Network Working Group                                          W. Kumari
Internet-Draft                                                    Google
Obsoletes: 7710 (if approved)                                   E. Kline
Intended status: Standards Track                                    Loon
Expires: July 15, 2020                                  January 3, 12, 2020                                    July 2, 2019

               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 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 of this document.

   [ 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. ]

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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  . . . . . . . . . . . . . . . . . . . .   4
     2.3.  The Captive-Portal IPv6 RA Option . . . . . . . . . . . .   5
   3.  The Captive-Portal Link Relation Type . . . . . . . . . . . .   6
   4.  Precedence of API URIs  . . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  IETF params Registration  . . . . . . . . . . . . . . . .   6
       4.1.1.  Registry name: Captive Portal Unrestricted Identifier   6
       5.1.2.  Registry name: Captive Portal API Link Relation Type    7
     4.2.  BOOTP Vendor Extensions and DHCP Options Code Change  . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  Changes / Author Notes.  . . . . . . . . . . . . . .   9  10
   Appendix B.  Changes from RFC 7710  . . . . . . . . . . . . . . .   9  10
   Appendix C.  Observations From IETF 106 Network Experiment  . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10  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.  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
   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 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 getting them to the captive portal
   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

   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 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 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] (i.e. the URI SHOULD contain a DNS name and
   SHOULD reference a secure transport, e.g. https).

   A captive portal MAY redirect requests that do not have an Accept
   header field ([RFC7231] Section 5.3) containing a field item whose
   content-type is "application/capport+json" to the URL conveyed in the
   "user-portal-url" API key.  When performing such content negotiation
   ([RFC7231] Section 3.4), 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

   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.

   The URI SHOULD NOT contain an IP address literal.  The URI parameter
   is not null terminated.

   Networks with no captive portals MAY explicitly indicate this
   condition by using this option with the IANA-assigned URI for this
   purpose (see Section 5.1.1). 4.1.1).  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.

       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 URI for the captive portal API endpoint to which the user
      should connect (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 (103) (two octets)

   o  option-len: The 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

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 NULL (0x00) to make the
      total option length (including the Type and Length fields) a
      multiple of 8 bytes.

3.  The Captive-Portal Link Relation Type

   Some captive portal network deployments may be unable to change, or
   unwilling to risk changing, the network infrastructure necessary to
   use any of the above options.  In such deployments, when clear text
   HTTP intercept and redirection are used, a Link relation header
   ([RFC8288], Section 3.3) MAY be inserted to convey to a HTTP client
   (user agent) the associated Captive Portal API URI.

   HTTP user agents MUST ignore this link relation in any context other
   than when explicitly probing to detect the presence of a captive
   portal.  Failure to do so could allow an attacker to inject a Captive
   Portal API URI other than the correct URI for a given network or for
   networks where there is no captive portal present at all.

4.  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.  It is a network
   configuration error if the learned URIs are not all identical.

   However, if the URIs learned are not in fact all identical the
   captive device MUST prioritize URIs learned from network provisioning
   or configuration mechanisms before all other URIs.  Specifically,
   URIs learned via any of the options in Section 2 should take
   precedence over any URI learned via some other mechanism, such as a mechanism like the one
   described in Section 3.

   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.  URI precedence in this situation is not
   specified by this document.


4.  IANA Considerations

   This document requests two new IETF URN protocol parameter
   ([RFC3553]) entries.  This document also requests a reallocation of
   DHCPv4 option codes (see Appendix C for background).

   Thanks IANA!


4.1.  IETF params Registration


4.1.1.  Registry name: Captive Portal Unrestricted Identifier

   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.

5.1.2.  Registry name: Captive Portal API Link Relation Type

   Registry name: Captive Portal API Link Relation Type

   URN: urn:ietf:params:capport-api

   Specification: RFC TBD (this document)


4.2.  BOOTP Vendor Extensions and DHCP Options Code Change

   [ RFC TBD (this document)

   Index value: Only one value Ed: Please remove before publication: RFC7710 uses DHCP Code
   160 -- unfortunately, it was discovered that this option code is defined
   already widely used by Polycom (see URN above).  No hierarchy appendix).  Option 114 (URL) is defined
   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 therefore no sub-namespace registrations are possible.

6. 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: REMOVED/Unassigned
      Data Length:
      Reference: [RFC7710][Deprecated]

5.  Security Considerations

   An attacker with the ability to inject DHCP messages, RAs, messages or HTTP
   headers into cleartext HTTP communications RAs could
   include an option or
   link relation from this document and so 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 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 using which is protected with
   TLS, the captive portal operator can attempt to reassure the user
   that the captive portal is not malicious.

   Operating systems should conduct all interactions with the API in a
   sand-boxed environment and with a configuration that minimizes
   tracking risks.


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 Lorenzo Colitti,
   Remi Nguyen Van, 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, <https://www.rfc-

   [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, <>.

   [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, <https://www.rfc-

   [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, <https://www.rfc-

   [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,

   [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, <>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017, <https://www.rfc-

7.2.  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 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.  Added urn:ietf:params:capport-api URN.

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