Network Working Group W. Kumari Internet-Draft Google Obsoletes: 7710 (if approved) E. Kline Intended status: Standards Track Loon Expires: October 30, 2020 April 28, 2020 Captive-Portal Identification in DHCP / RA draft-ietf-capport-rfc7710bis-04 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 Router Advertisement (RA) extension) to inform clients that they are behind some sort of captive-portal enforcement 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. This document replaces RFC 7710. RFC 7710 used DHCP code point 160. Due to a conflict, this document specifies 114. [ This document is being collaborated on in Github at: https://github.com/capport-wg/7710bis. The most recent version of the document, open issues, etc should all be available here. The authors (gratefully) accept pull requests. Text in square brackets will be removed before publication. ] 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 https://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 Kumari & Kline Expires October 30, 2020 [Page 1] Internet-Draft DHCP Captive-Portal April 2020 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 October 30, 2020. Copyright Notice Copyright (c) 2020 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 (https://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 . . . . . . . . . . . . . . . . . . . . 4 2.2. IPv6 DHCP Option . . . . . . . . . . . . . . . . . . . . 5 2.3. The Captive-Portal IPv6 RA Option . . . . . . . . . . . . 5 3. Precedence of API URIs . . . . . . . . . . . . . . . . . . . 6 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 4.1. Captive Portal Unrestricted Identifier . . . . . . . . . 7 4.2. BOOTP Vendor Extensions and DHCP Options Code Change . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 7.1. Normative References . . . . . . . . . . . . . . . . . . 9 7.2. Informative References . . . . . . . . . . . . . . . . . 10 Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 10 Appendix B. Changes from RFC 7710 . . . . . . . . . . . . . . . 11 Appendix C. Observations From IETF 106 Network Experiment . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 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. Regardless of how that mechanism operates, this document provides functionality Kumari & Kline Expires October 30, 2020 [Page 2] Internet-Draft DHCP Captive-Portal April 2020 to allow the client to know when it is behind a captive portal and how to contact it. In order to present users with the payment or AUP pages, presently a captive-portal enforcement device has to intercept the user's connections and redirect the user to a captive portal server, 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. This document describes a DHCP ([RFC2131]) option (Captive-Portal) and an IPv6 Router Advertisement (RA) ([RFC4861]) extension that informs clients that they are behind a captive-portal enforcement device and how to contact an API for more information. This document replaces RFC 7710 [RFC7710]. 1.1. Requirements Notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 2. The Captive-Portal Option The Captive Portal DHCP / RA Option informs the client that it may be behind a captive portal and provides the URI to access an API as defined by [draft-ietf-capport-api]. This is primarily intended to improve the user experience by showing the user the captive portal information faster and more reliably. Note that, 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. Clients that support the Captive Portal DHCP option SHOULD include the option in the Parameter Request List in DHCPREQUEST messages. DHCP servers MAY send the Captive Portal option without any explicit request. In order to support multiple "classes" of clients (e.g. IPv4 only, IPv6 only with DHCPv6 ([RFC8415]), and IPv6 only with RA) the captive network can provision the client with the URI via multiple methods (IPv4 DHCP, IPv6 DHCP, and IPv6 RA). The captive portal operator SHOULD ensure that the URIs provisioned by each method are equivalent to reduce the chance of operational problems. The maximum length of Kumari & Kline Expires October 30, 2020 [Page 3] Internet-Draft DHCP Captive-Portal April 2020 the URI that can be carried in IPv4 DHCP is 255 bytes, so URIs longer than 255 bytes should not be provisioned via IPv6 DHCP nor IPv6 RA options. In all variants of this option, the URI MUST be that of the captive portal API endpoint, conforming to the recommendations for such URIs [draft-ietf-capport-api]. A captive portal MAY do content negotiation ([RFC7231] section 3.4) and attempt to redirect clients querying without an explicit indication of support for the captive portal API content type (i.e. without application/capport+json listed explicitly anywhere within an Accept header vis. [RFC7231] section 5.3). In so doing, the captive portal SHOULD redirect the client to the value associated with the "user-portal-url" API key. When performing such content negotiation ([RFC7231] Section 3.4), implementors of captive portals need to keep in mind that such responses might be cached, and therefore SHOULD include an appropriate Vary header field ([RFC7231] Section 7.1.4) or mark them explicitly uncacheable (for example, using Cache-Control: no-store [RFC7234] Section 5.2.2.3). The URI SHOULD NOT contain an IP address literal. Exceptions to this might include networks with only one operational IP address family where DNS is either not available or not fully functional until the captive portal has been satisfied. Networks with no captive portals MAY explicitly indicate this condition by using this option with the IANA-assigned URI for this purpose. Clients observing the URI value "urn:ietf:params:capport:unrestricted" MAY forego time-consuming forms of captive portal detection. 2.1. IPv4 DHCP Option The format of the IPv4 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Len | URI (variable length) ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . ...URI continued... . | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ o Code: The Captive-Portal DHCPv4 Option (114) (one octet) o Len: The length (one octet), in octets of the URI. Kumari & Kline Expires October 30, 2020 [Page 4] Internet-Draft DHCP Captive-Portal April 2020 o URI: The URI for the captive portal API endpoint to which the user should connect (encoded following the rules in [RFC3986]). See [RFC2132], Section 2 for more on the format of IPv4 DHCP options. Note that the URI parameter is not null terminated. 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 (103) (two octets) o option-len: The unsigned 16-bit length, in octets, of the URI. o URI: The URI for the captive portal API endpoint to which the user should connect (encoded following the rules in [RFC3986]). See [RFC7227], Section 5.7 for more examples of DHCP Options with URIs. See [RFC8415], Section 21.1 for more on the format of IPv6 DHCP options. Note that the URI parameter is not null terminated. The maximum length of the URI that can be carried in IPv4 DHCP is 255 bytes, so URIs longer than 255 bytes should not be provisioned via IPv6 DHCP options. 2.3. The Captive-Portal IPv6 RA Option This section describes the Captive-Portal Router Advertisement option. Kumari & Kline Expires October 30, 2020 [Page 5] Internet-Draft DHCP Captive-Portal April 2020 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 37 Length 8-bit unsigned integer. The length of the option (including the Type and Length fields) in units of 8 bytes. URI The URI for the captive portal API endpoint to which the user should connect. This MUST be padded with NULL (0x00) to make the total option length (including the Type and Length fields) a multiple of 8 bytes. Note that the URI parameter is not guaranteed to be null terminated. The maximum length of the URI that can be carried in IPv4 DHCP is 255 bytes, so URIs longer than 255 bytes should not be provisioned via IPv6 RA options. 3. Precedence of API URIs A device may learn about Captive Portal API URIs through more than one of (or indeed all of) the above options. Implementations can select their own precedence order (e.g., prefer one of the IPv6 options before the DHCPv4 option, or vice versa, et cetera). If the URIs learned via more than one option described in Section 2 are not all identical, this condition should be logged for the device owner or administrator; it is a network configuration error if the learned URIs are not all identical. 4. IANA Considerations This document requests one new IETF URN protocol parameter ([RFC3553]) entry. This document also requests a reallocation of DHCPv4 option codes (see Appendix C for background). Thanks IANA! Kumari & Kline Expires October 30, 2020 [Page 6] Internet-Draft DHCP Captive-Portal April 2020 4.1. Captive Portal Unrestricted Identifier This document registers a new entry under the IETF URN Sub-namespace defined in [RFC3553]: Registry name: Captive Portal Unrestricted Identifier URN: urn:ietf:params:capport:unrestricted Specification: RFC TBD (this document) Repository: RFC TBD (this document) Index value: Only one value is defined (see URN above). No hierarchy is defined and therefore no sub-namespace registrations are possible. 4.2. BOOTP Vendor Extensions and DHCP Options Code Change [ RFC Ed: Please remove before publication: RFC7710 uses DHCP Code 160 -- unfortunately, it was discovered that this option code is already widely used by Polycom (see appendix). Option 114 (URL) is currently assigned to Apple (RFC3679, Section 3.2.3 - Contact: Dieter Siegmund, dieter@apple.com - Reason to recover: Never published in an RFC) Tommy Pauly (Apple) and Dieter Siegmund confirm that this codepoint hasn't been used, and Apple is willing to relinquish it for use in CAPPORT. Please see thread: https://mailarchive.ietf.org/arch/msg/captive-portals/ TmqQz6Ma_fznD3XbhwkH9m2dB28 for more background. ] The IANA is requested to update the "BOOTP Vendor Extensions and DHCP Options" registry (https://www.iana.org/assignments/bootp-dhcp- parameters/bootp-dhcp-parameters.xhtml) as follows. Tag: 114 Name: DHCP Captive-Portal Data Length: N Meaning: DHCP Captive-Portal Reference: [THIS-RFC] Tag: 160 Name: REMOVED/Unassigned Data Length: Meaning: Reference: [THIS-RFC][RFC7710] Kumari & Kline Expires October 30, 2020 [Page 7] Internet-Draft DHCP Captive-Portal April 2020 5. Security Considerations By removing or reducing the need for captive portals to perform MITM hijacking, this mechanism improves security by making the portal and its actions visible, rather than hidden, and reduces the likelihood that users will 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. By handing out a URI which is protected with TLS, the captive portal operator can attempt to reassure the user that the captive portal is not malicious. An attacker with the ability to inject DHCP messages or RAs could include an option from this document to 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 presently common captive portal mechanisms, so this technique does not give the attackers more capabilities. Captive portals are increasingly hijacking TLS connections to force browsers to talk to the portal. Providing the portal's URI via a DHCP or RA option is a cleaner technique, and reduces user expectations of being hijacked - this may improve security by making users more reluctant to accept TLS hijacking, which can be performed from beyond the network associated with the captive portal. 6. Acknowledgements This document is a -bis of RFC7710. Thanks to all of the original authors (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, Steve Sheng), and original contributors. Kumari & Kline Expires October 30, 2020 [Page 8] Internet-Draft DHCP Captive-Portal April 2020 Also thanks to the CAPPORT WG for all of the discussion and improvements including contributions and review from Joe Clarke, Lorenzo Colitti, Dave Dolson, Hans Kuhn, Kyle Larose, Clemens Schimpe, Martin Thomson, Michael Richardson, Remi Nguyen Van, Subash Tirupachur Comerica, Bernie Volz, and Tommy Pauly. 7. References 7.1. 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, . [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997, . [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An IETF URN Sub-namespace for Registered Protocol Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 2003, . [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, . [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, . [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, . Kumari & Kline Expires October 30, 2020 [Page 9] Internet-Draft DHCP Captive-Portal April 2020 [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., Richardson, M., Jiang, S., Lemon, T., and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, November 2018, . 7.2. Informative References [RFC7710] Kumari, W., Gudmundsson, O., Ebersman, P., and S. Sheng, "Captive-Portal Identification Using DHCP or Router Advertisements (RAs)", RFC 7710, DOI 10.17487/RFC7710, December 2015, . 7.3. URIs [1] https://tickets.meeting.ietf.org/wiki/IETF106network#Experiments [2] https://tickets.meeting.ietf.org/wiki/CAPPORT [3] https://community.polycom.com/t5/VoIP-SIP-Phones/DHCP- Standardization-160-vs-66/td-p/72577 Appendix A. Changes / Author Notes. [RFC Editor: Please remove this section before publication ] From initial to -00. o Import of RFC7710. From -00 to -01. o Remove link-relation text. o Clarify option should be in DHCPREQUEST parameter list. o Uppercase some SHOULDs. Kumari & Kline Expires October 30, 2020 [Page 10] Internet-Draft DHCP Captive-Portal April 2020 Appendix B. Changes from RFC 7710 This document incorporates the following changes from [RFC7710]. 1. Clarify that IP string literals are NOT RECOMMENDED. 2. Clarify that the option URI SHOULD be that of the captive portal API endpoint. 3. Clarify that captive portals MAY do content negotiation. 4. Added text about Captive Portal API URI precedence in the event of a network configuration error. 5. Added urn:ietf:params:capport:unrestricted URN. 6. Notes that the DHCP Code changed from 160 to 114. Appendix C. Observations From IETF 106 Network Experiment During IETF 106 in Singapore an experiment [1] enabling Captive Portal API compatible clients to discover a venue-info-url (see experiment description [2] for more detail) revealed that some Polycom devices on the same network made use of DHCPv4 option code 160 for other purposes [3]. The presence of DHCPv4 Option code 160 holding a value indicating the Captive Portal API URL caused these devices to not function as desired. For this reason, this document requests IANA deprecate option code 160 and reallocate different value to be used for the Captive Portal API URL. Authors' Addresses Warren Kumari Google 1600 Amphitheatre Parkway Mountain View, CA 94043 US Email: warren@kumari.net Kumari & Kline Expires October 30, 2020 [Page 11] Internet-Draft DHCP Captive-Portal April 2020 Erik Kline Loon 1600 Amphitheatre Parkway Mountain View, CA 94043 US Email: ek@loon.com Kumari & Kline Expires October 30, 2020 [Page 12]