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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group W. Kumari 3 Internet-Draft Google 4 Obsoletes: 7710 (if approved) E. Kline 5 Intended status: Standards Track Loon 6 Expires: October 30, 2020 April 28, 2020 8 Captive-Portal Identification in DHCP / RA 9 draft-ietf-capport-rfc7710bis-04 11 Abstract 13 In many environments offering short-term or temporary Internet access 14 (such as coffee shops), it is common to start new connections in a 15 captive portal mode. This highly restricts what the customer can do 16 until the customer has authenticated. 18 This document describes a DHCP option (and a Router Advertisement 19 (RA) extension) to inform clients that they are behind some sort of 20 captive-portal enforcement device, and that they will need to 21 authenticate to get Internet access. It is not a full solution to 22 address all of the issues that clients may have with captive portals; 23 it is designed to be used in larger solutions. The method of 24 authenticating to, and interacting with the captive portal is out of 25 scope of this document. 27 This document replaces RFC 7710. RFC 7710 used DHCP code point 160. 28 Due to a conflict, this document specifies 114. 30 [ This document is being collaborated on in Github at: 31 https://github.com/capport-wg/7710bis. The most recent version of 32 the document, open issues, etc should all be available here. The 33 authors (gratefully) accept pull requests. Text in square brackets 34 will be removed before publication. ] 36 Status of This Memo 38 This Internet-Draft is submitted in full conformance with the 39 provisions of BCP 78 and BCP 79. 41 Internet-Drafts are working documents of the Internet Engineering 42 Task Force (IETF). Note that other groups may also distribute 43 working documents as Internet-Drafts. The list of current Internet- 44 Drafts is at https://datatracker.ietf.org/drafts/current/. 46 Internet-Drafts are draft documents valid for a maximum of six months 47 and may be updated, replaced, or obsoleted by other documents at any 48 time. It is inappropriate to use Internet-Drafts as reference 49 material or to cite them other than as "work in progress." 51 This Internet-Draft will expire on October 30, 2020. 53 Copyright Notice 55 Copyright (c) 2020 IETF Trust and the persons identified as the 56 document authors. All rights reserved. 58 This document is subject to BCP 78 and the IETF Trust's Legal 59 Provisions Relating to IETF Documents 60 (https://trustee.ietf.org/license-info) in effect on the date of 61 publication of this document. Please review these documents 62 carefully, as they describe your rights and restrictions with respect 63 to this document. Code Components extracted from this document must 64 include Simplified BSD License text as described in Section 4.e of 65 the Trust Legal Provisions and are provided without warranty as 66 described in the Simplified BSD License. 68 Table of Contents 70 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 71 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3 72 2. The Captive-Portal Option . . . . . . . . . . . . . . . . . . 3 73 2.1. IPv4 DHCP Option . . . . . . . . . . . . . . . . . . . . 4 74 2.2. IPv6 DHCP Option . . . . . . . . . . . . . . . . . . . . 5 75 2.3. The Captive-Portal IPv6 RA Option . . . . . . . . . . . . 5 76 3. Precedence of API URIs . . . . . . . . . . . . . . . . . . . 6 77 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 78 4.1. Captive Portal Unrestricted Identifier . . . . . . . . . 7 79 4.2. BOOTP Vendor Extensions and DHCP Options Code Change . . 7 80 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 81 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 82 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 83 7.1. Normative References . . . . . . . . . . . . . . . . . . 9 84 7.2. Informative References . . . . . . . . . . . . . . . . . 10 85 Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 10 86 Appendix B. Changes from RFC 7710 . . . . . . . . . . . . . . . 11 87 Appendix C. Observations From IETF 106 Network Experiment . . . 11 88 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 90 1. Introduction 92 In many environments, users need to connect to a captive-portal 93 device and agree to an Acceptable Use Policy (AUP) and / or provide 94 billing information before they can access the Internet. Regardless 95 of how that mechanism operates, this document provides functionality 96 to allow the client to know when it is behind a captive portal and 97 how to contact it. 99 In order to present users with the payment or AUP pages, presently a 100 captive-portal enforcement device has to intercept the user's 101 connections and redirect the user to a captive portal server, using 102 methods that are very similar to man-in-the-middle (MITM) attacks. 103 As increasing focus is placed on security, and end nodes adopt a more 104 secure stance, these interception techniques will become less 105 effective and/or more intrusive. 107 This document describes a DHCP ([RFC2131]) option (Captive-Portal) 108 and an IPv6 Router Advertisement (RA) ([RFC4861]) extension that 109 informs clients that they are behind a captive-portal enforcement 110 device and how to contact an API for more information. 112 This document replaces RFC 7710 [RFC7710]. 114 1.1. Requirements Notation 116 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 117 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 118 "OPTIONAL" in this document are to be interpreted as described in BCP 119 14 [RFC2119] [RFC8174] when, and only when, they appear in all 120 capitals, as shown here. 122 2. The Captive-Portal Option 124 The Captive Portal DHCP / RA Option informs the client that it may be 125 behind a captive portal and provides the URI to access an API as 126 defined by [draft-ietf-capport-api]. This is primarily intended to 127 improve the user experience by showing the user the captive portal 128 information faster and more reliably. Note that, for the foreseeable 129 future, captive portals will still need to implement the interception 130 techniques to serve legacy clients, and clients will need to perform 131 probing to detect captive portals. 133 Clients that support the Captive Portal DHCP option SHOULD include 134 the option in the Parameter Request List in DHCPREQUEST messages. 135 DHCP servers MAY send the Captive Portal option without any explicit 136 request. 138 In order to support multiple "classes" of clients (e.g. IPv4 only, 139 IPv6 only with DHCPv6 ([RFC8415]), and IPv6 only with RA) the captive 140 network can provision the client with the URI via multiple methods 141 (IPv4 DHCP, IPv6 DHCP, and IPv6 RA). The captive portal operator 142 SHOULD ensure that the URIs provisioned by each method are equivalent 143 to reduce the chance of operational problems. The maximum length of 144 the URI that can be carried in IPv4 DHCP is 255 bytes, so URIs longer 145 than 255 bytes should not be provisioned via IPv6 DHCP nor IPv6 RA 146 options. 148 In all variants of this option, the URI MUST be that of the captive 149 portal API endpoint, conforming to the recommendations for such URIs 150 [draft-ietf-capport-api]. 152 A captive portal MAY do content negotiation ([RFC7231] section 3.4) 153 and attempt to redirect clients querying without an explicit 154 indication of support for the captive portal API content type (i.e. 155 without application/capport+json listed explicitly anywhere within an 156 Accept header vis. [RFC7231] section 5.3). In so doing, the captive 157 portal SHOULD redirect the client to the value associated with the 158 "user-portal-url" API key. When performing such content negotiation 159 ([RFC7231] Section 3.4), implementors of captive portals need to keep 160 in mind that such responses might be cached, and therefore SHOULD 161 include an appropriate Vary header field ([RFC7231] Section 7.1.4) or 162 mark them explicitly uncacheable (for example, using Cache-Control: 163 no-store [RFC7234] Section 5.2.2.3). 165 The URI SHOULD NOT contain an IP address literal. Exceptions to this 166 might include networks with only one operational IP address family 167 where DNS is either not available or not fully functional until the 168 captive portal has been satisfied. 170 Networks with no captive portals MAY explicitly indicate this 171 condition by using this option with the IANA-assigned URI for this 172 purpose. Clients observing the URI value 173 "urn:ietf:params:capport:unrestricted" MAY forego time-consuming 174 forms of captive portal detection. 176 2.1. IPv4 DHCP Option 178 The format of the IPv4 Captive-Portal DHCP option is shown below. 180 0 1 2 3 181 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 1 182 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 183 | Code | Len | URI (variable length) ... | 184 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 185 . ...URI continued... . 186 | ... | 187 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 189 o Code: The Captive-Portal DHCPv4 Option (114) (one octet) 191 o Len: The length (one octet), in octets of the URI. 193 o URI: The URI for the captive portal API endpoint to which the user 194 should connect (encoded following the rules in [RFC3986]). 196 See [RFC2132], Section 2 for more on the format of IPv4 DHCP options. 198 Note that the URI parameter is not null terminated. 200 2.2. IPv6 DHCP Option 202 The format of the IPv6 Captive-Portal DHCP option is shown below. 204 0 1 2 3 205 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 1 206 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 207 | option-code | option-len | 208 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 209 . URI (variable length) . 210 | ... | 211 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 213 o option-code: The Captive-Portal DHCPv6Option (103) (two octets) 215 o option-len: The unsigned 16-bit length, in octets, of the URI. 217 o URI: The URI for the captive portal API endpoint to which the user 218 should connect (encoded following the rules in [RFC3986]). 220 See [RFC7227], Section 5.7 for more examples of DHCP Options with 221 URIs. See [RFC8415], Section 21.1 for more on the format of IPv6 222 DHCP options. 224 Note that the URI parameter is not null terminated. 226 The maximum length of the URI that can be carried in IPv4 DHCP is 255 227 bytes, so URIs longer than 255 bytes should not be provisioned via 228 IPv6 DHCP options. 230 2.3. The Captive-Portal IPv6 RA Option 232 This section describes the Captive-Portal Router Advertisement 233 option. 235 0 1 2 3 236 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 1 237 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 238 | Type | Length | URI . 239 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . 240 . . 241 . . 242 . . 243 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 244 Figure 2: Captive-Portal RA Option Format 246 Type 37 248 Length 8-bit unsigned integer. The length of the option (including 249 the Type and Length fields) in units of 8 bytes. 251 URI The URI for the captive portal API endpoint to which the user 252 should connect. This MUST be padded with NULL (0x00) to make the 253 total option length (including the Type and Length fields) a 254 multiple of 8 bytes. 256 Note that the URI parameter is not guaranteed to be null terminated. 258 The maximum length of the URI that can be carried in IPv4 DHCP is 255 259 bytes, so URIs longer than 255 bytes should not be provisioned via 260 IPv6 RA options. 262 3. Precedence of API URIs 264 A device may learn about Captive Portal API URIs through more than 265 one of (or indeed all of) the above options. Implementations can 266 select their own precedence order (e.g., prefer one of the IPv6 267 options before the DHCPv4 option, or vice versa, et cetera). 269 If the URIs learned via more than one option described in Section 2 270 are not all identical, this condition should be logged for the device 271 owner or administrator; it is a network configuration error if the 272 learned URIs are not all identical. 274 4. IANA Considerations 276 This document requests one new IETF URN protocol parameter 277 ([RFC3553]) entry. This document also requests a reallocation of 278 DHCPv4 option codes (see Appendix C for background). 280 Thanks IANA! 282 4.1. Captive Portal Unrestricted Identifier 284 This document registers a new entry under the IETF URN Sub-namespace 285 defined in [RFC3553]: 287 Registry name: Captive Portal Unrestricted Identifier 289 URN: urn:ietf:params:capport:unrestricted 291 Specification: RFC TBD (this document) 293 Repository: RFC TBD (this document) 295 Index value: Only one value is defined (see URN above). No 296 hierarchy is defined and therefore no sub-namespace registrations 297 are possible. 299 4.2. BOOTP Vendor Extensions and DHCP Options Code Change 301 [ RFC Ed: Please remove before publication: RFC7710 uses DHCP Code 302 160 -- unfortunately, it was discovered that this option code is 303 already widely used by Polycom (see appendix). Option 114 (URL) is 304 currently assigned to Apple (RFC3679, Section 3.2.3 - Contact: Dieter 305 Siegmund, dieter@apple.com - Reason to recover: Never published in an 306 RFC) Tommy Pauly (Apple) and Dieter Siegmund confirm that this 307 codepoint hasn't been used, and Apple is willing to relinquish it for 308 use in CAPPORT. Please see thread: 309 https://mailarchive.ietf.org/arch/msg/captive-portals/ 310 TmqQz6Ma_fznD3XbhwkH9m2dB28 for more background. ] 312 The IANA is requested to update the "BOOTP Vendor Extensions and DHCP 313 Options" registry (https://www.iana.org/assignments/bootp-dhcp- 314 parameters/bootp-dhcp-parameters.xhtml) as follows. 316 Tag: 114 317 Name: DHCP Captive-Portal 318 Data Length: N 319 Meaning: DHCP Captive-Portal 320 Reference: [THIS-RFC] 322 Tag: 160 323 Name: REMOVED/Unassigned 324 Data Length: 325 Meaning: 326 Reference: [THIS-RFC][RFC7710] 328 5. Security Considerations 330 By removing or reducing the need for captive portals to perform MITM 331 hijacking, this mechanism improves security by making the portal and 332 its actions visible, rather than hidden, and reduces the likelihood 333 that users will disable useful security safeguards like DNSSEC 334 validation, VPNs, etc. In addition, because the system knows that it 335 is behind a captive portal, it can know not to send cookies, 336 credentials, etc. By handing out a URI which is protected with TLS, 337 the captive portal operator can attempt to reassure the user that the 338 captive portal is not malicious. 340 An attacker with the ability to inject DHCP messages or RAs could 341 include an option from this document to force users to contact an 342 address of his choosing. As an attacker with this capability could 343 simply list himself as the default gateway (and so intercept all the 344 victim's traffic); this does not provide them with significantly more 345 capabilities, but because this document removes the need for 346 interception, the attacker may have an easier time performing the 347 attack. As the operating systems and application that make use of 348 this information know that they are connecting to a captive-portal 349 device (as opposed to intercepted connections) they can render the 350 page in a sandboxed environment and take other precautions, such as 351 clearly labeling the page as untrusted. The means of sandboxing and 352 user interface presenting this information is not covered in this 353 document - by its nature it is implementation specific and best left 354 to the application and user interface designers. 356 Devices and systems that automatically connect to an open network 357 could potentially be tracked using the techniques described in this 358 document (forcing the user to continually authenticate, or exposing 359 their browser fingerprint). However, similar tracking can already be 360 performed with the presently common captive portal mechanisms, so 361 this technique does not give the attackers more capabilities. 363 Captive portals are increasingly hijacking TLS connections to force 364 browsers to talk to the portal. Providing the portal's URI via a 365 DHCP or RA option is a cleaner technique, and reduces user 366 expectations of being hijacked - this may improve security by making 367 users more reluctant to accept TLS hijacking, which can be performed 368 from beyond the network associated with the captive portal. 370 6. Acknowledgements 372 This document is a -bis of RFC7710. Thanks to all of the original 373 authors (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, Steve 374 Sheng), and original contributors. 376 Also thanks to the CAPPORT WG for all of the discussion and 377 improvements including contributions and review from Joe Clarke, 378 Lorenzo Colitti, Dave Dolson, Hans Kuhn, Kyle Larose, Clemens 379 Schimpe, Martin Thomson, Michael Richardson, Remi Nguyen Van, Subash 380 Tirupachur Comerica, Bernie Volz, and Tommy Pauly. 382 7. References 384 7.1. Normative References 386 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 387 Requirement Levels", BCP 14, RFC 2119, 388 DOI 10.17487/RFC2119, March 1997, 389 . 391 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 392 RFC 2131, DOI 10.17487/RFC2131, March 1997, 393 . 395 [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor 396 Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997, 397 . 399 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 400 IETF URN Sub-namespace for Registered Protocol 401 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 402 2003, . 404 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 405 Resource Identifier (URI): Generic Syntax", STD 66, 406 RFC 3986, DOI 10.17487/RFC3986, January 2005, 407 . 409 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 410 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 411 DOI 10.17487/RFC4861, September 2007, 412 . 414 [RFC7227] Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and 415 S. Krishnan, "Guidelines for Creating New DHCPv6 Options", 416 BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014, 417 . 419 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 420 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 421 DOI 10.17487/RFC7231, June 2014, 422 . 424 [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 425 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", 426 RFC 7234, DOI 10.17487/RFC7234, June 2014, 427 . 429 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 430 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 431 May 2017, . 433 [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., 434 Richardson, M., Jiang, S., Lemon, T., and T. Winters, 435 "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", 436 RFC 8415, DOI 10.17487/RFC8415, November 2018, 437 . 439 7.2. Informative References 441 [RFC7710] Kumari, W., Gudmundsson, O., Ebersman, P., and S. Sheng, 442 "Captive-Portal Identification Using DHCP or Router 443 Advertisements (RAs)", RFC 7710, DOI 10.17487/RFC7710, 444 December 2015, . 446 7.3. URIs 448 [1] https://tickets.meeting.ietf.org/wiki/IETF106network#Experiments 450 [2] https://tickets.meeting.ietf.org/wiki/CAPPORT 452 [3] https://community.polycom.com/t5/VoIP-SIP-Phones/DHCP- 453 Standardization-160-vs-66/td-p/72577 455 Appendix A. Changes / Author Notes. 457 [RFC Editor: Please remove this section before publication ] 459 From initial to -00. 461 o Import of RFC7710. 463 From -00 to -01. 465 o Remove link-relation text. 467 o Clarify option should be in DHCPREQUEST parameter list. 469 o Uppercase some SHOULDs. 471 Appendix B. Changes from RFC 7710 473 This document incorporates the following changes from [RFC7710]. 475 1. Clarify that IP string literals are NOT RECOMMENDED. 477 2. Clarify that the option URI SHOULD be that of the captive portal 478 API endpoint. 480 3. Clarify that captive portals MAY do content negotiation. 482 4. Added text about Captive Portal API URI precedence in the event 483 of a network configuration error. 485 5. Added urn:ietf:params:capport:unrestricted URN. 487 6. Notes that the DHCP Code changed from 160 to 114. 489 Appendix C. Observations From IETF 106 Network Experiment 491 During IETF 106 in Singapore an experiment [1] enabling Captive 492 Portal API compatible clients to discover a venue-info-url (see 493 experiment description [2] for more detail) revealed that some 494 Polycom devices on the same network made use of DHCPv4 option code 495 160 for other purposes [3]. 497 The presence of DHCPv4 Option code 160 holding a value indicating the 498 Captive Portal API URL caused these devices to not function as 499 desired. For this reason, this document requests IANA deprecate 500 option code 160 and reallocate different value to be used for the 501 Captive Portal API URL. 503 Authors' Addresses 505 Warren Kumari 506 Google 507 1600 Amphitheatre Parkway 508 Mountain View, CA 94043 509 US 511 Email: warren@kumari.net 512 Erik Kline 513 Loon 514 1600 Amphitheatre Parkway 515 Mountain View, CA 94043 516 US 518 Email: ek@loon.com