idnits 2.17.1 draft-ietf-dhc-dhcp-privacy-03.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (January 18, 2016) is 3015 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- -- Obsolete informational reference (is this intentional?): RFC 2629 (Obsoleted by RFC 7749) -- Obsolete informational reference (is this intentional?): RFC 3315 (Obsoleted by RFC 8415) -- Obsolete informational reference (is this intentional?): RFC 4941 (Obsoleted by RFC 8981) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 dhc S. Jiang 3 Internet-Draft Huawei Technologies Co., Ltd 4 Intended status: Informational S. Krishnan 5 Expires: July 21, 2016 Ericsson 6 T. Mrugalski 7 ISC 8 January 18, 2016 10 Privacy considerations for DHCPv4 11 draft-ietf-dhc-dhcp-privacy-03 13 Abstract 15 DHCP is a protocol that is used to provide addressing and 16 configuration information to IPv4 hosts. This document discusses the 17 various identifiers used by DHCP and the potential privacy issues. 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 July 21, 2016. 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. Requirements Language and Terminology . . . . . . . . . . . . 3 55 3. DHCP Options Carrying Identifiers . . . . . . . . . . . . . . 3 56 3.1. Client Identifier Option . . . . . . . . . . . . . . . . 4 57 3.2. Address Fields & Options . . . . . . . . . . . . . . . . 4 58 3.3. Client FQDN Option . . . . . . . . . . . . . . . . . . . 5 59 3.4. Parameter Request List Option . . . . . . . . . . . . . . 5 60 3.5. Vendor Class and Vendor-Identifying Vendor Class Options 5 61 3.6. Civic Location Option . . . . . . . . . . . . . . . . . . 5 62 3.7. Coordinate-Based Location Option . . . . . . . . . . . . 6 63 3.8. Client System Architecture Type Option . . . . . . . . . 6 64 3.9. Relay Agent Information Option and Sub-options . . . . . 6 65 4. Existing Mechanisms That Affect Privacy . . . . . . . . . . . 7 66 4.1. DNS Updates . . . . . . . . . . . . . . . . . . . . . . . 7 67 4.2. Allocation strategies . . . . . . . . . . . . . . . . . . 7 68 5. Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 69 5.1. Device type discovery . . . . . . . . . . . . . . . . . . 8 70 5.2. Operating system discovery . . . . . . . . . . . . . . . 9 71 5.3. Finding location information . . . . . . . . . . . . . . 9 72 5.4. Finding previously visited networks . . . . . . . . . . . 9 73 5.5. Finding a stable identity . . . . . . . . . . . . . . . . 9 74 5.6. Pervasive monitoring . . . . . . . . . . . . . . . . . . 9 75 5.7. Finding client's IP address or hostname . . . . . . . . . 10 76 5.8. Correlation of activities over time . . . . . . . . . . . 10 77 5.9. Location tracking . . . . . . . . . . . . . . . . . . . . 10 78 5.10. Leasequery & bulk leasequery . . . . . . . . . . . . . . 10 79 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 80 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 11 81 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 82 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 83 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 84 10.1. Normative References . . . . . . . . . . . . . . . . . . 11 85 10.2. Informative References . . . . . . . . . . . . . . . . . 12 86 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 88 1. Introduction 90 Dynamic Host Configuration Protocol (DHCP) [RFC2131] is a protocol 91 that is used to provide addressing and configuration information to 92 IPv4 hosts. The DHCP protocol uses several identifiers that could 93 become a source for gleaning information about the IPv4 host. This 94 information may include device type, operating system information, 95 location(s) that the device may have previously visited, etc. This 96 document discusses the various identifiers used by DHCP and the 97 potential privacy issues [RFC6973]. In particular, it also takes 98 into consideration the problem of pervasive monitoring [RFC7258]. 100 Future works may propose protocol changes to fix the privacy issues 101 that have been analyzed in this document. It is out of scope for 102 this document. 104 The primary focus of this document is around privacy considerations 105 for clients to support client mobility and connection to random 106 networks. The privacy of DHCP servers and relay agents are 107 considered less important as they are typically open for public 108 services. And, it is generally assumed that relay agent to server 109 communication is protected from casual snooping, as that 110 communication occurs in the provider's backbone. Nevertheless, the 111 topics involving relay agents and servers are explored to some 112 degree. However, future work may want to further explore privacy of 113 DHCP servers and relay agents. 115 2. Requirements Language and Terminology 117 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 118 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 119 document are to be interpreted as described in [RFC2119]. When these 120 words are not in ALL CAPS (such as "should" or "Should"), they have 121 their usual English meanings, and are not to be interpreted as 122 [RFC2119] key words. 124 In addition the following terminology is used: 126 Stable identifier - Any property disclosed by a DHCP client that 127 does not change over time or changes very infrequently and is 128 unique for said client in a given context. Examples may 129 include MAC address, client-id or a hostname. Some 130 identifiers may be considered stable only under certain 131 conditions, for example one client implementation may keep 132 its client-id stored in stable storage while other may 133 generate it on the fly and use a different one after each 134 boot. Stable identifier may or may not be globally unique. 136 3. DHCP Options Carrying Identifiers 138 In DHCP, there are a few options which contain identification 139 information or can be used to extract the identification information 140 about the client. This section enumerates various options and 141 identifiers conveyed in them, which can be used to disclose client 142 identification. They are targets of various attacks that would be 143 analyzed in Section 5. 145 3.1. Client Identifier Option 147 The Client Identifier Option [RFC2131] is used to pass an explicit 148 client identifier to a DHCP server. 150 The client identifier is an opaque key, which must be unique to that 151 client within the subnet to which the client is attached. It 152 typically remains stable after it has been initially generated. It 153 may contain a hardware address, identical to the contents of the 154 'chaddr' field, or another type of identifier, such as a DNS name. 155 [RFC3315] in Section 9.2 specifies DUID-LLT (Link-layer + time) as 156 the recommended DUID (DHCP Unique Identifier) type. [RFC4361], 157 Section 6.1 introduces this concept to DHCPv4. Those two document 158 recommend that client identifiers be generated by using the permanent 159 link-layer address of the network interface that the client is trying 160 to configure. [RFC4361] updates the recommendation of Client 161 Identifiers to be "consists of a type field whose value is normally 162 255, followed by a four-byte IA_ID field, followed by the DUID for 163 the client as defined in RFC 3315, section 9". This does not change 164 the lifecycle of the Client Identifiers. Clients are expected to 165 generate their Client Identifiers once (during first operation) and 166 store it in a non-volatile storage or use the same deterministic 167 algorithm to generate the same Client Identifier values again. 169 This means that most implementations will use the available link- 170 layer address during its first boot. Even if the administrator 171 enables link-layer address randomization, it is likely that it was 172 disabled during the first device boot. Hence the original, 173 unobfuscated link-layer address will likely end up being announced as 174 client identifier, even if the link- layer address has changed (or 175 even if being changed on a periodic basis). The exposure of the 176 original link-layer address in the client identifier will also 177 undermine other privacy extensions such as [RFC4941]. 179 3.2. Address Fields & Options 181 The 'yiaddr' field [RFC2131] in DHCP message is used to convey 182 allocated address from the server to the client. 184 The DHCPv4 specification [RFC2131] provides a way to specify the 185 client link-layer address in the DHCPv4 message header. A DHCPv4 186 message header has 'htype' and 'chaddr' fields to specify the client 187 link-layer address type and the link-layer address, respectively. 188 The 'chaddr' field is used both as a hardware address for 189 transmission of reply messages and as a client identifier. 191 The 'requested IP address' option [RFC2131] is used by client to 192 suggest that a particular IP address be assigned. 194 3.3. Client FQDN Option 196 The Client Fully Qualified Domain Name (FQDN) option [RFC4702] is 197 used by DHCP clients and servers to exchange information about the 198 client's fully qualified domain name and about who has the 199 responsibility for updating the DNS with the associated A and PTR 200 RRs. 202 A client can use this option to convey all or part of its domain name 203 to a DHCP server for the IP-address-to-FQDN mapping. In most case a 204 client sends its hostname as a hint for the server. The DHCP server 205 MAY be configured to modify the supplied name or to substitute a 206 different name. The server should send its notion of the complete 207 FQDN for the client in the Domain Name field. 209 3.4. Parameter Request List Option 211 The Parameter Request List option [RFC2131] is used to inform the 212 server about options the client wants the server to send to the 213 client. The content of a Parameter Request List option are the 214 option codes for an option requested by the client. 216 3.5. Vendor Class and Vendor-Identifying Vendor Class Options 218 The Vendor Class option [RFC2131], the Vendor-Identifying Vendor 219 Class option and Vendor-Identifying Vendor Information option 220 [RFC3925] are used by the DHCP client to identify the vendor that 221 manufactured the hardware on which the client is running. 223 The information contained in the data area of this option is 224 contained in one or more opaque fields that identify the details of 225 the hardware configuration of the host on which the client is 226 running, or of industry consortium compliance, for example, the 227 version of the operating system the client is running or the amount 228 of memory installed on the client. 230 3.6. Civic Location Option 232 DHCP servers use the Civic Location Option [RFC4776] to deliver of 233 the location information (the civic and postal addresses) to the DHCP 234 clients. It may refer to three locations: the location of the DHCP 235 server, the location of the network element believed to be closest to 236 the client, or the location of the client, identified by the "what" 237 element within the option. 239 3.7. Coordinate-Based Location Option 241 The GeoConf and GeoLoc options [RFC6225] is used by DHCP server to 242 provide the coordinate-based geographic location information to the 243 DHCP clients. It enables a DHCP client to obtain its geographic 244 location. 246 3.8. Client System Architecture Type Option 248 The Client System Architecture Type Option [RFC4578] is used by DHCP 249 client to send a list of supported architecture types to the DHCP 250 server. It is used by clients that must be booted using the network 251 rather than from local storage, so the server can decide which boot 252 file should be provided to the client. 254 3.9. Relay Agent Information Option and Sub-options 256 A DHCP relay agent includes a Relay Agent Information [RFC3046] to 257 identify the remote host end of the circuit. It contains a "circuit 258 ID" sub-option for the incoming circuit, which is an agent-local 259 identifier of the circuit from which a DHCP client-to-server packet 260 was received, and a "remote ID" sub-option which provides a trusted 261 identifier for the remote high-speed modem. 263 Possible encoding of "circuit ID" sub-option includes: router 264 interface number, switching hub port number, remote access server 265 port number, frame relay DLCI, ATM virtual circuit number, cable data 266 virtual circuit number, etc. 268 Possible encoding of the "remote ID" sub-option includes: a "caller 269 ID" telephone number for dial-up connection, a "user name" prompted 270 for by a remote access server, a remote caller ATM address, a "modem 271 ID" of a cable data modem, the remote IP address of a point-to-point 272 link, a remote X.25 address for X.25 connections, etc. 274 The link-selection sub-option [RFC3527] is used by any DHCP relay 275 agent that desires to specify a subnet/link for a DHCP client request 276 that it is relaying but needs the subnet/link specification to be 277 different from the IP address the DHCP server should use when 278 communicating with the relay agent. It contains an IP address, which 279 can identify the client's subnet/link. Also, assuming network 280 topology knowledge, it also reveals client location. 282 A DHCP relay includes a Subscriber-ID option [RFC3993] to associate 283 some provider-specific information with clients' DHCP messages that 284 is independent of the physical network configuration through which 285 the subscriber is connected. The "subscriber-id" assigned by the 286 provider is intended to be stable as customers connect through 287 different paths, and as network changes occur. The Subscriber-ID is 288 an ASCII string, which is assigned and configured by the network 289 provider. 291 4. Existing Mechanisms That Affect Privacy 293 This section describes deployed DHCP mechanisms that affect privacy. 295 4.1. DNS Updates 297 The Client FQDN (Fully Qualified Domain Name) Option [RFC4702] used 298 along with DNS Updates [RFC2136] defines a mechanism that allows both 299 clients and server to insert into the DNS domain information about 300 clients. Both forward (A) and reverse (PTR) resource records can be 301 updated. This allows other nodes to conveniently refer to a host, 302 despite the fact that its IP address may be changing. 304 This mechanism exposes two important pieces of information: current 305 address (which can be mapped to current location) and client's 306 hostname. The stable hostname can then be used to correlate the 307 client across different network attachments even when its IP 308 addresses keep changing. 310 4.2. Allocation strategies 312 A DHCP server running in typical, stateful mode is given a task of 313 managing one or more pools of IP address. When a client requests an 314 address, the server must pick an address out of configured pool. 315 Depending on the server's implementation, various allocation 316 strategies are possible. Choices in this regard may have privacy 317 implications. Note that the constraints in DHCPv4 and DHCPv6 are 318 radically different, but servers that allow allocation strategy 319 configuration may allow configuring them in both DHCPv4 and DHCPv6. 320 Not every allocation strategy is equally suitable for DHCPv4 and for 321 DHCPv6. 323 Iterative allocation - a server may choose to allocate addresses one 324 by one. That strategy has the benefit of being very fast, thus can 325 be favored in deployments that prefer performance. However, it makes 326 the allocated addresses very predictable. Also, since the addresses 327 allocated tend to be clustered at the beginning of available pool, it 328 makes scanning attacks much easier. 330 Identifier-based allocation - some server implementations may choose 331 to allocate an address that is based on one of available identifiers, 332 e.g. client identifier or MAC address. It is also convenient, as 333 returning client is very likely to get the same address. Those 334 properties are convenient for system administrators, so DHCP server 335 implementors are often requested to implement it. The downside of 336 such allocation is that the client has a very stable IP address. 337 That means that correlation of activities over time, location 338 tracking, address scanning and OS/vendor discovery apply. This is 339 certainly an issue in DHCPv6, but due to much smaller address space 340 is almost never a problem in DHCPv4. 342 Hash allocation - it's an extension of identifier based allocation. 343 Instead of using the identifier directly, it is being hashed first. 344 If the hash is implemented correctly, it removes the flaw of 345 disclosing the identifier, a property that eliminates susceptibility 346 to address scanning and OS/vendor discovery. If the hash is poorly 347 implemented (e.g. can be reverted), it introduces no improvement over 348 identifier-based allocation. 350 Random allocation - a server can pick a resource randomly out of 351 available pool. That strategy works well in scenarios where pool 352 utilization is small, as the likelihood of collision (resulting in 353 the server needing to repeat randomization) is small. With the pool 354 allocation increasing, the collision is disproportionally large, due 355 to birthday paradox. With high pool utilization (e.g. when 90% of 356 available resources being allocated already), the server will use 357 most computational resources to repeatedly pick a random resource, 358 which will degrade its performance. This allocation scheme 359 essentially prevents returning clients from getting the same address 360 again. On the other hand, it is beneficial from privacy perspective 361 as addresses generated that way are not susceptible to correlation 362 attacks, OS/vendor discovery attacks or identity discovery attacks. 363 Note that even though the address itself may be resilient to a given 364 attack, the client may still be susceptible if additional information 365 is disclosed other way, e.g. client's address can be randomized, but 366 it still can leak its MAC address in client-id option. 368 Other allocation strategies may be implemented. 370 Given the limited size of most IPv4 public address pools, allocation 371 mechanisms in IPv4 may not provide much privacy protection or leak 372 much useful information, if misused. 374 5. Attacks 376 5.1. Device type discovery 378 The type of device used by the client can be guessed by the attacker 379 using the Vendor Class Option, the 'chaddr' field, and by parsing the 380 Client ID Option. All of those options may contain an 381 Organizationally Unique Identifier (OUI) that represents the device's 382 vendor. That knowledge can be used for device-specific vulnerability 383 exploitation attacks. 385 5.2. Operating system discovery 387 The operating system running on a client can be guessed using the 388 Vendor Class option, the Client System Architecture Type option, or 389 by using fingerprinting techniques on the combination of options 390 requested using the Parameter Request List option. 392 5.3. Finding location information 394 The location information can be obtained by the attacker by many 395 means. The most direct way to obtain this information is by looking 396 into a message originating from the server that contains the Civic 397 Location, GeoConf, or GeoLoc options. It can also be indirectly 398 inferred using the Relay Agent Information option, with the remote ID 399 sub-option, the circuit ID option (e.g. if an access circuit on an 400 Access Node corresponds to a civic location), or the Subscriber ID 401 Option (if the attacker has access to subscriber info). 403 5.4. Finding previously visited networks 405 When DHCP clients connect to a network, they attempt to obtain the 406 same address they had used before they attached to the network. They 407 do this by putting the previously assigned address in the requested 408 IP address option. By observing these addresses, an attacker can 409 identify the network the client had previously visited. 411 5.5. Finding a stable identity 413 An attacker might use a stable identity gleaned from DHCP messages to 414 correlate activities of a given client on unrelated networks. The 415 Client FQDN option, the Subscriber ID Option and the Client ID 416 options can serve as long lived identifiers of DHCP clients. The 417 Client FQDN option can also provide an identity that can easily be 418 correlated with web server activity logs. 420 5.6. Pervasive monitoring 422 This is an enhancement, or a combination of most aforementioned 423 mechanisms. Operator who controls non-trivial number of access 424 points or network segments, may use obtained information about a 425 single client and observer client's habits. 427 5.7. Finding client's IP address or hostname 429 Many DHCP deployments use DNS Updates [RFC4702] that put client's 430 information (current IP address, client's hostname) into DNS, where 431 it is easily accessible by anyone interested. Client ID is also 432 disclosed, albeit in not easily accessible form (SHA-256 digest of 433 the client-id). As SHA-256 is considered irreversible, DHCID can't 434 be converted back to client-id. However, SHA-256 digest can be used 435 as an unique identifier that is accessible by any host. 437 5.8. Correlation of activities over time 439 As with other identifiers, an IP address can be used to correlate the 440 activities of a host for at least as long as the lifetime of the 441 address. If that address was generated from some other, stable 442 identifier and that generation scheme can be deducted by an attacker, 443 the duration of correlation attack extends to that identifier. In 444 many cases, its lifetime is equal to the lifetime of the device 445 itself. 447 5.9. Location tracking 449 If a stable identifier is used for assigning an address and such 450 mapping is discovered by an attacker, e.g. a hostname being put into 451 DNS, it can be used for tracking user. In particular both passive (a 452 service that the client connects to can log client's address and draw 453 conclusions regarding its location and movement patterns based on 454 address it is connecting from) and active (attacker can send ICMP 455 echo requests or other probe packets to networks of suspected client 456 locations) methods can be used. To give specific example, by 457 accessing a social portal from tomek- 458 laptop.coffee.somecity.com.example, tomek- 459 laptop.mycompany.com.example and tomek-laptop.myisp.example.com, the 460 portal administrator can draw conclusions about tomek-laptop's owner 461 current location and his habits. 463 5.10. Leasequery & bulk leasequery 465 Attackers may pretend as an access concentrator, either DHCP relay 466 agent or DHCP client, to obtain location information directly from 467 the DHCP server(s) using the DHCP leasequery [RFC4388] mechanism. 469 Location information is information needed by the access concentrator 470 to forward traffic to a broadband-accessible host. This information 471 includes knowledge of the host hardware address, the port or virtual 472 circuit that leads to the host, and/or the hardware address of the 473 intervening subscriber modem. 475 Furthermore, the attackers may use DHCP bulk leasequery [RFC6926] 476 mechanism to obtain bulk information about DHCP bindings, even 477 without knowing the target bindings. 479 Additionally, active leasequery [RFC7724] is a mechanism for 480 subscribing to DHCPv4 lease update changes in near real-time. The 481 intent of this mechanism is to update operator's database, but if 482 misused, an attacker could defeat server's authentication mechanisms 483 and subscribe to all updates. He then could continue receiving 484 updates, without any need for local presence. 486 6. Security Considerations 488 In current practice, the client privacy and the client authentication 489 are mutually exclusive. The client authentication procedure reveals 490 additional client information in their certificates/identifiers. 491 Full privacy for the clients may mean the clients are also anonymous 492 for the server and the network. 494 7. Privacy Considerations 496 This document at its entirety discusses privacy considerations in 497 DHCP. As such, no dedicated discussion is needed. 499 8. IANA Considerations 501 This draft does not request any IANA action. 503 9. Acknowledgements 505 The authors would like to thank the valuable comments made by Stephen 506 Farrell, Ted Lemon, Ines Robles, Russ White, Christian Huitema, 507 Bernie Volz, Jinmei Tatuya, Marcin Siodelski, Christian Schaefer and 508 other members of DHC WG. 510 This document was produced using the xml2rfc tool [RFC2629]. 512 10. References 514 10.1. Normative References 516 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 517 Requirement Levels", BCP 14, RFC 2119, 518 DOI 10.17487/RFC2119, March 1997, 519 . 521 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 522 RFC 2131, DOI 10.17487/RFC2131, March 1997, 523 . 525 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, 526 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 527 RFC 2136, DOI 10.17487/RFC2136, April 1997, 528 . 530 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 531 Morris, J., Hansen, M., and R. Smith, "Privacy 532 Considerations for Internet Protocols", RFC 6973, 533 DOI 10.17487/RFC6973, July 2013, 534 . 536 [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an 537 Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 538 2014, . 540 10.2. Informative References 542 [RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, 543 DOI 10.17487/RFC2629, June 1999, 544 . 546 [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", 547 RFC 3046, DOI 10.17487/RFC3046, January 2001, 548 . 550 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 551 C., and M. Carney, "Dynamic Host Configuration Protocol 552 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 553 2003, . 555 [RFC3527] Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy, 556 "Link Selection sub-option for the Relay Agent Information 557 Option for DHCPv4", RFC 3527, DOI 10.17487/RFC3527, April 558 2003, . 560 [RFC3925] Littlefield, J., "Vendor-Identifying Vendor Options for 561 Dynamic Host Configuration Protocol version 4 (DHCPv4)", 562 RFC 3925, DOI 10.17487/RFC3925, October 2004, 563 . 565 [RFC3993] Johnson, R., Palaniappan, T., and M. Stapp, "Subscriber-ID 566 Suboption for the Dynamic Host Configuration Protocol 567 (DHCP) Relay Agent Option", RFC 3993, 568 DOI 10.17487/RFC3993, March 2005, 569 . 571 [RFC4361] Lemon, T. and B. Sommerfeld, "Node-specific Client 572 Identifiers for Dynamic Host Configuration Protocol 573 Version Four (DHCPv4)", RFC 4361, DOI 10.17487/RFC4361, 574 February 2006, . 576 [RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration 577 Protocol (DHCP) Leasequery", RFC 4388, 578 DOI 10.17487/RFC4388, February 2006, 579 . 581 [RFC4578] Johnston, M. and S. Venaas, Ed., "Dynamic Host 582 Configuration Protocol (DHCP) Options for the Intel 583 Preboot eXecution Environment (PXE)", RFC 4578, 584 DOI 10.17487/RFC4578, November 2006, 585 . 587 [RFC4702] Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host 588 Configuration Protocol (DHCP) Client Fully Qualified 589 Domain Name (FQDN) Option", RFC 4702, 590 DOI 10.17487/RFC4702, October 2006, 591 . 593 [RFC4776] Schulzrinne, H., "Dynamic Host Configuration Protocol 594 (DHCPv4 and DHCPv6) Option for Civic Addresses 595 Configuration Information", RFC 4776, 596 DOI 10.17487/RFC4776, November 2006, 597 . 599 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 600 Extensions for Stateless Address Autoconfiguration in 601 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 602 . 604 [RFC6225] Polk, J., Linsner, M., Thomson, M., and B. Aboba, Ed., 605 "Dynamic Host Configuration Protocol Options for 606 Coordinate-Based Location Configuration Information", 607 RFC 6225, DOI 10.17487/RFC6225, July 2011, 608 . 610 [RFC6926] Kinnear, K., Stapp, M., Desetti, R., Joshi, B., Russell, 611 N., Kurapati, P., and B. Volz, "DHCPv4 Bulk Leasequery", 612 RFC 6926, DOI 10.17487/RFC6926, April 2013, 613 . 615 [RFC7724] Kinnear, K., Stapp, M., Volz, B., and N. Russell, "Active 616 DHCPv4 Lease Query", RFC 7724, DOI 10.17487/RFC7724, 617 December 2015, . 619 Authors' Addresses 621 Sheng Jiang 622 Huawei Technologies Co., Ltd 623 Q14, Huawei Campus, No.156 Beiqing Road 624 Hai-Dian District, Beijing, 100095 625 P.R. China 627 Email: jiangsheng@huawei.com 629 Suresh Krishnan 630 Ericsson 631 8400 Decarie Blvd. 632 Town of Mount Royal, QC 633 Canada 635 Phone: +1 514 345 7900 x42871 636 Email: suresh.krishnan@ericsson.com 638 Tomek Mrugalski 639 Internet Systems Consortium, Inc. 640 950 Charter Street 641 Redwood City, CA 94063 642 USA 644 Email: tomasz.mrugalski@gmail.com