Captive-Portal Identification in DHCP and Router Advertisements (RAs)
RFC 8910

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Internet Engineering Task Force (IETF)                         W. Kumari
Request for Comments: 8910                                        Google
Obsoletes: 7710                                                 E. Kline
Updates: 3679                                                       Loon
Category: Standards Track                                 September 2020
ISSN: 2070-1721

 Captive-Portal Identification in DHCP and Router Advertisements (RAs)

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 user can do
   until the user has satisfied the captive portal conditions.

   This document describes a DHCPv4 and DHCPv6 option and a Router
   Advertisement (RA) option to inform clients that they are behind some
   sort of captive portal enforcement device, and that they will need to
   satisfy the Captive Portal conditions 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 one component of a
   standardized approach for hosts to interact with such portals.  While
   this document defines how the network operator may convey the captive
   portal API endpoint to hosts, the specific methods of satisfying and
   interacting with the captive portal are out of scope of this
   document.

   This document replaces RFC 7710, which used DHCP code point 160.  Due
   to a conflict, this document specifies 114.  Consequently, this
   document also updates RFC 3679.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8910.

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
     1.1.  Requirements Notation
   2.  The Captive-Portal Option
     2.1.  IPv4 DHCP Option
     2.2.  IPv6 DHCP Option
     2.3.  The Captive-Portal IPv6 RA Option
   3.  Precedence of API URIs
   4.  IANA Considerations
     4.1.  Captive Portal Unrestricted Identifier
     4.2.  BOOTP Vendor Extensions and DHCP Options Code Change
     4.3.  Update DHCPv6 and IPv6 ND Options Registries
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Appendix A.  Changes from RFC 7710
   Appendix B.  Observations from IETF 106 Network Experiment
   Acknowledgements
   Authors' Addresses

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
   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, a captive
   portal enforcement device presently 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 DHCPv4 [RFC2131] and DHCPv6 [RFC8415]
   option (Captive-Portal) and an IPv6 Router Advertisement (RA)
   [RFC4861] option that informs clients that they are behind a captive
   portal enforcement device and the API endpoint that the host can
   contact for more information.

   This document replaces RFC 7710 [RFC7710], which used DHCP code point
   160.  Due to a conflict, this document specifies 114.  Consequently,
   this document also updates [RFC3679].

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 [RFC8908].  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 interception techniques to serve
   legacy clients, and clients will need to perform probing to detect
   captive portals; nonetheless, the mechanism provided by this document
   provides a more reliable and performant way to do so, and is
   therefore the preferred mechanism for captive portal detection.

   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
   identical to reduce the chance of operational problems.  As the
   maximum length of the URI that can be carried in IPv4 DHCP is 255
   bytes, URIs longer than this SHOULD NOT be provisioned by any of the
   IPv6 options described in this document.  In IPv6-only environments,
   this restriction can be relaxed.

   In all variants of this option, the URI MUST be that of the captive
   portal API endpoint ([RFC8908]).

   A captive portal MAY do content negotiation (Section 3.4 of
   [RFC7231]) 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 field as described in Section 5.3 of
   [RFC7231]).  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 (Section 3.4 of [RFC7231]),
   implementors of captive portals need to keep in mind that such
   responses might be cached, and therefore SHOULD include an
   appropriate Vary header field (Section 7.1.4 of [RFC7231]) or set the
   Cache-Control header field in any responses to "private" or a more
   restrictive value such as "no-store" (Section 5.2.2.3 of [RFC7234]).

   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.  Use of IP Address certificates
   ([RFC3779]) adds considerations that are out of scope for this
   document.

   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...                         .
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 1: Captive-Portal DHCPv4 Option Format

      Code:  The Captive-Portal DHCPv4 Option (114) (one octet).

      Len:  The length (one octet), in octets, of the URI.

      URI:  The URI for the captive portal API endpoint to which the
         user should connect (encoded following the rules in [RFC3986]).

   See Section 2 of [RFC2132] 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)                    .
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 2: Captive-Portal DHCPv6 Option Format

      option-code:  The Captive-Portal DHCPv6 Option (103) (two octets).

      option-len:  The unsigned 16-bit length, in octets, of the URI.

      URI:  The URI for the captive portal API endpoint to which the
         user should connect (encoded following the rules in [RFC3986]).

   See Section 5.7 of [RFC7227] for more examples of DHCP Options with
   URIs.  See Section 21.1 of [RFC8415] for more on the format of IPv6
   DHCP options.

   Note that the URI parameter is not null terminated.

   As the maximum length of the URI that can be carried in IPv4 DHCP is
   255 bytes, URIs longer than this 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.

       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 3: 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 NUL (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.

   As the maximum length of the URI that can be carried in IPv4 DHCP is
   255 bytes, URIs longer than this 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

   IANA has registered a new IETF URN protocol parameter ([RFC3553]).
   IANA has also reallocated two DHCPv4 option codes (see Appendix B for
   background) and updated the references for previously registered
   DHCPv6 and IPv6 ND options.

4.1.  Captive Portal Unrestricted Identifier

   IANA has registered a new entry in the "IETF URN Sub-namespace for
   Registered Protocol Parameter Identifiers" registry defined in
   [RFC3553]:

   Registered Parameter Identifier:  capport:unrestricted
   Reference:  RFC 8910
   IANA Registry Reference:  RFC 8910

   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

   IANA has updated the "BOOTP Vendor Extensions and DHCP Options"
   registry (https://www.iana.org/assignments/bootp-dhcp-parameters) as
   follows.

   Tag:  114
   Name:  DHCP Captive-Portal
   Data Length:  N
   Meaning:  DHCP Captive-Portal
   Reference:  RFC 8910

   Tag:  160
   Name:  Unassigned
   Data Length:
   Meaning:  Previously assigned by [RFC7710]; known to also be used by
      Polycom.
   Reference:  [RFC7710] RFC 8910

4.3.  Update DHCPv6 and IPv6 ND Options Registries

   IANA has updated the DHCPv6 (103 - DHCP Captive-Portal) and IPv6 ND
   (37 - DHCP Captive-Portal) options previously registered in [RFC7710]
   to reference this document.

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 order to interact with the captive portal.
   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 that is protected with TLS, the captive portal
   operator can attempt to reassure the user that the captive portal is
   not malicious.

   Clients processing these options SHOULD validate that the option's
   contents conform to the validation requirements for URIs, including
   those described in [RFC3986].

   Each of the options described in this document is presented to a node
   using the same protocols used to provision other information critical
   to the node's successful configuration on a network.  The security
   considerations applicable to each of these provisioning mechanisms
   also apply when the node is attempting to learn the information
   conveyed in these options.  In the absence of security measures like
   RA-Guard ([RFC6105], [RFC7113]) or DHCPv6-Shield [RFC7610], an
   attacker could inject, modify, or block DHCP messages or RAs.

   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 the attacker's choosing.  An attacker with this capability
   could simply list themselves 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.

   However, as the operating systems and application(s) that make use of
   this information know that they are connecting to a captive portal
   device (as opposed to intercepted connections where the OS/
   application may not know that they are connecting to a captive portal
   or hostile device), 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 a 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 resatisfy the Captive
   Portal conditions 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.  References

6.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
              <https://www.rfc-editor.org/info/rfc2132>.

   [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, <https://www.rfc-editor.org/info/rfc3553>.

   [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,
              <https://www.rfc-editor.org/info/rfc3986>.

   [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,
              <https://www.rfc-editor.org/info/rfc4861>.

   [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,
              <https://www.rfc-editor.org/info/rfc7227>.

   [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,
              <https://www.rfc-editor.org/info/rfc7231>.

   [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,
              <https://www.rfc-editor.org/info/rfc7234>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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,
              <https://www.rfc-editor.org/info/rfc8415>.

6.2.  Informative References

   [RFC3679]  Droms, R., "Unused Dynamic Host Configuration Protocol
              (DHCP) Option Codes", RFC 3679, DOI 10.17487/RFC3679,
              January 2004, <https://www.rfc-editor.org/info/rfc3679>.

   [RFC3779]  Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
              Addresses and AS Identifiers", RFC 3779,
              DOI 10.17487/RFC3779, June 2004,
              <https://www.rfc-editor.org/info/rfc3779>.

   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              DOI 10.17487/RFC6105, February 2011,
              <https://www.rfc-editor.org/info/rfc6105>.

   [RFC7113]  Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)", RFC 7113,
              DOI 10.17487/RFC7113, February 2014,
              <https://www.rfc-editor.org/info/rfc7113>.

   [RFC7610]  Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield:
              Protecting against Rogue DHCPv6 Servers", BCP 199,
              RFC 7610, DOI 10.17487/RFC7610, August 2015,
              <https://www.rfc-editor.org/info/rfc7610>.

   [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, <https://www.rfc-editor.org/info/rfc7710>.

   [RFC8908]  Pauly, T., Ed. and D. Thakore, Ed., "Captive Portal API",
              RFC 8908, DOI 10.17487/RFC8908, September 2020,
              <https://www.rfc-editor.org/info/rfc8908>.

Appendix A.  Changes from RFC 7710

   This document incorporates the following changes from [RFC7710].

   1.  Clarified that IP string literals are NOT RECOMMENDED.

   2.  Clarified that the option URI MUST be that of the captive portal
       API endpoint.

   3.  Clarified 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.  Noted that the DHCPv4 Option Code changed from 160 to 114.

Appendix B.  Observations from IETF 106 Network Experiment

   During IETF 106 in Singapore, an experiment
   (https://tickets.meeting.ietf.org/wiki/IETF106network#Experiments)
   enabling clients compatible with the Captive Portal API to discover a
   venue-info-url (see experiment description
   (https://tickets.meeting.ietf.org/wiki/CAPPORT) for more detail)
   revealed that some Polycom devices on the same network made use of
   DHCPv4 option code 160 for other purposes
   (https://community.polycom.com/t5/VoIP-SIP-Phones/DHCP-
   Standardization-160-vs-66/td-p/72577).

   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, IANA has deprecated option code 160 and
   allocated a different value to be used for the Captive Portal API
   URL.

Acknowledgements

   This document is a -bis of RFC 7710.  Thanks to all of the original
   authors (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, and Steve
   Sheng) and original contributors.

   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.

Authors' Addresses

   Warren Kumari
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America

   Email: warren@kumari.net

   Erik Kline
   Loon
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America

   Email: ek@loon.com