idnits 2.17.1 draft-ietf-dhc-dhcp-privacy-02.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 (December 30, 2015) is 3038 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 2, 2016 Ericsson 6 T. Mrugalski 7 ISC 8 December 30, 2015 10 Privacy considerations for DHCPv4 11 draft-ietf-dhc-dhcp-privacy-02 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 2, 2016. 36 Copyright Notice 38 Copyright (c) 2015 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. 144 3.1. Client Identifier Option 146 The Client Identifier Option [RFC2131] is used to pass an explicit 147 client identifier to a DHCP server. 149 The client identifier is an opaque key, which must be unique to that 150 client within the subnet to which the client is attached. It 151 typically remains stable after it has been initially generated. It 152 may contain a hardware address, identical to the contents of the 153 'chaddr' field, or another type of identifier, such as a DNS name. 154 [RFC3315] in Section 9.2 specifies DUID-LLT (Link-layer + time) as 155 the recommended DUID (DHCP Unique Identifier) type. [RFC4361], 156 Section 6.1 introduces this concept to DHCPv4. Those two document 157 recommend that client identifiers be generated by using the permanent 158 link-layer address of the network interface that the client is trying 159 to configure. [RFC4361] updates the recommendation of Client 160 Identifiers to be "consists of a type field whose value is normally 161 255, followed by a four-byte IA_ID field, followed by the DUID for 162 the client as defined in RFC 3315, section 9". This does not change 163 the lifecycle of the Client Identifiers. Clients are expected to 164 generate their Client Identifiers once (during first operation) and 165 store it in a non-volatile storage or use the same deterministic 166 algorithm to generate the same Client Identifier values again. 168 This means that most implementations will use the available link- 169 layer address during its first boot. Even if the administrator 170 enables link-layer address randomization, it is likely that it was 171 disabled during the first device boot. Hence the original, 172 unobfuscated link-layer address will likely end up being announced as 173 client identifier, even if the link- layer address has changed (or 174 even if being changed on a periodic basis). The exposure of the 175 original link-layer address in the client identifier will also 176 undermine other privacy extensions such as [RFC4941]. 178 3.2. Address Fields & Options 180 The 'yiaddr' field [RFC2131] in DHCP message is used to allocate 181 address from the server to the client. 183 The DHCPv4 specification [RFC2131] provides a way to specify the 184 client link-layer address in the DHCPv4 message header. A DHCPv4 185 message header has 'htype' and 'chaddr' fields to specify the client 186 link-layer address type and the link-layer address, respectively. 187 The 'chaddr' field is used both as a hardware address for 188 transmission of reply messages and as a client identifier. 190 The 'requested IP address' option [RFC2131] is used by client to 191 suggest that a particular IP address be assigned. 193 3.3. Client FQDN Option 195 The Client Fully Qualified Domain Name (FQDN) option [RFC4702] is 196 used by DHCP clients and servers to exchange information about the 197 client's fully qualified domain name and about who has the 198 responsibility for updating the DNS with the associated AAAA and PTR 199 RRs. 201 A client can use this option to convey all or part of its domain name 202 to a DHCP server for the IP-address-to-FQDN mapping. In most case a 203 client sends its hostname as a hint for the server. The DHCP server 204 MAY be configured to modify the supplied name or to substitute a 205 different name. The server should send its notion of the complete 206 FQDN for the client in the Domain Name field. 208 3.4. Parameter Request List Option 210 The Parameter Request List option [RFC2131] is used to inform the 211 server about options the client wants the server to send to the 212 client. The content of a Parameter Request List option are the 213 option codes for an option requested by the client. 215 3.5. Vendor Class and Vendor-Identifying Vendor Class Options 217 The Vendor Class option [RFC2131], the Vendor-Identifying Vendor 218 Class option and Vendor-Identifying Vendor Information option 219 [RFC3925] are used by the DHCP client to identify the vendor that 220 manufactured the hardware on which the client is running. 222 The information contained in the data area of this option is 223 contained in one or more opaque fields that identify the details of 224 the hardware configuration of the host on which the client is 225 running, or of industry consortium compliance, for example, the 226 version of the operating system the client is running or the amount 227 of memory installed on the client. 229 3.6. Civic Location Option 231 DHCP servers use the Civic Location Option [RFC4776] to deliver of 232 the location information (the civic and postal addresses) to the DHCP 233 clients. It may refer to three locations: the location of the DHCP 234 server, the location of the network element believed to be closest to 235 the client, or the location of the client, identified by the "what" 236 element within the option. 238 3.7. Coordinate-Based Location Option 240 The GeoConf and GeoLoc options [RFC6225] is used by DHCP server to 241 provide the coordinate-based geographic location information to the 242 DHCP clients. It enables a DHCP client to obtain its geographic 243 location. 245 3.8. Client System Architecture Type Option 247 The Client System Architecture Type Option [RFC4578] is used by DHCP 248 client to send a list of supported architecture types to the DHCP 249 server. It is used by clients that must be booted using the network 250 rather than from local storage, so the server can decide which boot 251 file should be provided to the client. 253 3.9. Relay Agent Information Option and Sub-options 255 A DHCP relay agent includes a Relay Agent Information [RFC3046] to 256 identify the remote host end of the circuit. It contains a "circuit 257 ID" sub-option for the incoming circuit, which is an agent-local 258 identifier of the circuit from which a DHCP client-to-server packet 259 was received, and a "remote ID" sub-option which provides a trusted 260 identifier for the remote high-speed modem. 262 Possible encoding of "circuit ID" sub-option includes: router 263 interface number, switching hub port number, remote access server 264 port number, frame relay DLCI, ATM virtual circuit number, cable data 265 virtual circuit number, etc. 267 Possible encoding of the "remote ID" sub-option includes: a "caller 268 ID" telephone number for dial-up connection, a "user name" prompted 269 for by a remote access server, a remote caller ATM address, a "modem 270 ID" of a cable data modem, the remote IP address of a point-to-point 271 link, a remote X.25 address for X.25 connections, etc. 273 The link-selection sub-option [RFC3527] is used by any DHCP relay 274 agent that desires to specify a subnet/link for a DHCP client request 275 that it is relaying but needs the subnet/link specification to be 276 different from the IP address the DHCP server should use when 277 communicating with the relay agent. It contains an IP address, which 278 can identify the client's subnet/link. Also, assuming network 279 topology knowledge, it also reveals client location. 281 A DHCP relay includes a Subscriber-ID option [RFC3993] to associate 282 some provider-specific information with clients' DHCP messages that 283 is independent of the physical network configuration through which 284 the subscriber is connected. The "subscriber-id" assigned by the 285 provider is intended to be stable as customers connect through 286 different paths, and as network changes occur. The Subscriber-ID is 287 an ASCII string, which is assigned and configured by the network 288 provider. 290 4. Existing Mechanisms That Affect Privacy 292 This section describes deployed DHCP mechanisms that affect privacy. 294 4.1. DNS Updates 296 The Client FQDN (Fully Qualified Domain Name) Option [RFC4702] used 297 along with DNS Updates [RFC2136] defines a mechanism that allows both 298 clients and server to insert into the DNS domain information about 299 clients. Both forward (A) and reverse (PTR) resource records can be 300 updated. This allows other nodes to conveniently refer to a host, 301 despite the fact that its IP address may be changing. 303 This mechanism exposes two important pieces of information: current 304 address (which can be mapped to current location) and client's 305 hostname. The stable hostname can then be used to correlate the 306 client across different network attachments even when its IP 307 addresses keep changing. 309 4.2. Allocation strategies 311 A DHCP server running in typical, stateful mode is given a task of 312 managing one or more pools of IP address. When a client requests an 313 address, the server must pick an address out of configured pool. 314 Depending on the server's implementation, various allocation 315 strategies are possible. Choices in this regard may have privacy 316 implications. Note that the constraints in DHCPv4 and DHCPv6 are 317 radically different, but servers that allow allocation strategy 318 configuration may allow configuring them in both DHCPv4 and DHCPv6. 319 Not every allocation strategy is equally suitable for DHCPv4 and for 320 DHCPv6. 322 Iterative allocation - a server may choose to allocate addresses one 323 by one. That strategy has the benefit of being very fast, thus can 324 be favored in deployments that prefer performance. However, it makes 325 the allocated addresses very predictable. Also, since the addresses 326 allocated tend to be clustered at the beginning of available pool, it 327 makes scanning attacks much easier. 329 Identifier-based allocation - some server implementations may choose 330 to allocate an address that is based on one of available identifiers, 331 e.g. client identifier or MAC address. It is also convenient, as 332 returning client is very likely to get the same address. Those 333 properties are convenient for system administrators, so DHCP server 334 implementors are often requested to implement it. The downside of 335 such allocation is that the client has a very stable IP address. 336 That means that correlation of activities over time, location 337 tracking, address scanning and OS/vendor discovery apply. This is 338 certainly an issue in DHCPv6, but due to much smaller address space 339 is almost never a problem in DHCPv4. 341 Hash allocation - it's an extension of identifier based allocation. 342 Instead of using the identifier directly, it is being hashed first. 343 If the hash is implemented correctly, it removes the flaw of 344 disclosing the identifier, a property that eliminates susceptibility 345 to address scanning and OS/vendor discovery. If the hash is poorly 346 implemented (e.g. can be reverted), it introduces no improvement over 347 identifier-based allocation. 349 Random allocation - a server can pick a resource randomly out of 350 available pool. That strategy works well in scenarios where pool 351 utilization is small, as the likelihood of collision (resulting in 352 the server needing to repeat randomization) is small. With the pool 353 allocation increasing, the collision is disproportionally large, due 354 to birthday paradox. With high pool utilization (e.g. when 90% of 355 available resources being allocated already), the server will use 356 most computational resources to repeatedly pick a random resource, 357 which will degrade its performance. This allocation scheme 358 essentially prevents returning clients from getting the same address 359 again. On the other hand, it is beneficial from privacy perspective 360 as addresses generated that way are not susceptible to correlation 361 attacks, OS/vendor discovery attacks or identity discovery attacks. 362 Note that even though the address itself may be resilient to a given 363 attack, the client may still be susceptible if additional information 364 is disclosed other way, e.g. client's address can be randomized, but 365 it still can leak its MAC address in client-id option. 367 Other allocation strategies may be implemented. 369 Given the limited size of most IPv4 public address pools, allocation 370 mechanisms in IPv4 may not provide much privacy protection or leak 371 much useful information, if misused. 373 5. Attacks 375 5.1. Device type discovery 377 The type of device used by the client can be guessed by the attacker 378 using the Vendor Class Option, the 'chaddr' field, and by parsing the 379 Client ID Option. All of those options may contain an 380 Organizationally Unique Identifier (OUI) that represents the device's 381 vendor. That knowledge can be used for device-specific vulnerability 382 exploitation attacks. 384 5.2. Operating system discovery 386 The operating system running on a client can be guessed using the 387 Vendor Class option, the Client System Architecture Type option, or 388 by using fingerprinting techniques on the combination of options 389 requested using the Parameter Request List option. 391 5.3. Finding location information 393 The location information can be obtained by the attacker by many 394 means. The most direct way to obtain this information is by looking 395 into a message originating from the server that contains the Civic 396 Location, GeoConf, or GeoLoc options. It can also be indirectly 397 inferred using the Relay Agent Information option, with the remote ID 398 sub-option, the circuit ID option (e.g. if an access circuit on an 399 Access Node corresponds to a civic location), or the Subscriber ID 400 Option (if the attacker has access to subscriber info). 402 5.4. Finding previously visited networks 404 When DHCP clients connect to a network, they attempt to obtain the 405 same address they had used before they attached to the network. They 406 do this by putting the previously assigned address in the requested 407 IP address option. By observing these addresses, an attacker can 408 identify the network the client had previously visited. 410 5.5. Finding a stable identity 412 An attacker might use a stable identity gleaned from DHCP messages to 413 correlate activities of a given client on unrelated networks. The 414 Client FQDN option, the Subscriber ID Option and the Client ID 415 options can serve as long lived identifiers of DHCP clients. The 416 Client FQDN option can also provide an identity that can easily be 417 correlated with web server activity logs. 419 5.6. Pervasive monitoring 421 This is an enhancement, or a combination of most aforementioned 422 mechanisms. Operator who controls non-trivial number of access 423 points or network segments, may use obtained information about a 424 single client and observer client's habits. 426 5.7. Finding client's IP address or hostname 428 Many DHCP deployments use DNS Updates [RFC4702] that put client's 429 information (current IP address, client's hostname) into DNS, where 430 it is easily accessible by anyone interested. Client ID is also 431 disclosed, albeit in not easily accessible form (SHA-256 digest of 432 the client-id). As SHA-256 is considered irreversible, DHCID can't 433 be converted back to client-id. However, SHA-256 digest can be used 434 as an unique identifier that is accessible by any host. 436 5.8. Correlation of activities over time 438 As with other identifiers, an IP address can be used to correlate the 439 activities of a host for at least as long as the lifetime of the 440 address. If that address was generated from some other, stable 441 identifier and that generation scheme can be deducted by an attacker, 442 the duration of correlation attack extends to that identifier. In 443 many cases, its lifetime is equal to the lifetime of the device 444 itself. 446 5.9. Location tracking 448 If a stable identifier is used for assigning an address and such 449 mapping is discovered by an attacker, e.g. a hostname being put into 450 DNS, it can be used for tracking user. In particular both passive (a 451 service that the client connects to can log client's address and draw 452 conclusions regarding its location and movement patterns based on 453 address it is connecting from) and active (attacker can send ICMP 454 echo requests or other probe packets to networks of suspected client 455 locations) methods can be used. To give specific example, by 456 accessing a social portal from tomek- 457 laptop.coffee.somecity.com.example, tomek- 458 laptop.mycompany.com.example and tomek-laptop.myisp.example.com, the 459 portal administrator can draw conclusions about tomek-laptop's owner 460 current location and his habits. 462 5.10. Leasequery & bulk leasequery 464 Attackers may pretend as an access concentrator, either DHCP relay 465 agent or DHCP client, to obtain location information directly from 466 the DHCP server(s) using the DHCP leasequery [RFC4388] mechanism. 468 Location information is information needed by the access concentrator 469 to forward traffic to a broadband-accessible host. This information 470 includes knowledge of the host hardware address, the port or virtual 471 circuit that leads to the host, and/or the hardware address of the 472 intervening subscriber modem. 474 Furthermore, the attackers may use DHCP bulk leasequery [RFC6926] 475 mechanism to obtain bulk information about DHCP bindings, even 476 without knowing the target bindings. 478 Additionally, active leasequery [RFC7724] is a mechanism for 479 subscribing to DHCPv4 lease update changes in near real-time. The 480 intent of this mechanism is to update operator's database, but if 481 misused, an attacker could defeat server's authentication mechanisms 482 and subscribe to all updates. He then could continue receiving 483 updates, without any need for local presence. 485 6. Security Considerations 487 In current practice, the client privacy and the client authentication 488 are mutually exclusive. The client authentication procedure reveals 489 additional client information in their certificates/identifiers. 490 Full privacy for the clients may mean the clients are also anonymous 491 for the server and the network. 493 7. Privacy Considerations 495 This document at its entirety discusses privacy considerations in 496 DHCP. As such, no dedicated discussion is needed. 498 8. IANA Considerations 500 This draft does not request any IANA action. 502 9. Acknowledgements 504 The authors would like to thank the valuable comments made by Stephen 505 Farrell, Ted Lemon, Ines Robles, Russ White, Christian Huitema, 506 Bernie Volz, Jinmei Tatuya, Marcin Siodelski, Christian Schaefer and 507 other members of DHC WG. 509 This document was produced using the xml2rfc tool [RFC2629]. 511 10. References 513 10.1. Normative References 515 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 516 Requirement Levels", BCP 14, RFC 2119, 517 DOI 10.17487/RFC2119, March 1997, 518 . 520 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 521 RFC 2131, DOI 10.17487/RFC2131, March 1997, 522 . 524 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, 525 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 526 RFC 2136, DOI 10.17487/RFC2136, April 1997, 527 . 529 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 530 Morris, J., Hansen, M., and R. Smith, "Privacy 531 Considerations for Internet Protocols", RFC 6973, 532 DOI 10.17487/RFC6973, July 2013, 533 . 535 [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an 536 Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 537 2014, . 539 10.2. Informative References 541 [RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, 542 DOI 10.17487/RFC2629, June 1999, 543 . 545 [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", 546 RFC 3046, DOI 10.17487/RFC3046, January 2001, 547 . 549 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 550 C., and M. Carney, "Dynamic Host Configuration Protocol 551 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 552 2003, . 554 [RFC3527] Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy, 555 "Link Selection sub-option for the Relay Agent Information 556 Option for DHCPv4", RFC 3527, DOI 10.17487/RFC3527, April 557 2003, . 559 [RFC3925] Littlefield, J., "Vendor-Identifying Vendor Options for 560 Dynamic Host Configuration Protocol version 4 (DHCPv4)", 561 RFC 3925, DOI 10.17487/RFC3925, October 2004, 562 . 564 [RFC3993] Johnson, R., Palaniappan, T., and M. Stapp, "Subscriber-ID 565 Suboption for the Dynamic Host Configuration Protocol 566 (DHCP) Relay Agent Option", RFC 3993, 567 DOI 10.17487/RFC3993, March 2005, 568 . 570 [RFC4361] Lemon, T. and B. Sommerfeld, "Node-specific Client 571 Identifiers for Dynamic Host Configuration Protocol 572 Version Four (DHCPv4)", RFC 4361, DOI 10.17487/RFC4361, 573 February 2006, . 575 [RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration 576 Protocol (DHCP) Leasequery", RFC 4388, 577 DOI 10.17487/RFC4388, February 2006, 578 . 580 [RFC4578] Johnston, M. and S. Venaas, Ed., "Dynamic Host 581 Configuration Protocol (DHCP) Options for the Intel 582 Preboot eXecution Environment (PXE)", RFC 4578, 583 DOI 10.17487/RFC4578, November 2006, 584 . 586 [RFC4702] Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host 587 Configuration Protocol (DHCP) Client Fully Qualified 588 Domain Name (FQDN) Option", RFC 4702, 589 DOI 10.17487/RFC4702, October 2006, 590 . 592 [RFC4776] Schulzrinne, H., "Dynamic Host Configuration Protocol 593 (DHCPv4 and DHCPv6) Option for Civic Addresses 594 Configuration Information", RFC 4776, 595 DOI 10.17487/RFC4776, November 2006, 596 . 598 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 599 Extensions for Stateless Address Autoconfiguration in 600 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 601 . 603 [RFC6225] Polk, J., Linsner, M., Thomson, M., and B. Aboba, Ed., 604 "Dynamic Host Configuration Protocol Options for 605 Coordinate-Based Location Configuration Information", 606 RFC 6225, DOI 10.17487/RFC6225, July 2011, 607 . 609 [RFC6926] Kinnear, K., Stapp, M., Desetti, R., Joshi, B., Russell, 610 N., Kurapati, P., and B. Volz, "DHCPv4 Bulk Leasequery", 611 RFC 6926, DOI 10.17487/RFC6926, April 2013, 612 . 614 [RFC7724] Kinnear, K., Stapp, M., Volz, B., and N. Russell, "Active 615 DHCPv4 Lease Query", RFC 7724, DOI 10.17487/RFC7724, 616 December 2015, . 618 Authors' Addresses 620 Sheng Jiang 621 Huawei Technologies Co., Ltd 622 Q14, Huawei Campus, No.156 Beiqing Road 623 Hai-Dian District, Beijing, 100095 624 P.R. China 626 Email: jiangsheng@huawei.com 628 Suresh Krishnan 629 Ericsson 630 8400 Decarie Blvd. 631 Town of Mount Royal, QC 632 Canada 634 Phone: +1 514 345 7900 x42871 635 Email: suresh.krishnan@ericsson.com 637 Tomek Mrugalski 638 Internet Systems Consortium, Inc. 639 950 Charter Street 640 Redwood City, CA 94063 641 USA 643 Email: tomasz.mrugalski@gmail.com