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Captive-Portal Identification Using DHCP or Router Advertisements (RAs)

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7710.
Authors Warren "Ace" Kumari , Ólafur Guðmundsson , P Ebersman , Steve Sheng
Last updated 2018-12-20 (Latest revision 2015-08-31)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Stream WG state Submitted to IESG for Publication
Document shepherd Ted Lemon
Shepherd write-up Show Last changed 2015-03-25
IESG IESG state Became RFC 7710 (Proposed Standard)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Joel Jaeggli
Send notices to (None)
IANA IANA review state IANA - Not OK
IANA action state RFC-Ed-Ack
Network Working Group                                          W. Kumari
Internet-Draft                                                    Google
Intended status: Standards Track                          O. Gudmundsson
Expires: March 3, 2016                                        CloudFlare
                                                             P. Ebersman
                                                                S. Sheng
                                                         August 31, 2015

               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 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: . ]

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

   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 March 3, 2016.

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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
   ( 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
     2.3.  The Captive-Portal IPv6 RA Option . . . . . . . . . . . .   4
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Changes / Author Notes.  . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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

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   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",
   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.  The maximum length of the URI that can be carried in IPv4
   DHCP is 255 byte, and so URIs longer than 255 bytes should not be
   used in IPv6 DHCP or IPv6 RA.

   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.  The URI paramter is not null

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.

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   o  URI: The contact URI for the captive portal that the user should
      connect to (encoded following the rules in [RFC3986]).

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 contact URI for the captive portal that the user should
      connect to (encoded following the rules in [RFC3986]).

   See [RFC7227], Section 5.7 for more examples of DHCP Options with

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  TBA3

   Length  8-bit unsigned integer.  The length of the option (including
      the Type and Length fields) in units of 8 bytes.

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   URI  The contact URI for the captive portal 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 be padded with NULL (0x0) to make the total option
      length (including the Type and Length fields) a multiple of 8

3.  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
   (h, as
   specified in [RFC2939].  It also requires assignment of an option
   code (TBA2) from the "DHCPv6 and DHCPv6 options" registry

   IANA is also requested to assign an IPv6 RA Option Type code (TBA3)
   from the "IPv6 Neighbor Discovery Option Formats" registry.  Thanks

4.  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.

   Captive portals are increasingly hijacking TLS connections to force >
   browsers to talk to the portal.  Providing the portal's URI via a

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   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.

   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.  By handing out a URI using which is protected with
   TLS, the captive portal operator can attempt to reassure the user
   that the captive portal is not malicious.

5.  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.  Thanks for David Black for review and providing text for
   the security considerations.  Also great thanks to Joel Jaeggli for
   providing feedback and text.

6.  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, <>.

   [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,

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   [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 15.1 to 16:

      Incorporated (missed) comments from David Black's GenART / OpsDir

   From 15 to 15.1:

   o  Incorporated Brian Haberman's IESG review comment: "I think you
      need to specify somewhere that the URIs are encoded following the
      rules in RFC 3986."

   From 14 to 15:

   o  Incorporated readability comment from Barry Leiba

   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

   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:

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   There was a Captive Portal Bar BoF held at the Dallas IETF meeting.
   See 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

   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.

   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

   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

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   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.

   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:

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   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
   1600 Amphitheatre Parkway
   Mountain View, CA  94043


   Olafur Gudmundsson
   San Francisco, CA  94107


   Paul Ebersman


   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

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