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Kumari 3 Internet-Draft Google 4 Updates: 7710 (if approved) E. Kline 5 Intended status: Standards Track Loon 6 Expires: July 19, 2019 January 15, 2019 8 Captive-Portal Identification in DHCP / RA 9 draft-ekwk-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 19, 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. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 74 4.1. IETF params Registration . . . . . . . . . . . . . . . . 6 75 4.1.1. Registry name: Captive Portal Unrestricted Identifier 6 76 4.1.2. Registry name: Captive Portal API Link Relation Type 6 77 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 78 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 79 7. Normative References . . . . . . . . . . . . . . . . . . . . 7 80 Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 8 81 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 83 1. Introduction 85 In many environments, users need to connect to a captive-portal 86 device and agree to an Acceptable Use Policy (AUP) and / or provide 87 billing information before they can access the Internet. It is 88 anticipated that the IETF will work on a more fully featured protocol 89 at some point, to ease interaction with Captive Portals. Regardless 90 of how that protocol operates, it is expected that this document will 91 provide needed functionality because the client will need to know 92 when it is behind a captive portal and how to contact it. 94 In order to present users with the payment or AUP pages, the captive- 95 portal device has to intercept the user's connections and redirect 96 the user to the captive portal, using methods that are very similar 97 to man-in-the-middle (MITM) attacks. As increasing focus is placed 98 on security, and end nodes adopt a more secure stance, these 99 interception techniques will become less effective and/or more 100 intrusive. 102 This document describes a DHCP ([RFC2131]) option (Captive-Portal) 103 and an IPv6 Router Advertisement (RA) ([RFC4861]) extension that 104 informs clients that they are behind a captive-portal device and how 105 to contact it. 107 1.1. Requirements Notation 109 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 110 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 111 document are to be interpreted as described in [RFC2119]. 113 2. The Captive-Portal Option 115 The Captive Portal DHCP / RA Option informs the client that it is 116 behind a captive portal and provides the URI to access an 117 authentication page. This is primarily intended to improve the user 118 experience by getting them to the captive portal faster; for the 119 foreseeable future, captive portals will still need to implement the 120 interception techniques to serve legacy clients, and clients will 121 need to perform probing to detect captive portals. 123 In order to support multiple "classes" of clients (e.g. IPv4 only, 124 IPv6 only with DHCPv6 ([RFC3315]), IPv6 only with RA) the captive 125 portal can provide the URI via multiple methods (IPv4 DHCP, IPv6 126 DHCP, IPv6 RA). The captive portal operator should ensure that the 127 URIs handed out are equivalent to reduce the chance of operational 128 problems. The maximum length of the URI that can be carried in IPv4 129 DHCP is 255 bytes, so URIs longer than 255 bytes should not be used 130 in IPv6 DHCP or IPv6 RA. 132 In all variants of this option, the URI SHOULD be that of the captive 133 portal API endpoint, conforming to the recommendations for such URIs 134 [cite:API] (i.e. the URI SHOULD contain a DNS name and SHOULD 135 reference a secure transport, e.g. https). A captive portal MAY do 136 content negotiation [citation?] and attempt to redirect clients 137 querying without an explicit indication of support for the captive 138 portal API content type (i.e. without application/capport+json listed 139 explicitly anywhere within an Accepts header [citation]). In so 140 doing, the captive portal SHOULD redirect the client to the value 141 associated with the "user-portal-url" API key. 143 The URI SHOULD NOT contain an IP address literal. 145 The URI parameter is not null terminated. 147 Networks with no captive portals MAY explicitly indicate this 148 condition by using this option with the IANA-assigned URI for this 149 purpose (see Section 4.1.1). Clients observing the URI value 150 "urn:ietf:params:capport-unrestricted" MAY forego time-consuming 151 forms of captive portal detection. 153 2.1. IPv4 DHCP Option 155 The format of the IPv4 Captive-Portal DHCP option is shown below. 157 Code Len Data 158 +------+------+------+------+------+-- --+-----+ 159 | code | len | URI ... | 160 +------+------+------+------+------+-- --+-----+ 162 o Code: The Captive-Portal DHCPv4 Option (160) (one octet) 164 o Len: The length, in octets of the URI. 166 o URI: The URI for the captive portal API endpoint to which the user 167 should connect (encoded following the rules in [RFC3986]). 169 2.2. IPv6 DHCP Option 171 The format of the IPv6 Captive-Portal DHCP option is shown below. 173 0 1 2 3 174 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 1 175 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 176 | option-code | option-len | 177 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 178 . URI (variable length) . 179 | ... | 180 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 182 o option-code: The Captive-Portal DHCPv6Option (103) (two octets) 184 o option-len: The length, in octets of the URI. 186 o URI: The URI for the captive portal API endpoint to which the user 187 should connect (encoded following the rules in [RFC3986]). 189 See [RFC7227], Section 5.7 for more examples of DHCP Options with 190 URIs. 192 2.3. The Captive-Portal IPv6 RA Option 194 This section describes the Captive-Portal Router Advertisement 195 option. 197 0 1 2 3 198 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 1 199 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 200 | Type | Length | URI . 201 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . 202 . . 203 . . 204 . . 205 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 206 Figure 2: Captive-Portal RA Option Format 208 Type 37 210 Length 8-bit unsigned integer. The length of the option (including 211 the Type and Length fields) in units of 8 bytes. 213 URI The URI for the captive portal API endpoint to which the user 214 should connect. This MUST be padded with NULL (0x00) to make the 215 total option length (including the Type and Length fields) a 216 multiple of 8 bytes. 218 3. The Captive-Portal Link Relation Type 220 Some captive portal network deployments may be unable to change, or 221 unwilling to risk changing, the network infrastructure necessary to 222 use any of the above options. In such deployments, when clear text 223 HTTP intercept and redirection are used, a Link relation header 224 ([RFC8288], Section 3.3) MAY be inserted to convey to a HTTP client 225 (user agent) the associated Captive Portal API URI. 227 HTTP user agents MUST ignore this link relation in any context other 228 than when explicitly probing to detect the presence of a captive 229 portal. Failure to do so could allow an attacker to inject a Captive 230 Portal API URI other than the correct URI for a given network or for 231 networks where there is no captive portal present at all. 233 4. IANA Considerations 235 This document requests two new IETF URN protocol parameter 236 ([RFC3553]) entries. 238 Thanks IANA! 240 4.1. IETF params Registration 242 4.1.1. Registry name: Captive Portal Unrestricted Identifier 244 Registry name: Captive Portal Unrestricted Identifier 246 URN: urn:ietf:params:capport-unrestricted 248 Specification: RFC TBD (this document) 250 Repository: RFC TBD (this document) 252 Index value: Only one value is defined (see URN above). No hierarchy 253 is defined and therefore no sub-namespace registrations are possible. 255 4.1.2. Registry name: Captive Portal API Link Relation Type 257 Registry name: Captive Portal API Link Relation Type 259 URN: urn:ietf:params:capport-api 261 Specification: RFC TBD (this document) 263 Repository: RFC TBD (this document) 265 Index value: Only one value is defined (see URN above). No hierarchy 266 is defined and therefore no sub-namespace registrations are possible. 268 5. Security Considerations 270 An attacker with the ability to inject DHCP messages could include 271 this option and so force users to contact an address of his choosing. 272 As an attacker with this capability could simply list himself as the 273 default gateway (and so intercept all the victim's traffic); this 274 does not provide them with significantly more capabilities, but 275 because this document removes the need for interception, the attacker 276 may have an easier time performing the attack. As the operating 277 systems and application that make use of this information know that 278 they are connecting to a captive-portal device (as opposed to 279 intercepted connections) they can render the page in a sandboxed 280 environment and take other precautions, such as clearly labeling the 281 page as untrusted. The means of sandboxing and user interface 282 presenting this information is not covered in this document - by its 283 nature it is implementation specific and best left to the application 284 and user interface designers. 286 Devices and systems that automatically connect to an open network 287 could potentially be tracked using the techniques described in this 288 document (forcing the user to continually authenticate, or exposing 289 their browser fingerprint). However, similar tracking can already be 290 performed with the standard captive portal mechanisms, so this 291 technique does not give the attackers more capabilities. 293 Captive portals are increasingly hijacking TLS connections to force 294 browsers to talk to the portal. Providing the portal's URI via a 295 DHCP or RA option is a cleaner technique, and reduces user 296 expectations of being hijacked - this may improve security by making 297 users more reluctant to accept TLS hijacking, which can be performed 298 from beyond the network associated with the captive portal. 300 By simplifying the interaction with the captive portal systems, and 301 doing away with the need for interception, we think that users will 302 be less likely to disable useful security safeguards like DNSSEC 303 validation, VPNs, etc. In addition, because the system knows that it 304 is behind a captive portal, it can know not to send cookies, 305 credentials, etc. By handing out a URI using which is protected with 306 TLS, the captive portal operator can attempt to reassure the user 307 that the captive portal is not malicious. 309 6. Acknowledgements 311 This document is a -bis of RFC7710. Thanks to all of the original 312 authors (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, Steve 313 Sheng), and original contributors. 315 Also thanks to the CAPPORT WG for all of the discussion and 316 improvements. 318 7. Normative References 320 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 321 Requirement Levels", BCP 14, RFC 2119, 322 DOI 10.17487/RFC2119, March 1997, . 325 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 326 RFC 2131, DOI 10.17487/RFC2131, March 1997, 327 . 329 [RFC2939] Droms, R., "Procedures and IANA Guidelines for Definition 330 of New DHCP Options and Message Types", BCP 43, RFC 2939, 331 DOI 10.17487/RFC2939, September 2000, . 334 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 335 C., and M. Carney, "Dynamic Host Configuration Protocol 336 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 337 2003, . 339 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 340 IETF URN Sub-namespace for Registered Protocol 341 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 342 2003, . 344 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 345 Resource Identifier (URI): Generic Syntax", STD 66, 346 RFC 3986, DOI 10.17487/RFC3986, January 2005, 347 . 349 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 350 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 351 DOI 10.17487/RFC4861, September 2007, . 354 [RFC7227] Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and 355 S. Krishnan, "Guidelines for Creating New DHCPv6 Options", 356 BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014, 357 . 359 [RFC8288] Nottingham, M., "Web Linking", RFC 8288, 360 DOI 10.17487/RFC8288, October 2017, . 363 Appendix A. Changes / Author Notes. 365 [RFC Editor: Please remove this section before publication ] 367 From initial to -00. 369 o Import of RFC7710. 371 Authors' Addresses 373 Warren Kumari 374 Google 375 1600 Amphitheatre Parkway 376 Mountain View, CA 94043 377 US 379 Email: warren@kumari.net 380 Erik Kline 381 Loon 382 1600 Amphitheatre Parkway 383 Mountain View, CA 94043 384 US 386 Email: ek@google.com