter should be incremented by 1, and 461 the algorithm should be restarted from the first step. 463 3.4. Generating Temporary Addresses 465 [RFC4862] describes the steps for generating a link-local address 466 when an interface becomes enabled as well as the steps for generating 467 addresses for other scopes. This document extends [RFC4862] as 468 follows. When processing a Router Advertisement with a Prefix 469 Information option carrying a prefix for the purposes of address 470 autoconfiguration (i.e., the A bit is set), the node MUST perform the 471 following steps: 473 1. Process the Prefix Information Option as defined in [RFC4862], 474 adjusting the lifetimes of existing temporary addresses. If a 475 received option may extend the lifetimes of temporary addresses, 476 with the overall constraint that no temporary addresses should 477 ever remain "valid" or "preferred" for a time longer than 478 (TEMP_VALID_LIFETIME) or (TEMP_PREFERRED_LIFETIME - 479 DESYNC_FACTOR) respectively. The configuration variables 480 TEMP_VALID_LIFETIME and TEMP_PREFERRED_LIFETIME correspond to 481 approximate target lifetimes for temporary addresses. 483 2. One way an implementation can satisfy the above constraints is to 484 associate with each temporary address a creation time (called 485 CREATION_TIME) that indicates the time at which the address was 486 created. When updating the preferred lifetime of an existing 487 temporary address, it would be set to expire at whichever time is 488 earlier: the time indicated by the received lifetime or 489 (CREATION_TIME + TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR). A 490 similar approach can be used with the valid lifetime. 492 3. If the node has not configured any temporary address for the 493 corresponding prefix, the node SHOULD create a new temporary 494 address for such prefix. 496 Note: 497 For example, a host might implement prefix-specific policies 498 such as not configuring temporary addresses for the Unique 499 Local IPv6 Unicast Addresses (ULA) [RFC4193] prefix. 501 4. When creating a temporary address, the lifetime values MUST be 502 derived from the corresponding prefix as follows: 504 * Its Valid Lifetime is the lower of the Valid Lifetime of the 505 prefix and TEMP_VALID_LIFETIME 507 * Its Preferred Lifetime is the lower of the Preferred Lifetime 508 of the prefix and TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR. 510 5. A temporary address is created only if this calculated Preferred 511 Lifetime is greater than REGEN_ADVANCE time units. In 512 particular, an implementation MUST NOT create a temporary address 513 with a zero Preferred Lifetime. 515 6. New temporary addresses MUST be created by appending a randomized 516 interface identifier (generates as described in Section 3.3 of 517 this document) to the prefix that was received. 519 7. The node MUST perform duplicate address detection (DAD) on the 520 generated temporary address. If DAD indicates the address is 521 already in use, the node MUST generate a new randomized interface 522 identifier, and repeat the previous steps as appropriate up to 523 TEMP_IDGEN_RETRIES times. If after TEMP_IDGEN_RETRIES 524 consecutive attempts no non-unique address was generated, the 525 node MUST log a system error and MUST NOT attempt to generate 526 temporary addresses for that interface. This allows hosts to 527 recover from ocassional DAD failures, or otherwhise log the 528 recurrent address collissions. 530 3.5. Expiration of Temporary Addresses 532 When a temporary address becomes deprecated, a new one MUST be 533 generated. This is done by repeating the actions described in 534 Section 3.4, starting at step 4). Note that, except for the 535 transient period when a temporary address is being regenerated, in 536 normal operation at most one temporary address per prefix should be 537 in a non-deprecated state at any given time on a given interface. 538 Note that if a temporary address becomes deprecated as result of 539 processing a Prefix Information Option with a zero Preferred 540 Lifetime, then a new temporary address MUST NOT be generated. To 541 ensure that a preferred temporary address is always available, a new 542 temporary address SHOULD be regenerated slightly before its 543 predecessor is deprecated. This is to allow sufficient time to avoid 544 race conditions in the case where generating a new temporary address 545 is not instantaneous, such as when duplicate address detection must 546 be run. The node SHOULD start the address regeneration process 547 REGEN_ADVANCE time units before a temporary address would actually be 548 deprecated. 550 As an optional optimization, an implementation MAY remove a 551 deprecated temporary address that is not in use by applications or 552 upper layers as detailed in Section 6. 554 3.6. Regeneration of Temporary Addresses 556 The frequency at which temporary addresses change depends on how a 557 device is being used (e.g., how frequently it initiates new 558 communication) and the concerns of the end user. The most egregious 559 privacy concerns appear to involve addresses used for long periods of 560 time (weeks to months to years). The more frequently an address 561 changes, the less feasible collecting or coordinating information 562 keyed on interface identifiers becomes. Moreover, the cost of 563 collecting information and attempting to correlate it based on 564 interface identifiers will only be justified if enough addresses 565 contain non-changing identifiers to make it worthwhile. Thus, having 566 large numbers of clients change their address on a daily or weekly 567 basis is likely to be sufficient to alleviate most privacy concerns. 569 There are also client costs associated with having a large number of 570 addresses associated with a node (e.g., in doing address lookups, the 571 need to join many multicast groups, etc.). Thus, changing addresses 572 frequently (e.g., every few minutes) may have performance 573 implications. 575 Nodes following this specification SHOULD generate new temporary 576 addresses on a periodic basis. This can be achieved by generating a 577 new temporary address at least once every (TEMP_PREFERRED_LIFETIME - 578 REGEN_ADVANCE - DESYNC_FACTOR) time units. As described above, 579 generating a new temporary address REGEN_ADVANCE time units before a 580 temporary address becomes deprecated produces addresses with a 581 preferred lifetime no larger than TEMP_PREFERRED_LIFETIME. The value 582 DESYNC_FACTOR is a random value (different for each client) that 583 ensures that clients don't synchronize with each other and generate 584 new addresses at exactly the same time. When the preferred lifetime 585 expires, a new temporary address MUST be generated using the new 586 randomized interface identifier. 588 Because the precise frequency at which it is appropriate to generate 589 new addresses varies from one environment to another, implementations 590 SHOULD provide end users with the ability to change the frequency at 591 which addresses are regenerated. The default value is given in 592 TEMP_PREFERRED_LIFETIME and is one day. In addition, the exact time 593 at which to invalidate a temporary address depends on how 594 applications are used by end users. Thus, the suggested default 595 value of one week (TEMP_VALID_LIFETIME) may not be appropriate in all 596 environments. Implementations SHOULD provide end users with the 597 ability to override both of these default values. 599 Finally, when an interface connects to a new (different) link, a new 600 set of temporary addresses MUST be generated immediately for use on 601 the new link. If a device moves from one link to another, generating 602 a new set of temporary addresses ensures that the device uses 603 different randomized interface identifiers for the temporary 604 addresses associated with the two links, making it more difficult to 605 correlate addresses from the two different links as being from the 606 same node. The node MAY follow any process available to it, to 607 determine that the link change has occurred. One such process is 608 described by "Simple Procedures for Detecting Network Attachment in 609 IPv6" [RFC6059]. Detecting link changes would prevent link down/up 610 events from causing temporary addresses to be (unnecessarily) 611 regenerated. 613 3.7. Implementation Considerations 615 Devices implementing this specification MUST provide a way for the 616 end user to explicitly enable or disable the use of temporary 617 addresses. In addition, a site might wish to disable the use of 618 temporary addresses in order to simplify network debugging and 619 operations. Consequently, implementations SHOULD provide a way for 620 trusted system administrators to enable or disable the use of 621 temporary addresses. 623 Additionally, sites might wish to selectively enable or disable the 624 use of temporary addresses for some prefixes. For example, a site 625 might wish to disable temporary address generation for "Unique local" 626 [RFC4193] prefixes while still generating temporary addresses for all 627 other global prefixes. Another site might wish to enable temporary 628 address generation only for the prefixes 2001:db8:1::/48 and 629 2001:db8:2::/48 while disabling it for all other prefixes. To 630 support this behavior, implementations SHOULD provide a way to enable 631 and disable generation of temporary addresses for specific prefix 632 subranges. This per-prefix setting SHOULD override the global 633 settings on the node with respect to the specified prefix subranges. 634 Note that the per-prefix setting can be applied at any granularity, 635 and not necessarily on a per subnet basis. 637 Use of the extensions defined in this document may complicate 638 debugging and other operational troubleshooting activities. 639 Consequently, it may be site policy that temporary addresses should 640 not be used. Consequently, implementations MUST provide a method for 641 the end user or trusted administrator to override the use of 642 temporary addresses. 644 3.8. Defined Constants 646 Constants defined in this document include: 648 TEMP_VALID_LIFETIME -- Default value: 1 week. Users should be able 649 to override the default value. 651 TEMP_PREFERRED_LIFETIME -- Default value: 1 day. Users should be 652 able to override the default value. 654 REGEN_ADVANCE -- 5 seconds 656 MAX_DESYNC_FACTOR -- 10 minutes. Upper bound on DESYNC_FACTOR. 658 DESYNC_FACTOR -- A random value within the range 0 - 659 MAX_DESYNC_FACTOR. It is computed once at system start (rather than 660 each time it is used) and must never be greater than 661 (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE). 663 TEMP_IDGEN_RETRIES -- Default value: 3 665 4. Implications of Changing Interface Identifiers 667 The desires of protecting individual privacy versus the desire to 668 effectively maintain and debug a network can conflict with each 669 other. Having clients use addresses that change over time will make 670 it more difficult to track down and isolate operational problems. 671 For example, when looking at packet traces, it could become more 672 difficult to determine whether one is seeing behavior caused by a 673 single errant machine, or by a number of them. 675 Network deployments are currently recommended to provide multiple 676 IPv6 addresses from each prefix to general-purpose hosts [RFC7934]. 677 However, in some scenarios, use of a large number of IPv6 addresses 678 may have negative implications on network devices that need to 679 maintain entries for each IPv6 address in some data structures (e.g., 680 [RFC7039]). Additionally, concurrent active use of multiple IPv6 681 addresses will increase neighbour discovery traffic if Neighbour 682 Caches in network devices are not large enough to store all addresses 683 on the link. This can impact performance and energy efficiency on 684 networks on which multicast is expensive (e.g. 685 [I-D.ietf-mboned-ieee802-mcast-problems]). 687 The use of temporary addresses may cause unexpected difficulties with 688 some applications. For example, some servers refuse to accept 689 communications from clients for which they cannot map the IP address 690 into a DNS name. That is, they perform a DNS PTR query to determine 691 the DNS name, and may then also perform an AAAA query on the returned 692 name to verify that the returned DNS name maps back into the address 693 being used. Consequently, clients not properly registered in the DNS 694 may be unable to access some services. As noted earlier, however, a 695 node's DNS name (if non-changing) serves as a constant identifier. 696 The wide deployment of the extension described in this document could 697 challenge the practice of inverse-DNS-based "authentication," which 698 has little validity, though it is widely implemented. In order to 699 meet server challenges, nodes could register temporary addresses in 700 the DNS using random names (for example, a string version of the 701 random address itself). 703 In addition, some applications may not behave robustly if temporary 704 addresses are used and an address expires before the application has 705 terminated, or if it opens multiple sessions, but expects them to all 706 use the same addresses. 708 5. Significant Changes from RFC4941 710 This section summarizes the changes in this document relative to RFC 711 4941 that an implementer of RFC 4941 should be aware of. 713 Broadly speaking, this document introduces the following changes: 715 o Addresses a number of flaws in the algorithm for generating 716 temporary addresses (see [RAID2015] and [RFC7721]). 718 o Allows hosts to employ only temporary addresses (i.e. 719 configuration of stable addresses is no longer implied or 720 required). 722 o Suggests that temporary addresses be enabled by default (in line 723 with [RFC7258]). 725 o Addresses all errata submitted for [RFC4941]. 727 6. Future Work 729 An implementation might want to keep track of which addresses are 730 being used by upper layers so as to be able to remove a deprecated 731 temporary address from internal data structures once no upper layer 732 protocols are using it (but not before). This is in contrast to 733 current approaches where addresses are removed from an interface when 734 they become invalid [RFC4862], independent of whether or not upper 735 layer protocols are still using them. For TCP connections, such 736 information is available in control blocks. For UDP-based 737 applications, it may be the case that only the applications have 738 knowledge about what addresses are actually in use. Consequently, an 739 implementation generally will need to use heuristics in deciding when 740 an address is no longer in use. 742 7. Security Considerations 744 If a very small number of nodes (say, only one) use a given prefix 745 for extended periods of time, just changing the interface identifier 746 part of the address may not be sufficient to address-based network 747 activity correlation, since the prefix acts as a constant identifier. 748 The procedures described in this document are most effective when the 749 prefix is reasonably non static or is used by a fairly large number 750 of nodes. 752 While this document discusses ways of obscuring a user's IP address, 753 the method described is believed to be ineffective against 754 sophisticated forms of traffic analysis. To increase effectiveness, 755 one may need to consider the use of more advanced techniques, such as 756 Onion Routing [ONION]. 758 Ingress filtering has been and is being deployed as a means of 759 preventing the use of spoofed source addresses in Distributed Denial 760 of Service (DDoS) attacks. In a network with a large number of 761 nodes, new temporary addresses are created at a fairly high rate. 762 This might make it difficult for ingress filtering mechanisms to 763 distinguish between legitimately changing temporary addresses and 764 spoofed source addresses, which are "in-prefix" (using a 765 topologically correct prefix and non-existent interface ID). This 766 can be addressed by using access control mechanisms on a per-address 767 basis on the network egress point. 769 8. Acknowledgments 771 The authors would like to thank (in alphabetical order) Fred Baker, 772 Brian Carpenter, Tim Chown, Lorenzo Colitti, David Farmer, Tom 773 Herbert, Bob Hinden, Christian Huitema, Erik Kline, Gyan Mishra, Dave 774 Plonka, Michael Richardson, Mark Smith, Pascal Thubert, Ole Troan, 775 Johanna Ullrich, and Timothy Winters, for providing valuable comments 776 on earlier versions of this document. 778 This document incorporates errata submitted for [RFC4941] by Jiri 779 Bohac and Alfred Hoenes. 781 This document is based on [RFC4941] (a revision of RFC3041). Suresh 782 Krishnan was the sole author of RFC4941. He would like to 783 acknowledge the contributions of the IPv6 working group and, in 784 particular, Jari Arkko, Pekka Nikander, Pekka Savola, Francis Dupont, 785 Brian Haberman, Tatuya Jinmei, and Margaret Wasserman for their 786 detailed comments. 788 Rich Draves and Thomas Narten were the authors of RFC 3041. They 789 would like to acknowledge the contributions of the IPv6 working group 790 and, in particular, Ran Atkinson, Matt Crawford, Steve Deering, 791 Allison Mankin, and Peter Bieringer. 793 9. References 795 9.1. Normative References 797 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 798 Requirement Levels", BCP 14, RFC 2119, 799 DOI 10.17487/RFC2119, March 1997, 800 . 802 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, 803 "Randomness Requirements for Security", BCP 106, RFC 4086, 804 DOI 10.17487/RFC4086, June 2005, 805 . 807 [RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast 808 Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005, 809 . 811 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 812 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 813 2006, . 815 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 816 Address Autoconfiguration", RFC 4862, 817 DOI 10.17487/RFC4862, September 2007, 818 . 820 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 821 Extensions for Stateless Address Autoconfiguration in 822 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 823 . 825 [RFC5453] Krishnan, S., "Reserved IPv6 Interface Identifiers", 826 RFC 5453, DOI 10.17487/RFC5453, February 2009, 827 . 829 [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, 830 "Default Address Selection for Internet Protocol Version 6 831 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, 832 . 834 [RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6 835 Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136, 836 February 2014, . 838 [RFC7217] Gont, F., "A Method for Generating Semantically Opaque 839 Interface Identifiers with IPv6 Stateless Address 840 Autoconfiguration (SLAAC)", RFC 7217, 841 DOI 10.17487/RFC7217, April 2014, 842 . 844 [RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu, 845 "Recommendation on Stable IPv6 Interface Identifiers", 846 RFC 8064, DOI 10.17487/RFC8064, February 2017, 847 . 849 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 850 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 851 May 2017, . 853 [RFC8190] Bonica, R., Cotton, M., Haberman, B., and L. Vegoda, 854 "Updates to the Special-Purpose IP Address Registries", 855 BCP 153, RFC 8190, DOI 10.17487/RFC8190, June 2017, 856 . 858 9.2. Informative References 860 [FIPS-SHS] 861 NIST, "Secure Hash Standard (SHS)", FIPS 862 Publication 180-4, August 2015, 863 . 866 [I-D.ietf-mboned-ieee802-mcast-problems] 867 Perkins, C., McBride, M., Stanley, D., Kumari, W., and J. 868 Zuniga, "Multicast Considerations over IEEE 802 Wireless 869 Media", draft-ietf-mboned-ieee802-mcast-problems-11 (work 870 in progress), December 2019. 872 [IANA-RESERVED-IID] 873 IANA, "Reserved IPv6 Interface Identifiers", 874 . 876 [ONION] Reed, MGR., Syverson, PFS., and DMG. Goldschlag, "Proxies 877 for Anonymous Routing", Proceedings of the 12th Annual 878 Computer Security Applications Conference, San Diego, CA, 879 December 1996. 881 [OPEN-GROUP] 882 The Open Group, "The Open Group Base Specifications Issue 883 7 / IEEE Std 1003.1-2008, 2016 Edition", 884 Section 4.16 Seconds Since the Epoch, 2016, 885 . 888 [RAID2015] 889 Ullrich, J. and E. Weippl, "Privacy is Not an Option: 890 Attacking the IPv6 Privacy Extension", International 891 Symposium on Recent Advances in Intrusion Detection 892 (RAID), 2015, . 895 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 896 DOI 10.17487/RFC1321, April 1992, 897 . 899 [RFC4472] Durand, A., Ihren, J., and P. Savola, "Operational 900 Considerations and Issues with IPv6 DNS", RFC 4472, 901 DOI 10.17487/RFC4472, April 2006, 902 . 904 [RFC5014] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6 905 Socket API for Source Address Selection", RFC 5014, 906 DOI 10.17487/RFC5014, September 2007, 907 . 909 [RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for 910 Detecting Network Attachment in IPv6", RFC 6059, 911 DOI 10.17487/RFC6059, November 2010, 912 . 914 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations 915 for the MD5 Message-Digest and the HMAC-MD5 Algorithms", 916 RFC 6151, DOI 10.17487/RFC6151, March 2011, 917 . 919 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 920 DOI 10.17487/RFC6265, April 2011, 921 . 923 [RFC7039] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed., 924 "Source Address Validation Improvement (SAVI) Framework", 925 RFC 7039, DOI 10.17487/RFC7039, October 2013, 926 . 928 [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an 929 Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 930 2014, . 932 [RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S., 933 Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit 934 Boundary in IPv6 Addressing", RFC 7421, 935 DOI 10.17487/RFC7421, January 2015, 936 . 938 [RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T., 939 Trammell, B., Huitema, C., and D. Borkmann, 940 "Confidentiality in the Face of Pervasive Surveillance: A 941 Threat Model and Problem Statement", RFC 7624, 942 DOI 10.17487/RFC7624, August 2015, 943 . 945 [RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6 946 Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016, 947 . 949 [RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy 950 Considerations for IPv6 Address Generation Mechanisms", 951 RFC 7721, DOI 10.17487/RFC7721, March 2016, 952 . 954 [RFC7934] Colitti, L., Cerf, V., Cheshire, S., and D. Schinazi, 955 "Host Address Availability Recommendations", BCP 204, 956 RFC 7934, DOI 10.17487/RFC7934, July 2016, 957 . 959 [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., 960 Richardson, M., Jiang, S., Lemon, T., and T. Winters, 961 "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", 962 RFC 8415, DOI 10.17487/RFC8415, November 2018, 963 . 965 Appendix A. Changes from RFC4941 [to be removed by the RFC-Editor 966 before publication 968 The following changes have been introduced by this document: 970 1. Discussion of IIDs based on IEEE identifiers has been removed, 971 since the current recommendation ([RFC8064]) is to employ 972 [RFC7217]). 974 2. The document employs the terminology from [RFC7721]. 976 3. Sections 2.2 and 2.3 of [RFC4941] have been removed since the 977 topic has been discussed in more detail in e.g. [RFC7721]. 979 4. The algorithm specified in Section 3.2.1 and 3.2.2 of [RFC4941] 980 was replaced by two possible alternative algorithms. 982 5. Generation of stable addresses is not implied or required by this 983 document. 985 6. Temporary addresses are enabled by default, in the light of 986 [RFC7258]. 988 7. Section 3.2.3 from [RFC4941] was removed, based on the 989 explanation of that very section of RFC4941. 991 8. All the verified errata for [RFC4941] has been incorporated. 993 Authors' Addresses 995 Fernando Gont 996 SI6 Networks / UTN-FRH 997 Evaristo Carriego 2644 998 Haedo, Provincia de Buenos Aires 1706 999 Argentina 1001 Phone: +54 11 4650 8472 1002 Email: fgont@si6networks.com 1003 URI: http://www.si6networks.com 1005 Suresh Krishnan 1006 Ericsson Research 1007 8400 Decarie Blvd. 1008 Town of Mount Royal, QC 1009 Canada 1011 Email: suresh.krishnan@ericsson.com 1013 Thomas Narten 1014 IBM Corporation 1015 P.O. Box 12195 1016 Research Triangle Park, NC 1017 USA 1019 Email: narten@us.ibm.com 1021 Richard Draves 1022 Microsoft Research 1023 One Microsoft Way 1024 Redmond, WA 1025 USA 1027 Email: richdr@microsoft.com