draft-ietf-dhc-dhcpv6-privacy.xml

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<rfc category="info" docName="draft-ietf-dhc-dhcpv6-privacy-05"
     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>

        <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 />

    <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 describes the
      privacy issues associated with the use of DHCPv6 by the Internet users.
      It is intended to be an analysis of the present situation and does not
      propose any solutions.</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. DHCPv6
      uses several identifiers that could become a source for gleaning
      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>. In particular, it also takes into
      consideration the problem of pervasive monitoring <xref
      target="RFC7258"></xref>.</t>

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

      <t>The primary focus of this document is around privacy considerations
      for clients to support client mobility and connection to random
      networks. The privacy of DHCPv6 servers and relay agents are considered
      less important as they are typically open for public services. And, it
      is generally assumed that relay agent to server communication is
      protected from casual snooping, as that communication occurs in the
      provider's backbone. Nevertheless, the topics involving relay agents and
      servers are explored to some degree. However, future work may want to
      further explore privacy of DHCPv6 servers and relay agents.</t>
    </section>

    <section title="Terminology">
      <t>Naming convention from <xref target="RFC3315"></xref> and related is
      used throughout this document. In addition the following terminology is
      used: <list hangIndent="8" style="hanging">
          <t hangText="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, and a hostname. Some identifiers may be
          considered stable only under certain conditions, for example one
          client implementation may keep its client-id stored in stable
          storage while another may generate it on the fly and use a different
          one after each boot. Stable identifiers may or may not be globally
          unique.</t>
        </list></t>
    </section>

    <section anchor="identifiers" title="Identifiers in DHCPv6 options and fields">
      <t>In DHCPv6, there are many options that include identification
      information or that can be used to extract identification
      information about the client. This section enumerates various
      options or fields and the identifiers conveyed in them, which
      can be used to disclose client identification. The attacks that
      are enabled by such disclosures are detailed in Section 5.</t>

      <section anchor="src-addr" title="Source IPv6 address">

        <t>Although IPv6 link-local address is technically not a part
        of DHCPv6, it appears in the DHCPv6 transmissions, so it is
        mentioned here for completeness.</t>

        <t>If the client does not use privacy extensions
        (see <xref target="RFC4941"/>) or similar solutions and its
        IPv6 link-local address is based on physical link-layer
        address, this information is disclosed to the DHCPv6 server
        and to anyone who manages to intercept this transmission.</t>

        <t>There are multiple cases where IPv6 link-local addresses
        are used in DHCPv6. Initial client transmissions are always
        sent from the IPv6 link-local addresses even when the server
        unicast option (see Sections 22.12 and 18 of <xref
        target="RFC3315"/> for details) is enabled. If there are relay
        agents, they forward client's traffic wrapped in Relay-forward
        and store original source IPv6 address in peer-address field.</t>
      </section>

      <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, defined in Section 9.2 of <xref
        target="RFC3315"></xref> is DUID-LLT that is based on link-layer
        address. It 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 link-layer address
        randomization, it is likely that it was not yet enabled 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). The exposure of the original link-layer address in DUID will
        also undermine other privacy extensions such as <xref
        target="RFC4941"></xref>.</t>
      </section>

      <section title="Client Identifier 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). 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 for a client, each IA_NA, IA_TA and IA_PD options have an
        IAID field with a unique value for a given IA type. 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="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
        options 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 and Vendor-specific Information Options">
        <t>The Vendor Class option, defined in Section 22.16 of <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 Vendor-specific Information option, defined in Section 22.17 of
        <xref target="RFC3315"></xref>, includes enterprise number, which
        identifies the client's vendor and often includes a number of
        additional parameters that are specific to a given vendor. That may
        include any type of information the vendor deems useful. It should be
        noted that this information may be present (and different) in both
        directions: client to server and server to client communications.</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 deliver location information (the civic
        and postal addresses) from the DHCPv6 server to 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> are used by
        DHCPv6 server to provide coordinate-based geographic location
        information to DHCPv6 clients. They enable a DHCPv6 client to obtain
        its location.</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 by
        clients that must be booted using the network rather than from local
        storage, so the server can decide which boot file should be provided
        to the client.</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 DHCPv6 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 title="Relay Agent Options">
        <t>A DHCPv6 relay agent may include a number of options. Those option
        contain information that can be used to identify the client. Those
        options are almost exclusively exchanged between the relay agent and
        the server, thus never leaving the operators network. In particular,
        they're almost never present in the last wireless hop in case of WiFi
        networks. The only exception to that rule is somewhat infrequently
        used Relay Supplied Options option <xref target="RFC6422"></xref>.
        This fact implies that the threat model related relay options is
        slightly different. Traffic sniffing at the last hop and related class
        of attacks typically do not apply. On the other hand, all attacks that
        involve operator's intfrastructure (either willing or coerced
        cooperation or infrastructure being compromised) usually apply.</t>

        <t>The following subsections describe various options inserted by the
        relay agents.</t>

        <section title="Subscriber ID Option">
          <t>A DHCPv6 relay may include 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="Interface ID Option">
          <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 sometimes 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="Remote ID Option">
          <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 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="Relay-ID Option">
          <t>Relay agent may include Relay-ID <xref target="RFC5460"></xref>,
          which contains a unique relay agent identifier. While its intended
          use is to provide additional information for the server, so it would
          be able to respond to leasequeries later, this information can be
          also used to identify client's location within the network.</t>
        </section>
      </section>
    </section>

    <section title="Existing Mechanisms That Affect Privacy">
      <t>This section describes deployed DHCPv6 mechanisms that can affect
      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 a number of serious issues, both related to protocol and
        its implementations, that make temporary addresses 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, in section 18.1.3 it explicitly mentions that
        temporary addresses can be renewed. Client implementations may
        mistakenly renew temporary addresses if they are not careful (i.e., by
        including the IA_TA with the same IAID in Renew or Rebind requests,
        rather than a new IAID - see <xref target="RFC3315"></xref> Section
        22.5), thus forfeiting short liveness. <xref target="RFC4704"></xref>
        does not explicitly prohibit servers to update DNS for assigned
        temporary addresses and there are implementations that can be
        configured to do that. However, this is not advised as publishing a
        client's IPv6 address in DNS that is publicly available is a major
        privacy breach.</t>
      </section>

      <section title="DNS Updates">
        <t>The Client FQDN Option<xref target="RFC4704"></xref> used along
        with DNS Update <xref target="RFC2136"></xref> defines a mechanism
        that allows both clients and server to insert into the 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
        being 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 an available pool,
        it makes scanning attacks much easier.</t>

        <t>Identifier-based allocation - some server implementations use a
        fixed identifier for a specific client, seemingly taken from the
        client's MAC address when available or some lower bits of client's
        source IPv6 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 a
        returning client is very likely to get the same address, even if the
        server does not retain 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. 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
        attacks apply.</t>

        <t>Hash allocation - it's an extension of identifier-based allocation.
        Instead of using the identifier directly, it is 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 reversed), it introduces no improvement over
        identifier-based allocation. Even a well implemented hash does not
        mitigate the threat of correlation over time.</t>

        <t>Random allocation - a server can pick a resource pseudo-randomly
        out of an available pool. 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., the client's address may be
        randomized, but it still can leak its MAC address in the 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, Vendor-specific Information
        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 Vendor-specific Information 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 physical location information can be obtained by the attacker
        by many means. The most direct way to obtain this information is by
        looking into a message originating from the server that contains the
        Civic Location or GeoLoc option. It can also be indirectly inferred
        using the Remote ID option, 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>

        <t>Another way to discover client's physical location is to use
        geolocation services. Those services typically map IP prefixes into
        geographical locations. Those services are usually based on known
        locations of the subnet, so they may reveal client's location as
        precise as they can locate a network it is connected to. They usually
        are not able to discover specific physical location within a network.
        That is not awlays true and it depends on the quality of the apriori
        information available in the geolocation services databases. It should
        be noted that this threat is general to the DHCPv6 mechanism.
        Regardless of the allocation strategy used by the DHCPv6 server
        implementation, the addresses assigned will always belong to the
        subnet the server is configured to manage. Cases of using ULA (Unique
        Local Addresses) assigned by the DHCPv6 server are out of scope for
        this document.</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). <xref target="RFC3315"></xref> does not exclude
        IA_TA in such a case, so it is possible that a client implementation
        includes an address contained in an IA_TA for the Confirm message. 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 option 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>

        <t>It should be noted that in general case, the MAC addresses as such
        are not available in the DHCPv6 packets. Therefore they cannot be used
        directly in a reliable way. However, they may become indirectly
        available using other mechanisms: client-id contains link-local
        address if DUID-LL or DUID-LLT types are used, source IPv6 address may
        use EUI-64 that contains MAC address, some access technologies may
        specify MAC address in dedicated options (e.g., cable modems use MAC
        addresses in DOCSIS options). Relay agents may insert additional
        information that are used to help the server to identify the client.
        This could be Remote-Id option, Subscriber-Id option, client
        link-layer address option or vendor specific information options.
        Options inserted by relay agents usually traverse only relay-server
        path, so they typically can't be eavesdropped by intercepting client's
        transmissions. This depends on the actual deployment model and used
        access technologies.</t>
      </section>

      <section title="Pervasive monitoring">
        <t>Pervasive Monitoring (PM) is widespread (and often covert)
        surveillance through intrusive gathering of protocol
        artefacts, including application content, or protocol metadata
        such as headers.  Active or passive wiretaps and traffic
        analysis, (e.g., correlation, timing or measuring packet
        sizes), or subverting the cryptographic keys used to secure
        protocols can also be used as part of pervasive monitoring. PM
        is distinguished by being indiscriminate and very large scale,
        rather than by introducing new types of technical
        compromise. See <xref target="RFC7258"></xref> for a
        discussion about PM.</t>

        <t>In the DHCPv6 context, PM approach can be used to collect
        any identifiers discussed in <xref
        target="identifiers"/>. DHCPv4 and DHCPv6 are especially
        susceptible as the initial message sent (SOLICIT in case of
        DHCPv6) is one of the very first packets sent when visiting a
        network. Furthermore, in certain cases this packet can be
        logged even on networks that do not support IPv6 (some
        implementations initiate DHCPv6 even without receiving RA with
        M or O bits set). This may be an easily overlooked attack
        vector when IPv6-capable device connects to an IPv4 only
        network, gains only IPv4 connectivity, but still leaks its
        stable identifiers over DHCPv6.</t>

        <t>Using PM approach, attacks discussed in Sections 5.1, 5.2,
        5.3, 5.4, 5.5, 5.7, 5.8 and possibly 5.9 apply.</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) into the DNS, where it is easily
        accessible by anyone interested. Client ID is also disclosed, albeit
        in not easily accessible form (SHA-256 digest of the client-id). As
        SHA-256 is considered irreversible, DHCID can't be converted back to
        client-id. However, SHA-256 digest can be used as an 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 deduced by an attacker,
        the duration of the correlation attack extends to that of the
        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 the client's address and draw conclusions regarding its location
        and movement patterns based on the prefix it is connecting from) and
        active (an attacker can send ICMPv6 echo requests or other probe
        packets to networks of suspected client locations) can be used. To
        give specific example, by accessing a social portal from
        tomek-laptop.coffee.somecity.com.example,
        tomek-laptop.mycompany.com.example and tomek-laptop.myisp.example.com,
        the portal administrator can draw conclusions about tomek-laptop's
        owner's current location and his habits.</t>
      </section>

      <section title="Leasequery &amp; bulk leasequery">
        <t>Attackers may masquerade to be an access concentrator, either as a
        DHCPv6 relay agent or as a DHCPv6 client, to obtain location
        information directly from the DHCPv6 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 the DHCPv6 bulk leasequery <xref
        target="RFC5460"></xref> mechanism to obtain bulk information about
        DHCPv6 bindings, even without knowing the target bindings.</t>

        <t>Additionally, active leasequery <xref target="RFC7653"></xref> is a
        mechanism for subscribing to DHCPv6 lease update changes in near
        real-time. The intent of this mechanism is to update an operator's
        database, but if misused, an attacker could defeat the server's
        authentication mechanisms and subscribe to all updates. He then could
        continue receiving updates, without any need for local presence.</t>
      </section>
    </section>

    <section anchor="security" title="Security Considerations">
      <t>In current practice, the client privacy and client authentication are
      mutually exclusive. The client authentication procedure reveals
      additional client information in their certificates/identifiers. Full
      privacy for the clients may mean the clients are also anonymous to the
      server and the network.</t>
    </section>

    <section anchor="privacy-consider" title="Privacy Considerations">
      <t>This document in its entirety discusses privacy considerations in
      DHCPv6. As such, no dedicated discussion 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 thank Stephen Farrell, Ted Lemon,
      Ines Robles, Russ White, Christian Schaefer, Jinmei Tatuya,
      Bernie Volz, Marcin Siodelski, Christian Huitema, Brian
      Haberman, Robert Sparks, Peter Yee, Ben Campbell and other
      members of DHC WG for their valuable comments.</t>

      <t>This document was produced using the xml2rfc tool <xref
      target="RFC7749"></xref>.</t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      <?rfc include='reference.RFC.3315'?>

      <?rfc include='reference.RFC.6973'?>

      <?rfc include='reference.RFC.7258'?>

      <?rfc include='reference.I-D.ietf-6man-ipv6-address-generation-privacy'?>
    </references>

    <references title="Informative References">
      <?rfc include='reference.RFC.2136'?>

      <?rfc include='reference.RFC.3633'?>

      <?rfc include='reference.RFC.4580'?>

      <?rfc include='reference.RFC.4649'?>

      <?rfc include='reference.RFC.4704'?>

      <?rfc include='reference.RFC.4776'?>

      <?rfc include='reference.RFC.4941'?>

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

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

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

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

      <?rfc include='reference.RFC.6355'?>

      <?rfc include='reference.RFC.6422'?>

      <?rfc include='reference.RFC.6939'?>

      <?rfc include='reference.RFC.7653'?>

      <?rfc include='reference.RFC.7749'?>
    </references>
  </back>
</rfc>