draft-ietf-dhc-dhcp-privacy.xml

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<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>
<?rfc toc="yes"?>
<?rfc symrefs="yes"?>
<?rfc sortrefs="yes"?>
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<rfc category="info" docName="draft-ietf-dhc-dhcp-privacy-01"
     ipr="trust200902">
  <front>
    <title abbrev="DHCP Privacy considerations">Privacy considerations for
    DHCPv4</title>

    <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, 100095</city>

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

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

    <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>

    <date />

    <area>Internet</area>

    <workgroup>dhc</workgroup>

    <keyword>DHCP(v4) Privacy</keyword>

    <abstract>
      <t>DHCP is a protocol that is used to provide addressing and
      configuration information to IPv4 hosts. This document discusses the
      various identifiers used by DHCP and the potential privacy issues.</t>
    </abstract>
  </front>

  <middle>
    <section anchor="intro" title="Introduction">
      <t>Dynamic Host Configuration Protocol (DHCP) <xref
      target="RFC2131"></xref> is a protocol that is used to provide
      addressing and configuration information to IPv4 hosts. The DHCP
      protocol uses several identifiers that could become a source for
      gleaning information about the IPv4 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 DHCP and the potential privacy issues <xref
      target="RFC6973"></xref>. In particular, it also takes into consideration
      the problem of pervasive monitoring <xref target="RFC7258"/>.</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>The primary focus of this document is around privacy considerations for
      clients to support client mobility and connection to random networks. The
      privacy or DHCP 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 DHCP servers and relay agents.</t>
    </section>

    <section title="Requirements Language and Terminology">
      <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>

      <t>In addition the following terminology is used:
      <list hangIndent="8" style="hanging">
          <t hangText="Stable identifier"> - Any property disclosed by a DHCP
          client that does not change over time or changes very infrequently and
          is unique for said client in a given context.  Examples may include
          MAC address, client-id or 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 other may generate it on the fly and use a different one after
          each boot. Stable identifier may or may not be globally unique.</t>
      </list></t>
    </section>

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

      <section title="Client ID Option">
        <t>The Client Identifier Option <xref target="RFC2131"></xref> is used
        to pass an explicit client identifier to a DHCP server.</t>

        <t>The client identifier is an opaque key, which must be unique to
        that client within the subnet to which the client is attached. It
        typically remains stable after it has been initially generated. It may
        contain a hardware address, identical to the contents of the 'chaddr'
        field, or another type of identifier, such as a DNS name. <xref
        target="RFC3315"/> in Section 9.2 specifies DUID-LLT (Link-layer + time)
        as the recommended DUID type. <xref target="RFC4361"/>, Section 6.1
        introduces this concept to DHCPv4. Those two document recommend that
        client identifiers be generated by using the
        permanent link-layer address of the network interface that the client
        is trying to configure. <xref target="RFC4361"></xref> updates the
        recommendation of Client Identifiers to be "consists of a type field
        whose value is normally 255, followed by a four-byte IA_ID field,
        followed by the DUID for the client as defined in RFC 3315, section
        9". This does not change the lifecycle of the Client Identifiers.
        Clients are expected to generate their Client Identifiers once (during
        first operation) and store it in a non-volatile storage or use the
        same deterministic algorithm to generate the same Client Identifier
        values again.</t>
      </section>

      <section title="Address Fields &amp; Options">
        <t>The 'yiaddr' field <xref target="RFC2131"></xref> in DHCP message
        is used to allocate address from the server to the client.</t>

        <t>The DHCPv4 specification <xref target="RFC2131"></xref> provides a
        way to specify the client link-layer address in the DHCPv4 message
        header. A DHCPv4 message header has 'htype' and 'chaddr' fields to
        specify the client link-layer address type and the link-layer address,
        respectively. The 'chaddr' field is used both as a hardware address
        for transmission of reply messages and as a client identifier.</t>

        <t>The 'requested IP address' option <xref target="RFC2131"></xref> is
        used by client to suggest that a particular IP address be
        assigned.</t>
      </section>

      <section title="Client FQDN Option">
        <t>The Client Fully Qualified Domain Name (FQDN) option <xref
        target="RFC4702"></xref> is used by DHCP 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 DHCP server for the IP-address-to-FQDN mapping. In most case
        a client sends its hostname as a hint for the server. The DHCP 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="Parameter Request List Option">
        <t>The Parameter Request List option <xref target="RFC2131"></xref> is
        used to inform the server about options the client wants the server to
        send to the client. The content of a Parameter Request List option are
        the option codes for an option requested by the client.</t>
      </section>

      <section title="Vendor Class and Vendor-Identifying Vendor Class Options">
        <t>The Vendor Class option <xref target="RFC2131"></xref>, the
        Vendor-Identifying Vendor Class option and Vendor-Identifying Vendor
        Information option <xref target="RFC3925"></xref> are used by the DHCP
        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 the details of
        the hardware configuration of the host on which the client is running,
        or of industry consortium compliance, 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>DHCP servers use the Civic Location Option <xref
        target="RFC4776"></xref> to delivery of the location information (the
        civic and postal addresses) to the DHCP clients. It may refer to three
        locations: the location of the DHCP 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 GeoConf and GeoLoc options <xref target="RFC6225"></xref> is
        used by DHCP server to provide the coordinate-based geographic
        location information to the DHCP clients. It enables a DHCP client to
        obtain its geographic location.</t>

        <t>After the relevant DHCP 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="RFC4578"></xref> is used by DHCP client to send a list of
        supported architecture types to the DHCP 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 Option and Client Machine 
        Identifier Option seems not in use. Together with the Client System 
        Architecture Type Option, they are used for the Intel Preboot eXecution 
        Environment (PXE). Client Network Interface Identifier Option provide 
        information about the level of UNDI support.--></t>
      </section>

      <section title="Relay Agent Information Option and Sub-options">
        <t>A DHCP relay agent includes a Relay Agent Information <xref
        target="RFC3046"></xref> to identify the remote host end of the
        circuit. It contains a "circuit ID" sub-option for the incoming
        circuit, which is an agent-local identifier of the circuit from which
        a DHCP client-to-server packet was received, and a "remote ID"
        sub-option which provides a trusted identifier for the remote
        high-speed modem.</t>

        <t>Possible encoding of "circuit ID" sub-option includes: router
        interface number, switching hub port number, remote access server port
        number, frame relay DLCI, ATM virtual circuit number, cable data
        virtual circuit number, etc.</t>

        <t>Possible encoding of the "remote ID" sub-option includes: a "caller
        ID" telephone number for dial-up connection, a "user name" prompted
        for by a remote access server, a remote caller ATM address, a "modem
        ID" of a cable data modem, the remote IP address of a point-to-point
        link, a remote X.25 address for X.25 connections, etc.</t>

        <t>The link-selection sub-option <xref target="RFC3527"></xref> is
        used by any DHCP relay agent that desires to specify a subnet/link for
        a DHCP client request that it is relaying but needs the subnet/link
        specification to be different from the IP address the DHCP server
        should use when communicating with the relay agent. It contains an IP
        address, which can identify the client's subnet/link. Also, assuming
        network topology knowledge, it also reveals client location.</t>

        <t>A DHCP relay includes a Subscriber-ID option <xref target="RFC3993"/>
        to associate some provider-specific information with clients' DHCP
        messages that is independent of the physical network configuration
        through which the subscriber is connected. The "subscriber-id" assigned
        by the provider is intended to be stable as customers connect through
        different paths, and as network changes occur. The Subscriber-ID is an
        ASCII string, which is assigned and configured by the network
        provider.</t>

      </section>

    </section>

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

      <section title="DNS Updates">
        <t>DNS Updates <xref target="RFC4702"></xref> defines a mechanism that
        allows both clients and server to insert into DNS domain information
        about clients. Both forward (A) and reverse (PTR) resource records
        can be updated. This allows other nodes to conveniently refer to a
        host, despite the fact that its IP 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 be used to correlate the client
        across different network attachments even when its IP addresses keep
        changing.</t>
      </section>

      <section title="Allocation strategies">
        <t>A DHCP server running in typical, stateful mode is given a task of
        managing one or more pools of IP address. When a client
        requests an address, the server must pick an address out of configured
        pool. Depending on the server's implementation, various allocation
        strategies are possible. Choices in this regard may have privacy
        implications. Note that the constraints in DHCPv4 and DHCPv6 are
        radically different, but servers that allow allocation strategy
        configuration may allow configuring them in both DHCPv4 and DHCPv6.
        Not every allocation strategy is equally suitable for DHCPv4 and for
        DHCPv6.</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
        allocated addresses very predictable. Also, since the addresses
        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. client
        identifier or MAC address. It is also convenient, as returning client
        is very likely to get the same address. Those properties are
        convenient for system administrators, so DHCP server implementors are
        often requested to implement it. On the other hand, the downside of
        such allocation is that the client has a very stable IP address. That
        means that correlation of activities over time, location tracking,
        address scanning and OS/vendor discovery apply. This is certainly an
        issue in DHCPv6, but due to much smaller address space is almost never
        a problem in DHCPv4.</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 again. On the
        other hand, it is beneficial from privacy perspective as addresses
        generated that way are not susceptible to correlation attacks,
        OS/vendor discovery attacks or identity discovery attacks. Note that
        even though the address 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>

        <t>Given the limited size of most IPv4 public address pools, allocation
        mechanisms in IPv4 may not provide much privacy protection or leak much
        useful information, if misused.</t>
      </section>
    </section>

    <section title="Attacks">
      <section title="Device type discovery">
        <t>The type of device used by the client can be guessed by the
        attacker using the Vendor Class Option, the 'chaddr' field, and by
        parsing the Client ID Option. All of those options may contain an
        Organizationally Unique Identifier (OUI) that represents the device's
        vendor. That knowledge can be used for device-specific vulnerability
        exploitation attacks.</t>
      </section>

      <section title="Operating system discovery">
        <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 Parameter Request List option.</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 message originating from the server that contains the Civic Location,
        GeoConf, or GeoLoc options. It can also be indirectly inferred using the
        Relay Agent Information option, with the remote ID sub-option, the
        circuit 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 DHCP 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 in the requested IP
        address option. 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 DHCP 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 DHCP 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 DHCP deployments use DNS Updates <xref
        target="RFC4702"></xref> that put client's information (current IP
        address, client's hostname) into 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 IP 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.</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 hostname being put into
        DNS, it can be used for tracking user. 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
        address it is connecting from) and active (attacker can send ICMP echo
        requests or other probe packets to networks of suspected client
        locations) methods 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
        current location and his habits.</t>
      </section>

      <section title="Leasequery &amp; bulk leasequery">
        <t>Attackers may pretend as an access concentrator, either DHCP relay
        agent or DHCP client, to obtain location information directly from the
        DHCP server(s) using the DHCP leasequery <xref
        target="RFC4388"></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 DHCP bulk leasequery <xref
        target="RFC6926"></xref> mechanism to obtain bulk information about
        DHCP bindings, even without knowing the target bindings.</t>

        <t>Additionally, active leasequery <xref
        target="I-D.ietf-dhc-dhcpv4-active-leasequery" /> is a mechanism for
        subscribing to DHCPv4 lease update changes in near real-time. The
        intent of this mechanism is to update operator's database, but if
        misused, an attacker could defeat 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 the 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 for the
      server and the network.</t>
    </section>

    <section anchor="privacy-consider" title="Privacy Considerations">
      <t>This document at its entirety discusses privacy considerations in
      DHCP. As such, no dedicated 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 Huitema,
      Bernie Volz 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">
      <?rfc include='reference.RFC.2119'?>

      <?rfc include='reference.RFC.2131'?>

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

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

    <references title="Informative References">

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

      <?rfc include='reference.RFC.3046'?>

      <?rfc include='reference.RFC.3315'?>

      <?rfc include='reference.RFC.3527'?>

      <?rfc include='reference.RFC.3925'?>

      <?rfc include='reference.RFC.3993'?>

      <?rfc include='reference.RFC.4361'?>

      <?rfc include='reference.RFC.4388'?>

      <?rfc include='reference.RFC.4578'?>

      <?rfc include='reference.RFC.4702'?>

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

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

      <?rfc include='reference.RFC.6926'?>

      <?rfc include='reference.I-D.ietf-dhc-dhcpv4-active-leasequery'?>
      
</references>
  </back>
</rfc>