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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: July 15, 2020 January 12, 2020 8 Captive-Portal Identification in DHCP / RA 9 draft-ietf-capport-rfc7710bis-01 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 device, and that they will need to authenticate to get 21 Internet access. It is not a full solution to address all of the 22 issues that clients may have with captive portals; it is designed to 23 be used in larger solutions. The method of authenticating to, and 24 interacting with the captive portal is out of scope of this document. 26 [ This document is being collaborated on in Github at: 27 https://github.com/wkumari/draft-ekwk-capport-rfc7710bis. The most 28 recent version of the document, open issues, etc should all be 29 available here. The authors (gratefully) accept pull requests. Text 30 in square brackets will be removed before publication. ] 32 Status of This Memo 34 This Internet-Draft is submitted in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at http://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on July 15, 2020. 49 Copyright Notice 51 Copyright (c) 2020 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents 56 (http://trustee.ietf.org/license-info) in effect on the date of 57 publication of this document. Please review these documents 58 carefully, as they describe your rights and restrictions with respect 59 to this document. Code Components extracted from this document must 60 include Simplified BSD License text as described in Section 4.e of 61 the Trust Legal Provisions and are provided without warranty as 62 described in the Simplified BSD License. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 67 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3 68 2. The Captive-Portal Option . . . . . . . . . . . . . . . . . . 3 69 2.1. IPv4 DHCP Option . . . . . . . . . . . . . . . . . . . . 4 70 2.2. IPv6 DHCP Option . . . . . . . . . . . . . . . . . . . . 4 71 2.3. The Captive-Portal IPv6 RA Option . . . . . . . . . . . . 5 72 3. Precedence of API URIs . . . . . . . . . . . . . . . . . . . 6 73 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 74 4.1. IETF params Registration . . . . . . . . . . . . . . . . 6 75 4.1.1. Registry name: Captive Portal Unrestricted Identifier 6 76 4.2. BOOTP Vendor Extensions and DHCP Options Code Change . . 6 77 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 78 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 79 7. Normative References . . . . . . . . . . . . . . . . . . . . 8 80 Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 10 81 Appendix B. Changes from RFC 7710 . . . . . . . . . . . . . . . 10 82 Appendix C. Observations From IETF 106 Network Experiment . . . 10 83 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 85 1. Introduction 87 In many environments, users need to connect to a captive-portal 88 device and agree to an Acceptable Use Policy (AUP) and / or provide 89 billing information before they can access the Internet. It is 90 anticipated that the IETF will work on a more fully featured protocol 91 at some point, to ease interaction with Captive Portals. Regardless 92 of how that protocol operates, it is expected that this document will 93 provide needed functionality because the client will need to know 94 when it is behind a captive portal and how to contact it. 96 In order to present users with the payment or AUP pages, the captive- 97 portal device has to intercept the user's connections and redirect 98 the user to the captive portal, using methods that are very similar 99 to man-in-the-middle (MITM) attacks. As increasing focus is placed 100 on security, and end nodes adopt a more secure stance, these 101 interception techniques will become less effective and/or more 102 intrusive. 104 This document describes a DHCP ([RFC2131]) option (Captive-Portal) 105 and an IPv6 Router Advertisement (RA) ([RFC4861]) extension that 106 informs clients that they are behind a captive-portal device and how 107 to contact it. 109 1.1. Requirements Notation 111 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 112 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 113 document are to be interpreted as described in [RFC2119]. 115 2. The Captive-Portal Option 117 The Captive Portal DHCP / RA Option informs the client that it may be 118 behind a captive portal and provides the URI to access an API as 119 defined by [draft-ietf-capport-api]. This is primarily intended to 120 improve the user experience by getting them to the captive portal 121 faster and more reliably. Note that, for the foreseeable future, 122 captive portals will still need to implement the interception 123 techniques to serve legacy clients, and clients will need to perform 124 probing to detect captive portals. 126 Clients that support the Captive Portal DHCP option SHOULD include 127 the option in the Parameter Request List in DHCPREQUEST messages. 128 DHCP servers MAY send the Captive Portal option without any explicit 129 request. 131 In order to support multiple "classes" of clients (e.g. IPv4 only, 132 IPv6 only with DHCPv6 ([RFC3315]), IPv6 only with RA) the captive 133 portal can provide the URI via multiple methods (IPv4 DHCP, IPv6 134 DHCP, IPv6 RA). The captive portal operator SHOULD ensure that the 135 URIs handed out are equivalent to reduce the chance of operational 136 problems. The maximum length of the URI that can be carried in IPv4 137 DHCP is 255 bytes, so URIs longer than 255 bytes should not be used 138 in IPv6 DHCP or IPv6 RA. 140 In all variants of this option, the URI MUST be that of the captive 141 portal API endpoint, conforming to the recommendations for such URIs 142 [draft-ietf-capport-api] (i.e. the URI SHOULD contain a DNS name and 143 SHOULD reference a secure transport, e.g. https). 145 A captive portal MAY redirect requests that do not have an Accept 146 header field ([RFC7231] Section 5.3) containing a field item whose 147 content-type is "application/capport+json" to the URL conveyed in the 148 "user-portal-url" API key. When performing such content negotiation 149 ([RFC7231] Section 3.4), captive portals need to keep in mind that 150 such responses might be cached, and therefore SHOULD include an 151 appropriate Vary header field ([RFC7231] Section 7.1.4) or mark them 152 explicitly uncacheable (for example, using Cache-Control: no-store 153 [RFC7234] Section 5.2.2.3). 155 A captive portal MAY do content negotiation ([RFC7231] section 3.4) 156 and attempt to redirect clients querying without an explicit 157 indication of support for the captive portal API content type (i.e. 158 without application/capport+json listed explicitly anywhere within an 159 Accept header vis. [RFC7231] section 5.3). In so doing, the captive 160 portal SHOULD redirect the client to the value associated with the 161 "user-portal-url" API key. 163 The URI SHOULD NOT contain an IP address literal. The URI parameter 164 is not null terminated. 166 Networks with no captive portals MAY explicitly indicate this 167 condition by using this option with the IANA-assigned URI for this 168 purpose (see Section 4.1.1). Clients observing the URI value 169 "urn:ietf:params:capport-unrestricted" MAY forego time-consuming 170 forms of captive portal detection. 172 2.1. IPv4 DHCP Option 174 The format of the IPv4 Captive-Portal DHCP option is shown below. 176 Code Len Data 177 +------+------+------+------+------+-- --+-----+ 178 | code | len | URI ... | 179 +------+------+------+------+------+-- --+-----+ 181 o Code: The Captive-Portal DHCPv4 Option (160) (one octet) 183 o Len: The length, in octets of the URI. 185 o URI: The URI for the captive portal API endpoint to which the user 186 should connect (encoded following the rules in [RFC3986]). 188 2.2. IPv6 DHCP Option 190 The format of the IPv6 Captive-Portal DHCP option is shown below. 192 0 1 2 3 193 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 1 194 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 195 | option-code | option-len | 196 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 197 . URI (variable length) . 198 | ... | 199 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 201 o option-code: The Captive-Portal DHCPv6Option (103) (two octets) 203 o option-len: The length, in octets of the URI. 205 o URI: The URI for the captive portal API endpoint to which the user 206 should connect (encoded following the rules in [RFC3986]). 208 See [RFC7227], Section 5.7 for more examples of DHCP Options with 209 URIs. 211 2.3. The Captive-Portal IPv6 RA Option 213 This section describes the Captive-Portal Router Advertisement 214 option. 216 0 1 2 3 217 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 1 218 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 219 | Type | Length | URI . 220 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . 221 . . 222 . . 223 . . 224 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 225 Figure 2: Captive-Portal RA Option Format 227 Type 37 229 Length 8-bit unsigned integer. The length of the option (including 230 the Type and Length fields) in units of 8 bytes. 232 URI The URI for the captive portal API endpoint to which the user 233 should connect. This MUST be padded with NULL (0x00) to make the 234 total option length (including the Type and Length fields) a 235 multiple of 8 bytes. 237 3. Precedence of API URIs 239 A device may learn about Captive Portal API URIs through more than 240 one of (or indeed all of) the above options. It is a network 241 configuration error if the learned URIs are not all identical. 243 However, if the URIs learned are not in fact all identical the 244 captive device MUST prioritize URIs learned from network provisioning 245 or configuration mechanisms before all other URIs. Specifically, 246 URIs learned via any of the options in Section 2 should take 247 precedence over any URI learned via some other mechanism, such as a 248 redirect. 250 If the URIs learned via more than one option described in Section 2 251 are not all identical, this condition should be logged for the device 252 owner or administrator. URI precedence in this situation is not 253 specified by this document. 255 4. IANA Considerations 257 This document requests two new IETF URN protocol parameter 258 ([RFC3553]) entries. This document also requests a reallocation of 259 DHCPv4 option codes (see Appendix C for background). 261 Thanks IANA! 263 4.1. IETF params Registration 265 4.1.1. Registry name: Captive Portal Unrestricted Identifier 267 Registry name: Captive Portal Unrestricted Identifier 269 URN: urn:ietf:params:capport-unrestricted 271 Specification: RFC TBD (this document) 273 Repository: RFC TBD (this document) 275 Index value: Only one value is defined (see URN above). No hierarchy 276 is defined and therefore no sub-namespace registrations are possible. 278 4.2. BOOTP Vendor Extensions and DHCP Options Code Change 280 [ RFC Ed: Please remove before publication: RFC7710 uses DHCP Code 281 160 -- unfortunately, it was discovered that this option code is 282 already widely used by Polycom (see appendix). Option 114 (URL) is 283 currently assigned to Apple (RFC3679, Section 3.2.3 - Contact: Dieter 284 Siegmund, dieter@apple.com - Reason to recover: Never published in an 285 RFC) Tommy Pauly (Apple) and Dieter Siegmund confirm that this 286 codepoint hasn't been used, and Apple is willing to relinquish it for 287 use in CAPPORT. Please see thread: 288 https://mailarchive.ietf.org/arch/msg/captive-portals/ 289 TmqQz6Ma_fznD3XbhwkH9m2dB28 for more background. ] 291 The IANA is requested to update the "BOOTP Vendor Extensions and DHCP 292 Options" registry (https://www.iana.org/assignments/bootp-dhcp- 293 parameters/bootp-dhcp-parameters.xhtml) as follows. 295 Tag: 114 296 Name: DHCP Captive-Portal 297 Data Length: N 298 Meaning: DHCP Captive-Portal 299 Reference: [THIS-RFC] 301 Tag: 160 302 Name: REMOVED/Unassigned 303 Data Length: 304 Meaning: 305 Reference: [RFC7710][Deprecated] 307 5. Security Considerations 309 An attacker with the ability to inject DHCP messages or RAs could 310 include an option from this document to force users to contact an 311 address of his choosing. As an attacker with this capability could 312 simply list himself as the default gateway (and so intercept all the 313 victim's traffic); this does not provide them with significantly more 314 capabilities, but because this document removes the need for 315 interception, the attacker may have an easier time performing the 316 attack. As the operating systems and application that make use of 317 this information know that they are connecting to a captive-portal 318 device (as opposed to intercepted connections) they can render the 319 page in a sandboxed environment and take other precautions, such as 320 clearly labeling the page as untrusted. The means of sandboxing and 321 user interface presenting this information is not covered in this 322 document - by its nature it is implementation specific and best left 323 to the application and user interface designers. 325 Devices and systems that automatically connect to an open network 326 could potentially be tracked using the techniques described in this 327 document (forcing the user to continually authenticate, or exposing 328 their browser fingerprint). However, similar tracking can already be 329 performed with the standard captive portal mechanisms, so this 330 technique does not give the attackers more capabilities. 332 Captive portals are increasingly hijacking TLS connections to force 333 browsers to talk to the portal. Providing the portal's URI via a 334 DHCP or RA option is a cleaner technique, and reduces user 335 expectations of being hijacked - this may improve security by making 336 users more reluctant to accept TLS hijacking, which can be performed 337 from beyond the network associated with the captive portal. 339 By simplifying the interaction with the captive portal systems, and 340 doing away with the need for interception, we think that users will 341 be less likely to disable useful security safeguards like DNSSEC 342 validation, VPNs, etc. In addition, because the system knows that it 343 is behind a captive portal, it can know not to send cookies, 344 credentials, etc. By handing out a URI using which is protected with 345 TLS, the captive portal operator can attempt to reassure the user 346 that the captive portal is not malicious. 348 Operating systems should conduct all interactions with the API in a 349 sand-boxed environment and with a configuration that minimizes 350 tracking risks. 352 6. Acknowledgements 354 This document is a -bis of RFC7710. Thanks to all of the original 355 authors (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, Steve 356 Sheng), and original contributors. 358 Also thanks to the CAPPORT WG for all of the discussion and 359 improvements including contributions and review from Lorenzo Colitti, 360 Remi Nguyen Van, and Tommy Pauly. 362 7. References 364 7.1. Normative References 366 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 367 Requirement Levels", BCP 14, RFC 2119, 368 DOI 10.17487/RFC2119, March 1997, . 371 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 372 RFC 2131, DOI 10.17487/RFC2131, March 1997, 373 . 375 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 376 C., and M. Carney, "Dynamic Host Configuration Protocol 377 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 378 2003, . 380 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 381 IETF URN Sub-namespace for Registered Protocol 382 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 383 2003, . 385 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 386 Resource Identifier (URI): Generic Syntax", STD 66, 387 RFC 3986, DOI 10.17487/RFC3986, January 2005, 388 . 390 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 391 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 392 DOI 10.17487/RFC4861, September 2007, . 395 [RFC7227] Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and 396 S. Krishnan, "Guidelines for Creating New DHCPv6 Options", 397 BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014, 398 . 400 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 401 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 402 DOI 10.17487/RFC7231, June 2014, . 405 [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 406 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", 407 RFC 7234, DOI 10.17487/RFC7234, June 2014, 408 . 410 [RFC7710] Kumari, W., Gudmundsson, O., Ebersman, P., and S. Sheng, 411 "Captive-Portal Identification Using DHCP or Router 412 Advertisements (RAs)", RFC 7710, DOI 10.17487/RFC7710, 413 December 2015, . 415 7.2. URIs 417 [1] https://tickets.meeting.ietf.org/wiki/IETF106network#Experiments 419 [2] https://tickets.meeting.ietf.org/wiki/CAPPORT 421 [3] https://community.polycom.com/t5/VoIP-SIP-Phones/DHCP- 422 Standardization-160-vs-66/td-p/72577 424 Appendix A. Changes / Author Notes. 426 [RFC Editor: Please remove this section before publication ] 428 From initial to -00. 430 o Import of RFC7710. 432 From -00 to -01. 434 o Remove link-relation text. 436 o Clarify option should be in DHCPREQUEST parameter list. 438 o Uppercase some SHOULDs. 440 Appendix B. Changes from RFC 7710 442 This document incorporates the following changes from [RFC7710]. 444 1. Clarify that IP string literals are NOT RECOMMENDED. 446 2. Clarify that the option URI SHOULD be that of the captive portal 447 API endpoint. 449 3. Clarify that captive portals MAY do content negotiation. 451 4. Added text about Captive Portal API URI precedence in the event 452 of a network configuration error. 454 5. Added urn:ietf:params:capport-unrestricted URN. 456 Appendix C. Observations From IETF 106 Network Experiment 458 During IETF 106 in Singapore an experiment [1] enabling Captive 459 Portal API compatible clients to discover a venue-info-url (see 460 experiment description [2] for more detail) revealed that some 461 Polycom devices on the same network made use of DHCPv4 option code 462 160 for other purposes [3]. 464 The presence of DHCPv4 Option code 160 holding a value indicating the 465 Captive Portal API URL caused these devices to not function as 466 desired. For this reason, this document requests IANA deprecate 467 option code 160 and reallocate different value to be used for the 468 Captive Portal API URL. 470 Authors' Addresses 472 Warren Kumari 473 Google 474 1600 Amphitheatre Parkway 475 Mountain View, CA 94043 476 US 478 Email: warren@kumari.net 480 Erik Kline 481 Loon 482 1600 Amphitheatre Parkway 483 Mountain View, CA 94043 484 US 486 Email: ek@loon.com