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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) == Unused Reference: 'RFC2939' is defined on line 350, but no explicit reference was found in the text ** Obsolete normative reference: RFC 3315 (Obsoleted by RFC 8415) ** Obsolete normative reference: RFC 7231 (Obsoleted by RFC 9110) ** Obsolete normative reference: RFC 7710 (Obsoleted by RFC 8910) Summary: 3 errors (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). 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: September 12, 2019 March 11, 2019 8 Captive-Portal Identification in DHCP / RA 9 draft-ekwk-capport-rfc7710bis-02 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 September 12, 2019. 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 . . . . . . . . . . . . 5 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 6 78 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 79 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 80 8. Normative References . . . . . . . . . . . . . . . . . . . . 8 81 Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 9 82 Appendix B. Differences 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 is 118 behind a captive portal and provides the URI to access an 119 authentication page. This is primarily intended to improve the user 120 experience by getting them to the captive portal faster; for the 121 foreseeable future, captive portals will still need to implement the 122 interception techniques to serve legacy clients, and clients will 123 need to perform probing to detect captive portals. 125 In order to support multiple "classes" of clients (e.g. IPv4 only, 126 IPv6 only with DHCPv6 ([RFC3315]), IPv6 only with RA) the captive 127 portal can provide the URI via multiple methods (IPv4 DHCP, IPv6 128 DHCP, IPv6 RA). The captive portal operator should ensure that the 129 URIs handed out are equivalent to reduce the chance of operational 130 problems. The maximum length of the URI that can be carried in IPv4 131 DHCP is 255 bytes, so URIs longer than 255 bytes should not be used 132 in IPv6 DHCP or IPv6 RA. 134 In all variants of this option, the URI SHOULD be that of the captive 135 portal API endpoint, conforming to the recommendations for such URIs 136 [cite:API] (i.e. the URI SHOULD contain a DNS name and SHOULD 137 reference a secure transport, e.g. https). A captive portal MAY do 138 content negotiation ([RFC7231] section 3.4) and attempt to redirect 139 clients querying without an explicit indication of support for the 140 captive portal API content type (i.e. without application/ 141 capport+json listed explicitly anywhere within an Accept header vis. 142 [RFC7231] section 5.3). In so doing, the captive portal SHOULD 143 redirect the client to the value associated with the "user-portal- 144 url" API key. 146 The URI SHOULD NOT contain an IP address literal. 148 The URI parameter is not null terminated. 150 Networks with no captive portals MAY explicitly indicate this 151 condition by using this option with the IANA-assigned URI for this 152 purpose (see Section 5.1.1). Clients observing the URI value 153 "urn:ietf:params:capport-unrestricted" MAY forego time-consuming 154 forms of captive portal detection. 156 2.1. IPv4 DHCP Option 158 The format of the IPv4 Captive-Portal DHCP option is shown below. 160 Code Len Data 161 +------+------+------+------+------+-- --+-----+ 162 | code | len | URI ... | 163 +------+------+------+------+------+-- --+-----+ 165 o Code: The Captive-Portal DHCPv4 Option (160) (one octet) 167 o Len: The length, in octets of the URI. 169 o URI: The URI for the captive portal API endpoint to which the user 170 should connect (encoded following the rules in [RFC3986]). 172 2.2. IPv6 DHCP Option 174 The format of the IPv6 Captive-Portal DHCP option is shown below. 176 0 1 2 3 177 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 1 178 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 179 | option-code | option-len | 180 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 181 . URI (variable length) . 182 | ... | 183 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 185 o option-code: The Captive-Portal DHCPv6Option (103) (two octets) 187 o option-len: The length, in octets of the URI. 189 o URI: The URI for the captive portal API endpoint to which the user 190 should connect (encoded following the rules in [RFC3986]). 192 See [RFC7227], Section 5.7 for more examples of DHCP Options with 193 URIs. 195 2.3. The Captive-Portal IPv6 RA Option 197 This section describes the Captive-Portal Router Advertisement 198 option. 200 0 1 2 3 201 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 1 202 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 203 | Type | Length | URI . 204 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . 205 . . 206 . . 207 . . 208 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 209 Figure 2: Captive-Portal RA Option Format 211 Type 37 213 Length 8-bit unsigned integer. The length of the option (including 214 the Type and Length fields) in units of 8 bytes. 216 URI The URI for the captive portal API endpoint to which the user 217 should connect. This MUST be padded with NULL (0x00) to make the 218 total option length (including the Type and Length fields) a 219 multiple of 8 bytes. 221 3. The Captive-Portal Link Relation Type 223 Some captive portal network deployments may be unable to change, or 224 unwilling to risk changing, the network infrastructure necessary to 225 use any of the above options. In such deployments, when clear text 226 HTTP intercept and redirection are used, a Link relation header 227 ([RFC8288], Section 3.3) MAY be inserted to convey to a HTTP client 228 (user agent) the associated Captive Portal API URI. 230 HTTP user agents MUST ignore this link relation in any context other 231 than when explicitly probing to detect the presence of a captive 232 portal. Failure to do so could allow an attacker to inject a Captive 233 Portal API URI other than the correct URI for a given network or for 234 networks where there is no captive portal present at all. 236 4. Precedence of API URIs 238 A device may learn about Captive Portal API URIs through more than 239 one of (or indeed all of) the above options. It is a network 240 configuration error if the learned URIs are not all identical. 242 However, if the URIs learned are not in fact all identical the 243 captive device MUST prioritize URIs learned from network provisioning 244 or configuration mechanisms before all other URIs. Specifically, 245 URIs learned via any of the options in Section 2 should take 246 precedence over any URI learned via a mechanism like the one 247 described in Section 3. 249 If the URIs learned via more than one option described in Section 2 250 are not all identical, this condition should be logged for the device 251 owner or administrator. URI precedence in this situation is not 252 specified by this document. 254 5. IANA Considerations 256 This document requests two new IETF URN protocol parameter 257 ([RFC3553]) entries. 259 Thanks IANA! 261 5.1. IETF params Registration 263 5.1.1. Registry name: Captive Portal Unrestricted Identifier 265 Registry name: Captive Portal Unrestricted Identifier 267 URN: urn:ietf:params:capport-unrestricted 269 Specification: RFC TBD (this document) 271 Repository: RFC TBD (this document) 273 Index value: Only one value is defined (see URN above). No hierarchy 274 is defined and therefore no sub-namespace registrations are possible. 276 5.1.2. Registry name: Captive Portal API Link Relation Type 278 Registry name: Captive Portal API Link Relation Type 280 URN: urn:ietf:params:capport-api 282 Specification: RFC TBD (this document) 283 Repository: RFC TBD (this document) 285 Index value: Only one value is defined (see URN above). No hierarchy 286 is defined and therefore no sub-namespace registrations are possible. 288 6. Security Considerations 290 An attacker with the ability to inject DHCP messages, RAs, or HTTP 291 headers into cleartext HTTP communications could include an option or 292 link relation from this document and so force users to contact an 293 address of his choosing. As an attacker with this capability could 294 simply list himself as the default gateway (and so intercept all the 295 victim's traffic); this does not provide them with significantly more 296 capabilities, but because this document removes the need for 297 interception, the attacker may have an easier time performing the 298 attack. As the operating systems and application that make use of 299 this information know that they are connecting to a captive-portal 300 device (as opposed to intercepted connections) they can render the 301 page in a sandboxed environment and take other precautions, such as 302 clearly labeling the page as untrusted. The means of sandboxing and 303 user interface presenting this information is not covered in this 304 document - by its nature it is implementation specific and best left 305 to the application and user interface designers. 307 Devices and systems that automatically connect to an open network 308 could potentially be tracked using the techniques described in this 309 document (forcing the user to continually authenticate, or exposing 310 their browser fingerprint). However, similar tracking can already be 311 performed with the standard captive portal mechanisms, so this 312 technique does not give the attackers more capabilities. 314 Captive portals are increasingly hijacking TLS connections to force 315 browsers to talk to the portal. Providing the portal's URI via a 316 DHCP or RA option is a cleaner technique, and reduces user 317 expectations of being hijacked - this may improve security by making 318 users more reluctant to accept TLS hijacking, which can be performed 319 from beyond the network associated with the captive portal. 321 By simplifying the interaction with the captive portal systems, and 322 doing away with the need for interception, we think that users will 323 be less likely to disable useful security safeguards like DNSSEC 324 validation, VPNs, etc. In addition, because the system knows that it 325 is behind a captive portal, it can know not to send cookies, 326 credentials, etc. By handing out a URI using which is protected with 327 TLS, the captive portal operator can attempt to reassure the user 328 that the captive portal is not malicious. 330 7. Acknowledgements 332 This document is a -bis of RFC7710. Thanks to all of the original 333 authors (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, Steve 334 Sheng), and original contributors. 336 Also thanks to the CAPPORT WG for all of the discussion and 337 improvements. 339 8. Normative References 341 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 342 Requirement Levels", BCP 14, RFC 2119, 343 DOI 10.17487/RFC2119, March 1997, . 346 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 347 RFC 2131, DOI 10.17487/RFC2131, March 1997, 348 . 350 [RFC2939] Droms, R., "Procedures and IANA Guidelines for Definition 351 of New DHCP Options and Message Types", BCP 43, RFC 2939, 352 DOI 10.17487/RFC2939, September 2000, . 355 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 356 C., and M. Carney, "Dynamic Host Configuration Protocol 357 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 358 2003, . 360 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 361 IETF URN Sub-namespace for Registered Protocol 362 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 363 2003, . 365 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 366 Resource Identifier (URI): Generic Syntax", STD 66, 367 RFC 3986, DOI 10.17487/RFC3986, January 2005, 368 . 370 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 371 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 372 DOI 10.17487/RFC4861, September 2007, . 375 [RFC7227] Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and 376 S. Krishnan, "Guidelines for Creating New DHCPv6 Options", 377 BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014, 378 . 380 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 381 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 382 DOI 10.17487/RFC7231, June 2014, . 385 [RFC7710] Kumari, W., Gudmundsson, O., Ebersman, P., and S. Sheng, 386 "Captive-Portal Identification Using DHCP or Router 387 Advertisements (RAs)", RFC 7710, DOI 10.17487/RFC7710, 388 December 2015, . 390 [RFC8288] Nottingham, M., "Web Linking", RFC 8288, 391 DOI 10.17487/RFC8288, October 2017, . 394 Appendix A. Changes / Author Notes. 396 [RFC Editor: Please remove this section before publication ] 398 From initial to -00. 400 o Import of RFC7710. 402 Appendix B. Differences from RFC 7710 404 This document incorporates the following differences from [RFC7710]. 406 o Clarify that IP string literals are NOT RECOMMENDED. 408 o Clarify that the option URI SHOULD be that of the captive portal 409 API endpoint. 411 o Clarify that captive portals MAY do content negotiation. 413 o Added text about Captive Portal API URI precedence in the event of 414 a network configuration error. 416 o Added urn:ietf:params:capport-unrestricted URN. 418 o Added urn:ietf:params:capport-api URN. 420 Authors' Addresses 422 Warren Kumari 423 Google 424 1600 Amphitheatre Parkway 425 Mountain View, CA 94043 426 US 428 Email: warren@kumari.net 430 Erik Kline 431 Loon 432 1600 Amphitheatre Parkway 433 Mountain View, CA 94043 434 US 436 Email: ek@google.com