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Checking references for intended status: Informational ---------------------------------------------------------------------------- ** Obsolete normative reference: RFC 3315 (Obsoleted by RFC 8415) ** Obsolete normative reference: RFC 4941 (Obsoleted by RFC 8981) -- Duplicate reference: RFC3972, mentioned in 'CGA-IPR', was also mentioned in 'RFC3972'. == Outdated reference: A later version (-08) exists of draft-ietf-opsec-ipv6-host-scanning-07 -- Obsolete informational reference (is this intentional?): RFC 1971 (Obsoleted by RFC 2462) -- Obsolete informational reference (is this intentional?): RFC 1972 (Obsoleted by RFC 2464) -- Obsolete informational reference (is this intentional?): RFC 3041 (Obsoleted by RFC 4941) -- Obsolete informational reference (is this intentional?): RFC 3484 (Obsoleted by RFC 6724) Summary: 2 errors (**), 0 flaws (~~), 2 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group A. Cooper 3 Internet-Draft Cisco 4 Intended status: Informational F. Gont 5 Expires: December 28, 2015 Huawei Technologies 6 D. Thaler 7 Microsoft 8 June 26, 2015 10 Privacy Considerations for IPv6 Address Generation Mechanisms 11 draft-ietf-6man-ipv6-address-generation-privacy-07.txt 13 Abstract 15 This document discusses privacy and security considerations for 16 several IPv6 address generation mechanisms, both standardized and 17 non-standardized. It evaluates how different mechanisms mitigate 18 different threats and the trade-offs that implementors, developers, 19 and users face in choosing different addresses or address generation 20 mechanisms. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on December 28, 2015. 39 Copyright Notice 41 Copyright (c) 2015 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 57 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 58 3. Weaknesses in IEEE-identifier-based IIDs . . . . . . . . . . 4 59 3.1. Correlation of activities over time . . . . . . . . . . . 5 60 3.2. Location tracking . . . . . . . . . . . . . . . . . . . . 6 61 3.3. Address scanning . . . . . . . . . . . . . . . . . . . . 6 62 3.4. Device-specific vulnerability exploitation . . . . . . . 6 63 4. Privacy and security properties of address generation 64 mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . 7 65 4.1. IEEE-identifier-based IIDs . . . . . . . . . . . . . . . 9 66 4.2. Static, manually configured IIDs . . . . . . . . . . . . 10 67 4.3. Constant, semantically opaque IIDs . . . . . . . . . . . 10 68 4.4. Cryptographically generated IIDs . . . . . . . . . . . . 10 69 4.5. Stable, semantically opaque IIDs . . . . . . . . . . . . 10 70 4.6. Temporary IIDs . . . . . . . . . . . . . . . . . . . . . 11 71 4.7. DHCPv6 generation of IIDs . . . . . . . . . . . . . . . . 12 72 4.8. Transition/co-existence technologies . . . . . . . . . . 12 73 5. Miscellaneous Issues with IPv6 addressing . . . . . . . . . . 13 74 5.1. Network Operation . . . . . . . . . . . . . . . . . . . . 13 75 5.2. Compliance . . . . . . . . . . . . . . . . . . . . . . . 13 76 5.3. Intellectual Property Rights (IPRs) . . . . . . . . . . . 13 77 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 78 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 79 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 80 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 81 9.1. Normative References . . . . . . . . . . . . . . . . . . 14 82 9.2. Informative References . . . . . . . . . . . . . . . . . 14 83 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 85 1. Introduction 87 IPv6 was designed to improve upon IPv4 in many respects, and 88 mechanisms for address assignment were one such area for improvement. 89 In addition to static address assignment and DHCP, stateless 90 autoconfiguration was developed as a less intensive, fate-shared 91 means of performing address assignment. With stateless 92 autoconfiguration, routers advertise on-link prefixes and hosts 93 generate their own interface identifiers (IIDs) to complete their 94 addresses. [RFC7136] clarifies that the IID should be treated as an 95 opaque value, while [RFC7421] provides an analysis of the 64-bit 96 boundary in IPv6 addressing (e.g. the implications of the IID length 97 on security and privacy). Over the years, many interface identifier 98 generation techniques have been defined, both standardized and non- 99 standardized: 101 o Manual configuration 103 * IPv4 address 105 * Service port 107 * Wordy 109 * Low-byte 111 o Stateless Address Auto-Cofiguration (SLAAC) 113 * IEEE 802 48-bit MAC or IEEE EUI-64 identifier [RFC2464] 115 * Cryptographically generated [RFC3972] 117 * Temporary (also known as "privacy addresses") [RFC4941] 119 * Constant, semantically opaque (also known as random) 120 [Microsoft] 122 * Stable, semantically opaque [RFC7217] 124 o DHCPv6-based [RFC3315] 126 o Specified by transition/co-existence technologies 128 * IPv4 address and port [RFC4380] 130 Deriving the IID from a globally unique IEEE identifier [RFC2464] 131 [RFC4862] was one of the earliest mechanisms developed (and 132 originally specified in [RFC1971] and [RFC1972]). A number of 133 privacy and security issues related to the IIDs derived from IEEE 134 identifiers were discovered after their standardization, and many of 135 the mechanisms developed later aimed to mitigate some or all of these 136 weaknesses. This document identifies four types of threats against 137 IEEE-identifier-based IIDs, and discusses how other existing 138 techniques for generating IIDs do or do not mitigate those threats. 140 2. Terminology 142 This section clarifies the terminology used throughout this document. 144 Public address: 146 An address that has been published in a directory or other public 147 location, such as the DNS, a SIP proxy, an application-specific 148 DHT, or a publicly available URI. A host's public addresses are 149 intended to be discoverable by third parties. 151 Stable address: 152 An address that does not vary over time within the same IPv6 link. 153 Note that [RFC4941] refers to these as "public" addresses, but 154 "stable" is used here for reasons explained in Section 4. 156 Temporary address: 157 An address that varies over time within the same IPv6 link. 159 Constant IID: 160 An IPv6 Interface Identifier that is globally stable. That is, 161 the Interface ID will remain constant even if the node moves from 162 one IPv6 link to another. 164 Stable IID: 165 An IPv6 Interface Identifier that is stable within some specified 166 context. For example, an Interface ID can be globally stable 167 (constant), or could be stable per IPv6 link (meaning that the 168 Interface ID will remain unchanged as long as a the node stays on 169 the same IPv6 link, but may change when the node moves from one 170 IPv6 link to another). 172 Temporary IID: 173 An IPv6 Interface Identifier that varies over time. 175 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 176 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 177 "OPTIONAL" in this document are to be interpreted as described in 178 [RFC2119]. These words take their normative meanings only when they 179 are presented in ALL UPPERCASE. 181 3. Weaknesses in IEEE-identifier-based IIDs 183 There are a number of privacy and security implications that exist 184 for hosts that use IEEE-identifier-based IIDs. This section 185 discusses four generic attack types: correlation of activities over 186 time, location tracking, address scanning, and device-specific 187 vulnerability exploitation. The first three of these rely on the 188 attacker first gaining knowledge of the target host's IID. This 189 could be achieved by a number of different entities: the operator of 190 a server to which the host connects, such as a web server or a peer- 191 to-peer server; an entity that connects to the same IPv6 link as the 192 target (such as a conference network or any public network); or an 193 entity that is on-path to the destinations with which the host 194 communicates, such as a network operator. 196 3.1. Correlation of activities over time 198 As with other identifiers, an IPv6 address can be used to correlate 199 the activities of a host for at least as long as the lifetime of the 200 address. The correlation made possible by IEEE-identifier-based IIDs 201 is of particular concern since they last roughly for the lifetime of 202 a device's network interface, allowing correlation on the order of 203 years. 205 As [RFC4941] explains, 207 "[t]he use of a non-changing interface identifier to form 208 addresses is a specific instance of the more general case where a 209 constant identifier is reused over an extended period of time and 210 in multiple independent activities. Anytime the same identifier 211 is used in multiple contexts, it becomes possible for that 212 identifier to be used to correlate seemingly unrelated activity. 213 ... The use of a constant identifier within an address is of 214 special concern because addresses are a fundamental requirement of 215 communication and cannot easily be hidden from eavesdroppers and 216 other parties. Even when higher layers encrypt their payloads, 217 addresses in packet headers appear in the clear." 219 IP addresses are just one example of information that can be used to 220 correlate activities over time. DNS names, cookies [RFC6265], 221 browser fingerprints [Panopticlick], and application-layer usernames 222 can all be used to link a host's activities together. Although IEEE- 223 identifier-based IIDs are likely to last at least as long or longer 224 than these other identifiers, IIDs generated in other ways may have 225 shorter or longer lifetimes than these identifiers depending on how 226 they are generated. Therefore, the extent to which a host's 227 activities can be correlated depends on whether the host uses 228 multiple identifiers together and the lifetimes of all of those 229 identifiers. Frequently refreshing an IPv6 address may not mitigate 230 correlation if an attacker has access to other longer lived 231 identifiers for a particular host. This is an important caveat to 232 keep in mind throughout the discussion of correlation in this 233 document. For further discussion of correlation, see Section 5.2.1 234 of [RFC6973]. 236 As noted in [RFC4941], in some cases correlation is just as feasible 237 for a host using an IPv4 address as for a host using an IEEE 238 identifier to generate its IID in its IPv6 address. Hosts that use 239 static IPv4 addressing or who are consistently allocated the same 240 address via DHCPv4 can be tracked as described above. However, the 241 widespread use of both NAT and DHCPv4 implementations that assign the 242 same host a different address upon lease expiration mitigates this 243 threat in the IPv4 case as compared to the IEEE identifier case in 244 IPv6. 246 3.2. Location tracking 248 Because the IPv6 address structure is divided between a topological 249 portion and an interface identifier portion, an interface identifier 250 that remains constant when a host connects to different IPv6 links 251 (as an IEEE-identifier-based IID does) provides a way for observers 252 to track the movements of that host. In a passive attack on a mobile 253 host, a server that receives connections from the same host over time 254 would be able to determine the host's movements as its prefix 255 changes. 257 Active attacks are also possible. An attacker that first learns the 258 host's interface identifier by being connected to the same IPv6 link, 259 running a server that the host connects to, or being on-path to the 260 host's communications could subsequently probe other networks for the 261 presence of the same interface identifier by sending a probe packet 262 (ICMPv6 Echo Request, or any other probe packet). Even if the host 263 does not respond, the first hop router will usually respond with an 264 ICMP Destination Unreachable/Address Unreachable (type 1, code 3) 265 when the host is not present, and be silent when the host is present. 267 Location tracking based on IP address is generally not possible in 268 IPv4 since hosts get assigned wholly new addresses when they change 269 networks. 271 3.3. Address scanning 273 The structure of IEEE-based identifiers used for address generation 274 can be leveraged by an attacker to reduce the target search space 275 [I-D.ietf-opsec-ipv6-host-scanning]. The 24-bit Organizationally 276 Unique Identifier (OUI) of MAC addresses, together with the fixed 277 value (0xff, 0xfe) used to form a Modified EUI-64 Interface 278 Identifier, greatly help to reduce the search space, making it easier 279 for an attacker to scan for individual addresses using widely-known 280 popular OUIs. This erases much of the protection against address 281 scanning that the larger IPv6 address space could provide as compared 282 to IPv4. 284 3.4. Device-specific vulnerability exploitation 286 IPv6 addresses that embed IEEE identifiers leak information about the 287 device (Network Interface Card vendor, or even Operating System and/ 288 or software type), which could be leveraged by an attacker with 289 knowledge of device/software-specific vulnerabilities to quickly find 290 possible targets. Attackers can exploit vulnerabilities in hosts 291 whose IIDs they have previously obtained, or scan an address space to 292 find potential targets. 294 4. Privacy and security properties of address generation mechanisms 296 Analysis of the extent to which a particular host is protected 297 against the threats described in Section 3 depends on how each of a 298 host's addresses is generated and used. In some scenarios, a host 299 configures a single global address and uses it for all 300 communications. In other scenarios, a host configures multiple 301 addresses using different mechanisms and may use any or all of them. 303 [RFC3041] (later obsoleted by [RFC4941]) sought to address some of 304 the problems described in Section 3 by defining "temporary addresses" 305 for outbound connections. Temporary addresses are meant to 306 supplement the other addresses that a device might use, not to 307 replace them. They use IIDs that are randomly generated and change 308 daily by default. The idea was for temporary addresses to be used 309 for outgoing connections (e.g., web browsing) while maintaining the 310 ability to use a stable address when more address stability is 311 desired (e.g., for IPv6 addresses published in the DNS). 313 [RFC3484] originally specified that stable addresses be used for 314 outbound connections unless an application explicitly prefers 315 temporary addresses. The default preference for stable addresses was 316 established to avoid applications potentially failing due to the 317 short lifetime of temporary addresses or the possibility of a reverse 318 look-up failure or error. However, [RFC3484] allowed that 319 "implementations for which privacy considerations outweigh these 320 application compatibility concerns MAY reverse the sense of this 321 rule" and instead prefer by default temporary addresses rather than 322 stable addresses. Indeed most implementations (notably including 323 Windows) chose to default to temporary addresses for outbound 324 connections since privacy was considered more important (and few 325 applications supported IPv6 at the time, so application compatibility 326 concerns were minimal). [RFC6724] then obsoleted [RFC3484] and 327 changed the default to match what implementations actually did. 329 The envisioned relationship in [RFC3484] between stability of an 330 address and its use in "public" can be misleading when conducting 331 privacy analysis. The stability of an address and the extent to 332 which it is linkable to some other public identifier are independent 333 of one another. For example, there is nothing that prevents a host 334 from publishing a temporary address in a public place, such as the 335 DNS. Publishing both a stable address and a temporary address in the 336 DNS or elsewhere where they can be linked together by a public 337 identifier allows the host's activities when using either address to 338 be correlated together. 340 Moreover, because temporary addresses were designed to supplement 341 other addresses generated by a host, the host may still configure a 342 more stable address even if it only ever intentionally uses temporary 343 addresses (as source addresses) for communication to off-link 344 destinations. An attacker can probe for the stable address even if 345 it is never used as such a source address or advertised (e.g., in DNS 346 or SIP) outside the link. 348 This section compares the privacy and security properties of a 349 variety of IID generation mechanisms and their possible usage 350 scenarios, including scenarios in which a single mechanism is used to 351 generate all of a host's IIDs and those in which temporary addresses 352 are used together with addresses generated using a different IID 353 generation mechanism. The analysis of the exposure of each IID type 354 to correlation assumes that IPv6 prefixes are shared by a reasonably 355 large number of nodes. As [RFC4941] notes, if a very small number of 356 nodes (say, only one) use a particular prefix for an extended period 357 of time, the prefix itself can be used to correlate the host's 358 activities regardless of how the IID is generated. For example, 359 [RFC3314] recommends that prefixes be uniquely assigned to mobile 360 handsets where IPv6 is used within GPRS. In cases where this advice 361 is followed and prefixes persist for extended periods of time (or get 362 reassigned to the same handsets whenever those handsets reconnect to 363 the same network router), hosts' activities could be correlatable for 364 longer periods than the analysis below would suggest. 366 The table below provides a summary of the whole analysis. 368 +--------------+-------------+----------+-------------+-------------+ 369 | Mechanism(s) | Correlation | Location | Address | Device | 370 | | | tracking | scanning | exploits | 371 +--------------+-------------+----------+-------------+-------------+ 372 | IEEE | For device | For | Possible | Possible | 373 | identifier | lifetime | device | | | 374 | | | lifetime | | | 375 | | | | | | 376 | Static | For address | For | Depends on | Depends on | 377 | manual | lifetime | address | generation | generation | 378 | | | lifetime | mechanism | mechanism | 379 | | | | | | 380 | Constant, | For address | For | No | No | 381 | semantically | lifetime | address | | | 382 | opaque | | lifetime | | | 383 | | | | | | 384 | CGA | For | No | No | No | 385 | | lifetime of | | | | 386 | | (modifier | | | | 387 | | block + | | | | 388 | | public key) | | | | 389 | | | | | | 390 | Stable, | Within | No | No | No | 391 | semantically | single IPv6 | | | | 392 | opaque | link | | | | 393 | | | | | | 394 | Temporary | For temp | No | No | No | 395 | | address | | | | 396 | | lifetime | | | | 397 | | | | | | 398 | DHCPv6 | For lease | No | Depends on | No | 399 | | lifetime | | generation | | 400 | | | | mechanism | | 401 +--------------+-------------+----------+-------------+-------------+ 403 Table 1: Privacy and security properties of IID generation mechanisms 405 4.1. IEEE-identifier-based IIDs 407 As discussed in Section 3, addresses that use IIDs based on IEEE 408 identifiers are vulnerable to all four threats. They allow 409 correlation and location tracking for the lifetime of the device 410 since IEEE identifiers last that long and their structure makes 411 address scanning and device exploits possible. 413 4.2. Static, manually configured IIDs 415 Because static, manually configured IIDs are stable, both correlation 416 and location tracking are possible for the life of the address. 418 The extent to which location tracking can be successfully performed 419 depends, to a some extent, on the uniqueness of the employed 420 Interface ID. For example, one would expect "low byte" Interface IDs 421 to be more widely reused than, for example, Interface IDs where the 422 whole 64-bits follow some pattern that is unique to a specific 423 organization. Widely reused Interface IDs will typically lead to 424 false positives when performing location tracking. 426 Whether manually configured addresses are vulnerable to address 427 scanning and device exploits depends on the specifics of how the IIDs 428 are generated. 430 4.3. Constant, semantically opaque IIDs 432 Although a mechanism to generate a constant, semantically opaque IID 433 has not been standardized, it has been in wide use for many years on 434 at least one platform (Windows). Windows uses the [RFC4941] random 435 generation mechanism in lieu of generating an IEEE-identifier-based 436 IID. This mitigates the device-specific exploitation and address 437 scanning attacks, but still allows correlation and location tracking 438 because the IID is constant across IPv6 links and time. 440 4.4. Cryptographically generated IIDs 442 Cryptographically generated addresses (CGAs) [RFC3972] bind a hash of 443 the host's public key to an IPv6 address in the SEcure Neighbor 444 Discovery (SEND) [RFC3971] protocol. CGAs may be regenerated for 445 each subnet prefix, but this is not required given that they are 446 computationally expensive to generate. A host using a CGA can be 447 correlated for as long as the lifetime of the combination of the 448 public key and the chosen modifier block, since it is possible to 449 rotate modifier blocks without generating new public keys. Because 450 the cryptographic hash of the host's public key uses the subnet 451 prefix as an input, even if the host does not generate a new public 452 key or modifier block when it moves to a different IPv6 link, its 453 location cannot be tracked via the IID. CGAs do not allow device- 454 specific exploitation or address scanning attacks. 456 4.5. Stable, semantically opaque IIDs 458 [RFC7217] specifies an algorithm that generates, for each network 459 interface, a unique random IID per IPv6 link. The aforementioned 460 algorithm is employed not only for global unicast addresses, but also 461 for unique local unicast addresses and link-local unicast addresses, 462 since these addresses may leak out via application protocols (e.g., 463 IPv6 addresses embedded in email headers). 465 A host that stays connected to the same IPv6 link could therefore be 466 tracked at length, whereas a mobile host's activities could only be 467 correlated for the duration of each network connection. Location 468 tracking is not possible with these addresses. They also do not 469 allow device-specific exploitation or address scanning attacks. 471 4.6. Temporary IIDs 473 A host that uses only a temporary address mitigates all four threats. 474 Its activities may only be correlated for the lifetime a single 475 temporary address. 477 A host that configures both an IEEE-identifier-based IID and 478 temporary addresses makes the host vulnerable to the same attacks as 479 if temporary addresses were not in use, although the viability of 480 some of them depends on how the host uses each address. An attacker 481 can correlate all of the host's activities for which it uses its 482 IEEE-identifier-based IID. Once an attacker has obtained the IEEE- 483 identifier-based IID, location tracking becomes possible on other 484 IPv6 links even if the host only makes use of temporary addresses on 485 those other IPv6 links; the attacker can actively probe the other 486 IPv6 links for the presence of the IEEE-identifier-based IID. 487 Device-specific vulnerabilities can still be exploited. Address 488 scanning is also still possible because the IEEE-identifier-based 489 address can be probed. 491 If the host instead generates a constant, semantically opaque IID to 492 use in a stable address for server-like connections together with 493 temporary addresses for outbound connections (as is the default in 494 Windows), it sees some improvements over the previous scenario. The 495 address scanning and device-specific exploitation attacks are no 496 longer possible because the OUI is no longer embedded in any of the 497 host's addresses. However, correlation of some activities across 498 time and location tracking are both still possible because the 499 semantically opaque IID is constant. And once an attacker has 500 obtained the host's semantically opaque IID, location tracking is 501 possible on any network by probing for that IID, even if the host 502 only uses temporary addresses on those networks. However, if the 503 host generates but never uses a constant, semantically opaque IID, it 504 mitigates all four threats. 506 When used together with temporary addresses, the stable, semantically 507 opaque IID generation mechanism [RFC7217] improves upon the previous 508 scenario by limiting the potential for correlation to the lifetime of 509 the stable address (which may still be lengthy for hosts that are not 510 mobile) and by eliminating the possibility for location tracking 511 (since a different IID is generated for each subnet prefix). As in 512 the previous scenario, a host that configures but does not use a 513 stable, semantically opaque address mitigates all four threats. 515 4.7. DHCPv6 generation of IIDs 517 The security/privacy implications of DHCPv6-based addresses will 518 typically depend on whether the client requests an IA_NA (Identity 519 Association for Non-temporary Addresses) or an IA_TA ( Identity 520 Association for Temporary Addresses) [RFC3315] and the specific 521 DHCPv6 server software being employed. 523 DHCPv6 temporary addresses have the same properties as SLAAC 524 temporary addresses Section 4.6 [RFC4941]. On the other hand, the 525 properties of DHCPv6 non-temporary addresses typically depend on the 526 specific DHCPv6 server software being employed. Recent releases of 527 most popular DHCPv6 server software typically lease random addresses 528 with a similar lease time as that of IPv4. Thus, these addresses can 529 be considered to be "stable, semantically opaque". 530 [I-D.ietf-dhc-stable-privacy-addresses] specifies an algorithm that 531 can be employed by DHCPv6 servers to generate "stable, semantically 532 opaque" addresses. 534 On the other hand, some DHCPv6 software leases sequential addresses 535 (typically low-byte addresses). These addresses can be considered to 536 be stable addresses. The drawback of this address generation scheme 537 compared to "stable, semantically opaque" addresses is that, since 538 they follow specific patterns, they enable IPv6 address scans. 540 4.8. Transition/co-existence technologies 542 Addresses specified based on transition/co-existence technologies 543 that embed an IPv4 address within an IPv6 address are not included in 544 Table 1 because their privacy and security properties are inherited 545 from the embedded address. For example, Teredo [RFC4380] specifies a 546 means to generate an IPv6 address from the underlying IPv4 address 547 and port, leaving many other bits set to zero. This makes it 548 relatively easy for an attacker to scan for IPv6 addresses by 549 guessing the Teredo client's IPv4 address and port (which for many 550 NATs is not randomized). For this reason, popular implementations 551 (e.g., Windows), began deviating from the standard by including 12 552 random bits in place of zero bits. This modification was later 553 standardized in [RFC5991]. 555 5. Miscellaneous Issues with IPv6 addressing 557 5.1. Network Operation 559 It is generally agreed that IPv6 addresses that vary over time in a 560 specific IPv6 link tend to increase the complexity of event logging, 561 trouble-shooting, enforcement of access controls and quality of 562 service, etc. As a result, some organizations disable the use of 563 temporary addresses [RFC4941] even at the expense of reduced privacy 564 [Broersma]. 566 5.2. Compliance 568 Some IPv6 compliance testing suites required (and might still 569 require) implementations to support IEEE-identifier-based IIDS in 570 order to be approved as compliant. This document recommends that 571 compliance testing suites be relaxed to allow other forms of address 572 generation that are more amenable to privacy. 574 5.3. Intellectual Property Rights (IPRs) 576 Some IPv6 addressing techniques might be covered by Intellectual 577 Property rights, which might limit their implementation in different 578 Operating Systems. [CGA-IPR] and [KAME-CGA] discuss the IPRs on 579 CGAs. 581 6. Security Considerations 583 This whole document concerns the privacy and security properties of 584 different IPv6 address generation mechanisms. 586 7. IANA Considerations 588 This document does not require actions by IANA. 590 8. Acknowledgements 592 The authors would like to thank Bernard Aboba, Brian Carpenter, Tim 593 Chown, Lorenzo Colitti, Rich Draves, Robert Hinden, Robert Moskowitz, 594 Erik Nordmark, Mark Smith, Ole Troan, and James Woodyatt for 595 providing valuable comments on earlier versions of this document. 597 9. References 598 9.1. Normative References 600 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 601 Requirement Levels", BCP 14, RFC 2119, March 1997. 603 [RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet 604 Networks", RFC 2464, December 1998. 606 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 607 and M. Carney, "Dynamic Host Configuration Protocol for 608 IPv6 (DHCPv6)", RFC 3315, July 2003. 610 [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure 611 Neighbor Discovery (SEND)", RFC 3971, March 2005. 613 [RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", 614 RFC 3972, March 2005. 616 [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through 617 Network Address Translations (NATs)", RFC 4380, February 618 2006. 620 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 621 Address Autoconfiguration", RFC 4862, September 2007. 623 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 624 Extensions for Stateless Address Autoconfiguration in 625 IPv6", RFC 4941, September 2007. 627 [RFC5991] Thaler, D., Krishnan, S., and J. Hoagland, "Teredo 628 Security Updates", RFC 5991, September 2010. 630 [RFC6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown, 631 "Default Address Selection for Internet Protocol Version 6 632 (IPv6)", RFC 6724, September 2012. 634 [RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6 635 Interface Identifiers", RFC 7136, February 2014. 637 [RFC7217] Gont, F., "A Method for Generating Semantically Opaque 638 Interface Identifiers with IPv6 Stateless Address 639 Autoconfiguration (SLAAC)", RFC 7217, April 2014. 641 9.2. Informative References 643 [Broersma] 644 Broersma, R., "IPv6 Everywhere: Living with a Fully 645 IPv6-enabled environment", Australian IPv6 Summit 2010, 646 Melbourne, VIC Australia, October 2010, October 2010, 647 . 650 [CGA-IPR] IETF, "Intellectual Property Rights on RFC 3972", 2005, 651 . 653 [I-D.ietf-dhc-stable-privacy-addresses] 654 Gont, F. and S. LIU, "A Method for Generating Semantically 655 Opaque Interface Identifiers with Dynamic Host 656 Configuration Protocol for IPv6 (DHCPv6)", draft-ietf-dhc- 657 stable-privacy-addresses-02 (work in progress), April 658 2015. 660 [I-D.ietf-opsec-ipv6-host-scanning] 661 Gont, F. and T. Chown, "Network Reconnaissance in IPv6 662 Networks", draft-ietf-opsec-ipv6-host-scanning-07 (work in 663 progress), April 2015. 665 [KAME-CGA] 666 KAME, "The KAME IPR policy and concerns of some 667 technologies which have IPR claims", 2005, 668 . 670 [Microsoft] 671 Microsoft, "IPv6 interface identifiers", 2013, . 675 [Panopticlick] 676 Electronic Frontier Foundation, "Panopticlick", 2011, 677 . 679 [RFC1971] Thomson, S. and T. Narten, "IPv6 Stateless Address 680 Autoconfiguration", RFC 1971, August 1996. 682 [RFC1972] Crawford, M., "A Method for the Transmission of IPv6 683 Packets over Ethernet Networks", RFC 1972, August 1996. 685 [RFC3041] Narten, T. and R. Draves, "Privacy Extensions for 686 Stateless Address Autoconfiguration in IPv6", RFC 3041, 687 January 2001. 689 [RFC3314] Wasserman, M., "Recommendations for IPv6 in Third 690 Generation Partnership Project (3GPP) Standards", RFC 691 3314, September 2002. 693 [RFC3484] Draves, R., "Default Address Selection for Internet 694 Protocol version 6 (IPv6)", RFC 3484, February 2003. 696 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 697 April 2011. 699 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 700 Morris, J., Hansen, M., and R. Smith, "Privacy 701 Considerations for Internet Protocols", RFC 6973, July 702 2013. 704 [RFC7421] Carpenter, B., Chown, T., Gont, F., Jiang, S., Petrescu, 705 A., and A. Yourtchenko, "Analysis of the 64-bit Boundary 706 in IPv6 Addressing", RFC 7421, January 2015. 708 Authors' Addresses 710 Alissa Cooper 711 Cisco 712 707 Tasman Drive 713 Milpitas, CA 95035 714 US 716 Phone: +1-408-902-3950 717 Email: alcoop@cisco.com 718 URI: https://www.cisco.com/ 720 Fernando Gont 721 Huawei Technologies 722 Evaristo Carriego 2644 723 Haedo, Provincia de Buenos Aires 1706 724 Argentina 726 Phone: +54 11 4650 8472 727 Email: fgont@si6networks.com 728 URI: http://www.si6networks.com 729 Dave Thaler 730 Microsoft 731 Microsoft Corporation 732 One Microsoft Way 733 Redmond, WA 98052 735 Phone: +1 425 703 8835 736 Email: dthaler@microsoft.com