dhc                                                             S. Jiang
Internet-Draft                              Huawei Technologies Co., Ltd
Intended status: Informational                               S. Krishnan
Expires: August 13, 2015 February 27, 2016                                      Ericsson
                                                            T. Mrugalski
                                                                     ISC
                                                        February 9,
                                                         August 26, 2015

                   Privacy considerations for DHCP
                     draft-ietf-dhc-dhcp-privacy-00 DHCPv4
                     draft-ietf-dhc-dhcp-privacy-01

Abstract

   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.

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   This Internet-Draft will expire on August 13, 2015. February 27, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language and Terminology . . . . . . . . . . . .   3
   3.  Identifiers in DHCP . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Client ID Option  . . . . . . . . . . . . . . . . . . . .   3   4
     3.2.  Address Fields & Options  . . . . . . . . . . . . . . . .   4
     3.3.  Subscriber-ID Option  . . . . . . . . . . . . . . . . . .   4
     3.4.  Relay Agent Information Option and Sub-options  . . . . .   4
     3.5.  Client FQDN Option  . . . . . . . . . . . . . . . . . . .   5
     3.6.   4
     3.4.  Parameter Request List Option . . . . . . . . . . . . . .   5
     3.7.
     3.5.  Vendor Class and Vendor-Identifying Vendor Class Options    5
     3.8.
     3.6.  Civic Location Option . . . . . . . . . . . . . . . . . .   6
     3.9.   5
     3.7.  Coordinate-Based Location Option  . . . . . . . . . . . .   6
     3.10.   5
     3.8.  Client System Architecture Type Option  . . . . . . . . .   6
     3.9.  Relay Agent Information Option and Sub-options  . . . . .   6
   4.  Existing Mechanisms That Affect Privacy . . . . . . . . . . .   6   7
     4.1.  DNS Updates . . . . . . . . . . . . . . . . . . . . . . .   6   7
     4.2.  Allocation strategies . . . . . . . . . . . . . . . . . .   7
   5.  Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Device type discovery . . . . . . . . . . . . . . . . . .   8
     5.2.  Operating system discovery  . . . . . . . . . . . . . . .   8   9
     5.3.  Finding location information  . . . . . . . . . . . . . .   8   9
     5.4.  Finding previously visited networks . . . . . . . . . . .   9
     5.5.  Finding a stable identity . . . . . . . . . . . . . . . .   9
     5.6.  Pervasive monitoring  . . . . . . . . . . . . . . . . . .   9
     5.7.  Finding client's IP address or hostname . . . . . . . . .   9
     5.8.  Correlation of activities over time . . . . . . . . . . .   9  10
     5.9.  Location tracking . . . . . . . . . . . . . . . . . . . .   9  10
     5.10. Leasequery & bulk leasequery  . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10  11
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10  11
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12  13

1.  Introduction

   Dynamic Host Configuration Protocol (DHCP) [RFC2131] 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 additional 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 [RFC6973].  In particular, it also takes
   into consideration the problem of pervasive monitoring [RFC7258].

   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.

   Editor notes: for now, the

   The primary focus of this document is mainly considering the around privacy
   of DHCP client. considerations
   for clients to support client mobility and connection to random
   networks.  The privacy of or DHCP server servers and relay agent agents are
   considered less important because 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, this future work may be a subject want to change if further study shows
   opposite result. explore privacy of
   DHCP servers and relay agents.

2.  Requirements Language and Terminology

   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 [RFC2119].  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
   [RFC2119] key words.

   In addition the following terminology is used:

   Stable identifier  any  - 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 that does not change 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.

3.  Identifiers in DHCP

   There are several identifiers used in DHCP.  This section provides an
   introduction to the various options that will be used further in the
   document.

3.1.  Client ID Option

   The Client Identifier Option [RFC2131] is used to pass an explicit
   client identifier to a DHCP 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).

   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.
   It is
   [RFC3315] in Section 9.2 specifies DUID-LLT (Link-layer + time) as
   the recommended DUID type.  [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.
   [RFC4361] 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 non-
   volatile storage or use the same deterministic algorithm to generate
   the same Client Identifier values again.

3.2.  Address Fields & Options

   The 'yiaddr' field [RFC2131] in DHCP message is used to allocate
   address from the server to the client.

   The DHCPv4 specification [RFC2131] 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.

   The 'requested IP address' option [RFC2131] is used by client to
   suggest that a particular IP address be assigned.

3.3.  Subscriber-ID  Client FQDN Option

   A DHCP relay includes a Subscriber-ID option [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.

3.4.  Relay Agent Information Option and Sub-options

   A DHCP relay agent includes a Relay Agent Information [RFC3046] 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.

   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.

   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.

   The link-selection sub-option [RFC3527] 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.

3.5.  Client FQDN Option

   The Client Fully Qualified Domain Name (FQDN)

   The Client Fully Qualified Domain Name (FQDN) option [RFC4702] 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.

   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.

3.6.

3.4.  Parameter Request List Option

   The Parameter Request List option [RFC2131] 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.

3.7.

3.5.  Vendor Class and Vendor-Identifying Vendor Class Options

   The Vendor Class option [RFC2131] and [RFC2131], the Vendor-Identifying Vendor
   Class option and Vendor-Identifying Vendor Information option
   [RFC3925] is are used by a the DHCP client to identify the vendor that
   manufactured the hardware on which the client is running.

   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.

3.8.

3.6.  Civic Location Option

   DHCP servers use the Civic Location Option [RFC4776] 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.

3.7.  Coordinate-Based Location Option

   The GeoConf and GeoLoc options [RFC6225] 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.

   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.

3.8.  Client System Architecture Type Option

   The Client System Architecture Type Option [RFC4578] 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.

3.9.  Relay Agent Information Option and Sub-options

   A DHCP servers use the Civic Location Option [RFC4776] relay agent includes a Relay Agent Information [RFC3046] to delivery of
   identify the location information (the civic and postal addresses) to remote host end of the DHCP
   clients. circuit.  It may refer to three locations: contains a "circuit
   ID" sub-option for the location incoming circuit, which is an agent-local
   identifier of the circuit from which a DHCP
   server, client-to-server packet
   was received, and a "remote ID" sub-option which provides a trusted
   identifier for the location remote high-speed modem.

   Possible encoding of the network element believed to be closest to
   the client, or the location "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.

   Possible encoding of the client, identified "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 "what"
   element within the option.

3.9.  Coordinate-Based Location Option remote IP address of a point-to-point
   link, a remote X.25 address for X.25 connections, etc.

   The GeoConf and GeoLoc options [RFC6225] link-selection sub-option [RFC3527] is used by any DHCP server relay
   agent that desires to
   provide specify a subnet/link for a DHCP client request
   that it is relaying but needs the coordinate-based geographic location information subnet/link specification to be
   different from the IP address the DHCP clients. server should use when
   communicating with the relay agent.  It enables a DHCP contains an IP address, which
   can identify the client's subnet/link.  Also, assuming network
   topology knowledge, it also reveals client to obtain its geographic location.

   After the relevant

   A DHCP exchanges have taken place, the location relay includes a Subscriber-ID option [RFC3993] to associate
   some provider-specific information with clients' DHCP messages that
   is stored on independent of the end device rather than somewhere else,
   where retrieving it might be difficult in practice.

3.10.  Client System Architecture Type Option

   The Client System Architecture Type Option [RFC4578] physical network configuration through which
   the subscriber is used connected.  The "subscriber-id" assigned by DHCP
   client to send a list of supported architecture types to the DHCP
   server.  It
   provider is used intended to provide configuration information for a node
   that must be booted using 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 rather than from local storage.
   provider.

4.  Existing Mechanisms That Affect Privacy

   This section describes available DHCP mechanisms that one can use to
   protect or enhance one's privacy.

4.1.  DNS Updates

   DNS Updates [RFC4704] [RFC4702] defines a mechanism that allows both clients
   and server to insert into DNS domain information about clients.  Both
   forward (AAAA) (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.

   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 be used to correlate the
   client across different network attachments even when its IP
   addresses keep changing.

4.2.  Allocation strategies

   A DHCP server running in typical, stateful mode is given a task of
   managing one or more pools of IP address resources. address.  When a client requests a resource, an
   address, the server must pick a resource 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.

   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 allocated addresses very predictable.  Also, since the resources addresses
   allocated tend to be clustered at the beginning of available pool, it
   makes scanning attacks much easier.

   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.

   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.

   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.

   Other allocation strategies may be implemented.

   However, giving

   Given the limited resource size of most IPv4 public address pool, pools, allocation mechanism
   mechanisms in IPv4 may not provide much protection, while
   in IPv6, the network has very large address space to distribute the
   address allocation. privacy protection or leak
   much useful information, if misused.

5.  Attacks

5.1.  Device type discovery

   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.

5.2.  Operating system discovery

   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.

5.3.  Finding location information

   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 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
   (e.g. using a telephone number),
   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).

5.4.  Finding previously visited networks

   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.

5.5.  Finding a stable identity

   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.

5.6.  Pervasive monitoring

   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.

5.7.  Finding client's IP address or hostname

   Many DHCP deployments use DNS Updates [RFC4702] that put client's
   information (current IP address, client's hostname). hostname) into DNS, where
   it is easily accessible by anyone interested.  Client ID is also
   disclosed, able it albeit in not easily accessible form (SHA-256 digest of
   the client-id).  Although  As SHA-256 is considered irreversible, so DHCID can't
   be converted back to client-id.  However, SHA-256 digest can be used
   as a an unique identifier that is accessible by any host.

5.8.  Correlation of activities over time

   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.

5.9.  Location tracking

   If a stable identifier is used for assigning an address and such
   mapping is discovered by an attacker. 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).
   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.

5.10.  Leasequery & bulk leasequery

   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 [RFC4388], [RFC6148] [RFC4388] mechanism.

   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.

   Furthermore, the attackers may use DHCP bulk leasequery [RFC6926]
   mechanism to obtain bulk information about DHCP bindings, even
   without knowing the target bindings.

   Additionally, active leasequery
   [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.

6.  Security Considerations

   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.

7.  Privacy Considerations

   This document at its entirety discusses privacy considerations in
   DHCP.  As such, no separate dedicated section about this is needed.

8.  IANA Considerations

   This draft does not request any IANA action.

9.  Acknowledgements

   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.

   This document was produced using the xml2rfc tool [RFC2629].

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997. 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997. 1997,
              <http://www.rfc-editor.org/info/rfc2131>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <http://www.rfc-editor.org/info/rfc6973>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <http://www.rfc-editor.org/info/rfc7258>.

10.2.  Informative References

   [I-D.ietf-dhc-dhcpv4-active-leasequery]
              Kinnear, K., Stapp, M., Volz, B., and N. Russell, "Active
              DHCPv4 Lease Query", draft-ietf-dhc-dhcpv4-active-
              leasequery-05 (work in progress), August 2015.

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              DOI 10.17487/RFC2629, June 1999,
              <http://www.rfc-editor.org/info/rfc2629>.

   [RFC3046]  Patrick, M., "DHCP Relay Agent Information Option",
              RFC 3046, DOI 10.17487/RFC3046, January 2001. 2001,
              <http://www.rfc-editor.org/info/rfc3046>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

   [RFC3527]  Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy,
              "Link Selection sub-option for the Relay Agent Information
              Option for DHCPv4", RFC 3527, DOI 10.17487/RFC3527, April 2003.
              2003, <http://www.rfc-editor.org/info/rfc3527>.

   [RFC3925]  Littlefield, J., "Vendor-Identifying Vendor Options for
              Dynamic Host Configuration Protocol version 4 (DHCPv4)",
              RFC 3925, DOI 10.17487/RFC3925, October 2004. 2004,
              <http://www.rfc-editor.org/info/rfc3925>.

   [RFC3993]  Johnson, R., Palaniappan, T., and M. Stapp, "Subscriber-ID
              Suboption for the Dynamic Host Configuration Protocol
              (DHCP) Relay Agent Option", RFC 3993,
              DOI 10.17487/RFC3993, March 2005. 2005,
              <http://www.rfc-editor.org/info/rfc3993>.

   [RFC4361]  Lemon, T. and B. Sommerfeld, "Node-specific Client
              Identifiers for Dynamic Host Configuration Protocol
              Version Four (DHCPv4)", RFC 4361, DOI 10.17487/RFC4361,
              February 2006. 2006, <http://www.rfc-editor.org/info/rfc4361>.

   [RFC4388]  Woundy, R. and K. Kinnear, "Dynamic Host Configuration
              Protocol (DHCP) Leasequery", RFC 4388,
              DOI 10.17487/RFC4388, February 2006. 2006,
              <http://www.rfc-editor.org/info/rfc4388>.

   [RFC4578]  Johnston, M. and S. Venaas, Ed., "Dynamic Host
              Configuration Protocol (DHCP) Options for the Intel
              Preboot eXecution Environment (PXE)", RFC 4578,
              DOI 10.17487/RFC4578, November 2006,
              <http://www.rfc-editor.org/info/rfc4578>.

   [RFC4702]  Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host
              Configuration Protocol (DHCP) Client Fully Qualified
              Domain Name (FQDN) Option", RFC 4702,
              DOI 10.17487/RFC4702, October 2006.

   [RFC4704]  Volz, B., "The Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
              Option", RFC 4704, October 2006. 2006,
              <http://www.rfc-editor.org/info/rfc4702>.

   [RFC4776]  Schulzrinne, H., "Dynamic Host Configuration Protocol
              (DHCPv4 and DHCPv6) Option for Civic Addresses
              Configuration Information", RFC 4776,
              DOI 10.17487/RFC4776, November 2006.

   [RFC6148]  Kurapati, P., Desetti, R., and B. Joshi, "DHCPv4 Lease
              Query by Relay Agent Remote ID", RFC 6148, February 2011. 2006,
              <http://www.rfc-editor.org/info/rfc4776>.

   [RFC6225]  Polk, J., Linsner, M., Thomson, M., and B. Aboba, Ed.,
              "Dynamic Host Configuration Protocol Options for
              Coordinate-Based Location Configuration Information",
              RFC 6225, DOI 10.17487/RFC6225, July 2011. 2011,
              <http://www.rfc-editor.org/info/rfc6225>.

   [RFC6926]  Kinnear, K., Stapp, M., Desetti, R., Joshi, B., Russell,
              N., Kurapati, P., and B. Volz, "DHCPv4 Bulk Leasequery",
              RFC 6926, DOI 10.17487/RFC6926, April 2013.

10.2.  Informative References

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [RFC4578]  Johnston, M. and S. Venaas, "Dynamic Host Configuration
              Protocol (DHCP) Options for the Intel Preboot eXecution
              Environment (PXE)", RFC 4578, November 2006.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973, July
              2013. 2013,
              <http://www.rfc-editor.org/info/rfc6926>.

Authors' Addresses

   Sheng Jiang
   Huawei Technologies Co., Ltd
   Q14, Huawei Campus, No.156 Beiqing Road
   Hai-Dian District, Beijing, 100095
   P.R. China

   Email: jiangsheng@huawei.com
   Suresh Krishnan
   Ericsson
   8400 Decarie Blvd.
   Town of Mount Royal, QC
   Canada

   Phone: +1 514 345 7900 x42871
   Email: suresh.krishnan@ericsson.com

   Tomek Mrugalski
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City, CA  94063
   USA

   Phone: +1 650 423 1345

   Email: tomasz.mrugalski@gmail.com