Network Working Group W. Kumari Internet-Draft Google Intended status: Standards Track O. Gudmundsson Expires: February 25, 2016 CloudFlare P. Ebersman Comcast S. Sheng ICANN August 24, 2015 Captive-Portal Identification in DHCP / RA draft-wkumari-dhc-capport-15 Abstract In many environments offering short-term or temporary Internet access (such as coffee shops), it is common to start new connections in a captive portal mode. This highly restricts what the customer can do until the customer has authenticated. This document describes a DHCP option (and a RA extension) to inform clients that they are behind some sort of captive portal device, and that they will need to authenticate to get Internet Access. It is not a full solution to address all of the issues that clients may have with captive portals; it is designed to be used in larger solutions. The method of authenticating to, and interacting with the captive portal is out of scope of this document. [ Ed note (remove): This document is being developed in github: https://github.com/wkumari/draft-wkumari-dhc-capport . ] Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on February 25, 2016. Kumari, et al. Expires February 25, 2016 [Page 1] Internet-Draft DHCP Captive-Portal August 2015 Copyright Notice Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3 2. The Captive-Portal Option . . . . . . . . . . . . . . . . . . 3 2.1. IPv4 DHCP Option . . . . . . . . . . . . . . . . . . . . 3 2.2. IPv6 DHCP Option . . . . . . . . . . . . . . . . . . . . 4 3. The Captive-Portal IPv6 RA Option . . . . . . . . . . . . . . 4 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 7. Normative References . . . . . . . . . . . . . . . . . . . . 6 Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction In many environments, users need to connect to a captive portal device and agree to an acceptable use policy (AUP) and / or provide billing information before they can access the Internet. It is anticipated that the IETF will work on a more fully featured protocol at some point, to ease interaction with Captive Portals. Regardless of how that protocol operates, it is expected that this document will provide needed functionality because the client will need to know when it is behind a CP and how to contact it. In order to present users with the payment or AUP pages, the captive portal device has to intercept the user's connections and redirect the user to the captive portal, using methods that are very similar to man-in-the-middle (MITM) attacks. As increasing focus is placed on security, and end nodes adopt a more secure stance, these interception techniques will become less effective and / or more intrusive. Kumari, et al. Expires February 25, 2016 [Page 2] Internet-Draft DHCP Captive-Portal August 2015 This document describe a DHCP ([RFC2131]) option (Captive Portal) and an IPv6 Router Advertisement (RA) ([RFC4861]) extension that informs clients that they are behind a captive portal device and how to contact it. 1.1. Requirements notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. The Captive-Portal Option The Captive Portal DHCP / RA Option informs the client that it is behind a captive portal and provides the URI to access an authentication page. This is primarily intended to improve the user experience by getting them to the captive portal faster; for the foreseeable future, captive portals will still need to implement the interception techniques to serve legacy clients, and clients will need to perform probing to detect captive portals. In order to support multiple "classes" of clients (e.g: IPv4 only, IPv6 only with DHCPv6([RFC3315]), IPv6 only with RA) the captive portal can provide the URI via multiple methods (IPv4 DHCP, IPv6 DHCP, IPv6 RA). The captive portal operator should ensure that the URIs handed out are equivalent to reduce the chance of operational problems. In order to avoid having to perform DNS interception, the URI SHOULD contain an address literal. If the captive portal allows the client to perform DNS requests to resolve the name, it is then acceptable for the URI to contain a DNS name. 2.1. IPv4 DHCP Option The format of the IPv4 Captive-Portal DHCP option is shown below. Code Len Data +------+------+------+------+------+-- --+-----+ | code | len | URI ... | +------+------+------+------+------+-- --+-----+ o Code: The Captive-Portal DHCPv4 Option (TBA1) (one octet) o Len: The length, in octets of the URI. o URI: The URI of the authentication page that the user should connect to. Kumari, et al. Expires February 25, 2016 [Page 3] Internet-Draft DHCP Captive-Portal August 2015 2.2. IPv6 DHCP Option The format of the IPv6 Captive-Portal DHCP option is shown below. 0 1 2 3 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | option-code | option-len | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . URI (variable length) . | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ o option-code: The Captive-Portal DHCPv6Option (TBA2) (two octets) o option-len: The length, in octets of the URI. o URI: The URI of the authentication page that the user should connect to. See [RFC7227], Section 5.7 for more examples of DHCP Options with URIs. 3. The Captive-Portal IPv6 RA Option This section describes the Captive-Portal Router Advertisement option. 0 1 2 3 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | URI . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Captive-Portal RA Option Format Type TBA3 Length 8-bit unsigned integer. The length of the option (including the Type and Length fields) in units of 8 bytes. URI The URI of the authentication page that the user should connect to. For the reasons described above, the implementer might want to use an IP address literal instead of a DNS name. This should Kumari, et al. Expires February 25, 2016 [Page 4] Internet-Draft DHCP Captive-Portal August 2015 be padded with NULL (0x0) to make the total option length (including the Type and Length fields) a multiple of 8 bytes. 4. IANA Considerations This document defines two DHCP Captive-Portal options, one for IPv4 and one for IPv6. It requires assignment of an option code (TBA1) to be assigned from "Bootp and DHCP options" registry (hhttp://www.iana.org/assignments/bootp-dhcp-parameters), as specified in [RFC2939]. It also requires assignment of an option code (TBA2) from the "DHCPv6 and DHCPv6 options" registry (http://www.iana.org/assignments/dhcpv6-parameters). IANA is also requested to assign an IPv6 RA Option Type code (TBA3) from the "IPv6 Neighbor Discovery Option Formats" registry. Thanks IANA! 5. Security Considerations An attacker with the ability to inject DHCP messages could include this option and so force users to contact an address of his choosing. As an attacker with this capability could simply list himself as the default gateway (and so intercept all the victim's traffic); this does not provide them with significantly more capabilities, but because this document removes the need for interception, the attacker may have an easier time performing the attack. As the operating systems and application that make use of this information know that they are connecting to a captive portal device (as opposed to intercepted connections) they can render the page in a sandboxed environment and take other precautions, such as clearly labeling the page as untrusted. The means of sandboxing and user interface presenting this information is not covered in this document - by its nature it is implementation specific and best left to the application and user interface designers. Devices and systems that automatically connect to an open network could potentially be tracked using the techniques described in this document (forcing the user to continually authenticate, or exposing their browser fingerprint). However, similar tracking can already be performed with the standard captive portal mechanisms, so this technique does not give the attackers more capabilities. By simplifying the interaction with the captive portal systems, and doing away with the need for interception, we think that users will be less likely to disable useful security safeguards like DNSSEC validation, VPNs, etc. In addition, because the system knows that it is behind a captive portal, it can know not to send cookies, credentials, etc. Redirection to a portal where TLS can be used Kumari, et al. Expires February 25, 2016 [Page 5] Internet-Draft DHCP Captive-Portal August 2015 without hijacking can ameliorate some of the implications of connecting to a potentially malicious captive portal. 6. Acknowledgements Thanks to Vint Cerf for the initial idea / asking me to write this. Thanks to Wes George for supplying the IPv6 text. Thanks to Lorenzo and Erik for the V6 RA kick in the pants. Thanks to Fred Baker, Paul Hoffman, Barry Leiba, Ted Lemon, Martin Nilsson, Ole Troan and Asbjorn Tonnesen for detailed review and comments. Also great thanks to Joel Jaeggli for providing feedback and text. 7. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ RFC2119, March 1997, . [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, March 1997, . [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 2003, . [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, . [RFC7227] Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and S. Krishnan, "Guidelines for Creating New DHCPv6 Options", BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014, . Appendix A. Changes / Author Notes. [RFC Editor: Please remove this section before publication ] From 14 to 15: o Incorporated readability comment from Barry Leiba Kumari, et al. Expires February 25, 2016 [Page 6] Internet-Draft DHCP Captive-Portal August 2015 From 13 to 14: o Added a bunch of disclaimers explaining that this is not a complete solution. We expect that the actual interaction bit should be done in CAPPORT. From 13.2 to 13(posted): o Shortened the document by removing most of the [Editors notes], Section 2, Section 5 and Appendix A. They were mainly background and have served their purpose. This change suggested by Paul Hoffman. From 13.1 to 13.2: o Moved all of the "what an OS could do with this info" to an Appendix, to make it even clearer that this is simply an example. From -12 to -13.1: There was a Captive Portal Bar BoF held at the Dallas IETF meeting. See https://github.com/httpwg/wiki/wiki/Captive-Portals for some details. This document was discussed, and I got a fair bit of feedback. Incorporating some of this in -13. o "In the text discussing why a captive portal notification might be useful (section 2.2 maybe?), perhaps you should say something about HSTS and HTTP2.0, since they will further erode the ability to use common captive portal redirection techniques." - Wes George. o Integrated a bunch of useful comments from Martin Nilsson From -11 to -12: o Integrated a whole bunch of comments from Ted Lemon, including missing references, track, missing size of DHCP option, From 10 to 11: o Updated Olafur's affiliation. From 09 to 10: o Ted Lemon and Joel Jaeggli: there's no benefit to insisting on an ordering. I think you should just say that the ordering is indeterminate, and if different mechanisms give non-equivalent answers, this is likely to cause operational problems in practice. Kumari, et al. Expires February 25, 2016 [Page 7] Internet-Draft DHCP Captive-Portal August 2015 From 08 to 09: o Put back the DHCPv6 option, and made the fact that is separate from the DHCPv4 option clearer (Ted Lemon) From 07 to 08: o Incorporated comments from Ted Lemon. Made the document much shorter. o Some cleanup. From 06 to 07: o Incoroprated a bunch of comments from Asbjorn Tonnesen o Clarified that this document is only for the DHCP bits, not everything. o CP's *can* do HTTP redirects to DNS names, as long as they allow access to all needed services. From 05 to 06: o Integrated comments from Joel, as below o Better introduction text, around the "kludgy hacks" section. o Better "neither condones nor condemns" text o Fingerprint text. o Some discussions on the v4 literal stuff. o More Security Consideration text. From 04 to 05: o Integrated comments, primarily from Fred Baker. From 03 to 04: o Some text cleanup for readability. o Some disclaimers about it working better on initial connection versus CP timeout. Kumari, et al. Expires February 25, 2016 [Page 8] Internet-Draft DHCP Captive-Portal August 2015 o Some more text explaining that CP interception is indistinguishable from an attack. o Connectivity Check test. o Posting just before the draft cutoff - "I love deadlines. I love the whooshing noise they make as they go by." -- Douglas Adams, The Salmon of Doubt From -02 to 03: o Removed the DHCPv6 stuff (as suggested / requested by Erik Kline) o Simplified / cleaned up text (I'm inclined to waffle on, then trim the fluff) o This was written on a United flight with in-flight WiFi - unfortunately I couldn't use it because their CP was borked. :-P From -01 to 02: o Added the IPv6 RA stuff. From -00 to -01: o Many nits and editorial changes. o Whole bunch of extra text and review from Wes George on v6. From initial to -00. o Nothing changed in the template! Authors' Addresses Warren Kumari Google 1600 Amphitheatre Parkway Mountain View, CA 94043 US Email: warren@kumari.net Kumari, et al. Expires February 25, 2016 [Page 9] Internet-Draft DHCP Captive-Portal August 2015 Olafur Gudmundsson CloudFlare San Francisco, CA 94107 USA Email: olafur@cloudflare.com Paul Ebersman Comcast Email: ebersman-ietf@dragon.net Steve Sheng Internet Corporation for Assigned Names and Numbers 12025 Waterfront Drive, Suite 300 Los Angeles 90094 United States of America Phone: +1.310.301.5800 Email: steve.sheng@icann.org Kumari, et al. Expires February 25, 2016 [Page 10]