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RFC 7710 - Captive-Portal Identification Using DHCP or Router Ad

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Internet Engineering Task Force (IETF)                         W. Kumari
Request for Comments: 7710                                        Google
Category: Standards Track                                 O. Gudmundsson
ISSN: 2070-1721                                               CloudFlare
                                                             P. Ebersman
                                                                S. Sheng
                                                           December 2015

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


   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 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 for this

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 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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
     2.3.  The Captive-Portal IPv6 RA Option . . . . . . . . . . . .   4
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   6.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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 captive portal 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

   This document describes a DHCP ([RFC2131]) option (Captive-Portal)
   and an IPv6 Router Advertisement (RA) ([RFC4861]) extension that
   inform 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 bytes, 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 parameter 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 (160) (one octet).

   o  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]).

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 DHCPv6 option (103) (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 Section 5.7 of [RFC7227] for more examples of DHCP options with

2.3.  The Captive-Portal IPv6 RA Option

   The format of the Captive-Portal Router Advertisement option is shown

    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              .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               .
   .                                                               .
   .                                                               .
   .                                                               .

   o  Type: 37

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

   o  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.  An option code (160) has been assigned from the
   "BOOTP Vendor Extensions and DHCP Options" registry
   (http://www.iana.org/assignments/bootp-dhcp-parameters), as specified
   in [RFC2939].  Also, an option code (103) has been assigned from the
   "Option Codes" registry under DHCPv6 parameters

   IANA also has assigned an IPv6 RA Option Type code (37) from the
   "IPv6 Neighbor Discovery Option Formats" registry under ICMPv6
   parameters (http://www.iana.org/assignments/icmpv6-parameters).
   Thanks, IANA!

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 the attacker 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 how the user
   interface presents this information are not covered in this document
   -- by their nature, those are 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
   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 that is protected with TLS,
   the captive-portal operator can attempt to reassure the user that the
   captive portal is not malicious.

5.  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, <http://www.rfc-editor.org/info/rfc3315>.

   [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,

6.  Informative References

   [RFC2939]  Droms, R., "Procedures and IANA Guidelines for Definition
              of New DHCP Options and Message Types", BCP 43, RFC 2939,
              DOI 10.17487/RFC2939, September 2000,


   Thanks to Vint Cerf for asking for this document to be written.
   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 reviews 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.

Authors' Addresses

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

   Email: warren@kumari.net

   Olafur Gudmundsson
   San Francisco, CA  94107
   United States

   Email: olafur@cloudflare.com

   Paul Ebersman

   Email: ebersman-ietf@dragon.net

   Steve Sheng
   Internet Corporation for Assigned Names and Numbers
   12025 Waterfront Drive, Suite 300
   Los Angeles, CA  90094
   United States
   Phone: +1.310.301.5800

   Email: steve.sheng@icann.org


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