draft-ietf-capport-rfc7710bis.xml

<?xml version="1.0" encoding="US-ASCII"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
<!ENTITY rfc2119 PUBLIC "" ".//reference.RFC.2119.xml">
<!ENTITY rfc8174 PUBLIC "" ".//reference.RFC.8174.xml">
]>
<!-- WK: Set category, IPR, docName -->
<rfc category="std" docName="draft-ietf-capport-rfc7710bis-11"
     ipr="trust200902" obsoletes="7710" updates="3679">
  <?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>

  <?rfc toc="yes" ?>

  <?rfc symrefs="yes" ?>

  <?rfc sortrefs="yes"?>

  <?rfc iprnotified="no" ?>

  <?rfc strict="yes"?>

  <?rfc compact="yes" ?>

  <front>
    <!--c WK: Set long title. -->

    <title abbrev="DHCP Captive-Portal">Captive-Portal Identification in DHCP
    / RA</title>

    <author fullname="Warren Kumari" initials="W." surname="Kumari">
      <organization>Google</organization>

      <address>
        <postal>
          <street>1600 Amphitheatre Parkway</street>

          <city>Mountain View, CA</city>

          <code>94043</code>

          <country>US</country>
        </postal>

        <email>warren@kumari.net</email>
      </address>
    </author>

    <author fullname="Erik Kline" initials="E." surname="Kline">
      <organization>Loon</organization>

      <address>
        <postal>
          <street>1600 Amphitheatre Parkway</street>

          <city>Mountain View, CA</city>

          <code>94043</code>

          <country>US</country>
        </postal>

        <phone/>

        <email>ek@loon.com</email>
      </address>
    </author>

    <date month="July" year="2020"/>

    <abstract>
      <t>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.</t>

      <t>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 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 document.</t>

      <t>This document replaces <xref target="RFC7710"/>.  <xref target="RFC7710"/>
      used DHCP code point 160.  Due to a conflict, this document specifies 114.
      Consequently, this document also updates <xref target="RFC3679"/>.</t>
    </abstract>
  </front>

  <middle>
    <section title="Introduction">
      <t>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.</t>

      <t>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.</t>

      <t>This document describes a DHCPv4 <xref target="RFC2131"/> and DHCPv6
      <xref target="RFC8415"/> option (Captive-Portal) and an IPv6
      Router Advertisement (RA) <xref target="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.</t>

      <t>This document replaces RFC 7710 <xref target="RFC7710"/>.</t>

      <section title="Requirements Notation">
        <t>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 <xref target="RFC2119"/> <xref target="RFC8174"/> when,
        and only when, they appear in all capitals, as shown here.</t>
      </section>
    </section>

    <section anchor="option" title="The Captive-Portal Option">
      <t>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 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.</t>

      <t>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.</t>

      <t>In order to support multiple "classes" of clients (e.g. IPv4 only,
      IPv6 only with DHCPv6 (<xref target="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.</t>

      <t>In all variants of this option, the URI MUST be that of the captive
      portal API endpoint [draft-ietf-capport-api].
      </t>

      <t>A captive portal MAY do content negotiation (<xref target="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. <xref target="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 (<xref target="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
      (<xref target="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" <xref target="RFC7234"/> Section 5.2.2.3).
      </t>

      <t>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 (<xref target="RFC3779"/>)
      adds considerations that are out of scope for this document.</t>

      <t>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.</t>

      <section anchor="dhcpv4opt" title="IPv4 DHCP Option">
        <t>The format of the IPv4 Captive-Portal DHCP option is shown
        below.<figure>
            <artwork><![CDATA[
    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...                         .
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
          </figure></t>

        <t><list style="symbols">
            <t>Code: The Captive-Portal DHCPv4 Option (114) (one octet)</t>

            <t>Len: The length (one octet), in octets, of the URI.</t>

            <t>URI: The URI for the captive portal API endpoint to which the
            user should connect (encoded following the rules in <xref
            target="RFC3986"/>).</t>
          </list>See <xref target="RFC2132"/>, Section 2 for more on the format
          of IPv4 DHCP options.</t>

        <t>Note that the URI parameter is not null terminated.</t>
      </section>

      <section anchor="dhcpv6opt" title="IPv6 DHCP Option">
        <t>The format of the IPv6 Captive-Portal DHCP option is shown below.
        <figure>
            <artwork><![CDATA[
    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)                    .
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
          </figure></t>

        <t><list style="symbols">
            <t>option-code: The Captive-Portal DHCPv6Option (103) (two
            octets)</t>

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

            <t>URI: The URI for the captive portal API endpoint to which the
            user should connect (encoded following the rules in <xref
            target="RFC3986"/>).</t>
          </list>See <xref target="RFC7227"/>, Section 5.7 for more examples
        of DHCP Options with URIs. See <xref target="RFC8415"/>, Section 21.1
        for more on the format of IPv6 DHCP options.</t>

        <t>Note that the URI parameter is not null terminated.</t>

        <t>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.</t>
      </section>

      <section anchor="v6ndopt" title="The Captive-Portal IPv6 RA Option">
        <t>This section describes the Captive-Portal Router Advertisement
        option.</t>

        <t><figure>
            <artwork><![CDATA[
    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]]></artwork>
          </figure></t>

        <t><list style="hanging">
            <t hangText="Type">37</t>

            <t hangText="Length">8-bit unsigned integer. The length of the
            option (including the Type and Length fields) in units of 8
            bytes.</t>

            <t hangText="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.</t>
          </list></t>

        <t>Note that the URI parameter is not guaranteed to be null terminated.</t>

        <t>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.</t>
      </section>
    </section>

    <section anchor="precedence" title="Precedence of API URIs">
      <t>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).</t>

      <t>If the URIs learned via more than one option described in <xref
      target="option"/> 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.</t>
    </section>

    <section anchor="iana" title="IANA Considerations">
      <t>This document requests one new IETF URN protocol parameter (<xref
      target="RFC3553"/>) entry. This document also requests a reallocation
      of DHCPv4 option codes (see <xref target="exp106"/> for background).</t>

      <t>Thanks IANA!</t>

      <section anchor="iana-urn"
               title="Captive Portal Unrestricted Identifier">
       <t>This document registers a new entry under the IETF URN
       Sub-namespace for Registered Protocol Parameter Identifiers
       defined in <xref target="RFC3553"/>:
       <list style="hanging">
         <t hangText="Registered Parameter Identifier:">capport:unrestricted</t>
         <t hangText="Reference:">RFC TBD (this document)</t>
         <t hangText="IANA Registry Reference:">RFC TBD (this document)</t>
      </list></t>

      <t>Only one value is defined (see URN above). No hierarchy is defined and
      therefore no sub-namespace registrations are possible.</t>
      </section>

      <section anchor="ietf_dhcpv4_option_code_change"
               title="BOOTP Vendor Extensions and DHCP Options Code Change">
        <t>
      [ 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, dieter@apple.com - 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: https://mailarchive.ietf.org/arch/msg/captive-portals/TmqQz6Ma_fznD3XbhwkH9m2dB28 for more background. ]
        </t>

        <t>The IANA is requested to update the
          "BOOTP Vendor Extensions and DHCP Options" registry
          (https://www.iana.org/assignments/bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml)
          as follows.</t>

        <t><figure><artwork><![CDATA[   Tag: 114
   Name: DHCP Captive-Portal
   Data Length: N
   Meaning: DHCP Captive-Portal
   Reference: [THIS-RFC]]]></artwork></figure></t>

        <t><figure><artwork><![CDATA[   Tag: 160
   Name: Unassigned
   Data Length:
   Meaning: Previously assigned by RFC7710; known to also be used by Polycom.
   Reference: [THIS-RFC][RFC7710]]]></artwork></figure></t>
      </section>

      <section anchor="iana_update_dhcv6_and_icmpv6"
               title="Update DHCPv6 and IPv6 ND Options Registries">
        <t>This document requests that the DHCPv6 and IPv6 ND options previously
        registered in <xref target="RFC7710"/> be updated to reference this
        document.</t>
      </section>

    </section>

    <section anchor="security" title="Security Considerations">
      <t>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.</t>

      <t>Clients processing these options SHOULD validate that the option's
      contents conform to the validation requirements for URIs, including
      <xref target="RFC3986"/>.</t>

      <t>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
      (<xref target="RFC6105"/>, <xref target="RFC7113"/>) or DHCP Shield
      <xref target="RFC7610"/>, an attacker could inject, modify, or block DHCP
      messages or RAs.</t>

      <t>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
      attack.</t>

      <t>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 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.</t>

      <t>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.</t>

      <t>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.</t>
    </section>

    <section title="Acknowledgements">
      <t>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.</t>

      <t>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.</t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      <?rfc include='reference.RFC.2131'?>

      <?rfc include='reference.RFC.2132'?>

      <?rfc include='reference.RFC.2119'?>

      <?rfc include='reference.RFC.3553'?>

      <?rfc include='reference.RFC.3986'?>

      <?rfc include='reference.RFC.4861'?>

      <?rfc include='reference.RFC.7227'?>

      <?rfc include='reference.RFC.7231'?>

      <?rfc include='reference.RFC.7234'?>

      <?rfc include='reference.RFC.8174'?>

      <?rfc include='reference.RFC.8415'?>
    </references>

    <references title="Informative References">
      <?rfc include='reference.RFC.3679'?>
      <?rfc include='reference.RFC.3779'?>
      <?rfc include='reference.RFC.6105'?>
      <?rfc include='reference.RFC.7113'?>
      <?rfc include='reference.RFC.7610'?>
      <?rfc include='reference.RFC.7710'?>
    </references>

    <section title="Changes / Author Notes.">
      <t>[RFC Editor: Please remove this section before publication ]</t>

      <t>From initial to -00.</t>

      <t><list style="symbols">
          <t>Import of RFC7710.</t>
        </list></t>

      <t>From -00 to -01.</t>

      <t><list style="symbols">
          <t>Remove link-relation text.</t>
          <t>Clarify option should be in DHCPREQUEST parameter list.</t>
          <t>Uppercase some SHOULDs.</t>
        </list></t>
    </section>

    <section anchor="diff7710" title="Changes from RFC 7710">
      <t>This document incorporates the following changes from <xref
      target="RFC7710"/>.</t>

      <t><list style="numbers">
          <t>Clarify that IP string literals are NOT RECOMMENDED.</t>

          <t>Clarify that the option URI MUST be that of the captive portal
          API endpoint.</t>

          <t>Clarify that captive portals MAY do content negotiation.</t>

          <t>Added text about Captive Portal API URI precedence in the event
          of a network configuration error.</t>

          <t>Added urn:ietf:params:capport:unrestricted URN.</t>

          <t>Notes that the DHCPv4 Option Code changed from 160 to 114.</t>
        </list></t>
    </section>

    <section anchor="exp106"
             title="Observations From IETF 106 Network Experiment">
      <t>During IETF 106 in Singapore an <eref
        target="https://tickets.meeting.ietf.org/wiki/IETF106network#Experiments"
          >experiment</eref> enabling Captive Portal API compatible clients to
        discover a venue-info-url (see <eref
        target="https://tickets.meeting.ietf.org/wiki/CAPPORT">
        experiment description</eref> for more detail) revealed that some
        Polycom devices on the same network made use of DHCPv4 option code 160
        for <eref
        target="https://community.polycom.com/t5/VoIP-SIP-Phones/DHCP-Standardization-160-vs-66/td-p/72577"
        >other purposes</eref>.</t>

      <t>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.</t>
    </section>
  </back>
</rfc>