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'IPSEC') (Obsoleted by RFC 4301) ** Downref: Normative reference to an Informational RFC: RFC 1321 (ref. 'MD5') -- Obsolete informational reference (is this intentional?): RFC 2965 (ref. 'COOKIES') (Obsoleted by RFC 6265) -- Obsolete informational reference (is this intentional?): RFC 3315 (ref. 'DHCPV6') (Obsoleted by RFC 8415) == Outdated reference: A later version (-12) exists of draft-ietf-dnsop-ipv6-dns-issues-10 -- Obsolete informational reference (is this intentional?): RFC 1750 (ref. 'RANDOM') (Obsoleted by RFC 4086) Summary: 7 errors (**), 0 flaws (~~), 5 warnings (==), 10 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IPv6 Working Group T. Narten 3 Internet-Draft IBM Corporation 4 Expires: November 25, 2005 R. Draves 5 Microsoft Research 6 S. Krishnan 7 Ericsson Research 8 May 24, 2005 10 Privacy Extensions for Stateless Address Autoconfiguration in IPv6 11 draft-ietf-ipv6-privacy-addrs-v2-04 13 Status of this Memo 15 By submitting this Internet-Draft, each author represents that any 16 applicable patent or other IPR claims of which he or she is aware 17 have been or will be disclosed, and any of which he or she becomes 18 aware will be disclosed, in accordance with Section 6 of BCP 79. 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF), its areas, and its working groups. Note that 22 other groups may also distribute working documents as Internet- 23 Drafts. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 The list of current Internet-Drafts can be accessed at 31 http://www.ietf.org/ietf/1id-abstracts.txt. 33 The list of Internet-Draft Shadow Directories can be accessed at 34 http://www.ietf.org/shadow.html. 36 This Internet-Draft will expire on November 25, 2005. 38 Copyright Notice 40 Copyright (C) The Internet Society (2005). 42 Abstract 44 Nodes use IPv6 stateless address autoconfiguration to generate 45 addresses using a combination of locally available information and 46 information advertised by routers. Addresses are formed by combining 47 network prefixes with an interface identifier. On interfaces that 48 contain embedded IEEE Identifiers, the interface identifier is 49 typically derived from it. On other interface types, the interface 50 identifier is generated through other means, for example, via random 51 number generation. This document describes an extension to IPv6 52 stateless address autoconfiguration for interfaces whose interface 53 identifier is derived from an IEEE identifier. Use of the extension 54 causes nodes to generate global scope addresses from interface 55 identifiers that change over time, even in cases where the interface 56 contains an embedded IEEE identifier. Changing the interface 57 identifier (and the global scope addresses generated from it) over 58 time makes it more difficult for eavesdroppers and other information 59 collectors to identify when different addresses used in different 60 transactions actually correspond to the same node. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 65 1.1 Conventions used in this document . . . . . . . . . . . . 4 66 1.2 Problem Statement . . . . . . . . . . . . . . . . . . . . 4 67 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5 68 2.1 Extended Use of the Same Identifier . . . . . . . . . . . 5 69 2.2 Address Usage in IPv4 Today . . . . . . . . . . . . . . . 6 70 2.3 The Concern With IPv6 Addresses . . . . . . . . . . . . . 7 71 2.4 Possible Approaches . . . . . . . . . . . . . . . . . . . 8 72 3. Protocol Description . . . . . . . . . . . . . . . . . . . . . 10 73 3.1 Assumptions . . . . . . . . . . . . . . . . . . . . . . . 10 74 3.2 Generation Of Randomized Interface Identifiers . . . . . . 12 75 3.2.1 When Stable Storage Is Present . . . . . . . . . . . . 12 76 3.2.2 In The Absence of Stable Storage . . . . . . . . . . . 13 77 3.2.3 Alternate approaches . . . . . . . . . . . . . . . . . 14 78 3.3 Generating Temporary Addresses . . . . . . . . . . . . . . 14 79 3.4 Expiration of Temporary Addresses . . . . . . . . . . . . 15 80 3.5 Regeneration of Randomized Interface Identifiers . . . . . 16 81 3.6 Deployment Considerations . . . . . . . . . . . . . . . . 17 82 4. Implications of Changing Interface Identifiers . . . . . . . . 19 83 5. Defined Constants . . . . . . . . . . . . . . . . . . . . . . 20 84 6. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 21 85 7. Significant Changes from RFC 3041 . . . . . . . . . . . . . . 22 86 8. Changes from version 00 . . . . . . . . . . . . . . . . . . . 23 87 9. Changes from version 01 . . . . . . . . . . . . . . . . . . . 24 88 10. Changes from version 02 . . . . . . . . . . . . . . . . . . 25 89 11. Changes from version 03 . . . . . . . . . . . . . . . . . . 26 90 12. Security Considerations . . . . . . . . . . . . . . . . . . 27 91 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28 92 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 93 14.1 Normative References . . . . . . . . . . . . . . . . . . . 29 94 14.2 Informative References . . . . . . . . . . . . . . . . . . 29 95 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30 96 Intellectual Property and Copyright Statements . . . . . . . . 32 98 1. Introduction 100 Stateless address autoconfiguration [ADDRCONF] defines how an IPv6 101 node generates addresses without the need for a DHCPv6 server. Some 102 types of network interfaces come with an embedded IEEE Identifier 103 (i.e., a link-layer MAC address), and in those cases stateless 104 address autoconfiguration uses the IEEE identifier to generate a 64- 105 bit interface identifier [ADDRARCH]. By design, the interface 106 identifier is likely to be globally unique when generated in this 107 fashion. The interface identifier is in turn appended to a prefix to 108 form a 128-bit IPv6 address. Note that an IPv6 identifier does not 109 necessarily have to be 64 bits in length, but the algorithm specified 110 in this document is targeted towards 64-bit interface identifiers. 112 All nodes combine interface identifiers (whether derived from an IEEE 113 identifier or generated through some other technique) with the 114 reserved link-local prefix to generate link-local addresses for their 115 attached interfaces. Additional addresses can then be created by 116 combining prefixes advertised in Router Advertisements via Neighbor 117 Discovery [DISCOVERY] with the interface identifier. 119 Not all nodes and interfaces contain IEEE identifiers. In such 120 cases, an interface identifier is generated through some other means 121 (e.g., at random), and the resultant interface identifier may not be 122 globally unique and may also change over time. The focus of this 123 document is on addresses derived from IEEE identifiers, because 124 tracking of individual devices, the concern being addressed here, is 125 possible only in those cases where the interface identifier is 126 globally unique and non-changing. The rest of this document assumes 127 that IEEE identifiers are being used, but the techniques described 128 may also apply to interfaces with other types of globally unique 129 and/or persistent identifiers. 131 This document discusses concerns associated with the embedding of 132 non-changing interface identifiers within IPv6 addresses and 133 describes extensions to stateless address autoconfiguration that can 134 help mitigate those concerns for individual users and in environments 135 where such concerns are significant. Section 2 provides background 136 information on the issue. Section 3 describes a procedure for 137 generating alternate interface identifiers and global scope 138 addresses. Section 4 discusses implications of changing interface 139 identifiers. The term "global scope addresses" is used in this 140 document to collectively refer to "Global unicast addresses" as 141 defined in [ADDRARCH] and "Unique local addresses" as defined in 142 [ULA] 144 1.1 Conventions used in this document 146 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 147 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 148 document are to be interpreted as described in [RFC2119]. 150 1.2 Problem Statement 152 Addresses generated using Stateless address autoconfiguration 153 [ADDRCONF]contain an embedded interface identifier, which remains 154 constant over time. Anytime a fixed identifier is used in multiple 155 contexts, it becomes possible to correlate seemingly unrelated 156 activity using this identifier. 158 The correlation can be performed by 160 o An attacker who is in the path between the node in question and 161 the peer(s) it is communicating to, and can view the IPv6 162 addresses present in the datagrams. 164 o An attacker who can access the communication logs of the peers 165 with which the node has communicated. 167 Since the identifier is embedded within the IPv6 address, which is a 168 fundamental requirement of communication, it cannot be easily hidden. 169 This document proposes a solution to this issue by generating 170 interface identifiers which vary over time. 172 Note that an attacker, who is on path, may be able to perform 173 significant correlation based on 175 o The payload contents of the packets on the wire 177 o The characteristics of the packets such as packet size and timing 179 Use of temporary addresses will not prevent such payload based 180 correlation. 182 2. Background 184 This section discusses the problem in more detail, provides context 185 for evaluating the significance of the concerns in specific 186 environments and makes comparisons with existing practices. 188 2.1 Extended Use of the Same Identifier 190 The use of a non-changing interface identifier to form addresses is a 191 specific instance of the more general case where a constant 192 identifier is reused over an extended period of time and in multiple 193 independent activities. Anytime the same identifier is used in 194 multiple contexts, it becomes possible for that identifier to be used 195 to correlate seemingly unrelated activity. For example, a network 196 sniffer placed strategically on a link across which all traffic to/ 197 from a particular host crosses could keep track of which destinations 198 a node communicated with and at what times. Such information can in 199 some cases be used to infer things, such as what hours an employee 200 was active, when someone is at home, etc. Although it might appear 201 that changing an address regularly in such environments would be 202 desirable to lessen privacy concerns, it should be noted that the 203 network prefix portion of an address also serves as a constant 204 identifier. All nodes at (say) a home, would have the same network 205 prefix, which identifies the topological location of those nodes. 206 This has implications for privacy, though not at the same granularity 207 as the concern that this document addresses. Specifically, all nodes 208 within a home could be grouped together for the purposes of 209 collecting information. If the network contains a very small number 210 of nodes, say just one, changing just the interface identifier will 211 not enhance privacy at all, since the prefix serves as a constant 212 identifier. 214 One of the requirements for correlating seemingly unrelated 215 activities is the use (and reuse) of an identifier that is 216 recognizable over time within different contexts. IP addresses 217 provide one obvious example, but there are more. Many nodes also 218 have DNS names associated with their addresses, in which case the DNS 219 name serves as a similar identifier. Although the DNS name 220 associated with an address is more work to obtain (it may require a 221 DNS query) the information is often readily available. In such 222 cases, changing the address on a machine over time would do little to 223 address the concerns raised in this document, unless the DNS name is 224 changed as well (see Section 4). 226 Web browsers and servers typically exchange "cookies" with each other 227 [COOKIES]. Cookies allow web servers to correlate a current activity 228 with a previous activity. One common usage is to send back targeted 229 advertising to a user by using the cookie supplied by the browser to 230 identify what earlier queries had been made (e.g., for what type of 231 information). Based on the earlier queries, advertisements can be 232 targeted to match the (assumed) interests of the end-user. 234 The use of a constant identifier within an address is of special 235 concern because addresses are a fundamental requirement of 236 communication and cannot easily be hidden from eavesdroppers and 237 other parties. Even when higher layers encrypt their payloads, 238 addresses in packet headers appear in the clear. Consequently, if a 239 mobile host (e.g., laptop) accessed the network from several 240 different locations, an eavesdropper might be able to track the 241 movement of that mobile host from place to place, even if the upper 242 layer payloads were encrypted. 244 2.2 Address Usage in IPv4 Today 246 Addresses used in today's Internet are often non-changing in practice 247 for extended periods of time. In an increasing number of sites, 248 addresses are assigned statically and typically change infrequently. 249 Over the last few years, sites have begun moving away from static 250 allocation to dynamic allocation via DHCP [DHCP]. In theory, the 251 address a client gets via DHCP can change over time, but in practice 252 servers often return the same address to the same client (unless 253 addresses are in such short supply that they are reused immediately 254 by a different node when they become free). Thus, even within sites 255 using DHCP, clients frequently end up using the same address for 256 weeks to months at a time. 258 For home users accessing the Internet over dialup lines, the 259 situation is generally different. Such users do not have permanent 260 connections and are often assigned temporary addresses each time they 261 connect to their ISP. Consequently, the addresses they use change 262 frequently over time and are shared among a number of different 263 users. Thus, an address does not reliably identify a particular 264 device over time spans of more than a few minutes. 266 A more interesting case concerns always-on connections (e.g., cable 267 modems, ISDN, DSL, etc.) that result in a home site using the same 268 address for extended periods of time. This is a scenario that is 269 just starting to become common in IPv4 and promises to become more of 270 a concern as always-on internet connectivity becomes widely 271 available. 273 Finally, it should be noted that nodes that need a (non-changing) DNS 274 name generally have static addresses assigned to them to simplify the 275 configuration of DNS servers. Although Dynamic DNS [DDNS] can be 276 used to update the DNS dynamically, it may not always be available 277 depending on the administrative policy. In addition, changing an 278 address but keeping the same DNS name does not really address the 279 underlying concern, since the DNS name becomes a non-changing 280 identifier. Servers generally require a DNS name (so clients can 281 connect to them), and clients often do as well (e.g., some servers 282 refuse to speak to a client whose address cannot be mapped into a DNS 283 name that also maps back into the same address). Section 4 describes 284 one approach to this issue. 286 2.3 The Concern With IPv6 Addresses 288 The division of IPv6 addresses into distinct topology and interface 289 identifier portions raises an issue new to IPv6 in that a fixed 290 portion of an IPv6 address (i.e., the interface identifier) can 291 contain an identifier that remains constant even when the topology 292 portion of an address changes (e.g., as the result of connecting to a 293 different part of the Internet). In IPv4, when an address changes, 294 the entire address (including the local part of the address) usually 295 changes. It is this new issue that this document addresses. 297 If addresses are generated from an interface identifier, a home 298 user's address could contain an interface identifier that remains the 299 same from one dialup session to the next, even if the rest of the 300 address changes. The way PPP is used today, however, PPP servers 301 typically unilaterally inform the client what address they are to use 302 (i.e., the client doesn't generate one on its own). This practice, 303 if continued in IPv6, would avoid the concerns that are the focus of 304 this document. 306 A more troubling case concerns mobile devices (e.g., laptops, PDAs, 307 etc.) that move topologically within the Internet. Whenever they 308 move they form new addresses for their current topological point of 309 attachment. This is typified today by the "road warrior" who has 310 Internet connectivity both at home and at the office. While the 311 node's address changes as it moves, however, the interface identifier 312 contained within the address remains the same (when derived from an 313 IEEE Identifier). In such cases, the interface identifier can be 314 used to track the movement and usage of a particular machine. For 315 example, a server that logs usage information together with a source 316 addresses, is also recording the interface identifier since it is 317 embedded within an address. Consequently, any data-mining technique 318 that correlates activity based on addresses could easily be extended 319 to do the same using the interface identifier. This is of particular 320 concern with the expected proliferation of next-generation network- 321 connected devices (e.g., PDAs, cell phones, etc.) in which large 322 numbers of devices are in practice associated with individual users 323 (i.e., not shared). Thus, the interface identifier embedded within 324 an address could be used to track activities of an individual, even 325 as they move topologically within the internet. 327 In summary, IPv6 addresses on a given interface generated via 328 Stateless Autoconfiguration contain the same interface identifier, 329 regardless of where within the Internet the device connects. This 330 facilitates the tracking of individual devices (and thus potentially 331 users). The purpose of this document is to define mechanisms that 332 eliminate this issue, in those situations where it is a concern. 334 2.4 Possible Approaches 336 One way to avoid having a static non-changing address is to use 337 DHCPv6 [DHCPV6] for obtaining addresses. The DHCPv6 server could be 338 configured to hand out addresses that change over time. But DHCPv6 339 will solve the privacy issue only if it frequently handed out 340 constantly changing addresses to the nodes or if the DHCPv6 client 341 moves from links to links frequently, being allocated independent 342 addresses from different DHCPv6 servers. However, the former does 343 not happen automatically, and is difficult to configure manually; the 344 latter cannot be assumed for static (not frequently moving) hosts. 345 Thus, DHCPv6 is not a self contained alternative for solving the 346 privacy issues addressed by this document. However, in the absence 347 of stateless address autoconfiguration, DHCPv6 can be used for 348 distributing temporary addresses to clients. 350 Another approach, compatible with the stateless address 351 autoconfiguration architecture, would be to change the interface 352 identifier portion of an address over time and generate new addresses 353 from the interface identifier for some address scopes. Changing the 354 interface identifier can make it more difficult to look at the IP 355 addresses in independent transactions and identify which ones 356 actually correspond to the same node, both in the case where the 357 routing prefix portion of an address changes and when it does not. 359 Many machines function as both clients and servers. In such cases, 360 the machine would need a DNS name for its use as a server. Whether 361 the address stays fixed or changes has little privacy implication 362 since the DNS name remains constant and serves as a constant 363 identifier. When acting as a client (e.g., initiating 364 communication), however, such a machine may want to vary the 365 addresses it uses. In such environments, one may need multiple 366 addresses: a "public" (i.e., non-secret) server address, registered 367 in the DNS, that is used to accept incoming connection requests from 368 other machines, and a "temporary" address used to shield the identity 369 of the client when it initiates communication. These two cases are 370 roughly analogous to telephone numbers and caller ID, where a user 371 may list their telephone number in the public phone book, but disable 372 the display of its number via caller ID when initiating calls. 374 To make it difficult to make educated guesses as to whether two 375 different interface identifiers belong to the same node, the 376 algorithm for generating alternate identifiers must include input 377 that has an unpredictable component from the perspective of the 378 outside entities that are collecting information. Picking 379 identifiers from a pseudo-random sequence suffices, so long as the 380 specific sequence cannot be determined by an outsider examining 381 information that is readily available or easily determinable (e.g., 382 by examining packet contents). This document proposes the generation 383 of a pseudo-random sequence of interface identifiers via an MD5 hash. 384 Periodically, the next interface identifier in the sequence is 385 generated, a new set of temporary addresses is created, and the 386 previous temporary addresses are deprecated to discourage their 387 further use. The precise pseudo-random sequence depends on both a 388 random component and the globally unique interface identifier (when 389 available), to increase the likelihood that different nodes generate 390 different sequences. 392 3. Protocol Description 394 The goal of this section is to define procedures that: 396 1. Do not result in any changes to the basic behavior of addresses 397 generated via stateless address autoconfiguration [ADDRCONF]. 399 2. Create additional addresses based on a random interface 400 identifier for the purpose of initiating outgoing sessions These 401 "random" or temporary addresses would be used for a short period 402 of time (hours to days) and would then be deprecated. Deprecated 403 address can continue to be used for already established 404 connections, but are not used to initiate new connections. New 405 temporary addresses are generated periodically to replace 406 temporary addresses that expire, with the exact time between 407 address generation a matter of local policy. 409 3. Produce a sequence of temporary global scope addresses from a 410 sequence of interface identifiers that appear to be random in the 411 sense that it is difficult for an outside observer to predict a 412 future address (or identifier) based on a current one and it is 413 difficult to determine previous addresses (or identifiers) 414 knowing only the present one. 416 4. By default, generate a set of addresses from the same 417 (randomized) interface identifier, one address for each prefix 418 for which a global address has been generated via stateless 419 address autoconfiguration. Using the same interface identifier 420 to generate a set of temporary addresses reduces the number of IP 421 multicast groups a host must join. Nodes join the solicited-node 422 multicast address for each unicast address they support, and 423 solicited-node addresses are dependent only on the low-order bits 424 of the corresponding address. This default behaviour was made to 425 address the concern that a node that joins a large number of 426 multicast groups may be required to put its interface into 427 promiscuous mode, resulting in possible reduced performance. 429 A node highly concerned about privacy MAY use different interface 430 identifiers on different prefixes, resulting in a set of global 431 addresses that cannot be easily tied to each other. For example 432 a node MAY create different interface identifiers I1,I2, and I3 433 for use with different prefixes P1,P2, and P3 on the same 434 interface. 436 3.1 Assumptions 438 The following algorithm assumes that each interface maintains an 439 associated randomized interface identifier. When temporary addresses 440 are generated, the current value of the associated randomized 441 interface identifier is used. While the same identifier can be used 442 to create more than one temporary address, the value SHOULD change 443 over time as described in Section 3.5. 445 The algorithm also assumes that for a given temporary address, an 446 implementation can determine the prefix from which it was generated. 447 When a temporary address is deprecated, a new temporary address is 448 generated. The specific valid and preferred lifetimes for the new 449 address are dependent on the corresponding lifetime values set for 450 the prefix from which it was generated. 452 Finally, this document assumes that when a node initiates outgoing 453 communication, temporary addresses can be given preference over 454 public addresses, when the device is configured to do so. 455 [ADDR_SELECT] mandates implementations to provide a mechanism, which 456 allows an application to configure its preference for temporary 457 addresses over public addresses. It also allows for an 458 implementation to prefer temporary addresses by default, so that the 459 connections initiated by the node can use temporary addresses without 460 requiring application-specific enablement. This document also 461 assumes that an API will exist that allows individual applications to 462 indicate whether they prefer to use temporary or public addresses and 463 override the system defaults. 465 3.2 Generation Of Randomized Interface Identifiers 467 We describe two approaches for the generation and maintenance of the 468 randomized interface identifier. The first assumes the presence of 469 stable storage that can be used to record state history for use as 470 input into the next iteration of the algorithm across system 471 restarts. A second approach addresses the case where stable storage 472 is unavailable and there is a need to generate randomized interface 473 identifiers without previous state. 475 The random interface identifier generation algorithm, as described in 476 this document, uses MD5 as the hash algorithm. The node MAY use 477 another algorithm instead of MD5 to produce the random interface 478 identifier. 480 3.2.1 When Stable Storage Is Present 482 The following algorithm assumes the presence of a 64-bit "history 483 value" that is used as input in generating a randomized interface 484 identifier. The very first time the system boots (i.e., out-of-the- 485 box), a random value SHOULD be generated using techniques that help 486 ensure the initial value is hard to guess [RANDOM]. Whenever a new 487 interface identifier is generated, a value generated by the 488 computation is saved in the history value for the next iteration of 489 the algorithm. 491 A randomized interface identifier is created as follows: 493 1. Take the history value from the previous iteration of this 494 algorithm (or a random value if there is no previous value) and 495 append to it the interface identifier generated as described in 496 [ADDRARCH]. 498 2. Compute the MD5 message digest [MD5] over the quantity created in 499 the previous step. 501 3. Take the left-most 64-bits of the MD5 digest and set bit 6 (the 502 left-most bit is numbered 0) to zero. This creates an interface 503 identifier with the universal/local bit indicating local 504 significance only. 506 4. Compare the generated identifier against a list of reserved 507 interface identifiers and to those already assigned to an address 508 on the local device. In the event that an unacceptable 509 identifier has been generated, the node MUST restart the process 510 at step 1 above, using the right-most 64 bits of the MD5 digest 511 obtained in step 2 in place of the history value in step 1. 513 5. Save the generated identifier as the associated randomized 514 interface identifier. 516 6. Take the rightmost 64-bits of the MD5 digest computed in step 2) 517 and save them in stable storage as the history value to be used 518 in the next iteration of the algorithm. 520 MD5 was chosen for convenience, and because its particular properties 521 were adequate to produce the desired level of randomization. IPv6 522 nodes are already required to implement MD5 as part of IPsec [IPSEC], 523 thus the code will already be present on IPv6 machines. 525 In theory, generating successive randomized interface identifiers 526 using a history scheme as above has no advantages over generating 527 them at random. In practice, however, generating truly random 528 numbers can be tricky. Use of a history value is intended to avoid 529 the particular scenario where two nodes generate the same randomized 530 interface identifier, both detect the situation via DAD, but then 531 proceed to generate identical randomized interface identifiers via 532 the same (flawed) random number generation algorithm. The above 533 algorithm avoids this problem by having the interface identifier 534 (which will often be globally unique) used in the calculation that 535 generates subsequent randomized interface identifiers. Thus, if two 536 nodes happen to generate the same randomized interface identifier, 537 they should generate different ones on the followup attempt. 539 3.2.2 In The Absence of Stable Storage 541 In the absence of stable storage, no history value will be available 542 across system restarts to generate a pseudo-random sequence of 543 interface identifiers. Consequently, the initial history value used 544 above SHOULD be generated at random. A number of techniques might be 545 appropriate. Consult [RANDOM] for suggestions on good sources for 546 obtaining random numbers. Note that even though machines may not 547 have stable storage for storing a history value, they will in many 548 cases have configuration information that differs from one machine to 549 another (e.g., user identity, security keys, serial numbers, etc.). 550 One approach to generating a random initial history value in such 551 cases is to use the configuration information to generate some data 552 bits (which may remain constant for the life of the machine, but will 553 vary from one machine to another), append some random data and 554 compute the MD5 digest as before. 556 3.2.3 Alternate approaches 558 Note that there are other approaches to generate random interface 559 identifiers, albeit with different goals and applicability. One such 560 approach is CGA [CGA], which generates a random interface identifier 561 based on the public key of the node. The goal of CGAs is to prove 562 ownership of an address and to prevent spoofing and stealing of 563 existing IPv6 addresses. They are used for securing neighbor 564 discovery using [SEND]. The CGA random interface identifier 565 generation algorithm may not be suitable for privacy addresses 566 because of the following properties 568 o It requires the node to have a public key. This means that the 569 node can still be identified by its public key 571 o The random interface identifier process is computationally 572 intensive and hence discourages frequent regeneration 574 3.3 Generating Temporary Addresses 576 [ADDRCONF] describes the steps for generating a link-local address 577 when an interface becomes enabled as well as the steps for generating 578 addresses for other scopes. This document extends [ADDRCONF] as 579 follows. When processing a Router Advertisement with a Prefix 580 Information option carrying a global scope prefix for the purposes of 581 address autoconfiguration (i.e., the A bit is set), the node MUST 582 perform the following steps: 584 1. Process the Prefix Information Option as defined in [ADDRCONF], 585 either creating a new public address or adjusting the lifetimes 586 of existing addresses, both public and temporary. If a received 587 option will extend the lifetime of a public address, the 588 lifetimes of temporary addresses should be extended, subject to 589 the overall constraint that no temporary addresses should ever 590 remain "valid" or "preferred" for a time longer than 591 (TEMP_VALID_LIFETIME - DESYNC_FACTOR) or (TEMP_PREFERRED_LIFETIME 592 - DESYNC_FACTOR) respectively. The configuration variables 593 TEMP_VALID_LIFETIME and TEMP_PREFERRED_LIFETIME correspond to 594 approximate target lifetimes for temporary addresses. 596 2. One way an implementation can satisfy the above constraints is to 597 associate with each temporary address a creation time (called 598 CREATION_TIME) that indicates the time at which the address was 599 created. When updating the preferred lifetime of an existing 600 temporary address, it would be set to expire at whichever time is 601 earlier: the time indicated by the received lifetime or 602 (CREATION_TIME + TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR). A 603 similar approach can be used with the valid lifetime. 605 3. When a new public address is created as described in [ADDRCONF], 606 the node SHOULD also create a new temporary address. 608 4. When creating a temporary address, the lifetime values MUST be 609 derived from the corresponding prefix as follows: 611 * Its Valid Lifetime is the lower of the Valid Lifetime of the 612 public address or TEMP_VALID_LIFETIME 614 * Its Preferred Lifetime is the lower of the Preferred Lifetime 615 of the prefix or TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR. 617 5. A temporary address is created only if this calculated Preferred 618 Lifetime is greater than REGEN_ADVANCE time units. In 619 particular, an implementation MUST NOT create a temporary address 620 with a zero Preferred Lifetime. 622 6. New temporary addresses MUST be created by appending the 623 interface's current randomized interface identifier to the prefix 624 that was received. 626 7. The node MUST Perform duplicate address detection (DAD) on the 627 generated temporary address. If DAD indicates the address is 628 already in use, the node MUST generate a new randomized interface 629 identifier as described in Section 3.2 above, and repeat the 630 previous steps as appropriate up to TEMP_IDGEN_RETRIES times. If 631 after TEMP_IDGEN_RETRIES consecutive attempts no non-unique 632 address was generated, the node MUST log a system error and MUST 633 NOT attempt to generate temporary addresses for that interface. 634 Note that DAD MUST be performed on every unicast address 635 generated from this randomized interface identifier. 637 3.4 Expiration of Temporary Addresses 639 When a temporary address becomes deprecated, a new one MUST be 640 generated. This is done by repeating the actions described in 641 Section 3.3, starting at step 3). Note that, except for the 642 transient period when a temporary address is being regenerated, in 643 normal operation at most one temporary address per prefix should be 644 in a non-deprecated state at any given time on a given interface. 645 Note that if a temporary address becomes deprecated as result of 646 processing a Prefix Information Option with a zero Preferred 647 Lifetime, then a new temporary address MUST NOT be generated. To 648 ensure that a preferred temporary address is always available, a new 649 temporary address SHOULD be regenerated slightly before its 650 predecessor is deprecated. This is to allow sufficient time to avoid 651 race conditions in the case where generating a new temporary address 652 is not instantaneous, such as when duplicate address detection must 653 be run. The node SHOULD start the address regeneration process 654 REGEN_ADVANCE time units before a temporary address would actually be 655 deprecated. 657 As an optional optimization, an implementation MAY remove a 658 deprecated temporary address that is not in use by applications or 659 upper-layers as detailed in Section 6. 661 3.5 Regeneration of Randomized Interface Identifiers 663 The frequency at which temporary addresses changes depends on how a 664 device is being used (e.g., how frequently it initiates new 665 communication) and the concerns of the end user. The most egregious 666 privacy concerns appear to involve addresses used for long periods of 667 time (weeks to months to years). The more frequently an address 668 changes, the less feasible collecting or coordinating information 669 keyed on interface identifiers becomes. Moreover, the cost of 670 collecting information and attempting to correlate it based on 671 interface identifiers will only be justified if enough addresses 672 contain non-changing identifiers to make it worthwhile. Thus, having 673 large numbers of clients change their address on a daily or weekly 674 basis is likely to be sufficient to alleviate most privacy concerns. 676 There are also client costs associated with having a large number of 677 addresses associated with a node (e.g., in doing address lookups, the 678 need to join many multicast groups, etc.). Thus, changing addresses 679 frequently (e.g., every few minutes) may have performance 680 implications. 682 Nodes following this specification SHOULD generate new temporary 683 addresses on a periodic basis. This can be achieved automatically by 684 generating a new randomized interface identifier at least once every 685 (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE - DESYNC_FACTOR) time units. 686 As described above, generating a new temporary address REGEN_ADVANCE 687 time units before a temporary address becomes deprecated produces 688 addresses with a preferred lifetime no larger than 689 TEMP_PREFERRED_LIFETIME. The value DESYNC_FACTOR is a random value 690 (different for each client) that ensures that clients don't 691 synchronize with each other and generate new addresses at exactly the 692 same time. When the preferred lifetime expires, a new temporary 693 address MUST be generated using the new randomized interface 694 identifier. 696 Because the precise frequency at which it is appropriate to generate 697 new addresses varies from one environment to another, implementations 698 SHOULD provide end users with the ability to change the frequency at 699 which addresses are regenerated. The default value is given in 700 TEMP_PREFERRED_LIFETIME and is one day. In addition, the exact time 701 at which to invalidate a temporary address depends on how 702 applications are used by end users. Thus, the suggested default 703 value of one week (TEMP_VALID_LIFETIME) may not be appropriate in all 704 environments. Implementations SHOULD provide end users with the 705 ability to override both of these default values. 707 Finally, when an interface connects to a new link, a new randomized 708 interface identifier SHOULD be generated immediately together with a 709 new set of temporary addresses. If a device moves from one ethernet 710 to another, generating a new set of temporary addresses from a 711 different randomized interface identifier ensures that the device 712 uses different randomized interface identifiers for the temporary 713 addresses associated with the two links, making it more difficult to 714 correlate addresses from the two different links as being from the 715 same node. The node MAY follow any process available to it, to 716 determine that the link change has occurred. One such process is 717 described by Detecting Network Attachment [DNA]. 719 3.6 Deployment Considerations 721 Devices implementing this specification MUST provide a way for the 722 end user to explicitly enable or disable the use of temporary 723 addresses. In addition, a site might wish to disable the use of 724 temporary addresses in order to simplify network debugging and 725 operations. Consequently, implementations SHOULD provide a way for 726 trusted system administrators to enable or disable the use of 727 temporary addresses. 729 Additionally, sites might wish to selectively enable or disable the 730 use of temporary addresses for some prefixes. For example, a site 731 might wish to disable temporary address generation for "Unique local" 732 [ULA] prefixes while still generating temporary addresses for all 733 other global prefixes. Another site might wish to enable temporary 734 address generation only for the prefixes 2001::/16 and 2002::/16 735 while disabling it for all other prefixes. To support this behavior, 736 implementations SHOULD provide a way to enable and disable generation 737 of temporary addresses for specific prefix subranges. This per- 738 prefix setting SHOULD override the global settings on the node with 739 respect to the specified prefix subranges. Note that the pre-prefix 740 setting can be applied at any granularity, and not necessarily on a 741 per subnet basis. 743 The use of temporary addresses may cause unexpected difficulties with 744 some applications. As described below, some servers refuse to accept 745 communications from clients for which they cannot map the IP address 746 into a DNS name. In addition, some applications may not behave 747 robustly if temporary addresses are used and an address expires 748 before the application has terminated, or if it opens multiple 749 sessions, but expects them to all use the same addresses. 750 Consequently, the use of temporary addresses SHOULD be disabled by 751 default in order to minimize potential disruptions. Individual 752 applications, which have specific knowledge about the normal duration 753 of connections, MAY override this as appropriate. 755 If a very small number of nodes (say only one) use a given prefix for 756 extended periods of time, just changing the interface identifier part 757 of the address may not be sufficient to ensure privacy, since the 758 prefix acts as a constant identifier. The procedures described in 759 this document are most effective when the prefix is reasonably non 760 static or is used by a fairly large number of nodes. 762 4. Implications of Changing Interface Identifiers 764 The IPv6 addressing architecture goes to some lengths to ensure that 765 interface identifiers are likely to be globally unique where easy to 766 do so. The widespread use of temporary addresses may result in a 767 significant fraction of Internet traffic not using addresses in which 768 the interface identifier portion is globally unique. Consequently, 769 usage of the algorithms in this document may complicate providing 770 such a future flexibility, if global uniqueness is necessary. 772 The desires of protecting individual privacy versus the desire to 773 effectively maintain and debug a network can conflict with each 774 other. Having clients use addresses that change over time will make 775 it more difficult to track down and isolate operational problems. 776 For example, when looking at packet traces, it could become more 777 difficult to determine whether one is seeing behavior caused by a 778 single errant machine, or by a number of them. 780 Some servers refuse to grant access to clients for which no DNS name 781 exists. That is, they perform a DNS PTR query to determine the DNS 782 name, and may then also perform an AAAA query on the returned name to 783 verify that the returned DNS name maps back into the address being 784 used. Consequently, clients not properly registered in the DNS may 785 be unable to access some services. As noted earlier, however, a 786 node's DNS name (if non-changing) serves as a constant identifier. 787 The wide deployment of the extension described in this document could 788 challenge the practice of inverse-DNS-based "authentication," which 789 has little validity, though it is widely implemented. In order to 790 meet server challenges, nodes could register temporary addresses in 791 the DNS using random names (for example a string version of the 792 random address itself). 794 Use of the extensions defined in this document may complicate 795 debugging and other operational troubleshooting activities. 796 Consequently, it may be site policy that temporary addresses should 797 not be used. Consequently, implementations MUST provide a method for 798 the end user or trusted administrator to override the use of 799 temporary addresses. 801 5. Defined Constants 803 Constants defined in this document include: 805 TEMP_VALID_LIFETIME -- Default value: 1 week. Users should be able 806 to override the default value. 808 TEMP_PREFERRED_LIFETIME -- Default value: 1 day. Users should be 809 able to override the default value. 811 REGEN_ADVANCE -- 5 seconds 813 MAX_DESYNC_FACTOR -- 10 minutes. Upper bound on DESYNC_FACTOR. 815 DESYNC_FACTOR -- A random value within the range 0 - 816 MAX_DESYNC_FACTOR. It is computed once at system start (rather than 817 each time it is used) and must never be greater than 818 (TEMP_VALID_LIFETIME - REGEN_ADVANCE). 820 TEMP_IDGEN_RETRIES -- Default value: 3 822 6. Future Work 824 An implementation might want to keep track of which addresses are 825 being used by upper layers so as to be able to remove a deprecated 826 temporary address from internal data structures once no upper layer 827 protocols are using it (but not before). This is in contrast to 828 current approaches where addresses are removed from an interface when 829 they become invalid [ADDRCONF], independent of whether or not upper 830 layer protocols are still using them. For TCP connections, such 831 information is available in control blocks. For UDP-based 832 applications, it may be the case that only the applications have 833 knowledge about what addresses are actually in use. Consequently, an 834 implementation generally will need to use heuristics in deciding when 835 an address is no longer in use. 837 The determination as to whether to use public versus temporary 838 addresses can in some cases only be made by an application. For 839 example, some applications may always want to use temporary 840 addresses, while others may want to use them only in some 841 circumstances or not at all. Suitable API extensions will likely 842 need to be developed to enable individual applications to indicate 843 with sufficient granularity their needs with regards to the use of 844 temporary addresses. Recommendations on DNS practices to avoid the 845 problem described in Section 4 when reverse DNS lookups fail may be 846 needed. [DNSOP] contains a more detailed discussion of the DNS 847 related issues. 849 While this document discusses ways of obscuring a user's permanent IP 850 address, the method described is believed to be ineffective against 851 sophisticated forms of traffic analysis. To increase effectiveness, 852 one may need to consider use of more advanced techniques, such as 853 Onion Routing [ONION]. 855 Open Issues 857 1) Implementations should allow system administrators to configure 858 the use of temporary addresses. We've considered the possibility of 859 using Router Advertisements to configure a host's use of temporary 860 addresses, but that has a major drawback: in some situations (for 861 example a home user receiving RAs from an ISP's router), the 862 administrator of the host and the administrator of the router may 863 have different opinions about the use of temporary addresses. Any 864 configuration mechanism that disables the use of temporary addresses 865 without input from the user MUST ensure that the host's administrator 866 has authorized the disabling. 868 7. Significant Changes from RFC 3041 870 This section summarizes the changes in this document relative to RFC 871 3041 that an implementer of RFC 3041 should be aware of. 873 1. Added wording to exclude certain interface identifiers from the 874 range of acceptable interface identifiers. Interface IDs such 875 as 0, those for reserved anycast addresses [RFC2526], etc. 877 2. Added a configuration knob that provides the end user with a way 878 to enable or disable the use of temporary addresses. 880 3. Under RFC 3041, RAs with short lifetimes (e.g., 1 hour) that 881 always send the same lifetime for long periods of time (e.g., 882 days to weeks) resulted in temporary addresses being created 883 with lifetimes of only 1 hour. Additional rules were added to 884 increase the Lifetime of temporary addresses when the advertised 885 lifetimes were short. 887 4. DAD is now run on all temporary addresses, not just the first 888 one generated from an interface identifier. 890 5. Changed the default setting for usage of temporary addresses to 891 be disabled. 893 6. Added a security considerations section to highlight the ingress 894 filtering issues which can be caused by the use of temporary 895 addresses as described in this document 897 7. Removed references to site-local addresses 899 8. Added a check for denial of service attacks using low valid 900 lifetimes in router advertisements 902 9. Changed the document to use RFC2119 language 904 10. The node is now allowed to generate different interface 905 identifiers for different prefixes, if it so desires. 907 8. Changes from version 00 909 This section summarizes the changes from version 00 of this draft 911 1. The algorithm used for generating random interface identifiers 912 is no longer restricted to just MD5 914 2. Added a problem statement 916 3. Classified the references into normative and informative 918 4. Reduced default number of retries to 3 from 5 and added a 919 configuration variable 921 5. Removed text about RA processing which is duplicated from 922 [ADDRCONF] 924 6. Added text about the privacy implications of a non-changing 925 prefix 927 7. Added a per-prefix enable/disable setting 929 8. Added text about the means of correlation 931 9. Clarified text about DHCPv6 933 10. Added reference to dnsop issues draft 935 9. Changes from version 01 937 This section summarizes the changes from version 01 of this draft 939 1. Clarifiying the length of interface identifier 941 2. Added a per-prefix enable/disable knob as a SHOULD to retain 942 backward compatibility 944 3. Removed normative reference to ISATAP to avoid downref problem 946 4. Added text for per-prefix knobs to be applied at any granularity 948 5. Moved RFC2526 to informative reference 950 10. Changes from version 02 952 This section summarizes the changes from version 02 of this draft 954 1. Explained briefly the concern that is being addressed in the 955 introduction 957 2. Removed reference to 64 bit identifiers in the ADDRCONF context 959 3. Added clarifying text for the usage of DHCPv6 as an alternate 960 approach 962 4. Moved RFC3484 to informative reference 964 5. Updated references for SEND, and CGA as they became RFCs 966 6. Updated draft versions for ULA, DNSOP issues, 2461bis, 2462bis 967 and DNA goals 969 11. Changes from version 03 971 This section summarizes the changes from version 03 of this draft 973 1. Added additional clarifying text regarding regeneration of 974 identifiers as proposed in the AD(Margaret Wasserman) review 975 comments. 977 2. Clarified confusing text which seemed to imply that the randomnly 978 generated identigiers could only be used with global scope 979 addresses. 981 3. Switched to the new IPR boilerplate 983 12. Security Considerations 985 Ingress filtering has been and is being deployed as a means of 986 preventing the use of spoofed source addresses in Distributed Denial 987 of Service(DDoS) attacks. In a network with a large number of nodes, 988 new temporary addresses are created at a fairly high rate. This 989 might make it difficult for ingress filtering mechanisms to 990 distinguish between legitimately changing temporary addresses and 991 spoofed source addresses, which are "in-prefix"(They use a 992 topologically correct prefix and non-existent interface ID). This 993 can be addressed by using access control mechanisms on a per address 994 basis on the network egress point. 996 13. Acknowledgements 998 The authors would like to acknowledge the contributions of the ipv6 999 working group and, in particular, Ran Atkinson, Matt Crawford, Steve 1000 Deering, Allison Mankin, Peter Bieringer, Jari Arkko, Pekka Nikander, 1001 Pekka Savola, Francis Dupont, Brian Haberman, Tatuya Jinmei and 1002 Margaret Wasserman for their detailed comments. 1004 14. References 1006 14.1 Normative References 1008 [ADDRARCH] 1009 Hinden, R. and S. Deering, "Internet Protocol Version 6 1010 (IPv6) Addressing Architecture", RFC 3513, April 2003. 1012 [ADDRCONF] 1013 Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 1014 Address Autoconfiguration", draft-ietf-ipv6-rfc2462bis-07 1015 (work in progress), December 2004. 1017 [DISCOVERY] 1018 Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 1019 "Neighbor Discovery for IP version 6 (IPv6)", 1020 draft-ietf-ipv6-2461bis-02 (work in progress), 1021 February 2005. 1023 [IPSEC] Kent, S. and R. Atkinson, "Security Architecture for the 1024 Internet Protocol", RFC 2401, November 1998. 1026 [MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 1027 April 1992. 1029 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1030 Requirement Levels", RFC 2119, March 1997. 1032 14.2 Informative References 1034 [ADDR_SELECT] 1035 Draves, R., "Default Address Selection for Internet 1036 Protocol version 6 (IPv6)", RFC 3484, February 2003. 1038 [CGA] Aura, T., "Cryptographically Generated Addresses (CGA)", 1039 RFC 3972, March 2005. 1041 [COOKIES] Kristol, D. and L. Montulli, "HTTP State Management 1042 Mechanism", RFC 2965, October 2000. 1044 [DDNS] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, 1045 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 1046 RFC 2136, April 1997. 1048 [DHCP] Droms, R., "Dynamic Host Configuration Protocol", 1049 RFC 2131, March 1997. 1051 [DHCPV6] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., 1052 and M. Carney, "Dynamic Host Configuration Protocol for 1053 IPv6 (DHCPv6)", RFC 3315, July 2003. 1055 [DNA] Choi, J. and G. Daley, "Detecting Network Attachment in 1056 IPv6 Goals", draft-ietf-dna-goals-04 (work in progress), 1057 December 2004. 1059 [DNSOP] Durand, A., Ihren, J., and P. Savola, "Operational 1060 Considerations and Issues with IPv6 DNS", 1061 draft-ietf-dnsop-ipv6-dns-issues-10 (work in progress), 1062 October 2004. 1064 [ONION] Reed, MGR., Syverson, PFS., and DMG. Goldschlag, "Proxies 1065 for Anonymous Routing", Proceedings of the 12th Annual 1066 Computer Security Applications Conference, San Diego, CA, 1067 December 1996. 1069 [RANDOM] Eastlake, D., Crocker, S., and J. Schiller, "Randomness 1070 Recommendations for Security", RFC 1750, December 1994. 1072 [RFC2526] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast 1073 Addresses", RFC 2526, March 1999. 1075 [SEND] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure 1076 Neighbor Discovery (SEND)", RFC 3971, March 2005. 1078 [ULA] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast 1079 Addresses", draft-ietf-ipv6-unique-local-addr-09 (work in 1080 progress), January 2005. 1082 Authors' Addresses 1084 Thomas Narten 1085 IBM Corporation 1086 P.O. Box 12195 1087 Research Triangle Park, NC 1088 USA 1090 Email: narten@raleigh.ibm.com 1091 Richard Draves 1092 Microsoft Research 1093 One Microsoft Way 1094 Redmond, WA 1095 USA 1097 Email: richdr@microsoft.com 1099 Suresh Krishnan 1100 Ericsson Research 1101 8400 Decarie Blvd. 1102 Town of Mount Royal, QC 1103 Canada 1105 Email: suresh.krishnan@ericsson.com 1107 Intellectual Property Statement 1109 The IETF takes no position regarding the validity or scope of any 1110 Intellectual Property Rights or other rights that might be claimed to 1111 pertain to the implementation or use of the technology described in 1112 this document or the extent to which any license under such rights 1113 might or might not be available; nor does it represent that it has 1114 made any independent effort to identify any such rights. 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