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Checking references for intended status: Informational ---------------------------------------------------------------------------- -- Obsolete informational reference (is this intentional?): RFC 4941 (Obsoleted by RFC 8981) == Outdated reference: A later version (-16) exists of draft-ietf-6man-default-iids-15 == Outdated reference: A later version (-08) exists of draft-ietf-6lo-6lobac-05 == Outdated reference: A later version (-09) exists of draft-ietf-6lo-dect-ule-05 == Outdated reference: A later version (-22) exists of draft-ietf-6lo-nfc-04 Summary: 0 errors (**), 0 flaws (~~), 5 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group D. Thaler 3 Internet-Draft Microsoft 4 Intended status: Informational September 13, 2016 5 Expires: March 17, 2017 7 Privacy Considerations for IPv6 over Networks of Resource-Constrained 8 Nodes 9 draft-ietf-6lo-privacy-considerations-03 11 Abstract 13 This document discusses how a number of privacy threats apply to 14 technologies designed for IPv6 over networks of resource-constrained 15 nodes, and provides advice to protocol designers on how to address 16 such threats in adaptation layer specifications for IPv6 over such 17 links. 19 Status of This Memo 21 This Internet-Draft is submitted in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF). Note that other groups may also distribute 26 working documents as Internet-Drafts. The list of current Internet- 27 Drafts is at http://datatracker.ietf.org/drafts/current/. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 This Internet-Draft will expire on March 17, 2017. 36 Copyright Notice 38 Copyright (c) 2016 IETF Trust and the persons identified as the 39 document authors. All rights reserved. 41 This document is subject to BCP 78 and the IETF Trust's Legal 42 Provisions Relating to IETF Documents 43 (http://trustee.ietf.org/license-info) in effect on the date of 44 publication of this document. Please review these documents 45 carefully, as they describe your rights and restrictions with respect 46 to this document. Code Components extracted from this document must 47 include Simplified BSD License text as described in Section 4.e of 48 the Trust Legal Provisions and are provided without warranty as 49 described in the Simplified BSD License. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 54 2. Amount of Entropy Needed in Global Addresses . . . . . . . . 3 55 3. Potential Approaches . . . . . . . . . . . . . . . . . . . . 4 56 3.1. IEEE-Identifier-Based Addresses . . . . . . . . . . . . . 5 57 3.2. Short Addresses . . . . . . . . . . . . . . . . . . . . . 6 58 4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 6 59 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 60 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 61 7. Informative References . . . . . . . . . . . . . . . . . . . 7 62 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 64 1. Introduction 66 RFC 6973 [RFC6973] discusses privacy considerations for Internet 67 protocols, and Section 5.2 in particular covers a number of privacy- 68 specific threats. In the context of IPv6 addresses, Section 3 of 69 [RFC7721] provides further elaboration on the applicability of the 70 privacy threats. 72 When interface identifiers (IIDs) are generated without sufficient 73 entropy compared to the link lifetime, devices and users can become 74 vulnerable to the various threats discussed there, including: 76 o Correlation of activities over time, if the same identifier is 77 used for traffic over period of time 79 o Location tracking, if the same interface identifier is used with 80 different prefixes as a device moves between different networks 82 o Device-specific vulnerability exploitation, if the identifier 83 helps identify a vendor or version or protocol and hence suggests 84 what types of attacks to try 86 o Address scanning, which enables all of the above attacks by off- 87 link attackers. (On some Non-Broadcast Multi-Access (NBMA) links 88 where all nodes aren't already privy to all on-link addresses, 89 address scans might also be done by on-link attackers, but in most 90 cases address scans are not an interesting threat from on-link 91 attackers and thus address scans generally apply only to routable 92 addresses.) 94 For example, for links that may last for years, "enough" bits of 95 entropy means at least 46 or so bits (see Section 2 for why) in a 96 routable address; ideally all 64 bits of the IID should be used, 97 although historically some bits have been excluded for reasons 98 discussed in [RFC7421]. Link-local addresses can also be susceptible 99 to the same privacy threats from off-link attackers, since experience 100 shows they are often leaked by upper-layer protocols such as SMTP, 101 SIP, or DNS. 103 For these reasons, [I-D.ietf-6man-default-iids] recommends using an 104 address generation scheme in [RFC7217], rather than addresses 105 generated from a fixed link-layer address. 107 Furthermore, to mitigate the threat of correlation of activities over 108 time on long-lived links, [RFC4941] specifies the notion of a 109 "temporary" address to be used for transport sessions (typically 110 locally-initiated outbound traffic to the Internet) that should not 111 be linkable to a more permanent identifier such as a DNS name, user 112 name, or fixed link-layer address. Indeed, the default address 113 selection rules [RFC6724] now prefer temporary addresses by default 114 for outgoing connections. If a device needs to simultaneously 115 support unlinkable traffic as well as traffic that is linkable to 116 such a stable identifier, this necessitates supporting simultaneous 117 use of multiple addresses per device. 119 2. Amount of Entropy Needed in Global Addresses 121 In terms of privacy threats discussed in [RFC7721], the one with the 122 need for the most entropy is address scans of routable addresses. To 123 mitigate address scans, one needs enough entropy to make the 124 probability of a successful address probe be negligible. Typically 125 this is measured in the length of time it would take to have a 50% 126 probability of getting at least one hit. Address scans often rely on 127 sending a packet such as a TCP SYN or ICMP Echo Request, and 128 determining whether the reply is an ICMP unreachable error (if no 129 host exists with that address) or a TCP response or ICMP Echo Reply 130 (if a host exists), or neither in which case nothing is known for 131 certain. 133 Many privacy-sensitive devices support a "stealth mode" as discussed 134 in Section 5 of [RFC7288] or are behind a network firewall that will 135 drop unsolicited inbound traffic (e.g., TCP SYNs, ICMP Echo Requests, 136 etc.) and thus no TCP RST or ICMP Echo Reply will be sent. In such 137 cases, and when the device does not listen on a well-known TCP or UDP 138 port known to the scanner, the effectiveness of an address scan is 139 limited by the ability to get ICMP unreachable errors, since the 140 attacker can only infer the presence of a host based on the absense 141 of an ICMP unreachable error. 143 Generation of ICMP unreachable errors is typically rate limited to 2 144 per second (the default in routers such as Cisco routers running IOS 145 12.0 or later). Such a rate results in taking about a year to 146 completely scan 26 bits of space. 148 The actual math is as follows. Let 2^N be the number of devices on 149 the subnet. Let 2^M be the size of the space to scan (i.e., M bits 150 of entropy). Let S be the number of scan attempts. The formula for 151 a 50% chance of getting at least one hit in S attempts is: P(at least 152 one success) = 1 - (1 - 2^N/2^M)^S = 1/2. Assuming 2^M >> S, this 153 simplifies to: S * 2^N/2^M = 1/2, giving S = 2^(M-N-1), or M = N + 1 154 + log_2(S). Using a scan rate of 2 per second, this results in the 155 following rule of thumb: 157 Bits of entropy needed = log_2(# devices per link) + log_2(seconds 158 of link lifetime) + 2 160 For example, for a network with at most 2^16 devices on the same 161 long-lived link, and the average lifetime of a link being 8 years 162 (2^28 seconds) or less, this results in a need for at least 46 bits 163 of entropy (16+28+2) so that an address scan would need to be 164 sustained for longer than the lifetime of the link to have a 50% 165 chance of getting a hit. 167 Although 46 bits of entropy may be enough to provide privacy in such 168 cases, 59 or more bits of entropy would be needed if addresses are 169 used to provide security against attacks such as spoofing, as CGAs 170 [RFC3972] and HBAs [RFC5535] do, since attacks are not limited by 171 ICMP rate limiting but by the processing power of the attacker. See 172 those RFCs for more discussion. 174 If, on the other hand, the devices being scanned for respond to 175 unsolicited inbound packets, then the address scan is not limited by 176 the ICMP unreachable rate limit in routers, since an adversary can 177 determine the presence of a host without them. In such cases, more 178 bits of entropy would be needed to provide the same level of 179 protection. 181 3. Potential Approaches 183 The table below shows the number of bits of entropy currently 184 available in various technologies: 186 +---------------+--------------------------+--------------------+ 187 | Technology | Reference | Bits of Entropy | 188 +---------------+--------------------------+--------------------+ 189 | 802.15.4 | [RFC4944] | 16+ or any EUI-64 | 190 | Bluetooth LE | [RFC7668] | 48 | 191 | DECT ULE | [I-D.ietf-6lo-dect-ule] | 40 or any EUI-48 | 192 | MS/TP | [I-D.ietf-6lo-6lobac] | 7 or 64 | 193 | ITU-T G.9959 | [RFC7428] | 8 | 194 | NFC | [I-D.ietf-6lo-nfc] | 6 or ??? | 195 +---------------+--------------------------+--------------------+ 197 Such technologies generally support either IEEE identifiers or so 198 called "Short Addresses", or both, as link layer addresses. We 199 discuss each in turn. 201 3.1. IEEE-Identifier-Based Addresses 203 Some technologies allow the use of IEEE EUI-48 or EUI-64 identifiers, 204 or allow using an arbitrary 64-bit identifier. Using such an 205 identifier to construct IPv6 addresses makes it easy to use the 206 normal LOWPAN_IPHC encoding with stateless compression, allowing such 207 IPv6 addresses to be fully elided in common cases. 209 Global addresses with interface identifiers formed from IEEE 210 identifiers can have insufficient entropy to mitigate address scans 211 unless the IEEE identifier itself has sufficient entropy, and enough 212 bits of entropy are carried over into the IPv6 address to 213 sufficiently mitigate the threats. Privacy threats other than 214 "Correlation over time" can be mitigated using per-network randomized 215 link-layer addresses with enough entropy compared to the link 216 lifetime. A number of such proposals can be found at 217 , and Section 10.8 of 218 [BTCorev4.1] specifies one for Bluetooth. Using routable IPv6 219 addresses derived from such link-layer addresses would be roughly 220 equivalent to those specified in [RFC7217]. 222 Correlation over time (for all addresses, not just routable 223 addresses) can be mitigated if the link-layer address itself changes 224 often enough, such as each time the link is established, if the link 225 lifetime is short. For further discussion, see 226 [I-D.huitema-6man-random-addresses]. 228 Another potential concern is that of efficiency, such as avoiding 229 Duplicate Address Detection (DAD) all together when IPv6 addresses 230 are IEEE-identifier-based. Appendix A of [RFC4429] provides an 231 analysis of address collision probability based on the number of bits 232 of entropy. A simple web search on "duplicate MAC addresses" will 233 show that collisions do happen with MAC addresses, and thus based on 234 the analysis in [RFC4429], using sufficient bits of entropy in random 235 addresses can provide greater protection against collision than using 236 MAC addresses. 238 3.2. Short Addresses 240 A routable IPv6 address with an interface identifier formed from the 241 combination of a "Short Address" and a set of well-known constant 242 bits (such as padding with 0's) lacks sufficient entropy to mitigate 243 address scanning unless the link lifetime is extremely short. 244 Furthermore, an adversary could also use statisical methods to 245 determine the size of the L2 address space and thereby make some 246 inference regarding the underlying technology on a given link, and 247 target further attacks accordingly. 249 When Short Addresses are desired on links that are not guaranteed to 250 have a short enough lifetime, the mechanism for constructing an IPv6 251 interface identifier from a Short Address could be designed to 252 sufficiently mitigate the problem. For example, if all nodes on a 253 given L2 network have a shared secret (such as the key needed to get 254 on the layer-2 network), the 64-bit IID might be generated using a 255 one-way hash that includes (at least) the shared secret together with 256 the Short Address. The use of such a hash would result in the IIDs 257 being spread out among the full range of IID address space, thus 258 mitigating address scans, while still allowing full stateless 259 compression/elision. 261 For long-lived links, "temporary" addresses might even be generated 262 in the same way by (for example) also including in the hash the 263 Version Number from the Authoritative Border Router Option 264 (Section 4.3 of [RFC6775]), if any. This would allow changing 265 temporary addresses whenever the Version Number is changed, even if 266 the set of prefix or context information is unchanged. 268 In summary, any specification using Short Addresses should carefully 269 construct an IID generation mechanism so as to provide sufficient 270 entropy compared to the link lifetime. 272 4. Recommendations 274 The following are recommended for adaptation layer specifications: 276 o Security (privacy) sections should say how address scans are 277 mitigated. An address scan might be mitigated by having a link 278 always be short-lived, or might be mitigated by having a large 279 number of bits of entropy in routable addresses, or some 280 combination. Thus, a specification should explain what the 281 maximum lifetime of a link is in practice, and show how the number 282 of bits of entropy is sufficient given that lifetime. 284 o Technologies should define a way to include sufficient bits of 285 entropy in the IPv6 interface identifier, based on the maximum 286 link lifetime. Specifying that randomized link-layer addresses 287 can be used is one easy way to do so, for technologies that 288 support such identifiers. 290 o Specifications should not simply construct an IPv6 interface 291 identifier by padding a short address with a set of other well- 292 known constant bits, unless the link lifetime is guaranteed to be 293 extremely short or the short address is allocated by the network 294 (rather than being constant in the node). This also applies to 295 link-local addresses if the same short address is used independent 296 of network and is unique enough to allow location tracking. 298 o Specifications should make sure that an IPv6 address can change 299 over long periods of time. For example, the interface identifier 300 might change each time a device connects to the network (if 301 connections are short), or might change each day (if connections 302 can be long). This is necessary to mitigate correlation over 303 time. 305 o If a device can roam between networks, and more than a few bits of 306 entropy exist in the IPv6 interface identifier, then make sure 307 that the interface identifier can vary per network as the device 308 roams. This is necessary to mitigate location tracking. 310 5. IANA Considerations 312 This document has no actions for IANA. 314 6. Security Considerations 316 This entire document is about security considerations and how to 317 specify possible mitigations. 319 7. Informative References 321 [RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", 322 RFC 3972, DOI 10.17487/RFC3972, March 2005, 323 . 325 [RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD) 326 for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006, 327 . 329 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 330 Extensions for Stateless Address Autoconfiguration in 331 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 332 . 334 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 335 "Transmission of IPv6 Packets over IEEE 802.15.4 336 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, 337 . 339 [RFC5535] Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535, 340 DOI 10.17487/RFC5535, June 2009, 341 . 343 [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, 344 "Default Address Selection for Internet Protocol Version 6 345 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, 346 . 348 [RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C. 349 Bormann, "Neighbor Discovery Optimization for IPv6 over 350 Low-Power Wireless Personal Area Networks (6LoWPANs)", 351 RFC 6775, DOI 10.17487/RFC6775, November 2012, 352 . 354 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 355 Morris, J., Hansen, M., and R. Smith, "Privacy 356 Considerations for Internet Protocols", RFC 6973, 357 DOI 10.17487/RFC6973, July 2013, 358 . 360 [RFC7217] Gont, F., "A Method for Generating Semantically Opaque 361 Interface Identifiers with IPv6 Stateless Address 362 Autoconfiguration (SLAAC)", RFC 7217, 363 DOI 10.17487/RFC7217, April 2014, 364 . 366 [RFC7288] Thaler, D., "Reflections on Host Firewalls", RFC 7288, 367 DOI 10.17487/RFC7288, June 2014, 368 . 370 [RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S., 371 Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit 372 Boundary in IPv6 Addressing", RFC 7421, 373 DOI 10.17487/RFC7421, January 2015, 374 . 376 [RFC7428] Brandt, A. and J. Buron, "Transmission of IPv6 Packets 377 over ITU-T G.9959 Networks", RFC 7428, 378 DOI 10.17487/RFC7428, February 2015, 379 . 381 [RFC7668] Nieminen, J., Savolainen, T., Isomaki, M., Patil, B., 382 Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low 383 Energy", RFC 7668, DOI 10.17487/RFC7668, October 2015, 384 . 386 [RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy 387 Considerations for IPv6 Address Generation Mechanisms", 388 RFC 7721, DOI 10.17487/RFC7721, March 2016, 389 . 391 [I-D.ietf-6man-default-iids] 392 Gont, F., Cooper, A., Thaler, D., and S. LIU, 393 "Recommendation on Stable IPv6 Interface Identifiers", 394 draft-ietf-6man-default-iids-15 (work in progress), August 395 2016. 397 [I-D.ietf-6lo-6lobac] 398 Lynn, K., Martocci, J., Neilson, C., and S. Donaldson, 399 "Transmission of IPv6 over MS/TP Networks", draft-ietf- 400 6lo-6lobac-05 (work in progress), June 2016. 402 [I-D.ietf-6lo-dect-ule] 403 Mariager, P., Petersen, J., Shelby, Z., Logt, M., and D. 404 Barthel, "Transmission of IPv6 Packets over DECT Ultra Low 405 Energy", draft-ietf-6lo-dect-ule-05 (work in progress), 406 May 2016. 408 [I-D.ietf-6lo-nfc] 409 Hong, Y. and J. Youn, "Transmission of IPv6 Packets over 410 Near Field Communication", draft-ietf-6lo-nfc-04 (work in 411 progress), July 2016. 413 [I-D.huitema-6man-random-addresses] 414 Huitema, C., "Implications of Randomized Link Layers 415 Addresses for IPv6 Address Assignment", draft-huitema- 416 6man-random-addresses-03 (work in progress), March 2016. 418 [BTCorev4.1] 419 Bluetooth Special Interest Group, "Bluetooth Core 420 Specification Version 4.1", December 2013, 421 . 424 Author's Address 426 Dave Thaler 427 Microsoft 428 One Microsoft Way 429 Redmond, WA 98052 430 USA 432 Email: dthaler@microsoft.com