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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 Updates: 7710 (if approved) E. Kline 5 Intended status: Standards Track Loon 6 Expires: January 3, 2020 July 2, 2019 8 Captive-Portal Identification in DHCP / RA 9 draft-ietf-capport-rfc7710bis-00 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 January 3, 2020. 49 Copyright Notice 51 Copyright (c) 2019 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. The Captive-Portal Link Relation Type . . . . . . . . . . . . 6 73 4. Precedence of API URIs . . . . . . . . . . . . . . . . . . . 6 74 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 75 5.1. IETF params Registration . . . . . . . . . . . . . . . . 6 76 5.1.1. Registry name: Captive Portal Unrestricted Identifier 6 77 5.1.2. Registry name: Captive Portal API Link Relation Type 7 78 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 79 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 80 8. Normative References . . . . . . . . . . . . . . . . . . . . 8 81 Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 9 82 Appendix B. Changes from RFC 7710 . . . . . . . . . . . . . . . 9 83 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 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 In order to support multiple "classes" of clients (e.g. IPv4 only, 127 IPv6 only with DHCPv6 ([RFC3315]), IPv6 only with RA) the captive 128 portal can provide the URI via multiple methods (IPv4 DHCP, IPv6 129 DHCP, IPv6 RA). The captive portal operator should ensure that the 130 URIs handed out are equivalent to reduce the chance of operational 131 problems. The maximum length of the URI that can be carried in IPv4 132 DHCP is 255 bytes, so URIs longer than 255 bytes should not be used 133 in IPv6 DHCP or IPv6 RA. 135 In all variants of this option, the URI MUST be that of the captive 136 portal API endpoint, conforming to the recommendations for such URIs 137 [cite:API] (i.e. the URI SHOULD contain a DNS name and SHOULD 138 reference a secure transport, e.g. https). 140 A captive portal MAY redirect requests that do not have an Accept 141 header field ([RFC7231] Section 5.3) containing a field item whose 142 content-type is "application/capport+json" to the URL conveyed in the 143 "user-portal-url" API key. When performing such content negotiation 144 ([RFC7231] Section 3.4), captive portals need to keep in mind that 145 such responses might be cached, and therefore SHOULD include an 146 appropriate Vary header field ([RFC7231] Section 7.1.4) or mark them 147 explicitly uncacheable (for example, using Cache-Control: no-store 148 [RFC7234] Section 5.2.2.3). 150 A captive portal MAY do content negotiation ([RFC7231] section 3.4) 151 and attempt to redirect clients querying without an explicit 152 indication of support for the captive portal API content type (i.e. 153 without application/capport+json listed explicitly anywhere within an 154 Accept header vis. [RFC7231] section 5.3). In so doing, the captive 155 portal SHOULD redirect the client to the value associated with the 156 "user-portal-url" API key. 158 The URI SHOULD NOT contain an IP address literal. The URI parameter 159 is not null terminated. 161 Networks with no captive portals MAY explicitly indicate this 162 condition by using this option with the IANA-assigned URI for this 163 purpose (see Section 5.1.1). Clients observing the URI value 164 "urn:ietf:params:capport-unrestricted" MAY forego time-consuming 165 forms of captive portal detection. 167 2.1. IPv4 DHCP Option 169 The format of the IPv4 Captive-Portal DHCP option is shown below. 171 Code Len Data 172 +------+------+------+------+------+-- --+-----+ 173 | code | len | URI ... | 174 +------+------+------+------+------+-- --+-----+ 176 o Code: The Captive-Portal DHCPv4 Option (160) (one octet) 178 o Len: The length, in octets of the URI. 180 o URI: The URI for the captive portal API endpoint to which the user 181 should connect (encoded following the rules in [RFC3986]). 183 2.2. IPv6 DHCP Option 185 The format of the IPv6 Captive-Portal DHCP option is shown below. 187 0 1 2 3 188 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 1 189 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 190 | option-code | option-len | 191 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 192 . URI (variable length) . 193 | ... | 194 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 196 o option-code: The Captive-Portal DHCPv6Option (103) (two octets) 198 o option-len: The length, in octets of the URI. 200 o URI: The URI for the captive portal API endpoint to which the user 201 should connect (encoded following the rules in [RFC3986]). 203 See [RFC7227], Section 5.7 for more examples of DHCP Options with 204 URIs. 206 2.3. The Captive-Portal IPv6 RA Option 208 This section describes the Captive-Portal Router Advertisement 209 option. 211 0 1 2 3 212 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 1 213 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 214 | Type | Length | URI . 215 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . 216 . . 217 . . 218 . . 219 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 220 Figure 2: Captive-Portal RA Option Format 222 Type 37 224 Length 8-bit unsigned integer. The length of the option (including 225 the Type and Length fields) in units of 8 bytes. 227 URI The URI for the captive portal API endpoint to which the user 228 should connect. This MUST be padded with NULL (0x00) to make the 229 total option length (including the Type and Length fields) a 230 multiple of 8 bytes. 232 3. The Captive-Portal Link Relation Type 234 Some captive portal network deployments may be unable to change, or 235 unwilling to risk changing, the network infrastructure necessary to 236 use any of the above options. In such deployments, when clear text 237 HTTP intercept and redirection are used, a Link relation header 238 ([RFC8288], Section 3.3) MAY be inserted to convey to a HTTP client 239 (user agent) the associated Captive Portal API URI. 241 HTTP user agents MUST ignore this link relation in any context other 242 than when explicitly probing to detect the presence of a captive 243 portal. Failure to do so could allow an attacker to inject a Captive 244 Portal API URI other than the correct URI for a given network or for 245 networks where there is no captive portal present at all. 247 4. Precedence of API URIs 249 A device may learn about Captive Portal API URIs through more than 250 one of (or indeed all of) the above options. It is a network 251 configuration error if the learned URIs are not all identical. 253 However, if the URIs learned are not in fact all identical the 254 captive device MUST prioritize URIs learned from network provisioning 255 or configuration mechanisms before all other URIs. Specifically, 256 URIs learned via any of the options in Section 2 should take 257 precedence over any URI learned via a mechanism like the one 258 described in Section 3. 260 If the URIs learned via more than one option described in Section 2 261 are not all identical, this condition should be logged for the device 262 owner or administrator. URI precedence in this situation is not 263 specified by this document. 265 5. IANA Considerations 267 This document requests two new IETF URN protocol parameter 268 ([RFC3553]) entries. 270 Thanks IANA! 272 5.1. IETF params Registration 274 5.1.1. Registry name: Captive Portal Unrestricted Identifier 276 Registry name: Captive Portal Unrestricted Identifier 278 URN: urn:ietf:params:capport-unrestricted 279 Specification: RFC TBD (this document) 281 Repository: RFC TBD (this document) 283 Index value: Only one value is defined (see URN above). No hierarchy 284 is defined and therefore no sub-namespace registrations are possible. 286 5.1.2. Registry name: Captive Portal API Link Relation Type 288 Registry name: Captive Portal API Link Relation Type 290 URN: urn:ietf:params:capport-api 292 Specification: RFC TBD (this document) 294 Repository: RFC TBD (this document) 296 Index value: Only one value is defined (see URN above). No hierarchy 297 is defined and therefore no sub-namespace registrations are possible. 299 6. Security Considerations 301 An attacker with the ability to inject DHCP messages, RAs, or HTTP 302 headers into cleartext HTTP communications could include an option or 303 link relation from this document and so force users to contact an 304 address of his choosing. As an attacker with this capability could 305 simply list himself as the default gateway (and so intercept all the 306 victim's traffic); this does not provide them with significantly more 307 capabilities, but because this document removes the need for 308 interception, the attacker may have an easier time performing the 309 attack. As the operating systems and application that make use of 310 this information know that they are connecting to a captive-portal 311 device (as opposed to intercepted connections) they can render the 312 page in a sandboxed environment and take other precautions, such as 313 clearly labeling the page as untrusted. The means of sandboxing and 314 user interface presenting this information is not covered in this 315 document - by its nature it is implementation specific and best left 316 to the application and user interface designers. 318 Devices and systems that automatically connect to an open network 319 could potentially be tracked using the techniques described in this 320 document (forcing the user to continually authenticate, or exposing 321 their browser fingerprint). However, similar tracking can already be 322 performed with the standard captive portal mechanisms, so this 323 technique does not give the attackers more capabilities. 325 Captive portals are increasingly hijacking TLS connections to force 326 browsers to talk to the portal. Providing the portal's URI via a 327 DHCP or RA option is a cleaner technique, and reduces user 328 expectations of being hijacked - this may improve security by making 329 users more reluctant to accept TLS hijacking, which can be performed 330 from beyond the network associated with the captive portal. 332 By simplifying the interaction with the captive portal systems, and 333 doing away with the need for interception, we think that users will 334 be less likely to disable useful security safeguards like DNSSEC 335 validation, VPNs, etc. In addition, because the system knows that it 336 is behind a captive portal, it can know not to send cookies, 337 credentials, etc. By handing out a URI using which is protected with 338 TLS, the captive portal operator can attempt to reassure the user 339 that the captive portal is not malicious. 341 Operating systems should conduct all interactions with the API in a 342 sand-boxed environment and with a configuration that minimizes 343 tracking risks. 345 7. Acknowledgements 347 This document is a -bis of RFC7710. Thanks to all of the original 348 authors (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, Steve 349 Sheng), and original contributors. 351 Also thanks to the CAPPORT WG for all of the discussion and 352 improvements. 354 8. Normative References 356 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 357 Requirement Levels", BCP 14, RFC 2119, 358 DOI 10.17487/RFC2119, March 1997, . 361 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 362 RFC 2131, DOI 10.17487/RFC2131, March 1997, 363 . 365 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 366 C., and M. Carney, "Dynamic Host Configuration Protocol 367 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 368 2003, . 370 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 371 IETF URN Sub-namespace for Registered Protocol 372 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 373 2003, . 375 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 376 Resource Identifier (URI): Generic Syntax", STD 66, 377 RFC 3986, DOI 10.17487/RFC3986, January 2005, 378 . 380 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 381 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 382 DOI 10.17487/RFC4861, September 2007, . 385 [RFC7227] Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and 386 S. Krishnan, "Guidelines for Creating New DHCPv6 Options", 387 BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014, 388 . 390 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 391 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 392 DOI 10.17487/RFC7231, June 2014, . 395 [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 396 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", 397 RFC 7234, DOI 10.17487/RFC7234, June 2014, 398 . 400 [RFC7710] Kumari, W., Gudmundsson, O., Ebersman, P., and S. Sheng, 401 "Captive-Portal Identification Using DHCP or Router 402 Advertisements (RAs)", RFC 7710, DOI 10.17487/RFC7710, 403 December 2015, . 405 [RFC8288] Nottingham, M., "Web Linking", RFC 8288, 406 DOI 10.17487/RFC8288, October 2017, . 409 Appendix A. Changes / Author Notes. 411 [RFC Editor: Please remove this section before publication ] 413 From initial to -00. 415 o Import of RFC7710. 417 Appendix B. Changes from RFC 7710 419 This document incorporates the following changes from [RFC7710]. 421 1. Clarify that IP string literals are NOT RECOMMENDED. 423 2. Clarify that the option URI SHOULD be that of the captive portal 424 API endpoint. 426 3. Clarify that captive portals MAY do content negotiation. 428 4. Added text about Captive Portal API URI precedence in the event 429 of a network configuration error. 431 5. Added urn:ietf:params:capport-unrestricted URN. 433 6. Added urn:ietf:params:capport-api URN. 435 Authors' Addresses 437 Warren Kumari 438 Google 439 1600 Amphitheatre Parkway 440 Mountain View, CA 94043 441 US 443 Email: warren@kumari.net 445 Erik Kline 446 Loon 447 1600 Amphitheatre Parkway 448 Mountain View, CA 94043 449 US 451 Email: ek@google.com