Network Working Group                                          W. Kumari
Internet-Draft                                                    Google
Obsoletes: 7710 (if approved)                                   E. Kline
Updates: 3679 (if approved)                                         Loon
Intended status: Standards Track                           June 24,                            July 1, 2020
Expires: December 26, 2020 January 2, 2021

               Captive-Portal Identification in DHCP / RA


   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 satified satisfied the Captive Portal 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 captive portal enforcement device, and that they will need to
   satify 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

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

   [ This document is being collaborated on in Github at:  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

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on December 26, 2020. January 2, 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3   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 . . . . . . . . . . . .   6   5
   3.  Precedence of API URIs  . . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7   6
     4.1.  Captive Portal Unrestricted Identifier  . . . . . . . . .   7
     4.2.  BOOTP Vendor Extensions and DHCP Options Code Change  . .   7
     4.3.  Update DHCPv6 and IPv6 ND Options Registries  . . . . . .   8   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11  10
   Appendix A.  Changes / Author Notes.  . . . . . . . . . . . . . .  11
   Appendix B.  Changes from RFC 7710  . . . . . . . . . . . . . . .  12
   Appendix C.  Observations From IETF 106 Network Experiment  . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   In many environments, users need to connect to a captive-portal 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, 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 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- captive
   portal enforcement device and the API endpoint that the host can
   contact for more information.

   This document replaces RFC 7710 [RFC7710].

1.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 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 interception
   techniques to serve legacy clients, and clients will need to perform
   probing to detect captive portals. 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

   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

   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 [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
   set the Cache-Control header field in any responses to "private", or
   a more restrictive value such as "no-store" [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 iPAddress certificates
   ([RFC3779]) adds considerations that are out of scope for this

   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.

   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.

   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

       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 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

   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!

4.1.  Captive Portal Unrestricted Identifier

   This document registers a new entry under the IETF URN Sub-namespace
   for Registered Protocol Parameter Identifiers defined in [RFC3553]:

   Registered Parameter Identifier:  capport:unrestricted

   Reference:  RFC TBD (this document)

   IANA Registry Reference:  [RFC3553]

   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, - 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:
   TmqQz6Ma_fznD3XbhwkH9m2dB28 for more background. ]

   The IANA is requested to update the "BOOTP Vendor Extensions and DHCP
   Options" registry (
   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: Unassigned
   Data Length:
   Meaning: Previously assigned by RFC7710; known to also be used by Polycom.
   Reference: [THIS-RFC][RFC7710]

4.3.  Update DHCPv6 and IPv6 ND Options Registries

   This document requests that the DHCPv6 and IPv6 ND options previously
   registered in [RFC7710] be updated 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 which 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

   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 DHCP 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 his choosing.  As 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

   However, as the operating systems and application(s) that make use of
   this information know that they are connecting to a captive-portal 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 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 re-satisfy 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.  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.

   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,

   [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

   [RFC3679]  Droms, R., "Unused Dynamic Host Configuration Protocol
              (DHCP) Option Codes", RFC 3679, DOI 10.17487/RFC3679,
              January 2004, <>.

   [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,

   [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,

   [RFC7113]  Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)", RFC 7113,
              DOI 10.17487/RFC7113, February 2014,

   [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,

   [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




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.

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 MUST 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 DHCPv4 Option 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
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   Erik Kline
   1600 Amphitheatre Parkway
   Mountain View, CA  94043