draft-krishnan-dhc-dhcpv6-privacy.xml

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<rfc category="std" docName="draft-krishnan-dhc-dhcpv6-privacy-00"
     ipr="trust200902">
  <front>
    <title abbrev="DHCPv6 Privacy considerations">Privacy considerations for
    DHCPv6</title>

    <author fullname="Suresh Krishnan" initials="S." surname="Krishnan">
      <organization>Ericsson</organization>

      <address>
        <postal>
          <street>8400 Decarie Blvd.</street>

          <city>Town of Mount Royal</city>

          <region>QC</region>

          <country>Canada</country>
        </postal>

        <phone>+1 514 345 7900 x42871</phone>

        <email>suresh.krishnan@ericsson.com</email>
      </address>
    </author>

    <author fullname="Tomek Mrugalski" initials="T." surname="Mrugalski">
      <organization abbrev="ISC">Internet Systems Consortium,
      Inc.</organization>

      <address>
        <postal>
          <street>950 Charter Street</street>

          <city>Redwood City</city>

          <region>CA</region>

          <code>94063</code>

          <country>USA</country>
        </postal>

        <phone>+1 650 423 1345</phone>

        <email>tomasz.mrugalski@gmail.com</email>
      </address>
    </author>

    <author fullname="Sheng Jiang" initials="S." surname="Jiang">
      <organization>Huawei Technologies Co., Ltd</organization>

      <address>
        <postal>
          <street>Q14, Huawei Campus, No.156 BeiQing Road</street>

          <city>Hai-Dian District, Beijing</city>

          <region></region>

          <code>100095</code>

          <country>P.R. China</country>
        </postal>

        <email>jiangsheng@huawei.com</email>
      </address>
    </author>

    <date year="2014" />

    <area>Internet</area>

    <workgroup>dhc</workgroup>

    <abstract>
      <t>DHCPv6 is a protocol that is used to provide addressing and
      configuration information to IPv6 hosts. This document discusses the
      various identifiers used by DHCPv6 and the potential privacy issues.</t>
    </abstract>
  </front>

  <middle>
    <section anchor="intro" title="Introduction">
      <t>DHCPv6 <xref target="RFC3315"></xref> is a protocol that is used to
      provide addressing and configuration information to IPv6 hosts. The
      DHCPv6 protocol uses several identifiers that could become a source for
      gleaning additional information about the IPv6 host. This information
      may include device type, operating system information, location(s) that
      the device may have previously visited, etc. This document discusses the
      various identifiers used by DHCPv6 and the potential privacy issues
      <xref target="RFC6973"></xref>.</t>

      <t>Future works may propose protocol changes to fix the privacy issues
      that have been analyzed in this document. It is out of scope for this
      document.</t>

      <t>Editor notes: for now, the document is mainly considering the privacy
      of DHCPv6 client. The privacy of DHCPv6 server and relay agent are
      considered less important because they are open for public services.
      However, this may be a subject to change if further study shows opposite
      result.</t>
    </section>

    <section title="Terminology">
      <t>This section clarifies the terminology used throughout this
      document.</t>

      <t>Stable identifier - any property disclosed by a DHCPv6 client that
      does not change over time or changes very infrequently and is unique for
      said client in a given context. Examples include MAC address, client-id
      that does not change or a hostname. Stable identifier may or may not be
      globally unique.</t>

      <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
      "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
      document are to be interpreted as described in <xref
      target="RFC2119"></xref>. When these words are not in ALL CAPS (such as
      "should" or "Should"), they have their usual English meanings, and are
      not to be interpreted as <xref target="RFC2119"></xref> key words.</t>
    </section>

    <section title="Identifiers in DHCPv6">
      <t>There are several identifiers used in DHCPv6. This section provides
      an introduction to the various options that will be used further in the
      document.</t>

      <section anchor="duid" title="DUID">
        <t>Each DHCPv6 client and server has a DHCPv6 Unique Identifier (DUID)
        <xref target="RFC3315"></xref>. The DUID is designed to be unique
        across all DHCPv6 clients and servers, and to remain stable after it
        has been initially generated. The DUID can be of different forms.
        Commonly used forms are based on the link-layer address of one of the
        device's network interfaces (with or without a timestamp), on the
        Universally Unique IDentifier (UUID) <xref target="RFC6355"></xref>.
        The default type, recommended by <xref target="RFC3315"></xref>, is
        DUID-LLT that is based on link-layer address, which is commonly
        implemented in most popular clients.</t>

        <t>It is important to understand DUID lifecycle. Clients and servers
        are expected to generate their DUID once (during first operation) and
        store it in a non-volatile storage or use the same deterministic
        algorithm to generate the same DUID value again. This means that most
        implementations will use the available link-layer address during its
        first boot. Even if the administrator enables privacy extensions (see
        <xref target="RFC4941"></xref>) and its equivalent for link-layer
        address randomization, it is likely that those privacy mechanisms were
        disabled during the first device boot. Hence the original,
        unobfuscated link-layer address will likely end up being announced as
        client DUID, even if the link-layer address has changed (or even if
        being changed on a periodic basis).</t>
      </section>

      <section title="Client ID Option">
        <t>The Client Identifier Option (OPTION_CLIENTID) <xref
        target="RFC3315"></xref> is used to carry the DUID of a DHCPv6 client
        between a client and a server. There is an analogous Server Identifier
        Option but it is not as interesting in the privacy context (unless a
        host can be convinced to start acting as a server). Client ID is an
        example of DUID. See <xref target="duid"></xref> for relevant
        discussion about DUIDs.</t>
      </section>

      <section title="IA_NA, IA_TA, IA_PD, IA Address and IA Prefix Options">
        <t>The Identity Association for Non-temporary Addresses (IA_NA) option
        <xref target="RFC3315"></xref> is used to carry the parameters and any
        non-temporary addresses associated with the given IA_NA. The Identity
        Association for Temporary Addresses (IA_TA) option <xref
        target="RFC3315"></xref> is analogous to the IA_NA option but for
        temporary addresses. The IA Address option <xref
        target="RFC3315"></xref> is used to specify IPv6 addresses associated
        with an IA_NA or an IA_TA and is encapsulated within the Options field
        of such an IA_NA or IA_TA option. The Identity Association for Prefix
        Delegation (IA_PD) <xref target="RFC3633"></xref> option is used to
        carry the prefixes that are assigned to the requesting router. IA
        Prefix option <xref target="RFC3633"></xref> is used to specify IPv6
        prefixes associated with an IA_PD and is encapsulated within the
        Options field of such an IA_PD option.</t>

        <t>To differentiate between instances of the same type of IA
        containers, each IA_NA, IA_TA and IA_PD options have an IAID field
        that is unique for each client/option type pair. It is up to the
        client to pick unique IAID values. At least one popular implementation
        uses last four octets of the link-layer address. In most cases, that
        means that merely two bytes are missing for a full link-layer address
        reconstruction. However, the first three octets in a typical
        link-layer address are vendor identifier. That can be determined with
        high level of certainty using other means, thus allowing full
        link-layer address discovery.</t>
      </section>

      <section title="Interface ID">
        <t>A DHCPv6 relay includes the Interface ID <xref
        target="RFC3315"></xref> option to identify the interface on which it
        received the client message that is being relayed.</t>

        <t>Although in principle Interface ID can be arbitrarily long with
        completely random values, it is often a text string that includes the
        relay agent name followed by interface name. This can be used for
        fingerprinting the relay or determining client's point of
        attachment.</t>
      </section>

      <section title="Subscriber ID">
        <t>A DHCPv6 relay includes a Subscriber ID option <xref
        target="RFC4580"></xref> to associate some provider-specific
        information with clients' DHCPv6 messages that is independent of the
        physical network configuration.</t>

        <t>In many deployments, the relay agent that inserts this option is
        configured to use client's link-layer address as Subscriber ID.</t>
      </section>

      <section title="Remote ID">
        <t>A DHCPv6 relay includes a Remote ID option <xref
        target="RFC4649"></xref> to identify the remote host end of the
        circuit.</t>

        <t>The remote-id is vendor specific, for which the vendor is indicated
        in the enterprise-number field. The remote-id field may encode the
        information that identified the DHCPv6 clients:</t>

        <t><list style="symbols">
            <t>a "caller ID" telephone number for dial-up connection</t>

            <t>a "user name" prompted for by a Remote Access Server</t>

            <t>a remote caller ATM address o a "modem ID" of a cable data
            modem</t>

            <t>the remote IP address of a point-to-point link</t>

            <t>an interface or port identifier</t>
          </list></t>
      </section>

      <section title="Client FQDN Option">
        <t>The Client Fully Qualified Domain Name (FQDN) option <xref
        target="RFC4704"></xref> is used by DHCPv6 clients and servers to
        exchange information about the client's fully qualified domain name
        and about who has the responsibility for updating the DNS with the
        associated AAAA and PTR RRs.</t>

        <t>A client can use this option to convey all or part of its domain
        name to a DHCPv6 server for the IPv6-address-to-FQDN mapping. In most
        case a client sends its hostname as a hint for the server. The DHCPv6
        server MAY be configured to modify the supplied name or to substitute
        a different name. The server should send its notion of the complete
        FQDN for the client in the Domain Name field.</t>
      </section>

      <section title="Client Link-layer Address Option">
        <t>The Client link-layer address option <xref target="RFC6939"></xref>
        is used by first-hop DHCPv6 relays to provide the client's link-layer
        address towards the server.</t>

        <t>DHCPv6 relay agents that receive messages originating from clients
        may include the link-layer source address of the received DHCPv6
        message in the Client Link-Layer Address option, in relayed DHCPv6
        Relay-Forward messages.</t>
      </section>

      <section title="Option Request Option">
        <t>DHCPv6 clients include an Option Request option <xref
        target="RFC3315"></xref> in DHCPv6 messages to inform the server about
        options the client wants the server to send to the client.</t>

        <t>The content of an Option Request option are the option codes for an
        option requested by the client. The client may additionally include
        instances of those options that are identified in the Option Request
        option, with data values as hints to the server about parameter values
        the client would like to have returned.</t>
      </section>

      <section title="Vendor Class Option">
        <t>This Vendor Class option <xref target="RFC3315"></xref> is used by
        a DHCPv6 client to identify the vendor that manufactured the hardware
        on which the client is running.</t>

        <t>The information contained in the data area of this option is
        contained in one or more opaque fields that identify details of the
        hardware configuration, for example, the version of the operating
        system the client is running or the amount of memory installed on the
        client.</t>
      </section>

      <section title="Civic Location Option">
        <t>DHCPv6 servers use the Civic Location option <xref
        target="RFC4776"></xref> to delivery of location information (the
        civic and postal addresses) from the DHCPv6 server to the DHCPv6
        clients. It may refer to three locations: the location of the DHCPv6
        server, the location of the network element believed to be closest to
        the client, or the location of the client, identified by the "what"
        element within the option.</t>
      </section>

      <section title="Coordinate-Based Location Option">
        <t>The GeoLoc options <xref target="RFC6225"></xref> is used by DHCPv6
        server to provide the coordinate- based geographic location
        information to the DHCPv6 clients. It enable a DHCPv6 client to obtain
        its location.</t>

        <t>After the relevant DHCPv6 exchanges have taken place, the location
        information is stored on the end device rather than somewhere else,
        where retrieving it might be difficult in practice.</t>
      </section>

      <section title="Client System Architecture Type Option">
        <t>The Client System Architecture Type option <xref
        target="RFC5970"></xref> is used by DHCPv6 client to send a list of
        supported architecture types to the DHCPv6 server. It is used to
        provide configuration information for a node that must be booted using
        the network rather than from local storage.</t>

        <t><!--Client Network Interface Identifier [RFC5970] seems not in use. It provides information about the level of UNDI support.--></t>
      </section>

      <!--<section title="Options for Home Network/Agent Dynamic Discovery">
        <t>A set of DHCP options <xref target="RFC6610"></xref> are used to
        deliver the home agent information (the home agent's IPv6 address,
        IPv6 home network prefix, or FQDN information) to the mobile node
        (DHCPv6 client), including MIPv6 Home Network ID FQDN option, Home
        Network Information options, MIPv6 Home Network Prefix option, MIPv6
        Home Agent Address option, MIPv6 Home Agent FQDN option. They contains
        the information of target home networks for which the client is
        requesting configuration information.</t>
      </section>-->
    </section>

    <section title="Existing Mechanisms That Affect Privacy">
      <t>This section describes available DHCPv6 mechanisms that one can use
      to protect or enhance one's privacy.</t>

      <section title="Temporary addresses">
        <t><xref target="RFC3315"></xref> defines a mechanism for a client to
        request temporary addresses. The idea behind temporary addresses is
        that a client can request a temporary address for a specific purpose,
        use it, and then never renew it. i.e. let it expire.</t>

        <t>There are number of serious issues, both protocolar and
        implementational, that make them nearly useless for their original
        goal. First, <xref target="RFC3315"></xref> does not include T1 and T2
        renewal timers in IA_TA (a container for temporary addresses).
        However, it mentions that temporary addresses can be renewed. Many
        client implementations renew those addresses during a renewal
        procedure initiated by other resources (non-temporary addresses or
        prefixes), thus forfeiting shortliveness. Second, <xref
        target="RFC4704"></xref> allows servers to update DNS for assigned
        temporary addresses. Publishing client's IPv6 address in DNS that is
        publicly available is a major privacy breach.</t>
      </section>

      <section title="DNS Updates">
        <t>DNS Updates <xref target="RFC4704"></xref> defines a mechanism that
        allows both clients and server to insert into DNS domain information
        about clients. Both forward (AAAA) and reverse (PTR) resource records
        can be updated. This allows other nodes to conveniently refer to a
        host, despite the fact that its IPv6 address may be changing.</t>

        <t>This mechanism exposes two important pieces of information: current
        address (which can be mapped to current location) and client's
        hostname. The stable hostname can then by used to correlate the client
        across different network attachments even when its IPv6 address keeps
        changing.</t>
      </section>

      <section title="Allocation strategies">
        <t>A DHCPv6 server running in typical, stateful mode is given a task
        of managing one or more pools of IPv6 resources (currently
        non-temporary addresses, temporary addresses and/or prefixes, but more
        resource types may be defined in the future). When a client requests a
        resource, server must pick a resource out of configured pool.
        Depending on the server's implementation, various allocation
        strategies are possible. Choices in this regard may have privacy
        implications.</t>

        <t>Iterative allocation - a server may choose to allocate addresses
        one by one. That strategy has the benefit of being very fast, thus can
        be favored in deployments that prefer performance. However, it makes
        the resources very predictable. Also, since the resources allocated
        tend to be clustered at the beginning of available pool, it makes
        scanning attacks much easier.</t>

        <t>Identifier-based allocation - a server may choose to allocate an
        address that is based on one of available identifiers, e.g. IID or MAC
        address. This has a property of being convenient for converting IP
        address to/from other identifiers, especially if the identifier is or
        contains MAC address. It is also convenient, as returning client is
        very likely to get the same address, even if the server does not store
        previous client's address. Those properties are convenient for system
        administrators, so DHCPv6 server implementors are sometimes requested
        to implement it. There is at least one implementation that supports
        it. On the other hand, the downside of such allocation is that the
        client now discloses its identifier in its IPv6 address to all
        services it connects to. That means that correlation of activities
        over time, location tracking, address scanning and OS/vendor discovery
        apply.</t>

        <t>Hash allocation - it's an extension of identifier based allocation.
        Instead of using the identifier directly, it is being hashed first. If
        the hash is implemented correctly, it removes the flaw of disclosing
        the identifier, a property that eliminates susceptibility to address
        scanning and OS/vendor discovery. If the hash is poorly implemented
        (e.g. can be reverted), it introduces no improvement over
        identifier-based allocation.</t>

        <t>Random allocation - a server can pick a resource randomly out of
        available pool. That strategy works well in scenarios where pool
        utilization is small, as the likelihood of collision (resulting in the
        server needing to repeat randomization) is small. With the pool
        allocation increasing, the collision is disproportionally large, due
        to birthday paradox. With high pool utilization (e.g. when 90% of
        available resources being allocated already), the server will use most
        computational resources to repeatedly pick a random resource, which
        will degrade its performance. This allocation scheme essentially
        prevents returning clients from getting the same address or prefix
        again. On the other hand, it is beneficial from privacy perspective as
        addresses and prefixes generated that way are not susceptible to
        correlation attacks, OS/vendor discovery attacks or identity discovery
        attacks. Note that even though the address or prefix itself may be
        resilient to a given attack, the client may still be susceptible if
        additional information is disclosed other way, e.g. client's address
        can be randomized, but it still can leak its MAC address in client-id
        option.</t>

        <t>Other allocation strategies may be implemented.</t>
      </section>
    </section>

    <section title="Attacks">
      <section title="Device type discovery (fingerprinting)">
        <t>The type of device used by the client can be guessed by the
        attacker using the Vendor Class option, the Client Link-layer Address
        option, and by parsing the Client ID option. All of those options may
        contain OUI (Organizationally Unique Identifier) that represents the
        device's vendor. That knowledge can be used for device-specific
        vulnerability exploitation attacks. See Section 3.4 of <xref
        target="I-D.ietf-6man-ipv6-address-generation-privacy"></xref> for a
        discussion about this type of attack.</t>
      </section>

      <section title="Operating system discovery (fingerprinting)">
        <t>The operating system running on a client can be guessed using the
        Vendor Class option, the Client System Architecture Type option, or by
        using fingerprinting techniques on the combination of options
        requested using the Option Request option. See Section 3.4 of <xref
        target="I-D.ietf-6man-ipv6-address-generation-privacy"></xref> for a
        discussion about this type of attack.</t>
      </section>

      <section title="Finding location information">
        <t>The location information can be obtained by the attacker by many
        means. The most direct way to obtain this information is by looking
        into a server initiated message that contains the Civic Location or
        GeoLoc option. It can also be indirectly inferred using the Remote ID
        Option (e.g. using a telephone number), the Interface ID option (e.g.
        if an access circuit on an Access Node corresponds to a civic
        location), or the Subscriber ID Option (if the attacker has access to
        subscriber info).</t>
      </section>

      <section title="Finding previously visited networks">
        <t>When DHCPv6 clients connect to a network, they attempt to obtain
        the same address they had used before they attached to the network.
        They do this by putting the previously assigned address(es) in the IA
        Address Option(s) inside the IA_NA, IA_TA. By observing these
        addresses, an attacker can identify the network the client had
        previously visited.</t>
      </section>

      <section title="Finding a stable identity">
        <t>An attacker might use a stable identity gleaned from DHCPv6
        messages to correlate activities of a given client on unrelated
        networks. The Client FQDN option, the Subscriber ID Option and the
        Client ID options can serve as long lived identifiers of DHCPv6
        clients. The Client FQDN option can also provide an identity that can
        easily be correlated with web server activity logs.</t>
      </section>

      <section title="Pervasive monitoring">
        <t>This is an enhancement, or a combination of most aforementioned
        mechanisms. Operator, who controls non-trivial number of access points
        or network segments, may use obtained information about a single
        client and observer client's habits.</t>
      </section>

      <section title="Finding client's IP address or hostname">
        <t>Many DHCPv6 deployments use DNS Updates <xref
        target="RFC4704"></xref> that put client's information (current IP
        address, client's hostname). Client ID is also disclosed, able it in
        not easily accessible form (SHA-256 digest of the client-id). Although
        SHA-256 is irreversible, so DHCPv6 client ID can't be converted back
        to client-id. However, SHA-256 digest can be used as a unique
        identifier that is accessible by any host.</t>
      </section>

      <section title="Correlation of activities over time">
        <t>As with other identifiers, an IPv6 address can be used to correlate
        the activities of a host for at least as long as the lifetime of the
        address. If that address was generated from some other, stable
        identifier and that generation scheme can be deducted by an attacker,
        the duration of correlation attack extends to that identifier. In many
        cases, its lifetime is equal to the lifetime of the device itself. See
        Section 3.1 of <xref
        target="I-D.ietf-6man-ipv6-address-generation-privacy"></xref> for
        detailed discussion.</t>
      </section>

      <section title="Location tracking">
        <t>If a stable identifier is used for assigning an address and such
        mapping is discovered by an attacker (e.g. a server that uses
        IEEE-identifier-based IID to generate IPv6 address), all scenarios
        discussed in Section 3.2 of <xref
        target="I-D.ietf-6man-ipv6-address-generation-privacy"></xref> apply.
        In particular both passive (a service that the client connects to can
        log client's address and draw conclusions regarding its location and
        movement patterns based on prefix it is connecting from) and active
        (attacker can send ICMPv6 echo requests or other probe packets to
        networks of suspected client locations).</t>
      </section>

      <section title="Leasequery &amp; bulk leasequery">
        <t>Attackers may pretend as an access concentrator, either DHCPv6
        relay agent or DHCPv6 client, to obtain location information directly
        from the DHCP server(s) using the DHCPv6 Leasequery <xref
        target="RFC5007"></xref> mechanism.</t>

        <t>Location information is information needed by the access
        concentrator to forward traffic to a broadband-accessible host. This
        information includes knowledge of the host hardware address, the port
        or virtual circuit that leads to the host, and/or the hardware address
        of the intervening subscriber modem.</t>

        <t>Furthermore, the attackers may use DHCPv6 bulk leasequery <xref
        target="RFC5460"></xref> mechanism to obtain bulk information about
        DHCPv6 bindings, even without knowing the target bindings.</t>
      </section>
    </section>

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

    <section anchor="privacy-consider" title="Privacy Considerations">
      <t>This document at its entirety discusses privacy considerations in
      DHCPv6. As such, no separate section about this is needed.</t>
    </section>

    <section title="IANA Considerations">
      <t>This draft does not request any IANA action.</t>
    </section>

    <section anchor="acks" title="Acknowledgements">
      <t>The authors would like to thanks the valuable comments made by
      Stephen Farrell, Ted Lemon, Ines Robles, Russ White, Christian Schaefer
      and other members of DHC WG.</t>

      <t>This document was produced using the xml2rfc tool <xref
      target="RFC2629"></xref>.</t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      &rfc2119;

      &rfc3315;
    </references>

    <references title="Informative References">
      &rfc3633;

      <?rfc include='reference.RFC.2629'?>

      &rfc4580;

      &rfc4649;

      &rfc4704;

      &rfc4776;

      &rfc4941;

      <?rfc include='reference.RFC.5007'?>

      <?rfc include='reference.RFC.5460'?>

      <?rfc include='reference.RFC.5970'?>

      <?rfc include='reference.RFC.6225'?>

      &rfc6355;

      &rfc6939;

      &rfc6973;

      &I-D.ietf-6man-ipv6-address-generation-privacy;
    </references>
  </back>
</rfc>