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Kumari 3 Internet-Draft Google 4 Updates: 7710 (if approved) E. Kline 5 Intended status: Standards Track Google Japan 6 Expires: January 16, 2019 July 15, 2018 8 Captive-Portal Identification in DHCP / RA 9 draft-ekwk-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 16, 2019. 49 Copyright Notice 51 Copyright (c) 2018 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. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 73 3.1. IETF params Registration . . . . . . . . . . . . . . . . 5 74 4. Security Considerations . . . . . . . . . . . . . . . . . . . 6 75 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 76 6. Normative References . . . . . . . . . . . . . . . . . . . . 7 77 Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 8 78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 80 1. Introduction 82 In many environments, users need to connect to a captive-portal 83 device and agree to an Acceptable Use Policy (AUP) and / or provide 84 billing information before they can access the Internet. It is 85 anticipated that the IETF will work on a more fully featured protocol 86 at some point, to ease interaction with Captive Portals. Regardless 87 of how that protocol operates, it is expected that this document will 88 provide needed functionality because the client will need to know 89 when it is behind a captive portal and how to contact it. 91 In order to present users with the payment or AUP pages, the captive- 92 portal device has to intercept the user's connections and redirect 93 the user to the captive portal, using methods that are very similar 94 to man-in-the-middle (MITM) attacks. As increasing focus is placed 95 on security, and end nodes adopt a more secure stance, these 96 interception techniques will become less effective and/or more 97 intrusive. 99 This document describes a DHCP ([RFC2131]) option (Captive-Portal) 100 and an IPv6 Router Advertisement (RA) ([RFC4861]) extension that 101 informs clients that they are behind a captive-portal device and how 102 to contact it. 104 1.1. Requirements Notation 106 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 107 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 108 document are to be interpreted as described in [RFC2119]. 110 2. The Captive-Portal Option 112 The Captive Portal DHCP / RA Option informs the client that it is 113 behind a captive portal and provides the URI to access an 114 authentication page. This is primarily intended to improve the user 115 experience by getting them to the captive portal faster; for the 116 foreseeable future, captive portals will still need to implement the 117 interception techniques to serve legacy clients, and clients will 118 need to perform probing to detect captive portals. 120 In order to support multiple "classes" of clients (e.g. IPv4 only, 121 IPv6 only with DHCPv6 ([RFC3315]), IPv6 only with RA) the captive 122 portal can provide the URI via multiple methods (IPv4 DHCP, IPv6 123 DHCP, IPv6 RA). The captive portal operator should ensure that the 124 URIs handed out are equivalent to reduce the chance of operational 125 problems. The maximum length of the URI that can be carried in IPv4 126 DHCP is 255 bytes, so URIs longer than 255 bytes should not be used 127 in IPv6 DHCP or IPv6 RA. 129 In all variants of this option, the URI SHOULD be that of the captive 130 portal API endpoint, conforming to the recommendations for such URIs 131 [cite:API] (i.e. the URI SHOULD contain a DNS name and SHOULD 132 reference a secure transport, e.g. https). A captive portal MAY do 133 content negotiation [citation?] and attempt to redirect clients 134 querying without an explicit indication of support for the captive 135 portal API content type (i.e. without application/capport+json listed 136 explicitly anywhere within an Accepts header [citation]). In so 137 doing, the captive portal SHOULD redirect the client to the value 138 associated with the "user-portal-url" API key. 140 The URI SHOULD NOT contain an IP address literal. 142 The URI parameter is not null terminated. 144 Networks with no captive portals MAY explicitly indicate this 145 condition by using this option with the IANA-assigned URI for this 146 purpose . Clients observing the URI value 147 "urn:ietf:params:capport-unrestricted" MAY forego time-consuming 148 forms of captive portal detection. 150 2.1. IPv4 DHCP Option 152 The format of the IPv4 Captive-Portal DHCP option is shown below. 154 Code Len Data 155 +------+------+------+------+------+-- --+-----+ 156 | code | len | URI ... | 157 +------+------+------+------+------+-- --+-----+ 159 o Code: The Captive-Portal DHCPv4 Option (160) (one octet) 161 o Len: The length, in octets of the URI. 163 o URI: The URI for the captive portal API endpoint to which the user 164 should connect (encoded following the rules in [RFC3986]). 166 2.2. IPv6 DHCP Option 168 The format of the IPv6 Captive-Portal DHCP option is shown below. 170 0 1 2 3 171 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 1 172 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 173 | option-code | option-len | 174 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 175 . URI (variable length) . 176 | ... | 177 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 179 o option-code: The Captive-Portal DHCPv6Option (103) (two octets) 181 o option-len: The length, in octets of the URI. 183 o URI: The URI for the captive portal API endpoint to which the user 184 should connect (encoded following the rules in [RFC3986]). 186 See [RFC7227], Section 5.7 for more examples of DHCP Options with 187 URIs. 189 2.3. The Captive-Portal IPv6 RA Option 191 This section describes the Captive-Portal Router Advertisement 192 option. 194 0 1 2 3 195 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 1 196 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 197 | Type | Length | URI . 198 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . 199 . . 200 . . 201 . . 202 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 203 Figure 2: Captive-Portal RA Option Format 205 Type 37 207 Length 8-bit unsigned integer. The length of the option (including 208 the Type and Length fields) in units of 8 bytes. 210 URI The URI for the captive portal API endpoint to which the user 211 should connect. This MUST be padded with NULL (0x00) to make the 212 total option length (including the Type and Length fields) a 213 multiple of 8 bytes. 215 3. IANA Considerations 217 This document requests one new IETF URN protocol parameter 218 ([RFC3553]) entry. 220 Thanks IANA! 222 3.1. IETF params Registration 224 Registry name: Captive Portal Unrestricted Identifier 226 URN: urn:ietf:params:capport-unrestricted 228 Specification: RFC TBD (this document) 230 Repository: RFC TBD (this document) 232 Index value: Only one value is defined (see URN above). No hierarchy 233 is defined and therefore no sub-namespace registrations are possible. 235 4. Security Considerations 237 An attacker with the ability to inject DHCP messages could include 238 this option and so force users to contact an address of his choosing. 239 As an attacker with this capability could simply list himself as the 240 default gateway (and so intercept all the victim's traffic); this 241 does not provide them with significantly more capabilities, but 242 because this document removes the need for interception, the attacker 243 may have an easier time performing the attack. As the operating 244 systems and application that make use of this information know that 245 they are connecting to a captive-portal device (as opposed to 246 intercepted connections) they can render the page in a sandboxed 247 environment and take other precautions, such as clearly labeling the 248 page as untrusted. The means of sandboxing and user interface 249 presenting this information is not covered in this document - by its 250 nature it is implementation specific and best left to the application 251 and user interface designers. 253 Devices and systems that automatically connect to an open network 254 could potentially be tracked using the techniques described in this 255 document (forcing the user to continually authenticate, or exposing 256 their browser fingerprint). However, similar tracking can already be 257 performed with the standard captive portal mechanisms, so this 258 technique does not give the attackers more capabilities. 260 Captive portals are increasingly hijacking TLS connections to force 261 browsers to talk to the portal. Providing the portal's URI via a 262 DHCP or RA option is a cleaner technique, and reduces user 263 expectations of being hijacked - this may improve security by making 264 users more reluctant to accept TLS hijacking, which can be performed 265 from beyond the network associated with the captive portal. 267 By simplifying the interaction with the captive portal systems, and 268 doing away with the need for interception, we think that users will 269 be less likely to disable useful security safeguards like DNSSEC 270 validation, VPNs, etc. In addition, because the system knows that it 271 is behind a captive portal, it can know not to send cookies, 272 credentials, etc. By handing out a URI using which is protected with 273 TLS, the captive portal operator can attempt to reassure the user 274 that the captive portal is not malicious. 276 5. Acknowledgements 278 This document is a -bis of RFC7710. Thanks to all of the original 279 authors (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, Steve 280 Sheng), and original contributors. 282 Also thanks to the CAPPORT WG for all of the discussion and 283 improvements. 285 6. Normative References 287 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 288 Requirement Levels", BCP 14, RFC 2119, 289 DOI 10.17487/RFC2119, March 1997, . 292 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 293 RFC 2131, DOI 10.17487/RFC2131, March 1997, 294 . 296 [RFC2939] Droms, R., "Procedures and IANA Guidelines for Definition 297 of New DHCP Options and Message Types", BCP 43, RFC 2939, 298 DOI 10.17487/RFC2939, September 2000, . 301 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 302 C., and M. Carney, "Dynamic Host Configuration Protocol 303 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 304 2003, . 306 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 307 IETF URN Sub-namespace for Registered Protocol 308 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 309 2003, . 311 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 312 Resource Identifier (URI): Generic Syntax", STD 66, 313 RFC 3986, DOI 10.17487/RFC3986, January 2005, 314 . 316 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 317 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 318 DOI 10.17487/RFC4861, September 2007, . 321 [RFC7227] Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and 322 S. Krishnan, "Guidelines for Creating New DHCPv6 Options", 323 BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014, 324 . 326 Appendix A. Changes / Author Notes. 328 [RFC Editor: Please remove this section before publication ] 330 From initial to -00. 332 o Import of RFC7710. 334 Authors' Addresses 336 Warren Kumari 337 Google 338 1600 Amphitheatre Parkway 339 Mountain View, CA 94043 340 US 342 Email: warren@kumari.net 344 Erik Kline 345 Google Japan 346 Roppongi 6-10-1, 44th Floor 347 Minato, Tokyo 106-6144 348 Japan