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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Missing Reference: 'RFC2939' is mentioned on line 353, but not defined Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). 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 Intended status: Informational O. Gudmundsson 5 Expires: July 31, 2015 Shinkuro Inc. 6 P. Ebersman 7 Comcast 8 S. Sheng 9 ICANN 10 January 27, 2015 12 Captive-Portal identification in DHCP / RA 13 draft-wkumari-dhc-capport-09 15 Abstract 17 In many environments offering short-term or temporary Internet access 18 (such as coffee shops), it is common to start new connections in a 19 captive portal mode. This highly restricts what the customer can do 20 until the customer has authenticated. 22 This document describes a DHCP option (and a RA extension) to inform 23 clients that they are behind some sort of captive portal device, and 24 that they will need to authenticate to get Internet Access. 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at http://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on July 31, 2015. 43 Copyright Notice 45 Copyright (c) 2015 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (http://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3 62 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 2.1. DNS Redirection . . . . . . . . . . . . . . . . . . . . . 4 64 2.2. HTTP Redirection . . . . . . . . . . . . . . . . . . . . 4 65 2.3. IP Hijacking . . . . . . . . . . . . . . . . . . . . . . 4 66 3. The Captive-Portal DHCP Option . . . . . . . . . . . . . . . 5 67 3.1. IPv4 DHCP Option . . . . . . . . . . . . . . . . . . . . 5 68 3.2. IPv6 DHCP Option . . . . . . . . . . . . . . . . . . . . 5 69 4. The Captive-Portal IPv6 RA Option . . . . . . . . . . . . . . 6 70 5. Use of the Captive-Portal Option . . . . . . . . . . . . . . 6 71 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 72 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 73 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 74 9. Normative References . . . . . . . . . . . . . . . . . . . . 9 75 Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 9 76 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 78 1. Introduction 80 In many environments, users need to connect to a captive portal 81 device and agree to an acceptable use policy and / or provide billing 82 information before they can access the Internet. 84 Many devices perform DNS, HTTP, and / or IP hijacks in order to 85 present the user with the captive portal web page. These workarounds 86 and techniques resemble attacks that DNSSEC and TLS are intended to 87 protect against. This document describe a DHCP option (Captive 88 Portal) and an IPv6 Router Advertisement (RA) extension that informs 89 clients that they are behind a captive portal device and how to 90 contact it. 92 This document neither condones nor condemns the use of captive 93 portals; instead, it recognises that their apparent necessity, and 94 attempts to improve the user experience. 96 [ Ed note: This solution is somewhat similar / complements 802.11u / 97 WiFi Passpoint Online Sign-up, but is much simpler, easier to deploy, 98 and works on wired as well ] 100 1.1. Requirements notation 102 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 103 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 104 document are to be interpreted as described in [RFC2119]. 106 2. Background 108 Some ISPs implement a captive portal (CP) - a system that intercepts 109 user requests and redirects them to an interstitial login page - in 110 order to require the user accept an Acceptable Use Policy (AUP), 111 provide billing information, or otherwise authenticate a user prior 112 to allowing them to access the Internet. 114 Captive portals intercept and redirect user requests in a number of 115 ways, including: 117 o DNS Redirection 119 o IP Redirection 121 o HTTP Redirection 123 o Restricted scope addresses 125 o Traffic blocking (until the user is authenticated) 127 In order to ensure that the user is unable to access the Internet 128 until they have satisfied the requirements, captive portals usually 129 implement IP based filters, or place the user into a restricted VLAN 130 (or restricted IP range) until after they have been authorized / 131 satisfied. 133 These techniques are very similar to attacks that protocols (such as 134 VPNs, DNSSEC, TLS) are designed to protect against. The interaction 135 of these protections and the interception leads to poor user 136 experiences, such as long timeouts, inability to reach the captive 137 portal web page, etc. The interception may also leak user 138 information (for example, if the captive portal intercepts and logs 139 an HTTP Cookie, or URL of the form http://fred:password@example.com). 140 The user is often unaware of what is causing the issue (their browser 141 appears to hang, saying something like "Downloading Proxy Script", or 142 simply "The Internet doesn't work"), and they become frustrated. 143 This may result in them not purchasing the Internet access provided 144 by the captive portal. The connectivity attempts may also facilitate 145 OS fingerprinting even before a client attempts to connect to the 146 portal itself. 148 2.1. DNS Redirection 150 The CP either intercepts all DNS traffic or advertises itself (for 151 example using DHCP) as the recursive server for the network. Until 152 the user has authenticated to the captive portal, the CP responds to 153 all DNS requests listing the address of its web portal. Once the 154 user has authenticated, the CP returns the "correct" addresses. 156 This technique has many shortcomings. It fails if the client is 157 performing DNSSEC validation, is running their own resolver, is using 158 a VPN, or already has the DNS information cached. 160 2.2. HTTP Redirection 162 In this implementation, the CP acts like a transparent HTTP proxy; 163 but when it sees an HTTP request from an unauthenticated client using 164 HTTP/1.0, it intercepts the request and responds with an HTTP status 165 code 302 to redirect the client to the Captive Portal Login. If the 166 client is using HTTP/1.1, we respond with a status code 303 See 167 Other. 169 This technique has a number of issues, including: 171 o It fails if the user is only using HTTPS. 173 o It exposes various private user information, such as HTTP Cookies, 174 etc. 176 o It doesn't work if the client has a VPN and / or proxies their web 177 traffic to an external web proxy. 179 2.3. IP Hijacking 181 In this scenario, the captive portal intercepts connections to any IP 182 address. It spoofs the destination IP address and "pretends" to be 183 whatever the user tried to access. 185 This technique has issues similar to the HTTP solution, but may also 186 break other protocols, and may expose more of the user's private 187 information. 189 3. The Captive-Portal DHCP Option 191 The Captive Portal DHCP Option informs the client that it is behind a 192 captive portal and provides the URI to access an authentication page. 193 This is primarily intended to improve the user experience; for the 194 foreseeable future (until such time that most systems implement this 195 technique) captive portals will still need to implement the 196 interception techniques to serve legacy clients. 198 In order to avoid having to perform DNS interception, the URI SHOULD 199 contain an address literal, but MAY contain a DNS name if the captive 200 portal allows the client to perform DNS requests to resolve the name. 202 [ED NOTE: Using an address literal is less than ideal, but better 203 than the alternatives. Recommending a DNS name means that the CP 204 would need to allow DNS from unauthenticated clients (as we don't 205 want to force users to use the CP's provided DNS) and some users 206 would use this to DNS Tunnel out, which may make the CP admin block 207 external recursives). DNS is needed to allow operators to serve SSL/ 208 TLS for e.g billing (certificates with IP addresses are frowned upon 209 :-))] 211 3.1. IPv4 DHCP Option 213 The format of the IPv4 Captive-Portal DHCP option is shown below. 215 Code Len Data 216 +------+------+------+------+------+-- --+-----+ 217 | code | len | URI ... | 218 +------+------+------+------+------+-- --+-----+ 220 o Code: The Captive-Portal DHCPv4 Option (TBA1) 222 o Len: The length, in octets of the URI. 224 o URI: The URI of the authentication page that the user should 225 connect to. 227 3.2. IPv6 DHCP Option 229 The format of the IPv6 Captive-Portal DHCP option is shown below. 230 Other than the code it is identical to the IPv4 DHCP option. 232 Code Len Data 233 +------+------+------+------+------+-- --+-----+ 234 | code | len | URI ... | 235 +------+------+------+------+------+-- --+-----+ 237 o Code: The Captive-Portal DHCPv6Option (TBA2) 239 o Len: The length, in octets of the URI. 241 o URI: The URI of the authentication page that the user should 242 connect to. 244 4. The Captive-Portal IPv6 RA Option 246 This section describes the Captive-Portal Router Advertisement 247 option. 249 0 1 2 3 250 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 1 251 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 252 | Type | Length | URI . 253 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . 254 . . 255 . . 256 . . 257 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 258 Figure 2: Captive-Portal RA Option Format 260 Type TBA3 262 Length 8-bit unsigned integer. The length of the option (including 263 the Type and Length fields) in units of 8 bytes. 265 URI The URI of the authentication page that the user should connect 266 to. For the reasons described above, the implementer might want 267 to use an IP address literal instead of a DNS name. This should 268 be padded with NULL (0x0) to make the total option length 269 (including the Type and Length fields) a multiple of 8 bytes. 271 5. Use of the Captive-Portal Option 273 [ED NOTE: This option provides notice to the OS / User applications 274 that there is a CP. Because of differences in UI design between 275 Operating Systems, the exact behaviour by OS and Applications is left 276 to the OS vendor/Application Developer.] 278 The purpose of the Captive-Portal Option is to inform the operating 279 system and applications that they are behind a captive portal type 280 device and will need to authenticate before getting network access 281 (and how to reach the authentication page). What is done with this 282 information is left up to the operating system and application 283 vendors. This document provides a very high level example of what 284 could be done with this information. 286 Many operating systems / applications already include a "connectivity 287 test" to determine if they are behind a captive portal (for example, 288 attempting to fetch a specific URL and looking for a specific string 289 (such as "Success"). These tests sometimes fail or take a long time 290 to determine when they are behind a CP, but are usually effective for 291 determining that the captive portal has been satisfied. These tests 292 will continue to be needed, because there is currently no definitive 293 signal from the captive portal that it has been satisfied. [ Editor 294 note: It may be useful to write another document that specifies how a 295 client can determine that it has passed the CP. This document could 296 also contain advice to implementors on only intercepting actually 297 needed ports, how to advertise that the CP needs to be satisfied 298 *again*, etc. This should not be done in this document though. ] The 299 connectivity test may also need to be used if the captive portal 300 times out the user session and needs the user to re-authenticate. 301 The operating system may still find the information about the captive 302 portal URI useful in this case. 304 If the device gets different URIs (for example, via DHCPv6 and IPv6 305 RA) it should try them in the following order: DHCPv4, DHCPv6, RA. 307 [Ed note: This ordering is somewhat arbitrary - this order was chosen 308 because this is the order I expect the code to be implemented by OS 309 vendors, and I'd like the same behavior from newer and older devices 310 to make troubleshooting easier.] 312 When the device is informed that it is behind a captive portal it 313 should: 315 1. Not initiate new IP connections until the CP has been satisfied 316 (other than those to the captive portal browser session and 317 connectivity checks). Existing connections should be quiesced 318 (this will happen more often than some expect -- for example, the 319 user purchases 1 hour of Internet at a cafe and stays there for 3 320 hours -- this will "interrupt" the user a few times). 322 2. Present a dialog box to the user informing them that they are 323 behind a captive portal and ask if they wish to proceed. 325 3. If the user elects to proceed, the device should initiate a 326 connection to the captive portal login page using a web browser 327 configured with a separate cookie store, and without a proxy 328 server. If there is a VPN in place, this connection should be 329 made outside of the VPN and the user should be informed that 330 connection is outside the VPN. Some captive portals send the 331 user a cookie when they authenticate (so that the user can re- 332 authenticate more easily in the future) - the browser should keep 333 these CP cookies separate from other cookies. 335 4. Once the user has authenticated, normal IP connectivity should 336 resume. The CP success page should contain a string, e.g 337 "CP_SATISFIED." The OS can then use this string to provide 338 further information to the user. 340 5. The device should (using an OS dependent method) expose to the 341 user / user applications that they have connected though a 342 captive portal (for example by creating a file in /proc/net/ 343 containing the interface and captive portal URI). This should 344 continue until the network changes, or a new DHCP message without 345 the CP is received. 347 6. IANA Considerations 349 This document defines two DHCP Captive-Portal options, one for IPv6 350 and one for IPv6. It requires assignment of an option code (TBA1) to 351 be assigned from "Bootp and DHCP options" registry (http://www.iana 352 .org/assignments/ bootp-dhcp-parameters/bootp-dhcp-parameters.xml), 353 as specified in [RFC2939]. It also requires assignment of an option 354 code (TBA2) from the "DHCPv6 and DHCPv6 options" registry 355 (http://www.iana.org/assignments/dhcpv6-parameters/ 356 dhcpv6-parameters.xml). 358 IANA is also requested to assign an IPv6 RA Option Type code (TBA2) 359 from the "IPv6 Neighbor Discovery Option Formats" registry. Thanks 360 IANA! 362 7. Security Considerations 364 An attacker with the ability to inject DHCP messages could include 365 this option and so force users to contact an address of his choosing. 366 As an attacker with this capability could simply list himself as the 367 default gateway (and so intercept all the victim's traffic), this 368 does not provide them with significantly more capabilities. Fake 369 DHCP servers / fake RAs are currently a security concern - this 370 doesn't make them any better or worse. 372 Devices and systems that automatically connect to an open network 373 could potentially be tracked using the techniques described in this 374 document (forcing the user to continually authenticate, or exposing 375 their browser fingerprint.) However, similar tracking can already be 376 performed with the standard captive portal mechanisms, so this 377 technique does not give the attackers more capabilities. 379 By simplifying the interaction with the captive portal systems, and 380 doing away with the need for interception, we think that users will 381 be less likely to disable useful security safeguards like DNSSEC 382 validation, VPNs, etc. In addition, because the system knows that it 383 is behind a captive portal, it can know not to send cookies, 384 credentials, etc. Redirection to a portal where TLS can be used 385 without hijacking can ameliorate some of the implications of 386 connecting to a potentially malicious captive portal. 388 8. Acknowledgements 390 Thanks to Vint Cerf for the initial idea / asking me to write this. 391 Thanks to Wes George for supplying the IPv6 text. Thanks to Lorenzo 392 and Erik for the V6 RA kick in the pants. 394 Thanks to Fred Baker, Ted Lemon, Ole Troan and Asbjorn Tonnesen for 395 detailed review and comments. Also great thanks to Joel Jaeggli for 396 providing feedback and text. 398 9. Normative References 400 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 401 Requirement Levels", BCP 14, RFC 2119, March 1997. 403 Appendix A. Changes / Author Notes. 405 [RFC Editor: Please remove this section before publication ] 407 From 08 to 09: 409 o Put back the DHCPv6 option, and made the fact that is separate 410 from the DHCPv4 option clearer (Ted Lemon) 412 From 07 to 08: 414 o Incorporated comments from Ted Lemon. Made the document much 415 shorter. 417 o Some cleanup. 419 From 06 to 07: 421 o Incoroprated a bunch of comments from Asbjorn Tonnesen 423 o Clarified that this document is only for the DHCP bits, not 424 everything. 426 o CP's *can* do HTTP redirects to DNS banes, as long as they allow 427 access to all needed services. 429 From 05 to 06: 431 o Integrated comments from Joel, as below 433 o Better introduction text, around the "kludgy hacks" section. 435 o Better "neither condones nor condems" text 437 o Fingerprint text. 439 o Some discussions on the v4 literal stuff. 441 o More Security Consideration text. 443 From 04 to 05: 445 o Integrated comments, primarily from Fred Baker. 447 From 03 to 04: 449 o Some text cleanup for readability. 451 o Some disclaimers about it working better on initial connection 452 versus CP timeout. 454 o Some more text explaining that CP interception is 455 indistinguishable from an attack. 457 o Connectivity Check test. 459 o Posting just before the draft cutoff - "I love deadlines. I love 460 the whooshing noise they make as they go by." -- Douglas Adams, 461 The Salmon of Doubt 463 From -02 to 03: 465 o Removed the DHCPv6 stuff (as suggested / requested by Erik Kline) 467 o Simplified / cleaned up text (I'm inclined to waffle on, then trim 468 the fluff) 470 o This was written on a United flight with in-flight WiFi - 471 unfortunately I couldn't use it because their CP was borked. :-P 473 From -01 to 02: 475 o Added the IPv6 RA stuff. 477 From -00 to -01: 479 o Many nits and editorial changes. 481 o Whole bunch of extra text and review from Wes George on v6. 483 From initial to -00. 485 o Nothing changed in the template! 487 Authors' Addresses 489 Warren Kumari 490 Google 491 1600 Amphitheatre Parkway 492 Mountain View, CA 94043 493 US 495 Email: warren@kumari.net 497 Olafur Gudmundsson 498 Shinkuro Inc. 499 4922 Fairmont Av, Suite 250 500 Bethesda, MD 20814 501 USA 503 Email: ogud@ogud.com 505 Paul Ebersman 506 Comcast 508 Email: ebersman-ietf@dragon.net 510 Steve Sheng 511 Internet Corporation for Assigned Names and Numbers 512 12025 Waterfront Drive, Suite 300 513 Los Angeles 90094 514 United States of America 516 Phone: +1.310.301.5800 517 Email: steve.sheng@icann.org