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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Dynamic Host Configuration L. Colitti 3 Internet-Draft J. Linkova 4 Updates: 2563 (if approved) Google 5 Intended status: Standards Track M. Richardson 6 Expires: February 7, 2021 Sandelman 7 T. Mrugalski 8 ISC 9 August 6, 2020 11 IPv6-Only-Preferred Option for DHCPv4 12 draft-ietf-dhc-v6only-06 14 Abstract 16 This document specifies a DHCPv4 option to indicate that a host 17 supports an IPv6-only mode and willing to forgo obtaining an IPv4 18 address if the network provides IPv6 connectivity. It also updates 19 RFC2563 to specify the DHCPv4 server behavior when the server 20 receives a DHCPDISCOVER not containing the Auto-Configure option. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at https://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on February 7, 2021. 39 Copyright Notice 41 Copyright (c) 2020 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (https://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 57 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 58 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 59 2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets . . . . 5 60 3. IPv6-Only Preferred Option . . . . . . . . . . . . . . . . . 5 61 3.1. Option format . . . . . . . . . . . . . . . . . . . . . . 5 62 3.2. DHCPv4 Client Behavior . . . . . . . . . . . . . . . . . 6 63 3.3. DHCPv4 Server Behavior . . . . . . . . . . . . . . . . . 7 64 3.3.1. Interaction with RFC2563 . . . . . . . . . . . . . . 8 65 3.4. Constants and Configuration Variables . . . . . . . . . . 9 66 4. IPv6-Only Transition Technologies Considerations . . . . . . 10 67 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 68 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 69 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 70 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 71 8.1. Normative References . . . . . . . . . . . . . . . . . . 12 72 8.2. Informative References . . . . . . . . . . . . . . . . . 12 73 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 75 1. Introduction 77 One of the biggest challenges of deploying IPv6-only LANs is that 78 such networks might contain rather heterogeneous collection of hosts. 79 While some hosts are capable of operating in IPv6-only mode (either 80 because the OS and all applications are IPv6-only capable or because 81 the host has some form of 464XLAT [RFC6877] deployed), others might 82 still have IPv4 dependencies and need IPv4 addresses to operate 83 properly. To incrementally rollout IPv6-only, network operators 84 might need to provide IPv4 on demand whereby a host receives an IPv4 85 address if it needs it, while IPv6-only capable hosts (such as modern 86 mobile devices) are not allocated IPv4 addresses. Traditionally that 87 goal is achieved by placing IPv6-only capable devices into a 88 dedicated IPv6-only network segment or WiFi SSID, while dual-stack 89 devices reside in another network with IPv4 and DHCPv4 enabled. 90 However such approach has a number of drawbacks, including but not 91 limited to: 93 o Doubling the number of network segments leads to operational 94 complexity and performance impact, for instance due to high memory 95 utilization caused by an increased number of ACL entries. 97 o Placing a host into the correct network segment is problematic. 98 For example, in the case of 802.11 Wi-Fi the user might select the 99 wrong SSID. In the case of wired 802.1x authentication the 100 authentication server might not have all the information required 101 to make the correct decision and choose between an IPv6-only and a 102 dual-stack VLAN. 104 It would be beneficial for IPv6 deployment if operators could 105 implement IPv6-mostly (or IPv4-on-demand) segments where IPv6-only 106 hosts co-exist with legacy dual-stack devices. The trivial solution 107 of disabling IPv4 stack on IPv6-only capable hosts is not feasible as 108 those clients must be able to operate on IPv4-only networks as well. 109 While IPv6-only capable devices might use a heuristic approach to 110 learning if the network provides IPv6-only functionality and stop 111 using IPv4 if it does, such approach might be practically 112 undesirable. One important reason is that when a host connects to a 113 network, it does not know if the network is IPv4-only, dual-stack or 114 IPv6-only. To ensure that the connectivity over whatever protocol is 115 present becomes available as soon as possible the host usually starts 116 configuring both IPv4 and IPv6 immediately. If hosts were to delay 117 requesting IPv4 until IPv6 reachability is confirmed, that would 118 penalize IPv4-only and dual-stack networks, which does not seem 119 practical. Requesting IPv4 and then releasing it later, after IPv6 120 reachability is confirmed, might cause user-visible errors as it 121 would be disruptive for applications which have started using the 122 assigned IPv4 address already. Instead it would be useful to have a 123 mechanism which would allow a host to indicate that its request for 124 an IPv4 address is optional and a network to signal that IPv6-only 125 functionality (such as NAT64, [RFC6146]) is available. The proposed 126 solution is to introduce a new DHCPv4 option which a client uses to 127 indicate that it does not need an IPv4 address if the network 128 provides IPv6-only connectivity (as NAT64 and DNS64). If the 129 particular network segment provides IPv4-on-demand such clients would 130 not be supplied with IPv4 addresses, while on IPv4-only or dual-stack 131 segments without NAT64 services IPv4 addresses will be provided. 133 [RFC2563] introduces the Auto-Configure DHCPv4 option and describes 134 DHCPv4 servers behavior if no address is chosen for a host. This 135 document updates [RFC2563] to modify the server behavior if the 136 DHCPOFFER contains the IPv6-only Preferred option. 138 1.1. Requirements Language 140 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 141 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 142 "OPTIONAL" in this document are to be interpreted as described in BCP 143 14 [RFC2119] [RFC8174] when, and only when, they appear in all 144 capitals, as shown here. 146 1.2. Terminology 148 IPv6-only capable host: a host which does not require an IPv4 address 149 and can operate on IPv6-only networks. Strictly speaking IPv6-only 150 capability is specific to a given interface of the host: if some 151 application on a host require IPv4 and 464XLAT CLAT [RFC6877] is only 152 enabled on one interface, the host is IPv6-only capable if connected 153 to a NAT64 network via that interface. This document currently 154 implies that IPv6-only capable hosts reach IPv4-only destinations via 155 NAT64 service provided by the network. Section 4 discusses 156 hypothetical scenarios of other transition technologies being used. 158 IPv4-requiring host: a host which is not IPv6-only capable and can 159 not operate in IPv6-only network providing NAT64 service. 161 IPv4-on-demand: a deployment scenario when end hosts are expected to 162 operate in IPv6-only mode by default and IPv4 addresses can be 163 assigned to some hosts if those hosts explicitly opt-in to receiving 164 IPv4 addresses. 166 IPv6-mostly network: a network which provides NAT64 (possibly with 167 DNS64) service as well as IPv4 connectivity and allows coexistence of 168 IPv6-only, dual-stack and IPv4-only hosts on the same segment. Such 169 deployment scenario allows operators to incrementally turn off IPv4 170 on end hosts, while still providing IPv4 to devices which require 171 IPv4 to operate. But, IPv6-only capable devices need not be assigned 172 IPv4 addresses. 174 IPv6-only mode: a mode of operation when a host acts as an IPv6-only 175 capable host and does not have IPv4 addresses assigned (except that 176 IPv4 link-local addresses [RFC3927] may have been configured). 178 IPv6-Only network: a network which does not provide routing 179 functionality for IPv4 packets. Such networks may or may not allow 180 intra-LAN IPv4 connectivity. IPv6-Only network usually provides 181 access to IPv4-only resources via NAT64 [RFC6146]. 183 NAT64: Network Address and Protocol Translation from IPv6 Clients to 184 IPv4 Servers [RFC6146]. 186 RA: Router Advertisement, a message used by IPv6 routers to advertise 187 their presence together with various link and Internet parameters 188 [RFC4861]. 190 DNS64: a mechanism for synthesizing AAAA records from A records 191 [RFC6147]. 193 2. Reasons to Signal IPv6-Only Support in DHCPv4 Packets 195 For networks which contain a mix of both IPv6-only capable hosts and 196 IPv4-requiring hosts, and which utilize DHCPv4 for configuring the 197 IPv4 network stack on hosts, it seems natural to leverage the same 198 protocol to signal that IPv4 is discretional on a given segment. An 199 ability to remotely disable IPv4 on a host can be seen as a new 200 denial-of-service attack vector. The proposed approach limits the 201 attack surface to DHCPv4-related attacks without introducing new 202 vulnerable elements. 204 Another benefit of using DHCPv4 for signaling is that IPv4 will be 205 disabled only if both the client and the server indicate IPv6-only 206 capability. It allows IPv6-only capable hosts to turn off IPv4 only 207 upon receiving an explicit signal from the network and operate in 208 dual-stack or IPv4-only mode otherwise. In addition, the proposed 209 mechanism does not introduce any additional delays to the process of 210 configuring IP stack on hosts. If the network does not support IPv6- 211 only/IPv4-on-demand mode, an IPv6-only capable host would configure 212 an IPv4 address as quickly as on any other host. 214 Being a client/server protocol, DHCPv4 allows IPv4 to be selectively 215 disabled on a per-host basis on a given network segment. Coexistence 216 of IPv6-only, dual-stack and even IPv4-only hosts on the same LAN 217 would not only allow network administrators to preserve scarce IPv4 218 addresses but would also drastically simplify incremental deployment 219 of IPv6-only networks, positively impacting IPv6 adoption. 221 3. IPv6-Only Preferred Option 223 3.1. Option format 225 0 1 2 3 226 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 227 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 228 | Code | Length | Value | 229 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 230 | Value (contd) | 231 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 233 Figure 1: IPv6-Only Preferred Option Format 235 Fields: 237 Code: 8-bit identifier of the IPv6-Only Preferred option code as 238 assigned by IANA: TBD. 239 The client includes the Code in the Parameter Request List in 240 DHCPDISCOVER and DHCPREQUEST messages as described in 241 Section 3.2. 242 Length: 8-bit unsigned integer. The length of the option excluding 243 the Code and Length Fields. The server MUST set the length 244 field to 4. The client MUST ignore the IPv6-Only Preferred 245 option if the length field value is not 4. 246 Value: 32-bit unsigned integer. 247 The number of seconds the client should disable DHCPv4 for 248 (V6ONLY_WAIT configuration variable). 249 If the server pool is explicitly configured with a 250 V6ONLY_WAIT timer the server MUST set the field to that 251 configured value. Otherwise the server MUST set it to zero. 252 The client MUST process that field as described in 253 Section 3.2. 255 3.2. DHCPv4 Client Behavior 257 A DHCPv4 client SHOULD allow a device administrator to configure 258 IPv6-only preferred mode either for a specific interface (to indicate 259 that the device is IPv6-only capable if connected to a NAT64 network 260 via that interface) or for all interfaces. If only a specific 261 interface is configured as IPv6-only capable the DHCPv4 client MUST 262 NOT consider the host to be an IPv6-only capable for the purpose of 263 sending/receiving DHCPv4 packets over any other interfaces. 265 The DHCPv4 client on an IPv4-requiring host MUST NOT include the 266 IPv6-only Preferred option in the Parameter Request List of any 267 DHCPv4 packets and MUST ignore that option in packets received from 268 DHCPv4 servers. 270 DHCPv4 clients running on IPv6-only capable hosts SHOULD include the 271 IPv6-only Preferred option code in the Parameter Request List in 272 DHCPDISCOVER and DHCPREQUEST messages for interfaces so enabled and 273 follow the processing as described below on a per interface enabled 274 basis. 276 If the client did not include the IPv6-only Preferred option code in 277 the Parameter Request List option in the DHCPDISCOVER or DHCPREQUEST 278 message it MUST ignore the IPv6-only Preferred option in any messages 279 received from the server. 281 If the client includes the IPv6-only Preferred option in the 282 Parameter Request List and the DHCPOFFER message from the server 283 contains a valid IPv6-only Preferred option, the client SHOULD NOT 284 request the IPv4 address provided in the DHCPOFFER. If the IPv6-only 285 Preferred option returned by the server contains a value greater or 286 equal to MIN_V6ONLY_WAIT, the client SHOULD set the V6ONLY_WAIT timer 287 to that value. Otherwise, the client SHOULD set the V6ONLY_WAIT 288 timer to MIN_V6ONLY_WAIT. The client SHOULD stop the DHCPv4 289 configuration process for at least V6ONLY_WAIT seconds or until a 290 network attachment event happens. The host MAY disable the IPv4 291 stack completely for V6ONLY_WAIT seconds or until the network 292 disconnection event happens. 294 The IPv6-only Preferred option SHOULD be included in the Parameter 295 Request List option in DHCPREQUEST messages (after receiving a 296 DHCPOFFER without this option, for a INIT-REBOOT, or when renewing or 297 rebinding a leased address). If the DHCPv4 server responds with a 298 DHCPACK that includes the IPv6-only Preferred option, the client 299 behaviour depends on the client's state. If the client is in the 300 INIT-REBOOT state it SHOULD stop the DHCPv4 configuration process or 301 disable IPv4 stack completely for V6ONLY_WAIT seconds or until the 302 network disconnection event happens. It also MAY send a DHCPRELEASE 303 message. If the client is in any other state it SHOULD continue to 304 use the assigned IPv4 address until further DHCPv4 reconfiguration 305 events. 307 If the client includes the IPv6-only Preferred option in the 308 Parameter Request List and the server responds with DHCPOFFER message 309 without a valid IPv6-only Preferred option, the client MUST proceed 310 as normal with a DHCPREQUEST. 312 If the client waits for multiple DHCPOFFER responses and selects one 313 of them, it MUST follow the processing for the IPv6-only Preferred 314 option based on the selected response. A client MAY use the presence 315 of the IPv6-only Preferred option as a selection criteria. 317 When an IPv6-only capable client receives the IPv6-Only Preferred 318 option from the server, the client MAY configure IPv4 link-local 319 address [RFC3927]. In that case IPv6-Only capable devices might 320 still be able to communicate over IPv4 to other devices on the link. 321 The Auto-Configure Option [RFC2563] can be used to control IPv4 link- 322 local addresses autoconfiguration. Section 3.3.1 discusses the 323 interaction between the IPv6-only Preferred and the Auto-Configure 324 options. 326 3.3. DHCPv4 Server Behavior 328 The DHCPv4 server SHOULD be able to configure certain pools to 329 include the IPv6-only preferred option in DHCPv4 responses if the 330 client included the option code in the Parameter Request List option. 331 The DHCPv4 server MAY have a configuration option to specify 332 V6ONLY_WAIT timer for all or individual IPv6-mostly pools. 334 The server MUST NOT include the IPv6-only Preferred option in the 335 DHCPOFFER or DHCPACK message if the YIADDR field in the message does 336 not belong to a pool configured as IPv6-mostly. The server MUST NOT 337 include the IPv6-only Preferred option in the DHCPOFFER or DHCPACK 338 message if the option was not present in the Parameter Request List 339 sent by the client. 341 If the IPv6-only Preferred option is present in the Parameter Request 342 List received from the client and the corresponding DHCPv4 pool is 343 explicitly configured as belonging to an IPv6-mostly network segment, 344 the server MUST include the IPv6-only Preferred option when 345 responding with the DHCPOFFER or DHCPACK message. If the server 346 responds with the IPv6-only Preferred option and the V6ONLY_WAIT 347 timer is configured for the pool, the server MUST copy the configured 348 value to the IPv6-only Preferred option value field. Otherwise it 349 MUST set the field to zero. The server SHOULD NOT assign an address 350 for the pool. Instead it SHOULD return 0.0.0.0 as the offered 351 address. Alternatively, the server MAY include an available IPv4 352 address from the pool into the DHCPOFFER as per recommendations in 353 [RFC2131]. In this case, the offered address MUST be a valid address 354 that is not committed to any other client. Because the client is not 355 expected ever to request this address, the server SHOULD NOT reserve 356 the address and SHOULD NOT verify its uniqueness. If the client then 357 issues a DHCPREQUEST for the address, the server MUST process it per 358 [RFC2131], including replying with a DHCPACK for the address if in 359 the meantime it has not been committed to another client. 361 If a client includes both a Rapid-Commit option [RFC4039] and 362 IPv6-Only Preferred option in the DHCPDISCOVER message the server 363 SHOULD NOT honor the Rapid-Commit option if the response would 364 contain the IPv6-only Preferred option to the client. It SHOULD 365 instead respond with a DHCPOFFER as indicated above. 367 If the server receives a DHCPREQUEST containing the IPv6-only 368 Preferred option for the address from a pool configured as 369 IPv6-mostly, the server MUST process it per [RFC2131]. 371 3.3.1. Interaction with RFC2563 373 [RFC2563] defines an Auto-Configure DHCPv4 option to disable IPv4 374 link-local address configuration for IPv4 clients. Clients can 375 support both, neither or just one of IPv6-Only Preferred and Auto- 376 Configure options. If a client sends both IPv6-Only Preferred and 377 Auto-Configure options the network administrator can prevent the host 378 from configuring an IPv4 link-local address on IPv6-mostly network. 379 To achieve this the server needs to send DHCPOFFER which contains a 380 'yiaddr' of 0x00000000, and the Auto-Configure flag saying 381 "DoNotAutoConfigure". 383 However special care should be taken in a situation when a server 384 supports both options and receives just IPv6-Only Preferred option 385 from a client. Section 2.3 of [RFC2563] states that if no address is 386 chosen for the host (which would be the case for IPv6-only capable 387 clients on IPv6-mostly network) then: "If the DHCPDISCOVER does not 388 contain the Auto-Configure option, it is not answered." Such 389 behavior would be undesirable for clients supporting the IPv6-Only 390 Preferred option w/o supporting the Auto-Configure option as they 391 would not receive any response from the server and would keep asking, 392 instead of disabling DHCPv4 for V6ONLY_WAIT seconds. Therefore the 393 following update is proposed to Section 2.3 of [RFC2563]" 395 OLD TEXT: 397 --- 399 However, if no address is chosen for the host, a few additional steps 400 MUST be taken. 402 If the DHCPDISCOVER does not contain the Auto-Configure option, it is 403 not answered. 405 --- 407 NEW TEXT: 409 --- 411 However, if no address is chosen for the host, a few additional steps 412 MUST be taken. 414 If the DHCPDISCOVER does not contain the Auto-Configure option and 415 the IPv6-Only Preferred option is not present, it is not answered. 416 If the DHCPDISCOVER does not contain the Auto-Configure option but 417 contains the IPv6-Only Preferred option, the processing rules for the 418 IPv6-Only Preferred option apply. 420 --- 422 3.4. Constants and Configuration Variables 424 V6ONLY_WAIT The minimum time the client SHOULD stop the DHCPv4 425 configuration process for. The value MUST NOT be less 426 than MIN_V6ONLY_WAIT seconds. Default: 1800 seconds 427 MIN_V6ONLY_WAIT The lower boundary for V6ONLY_WAIT. Value: 300 428 seconds 430 4. IPv6-Only Transition Technologies Considerations 432 Until IPv6 adoption in the Internet reaches 100%, communication 433 between an IPv6-only host and IPv4-only destination requires some 434 form of transition mechanism deployed in the network. At the time of 435 writing, the only such mechanism that is widely supported by end 436 hosts is NAT64 [RFC6146] (either with or without 464XLAT). Therefore 437 the IPv6-only Preferred option is only sent by hosts capable of 438 operating on NAT64 networks. In a typical deployment scenario, a 439 network administrator would not configure the DHCPv4 server to return 440 the IPv6-only Preferred option unless the network provides NAT64 441 service. 443 Hypothetically it is possible for multiple transition technologies to 444 coexist. In such scenario some form of negotiation would be required 445 between a client and a server to ensure that the transition 446 technology supported by the client is the one the network provides. 447 However it seems unlikely that any new transition technology would 448 arise and be widely adopted in any foreseeable future. Therefore 449 adding support for non-existing technologies seems to be suboptimal 450 and the proposed mechanism implies that NAT64 is used to facilitate 451 connectivity between IPv6 and IPv4. In the unlikely event that a new 452 transition mechanism becomes widely deployed, the applicability of 453 the IPv6-Only-Preferred option to that mechanism will depend on the 454 nature of the new mechanism. If the new mechanism is designed in 455 such a way that it's fully transparent for hosts that support NAT64 456 and the IPv6-Only-Preferred option, then the option can continue to 457 be used with the new mechanism. If the new mechanism is not 458 compatible with NAT64, and implementation on the host side is 459 required to support it, then a new DHCPv4 option needs to be defined. 461 It should be also noted that declaring a host or (strictly speaking, 462 a host interface) IPv6-only capable is a policy decision. For 463 example, 465 o An operating system vendor may make such decision and configure 466 their DHCPv4 clients to send the IPv6-Only Preferred option by 467 default if the OS has 464XLAT CLAT [RFC6877] enabled. 469 o An enterprise network administrator may provision the corporate 470 hosts as IPv6-only capable if all applications users are supposed 471 to run have been tested in IPv6-only environment (or if 464XLAT 472 CLAT is enabled on the devices). 474 o IoT devices may be shipped in IPv6-only capable mode if they are 475 designed to connect to IPv6-enabled cloud destination only. 477 5. IANA Considerations 479 The IANA is requested to assign a new DHCPv4 Option code for the 480 IPv6-Only Preferred option from the BOOTP Vendor Extensions and 481 DHCPv4 Options registry, located at https://www.iana.org/assignments/ 482 bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml#options . If 483 possible, please assign option code 108. 485 +----------------------------+-------+ 486 | Option Name | Code | 487 +----------------------------+-------+ 488 | IPv6-only Preferred option | (TBD) | 489 +----------------------------+-------+ 491 Table 1 493 6. Security Considerations 495 An attacker might send a spoofed DHCPOFFER containing IPv6-only 496 Preferred option with the value field set to 0xffffffff, disabling 497 DHCPv4 on clients supporting the option. If the network is IPv4-only 498 such clients would lose connectivity, while on a dual-stack network 499 without NAT64 service only connectivity to IPv4-only destinations 500 would be affected. The recovery would require triggering a network 501 attachment event. However it should be noted that if the network 502 does not provide protection from a rogue DHCPv4 server the similar 503 attack vector can be executed by offering an invalid address and 504 setting the Lease Time option value field to 0xffffffff. The latter 505 attack would affect all hosts, not just hosts that support the 506 IPv6-only Preferred option. Therefore the security measures against 507 rogue DHCPv4 servers would be sufficient to prevent the attacks 508 specific to IPv6-only Preferred option. 510 It should be noted that disabling IPv4 on a host upon receiving the 511 IPv6-only Preferred option from the DHCPv4 server protects the host 512 from IPv4-related attacks and therefore could be considered a 513 security feature. 515 7. Acknowledgements 517 Thanks to the following people (in alphabetical order) for their 518 review and feedback: Mohamed Boucadair, Martin Duke, Russ Housley, 519 Sheng Jiang, Ted Lemon, Roy Marples, Bjorn Mork, Peng Shuping, Pascal 520 Thubert, Bernie Volz, Eric Vyncke. Authors would like to thank Bob 521 Hinden and Brian Carpenter for the initial idea of signaling 522 IPv6-only capability to hosts. Special thanks to Erik Kline, Mark 523 Townsley and Maciej Zenczykowski for the discussion which led to the 524 idea of signalling IPv6-only capability over DHCPv4. 526 8. References 528 8.1. Normative References 530 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 531 Requirement Levels", BCP 14, RFC 2119, 532 DOI 10.17487/RFC2119, March 1997, 533 . 535 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 536 RFC 2131, DOI 10.17487/RFC2131, March 1997, 537 . 539 [RFC2563] Troll, R., "DHCP Option to Disable Stateless Auto- 540 Configuration in IPv4 Clients", RFC 2563, 541 DOI 10.17487/RFC2563, May 1999, 542 . 544 [RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic 545 Configuration of IPv4 Link-Local Addresses", RFC 3927, 546 DOI 10.17487/RFC3927, May 2005, 547 . 549 [RFC4039] Park, S., Kim, P., and B. Volz, "Rapid Commit Option for 550 the Dynamic Host Configuration Protocol version 4 551 (DHCPv4)", RFC 4039, DOI 10.17487/RFC4039, March 2005, 552 . 554 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 555 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 556 DOI 10.17487/RFC4861, September 2007, 557 . 559 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 560 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 561 May 2017, . 563 8.2. Informative References 565 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 566 NAT64: Network Address and Protocol Translation from IPv6 567 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146, 568 April 2011, . 570 [RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van 571 Beijnum, "DNS64: DNS Extensions for Network Address 572 Translation from IPv6 Clients to IPv4 Servers", RFC 6147, 573 DOI 10.17487/RFC6147, April 2011, 574 . 576 [RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT: 577 Combination of Stateful and Stateless Translation", 578 RFC 6877, DOI 10.17487/RFC6877, April 2013, 579 . 581 Authors' Addresses 583 Lorenzo Colitti 584 Google 585 Shibuya 3-21-3 586 Shibuya, Tokyo 150-0002 587 JP 589 Email: lorenzo@google.com 591 Jen Linkova 592 Google 593 1 Darling Island Rd 594 Pyrmont, NSW 2009 595 AU 597 Email: furry@google.com 599 Michael C. Richardson 600 Sandelman Software Works 602 Email: mcr+ietf@sandelman.ca 603 URI: http://www.sandelman.ca/ 605 Tomek Mrugalski 606 Internet Systems Consortium, Inc. 607 950 Charter Street 608 Redwood City, CA 94063 609 USA 611 Email: tomasz.mrugalski@gmail.com