DHCPv4 Client ID: Node-specific Client Identifiers for Dynamic Host Configuration Protocol Version Four (DHCPv4)

Network Working Group                                           T. Lemon
Request for Comments: 4361                                       Nominum
Updates: 2131, 2132, 3315                                 B. Sommerfield
Category: Standards Track                               Sun Microsystems
                                                           February 2006


                  Node-specific Client Identifiers for
       Dynamic Host Configuration Protocol Version Four (DHCPv4)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document specifies the format that is to be used for encoding
   Dynamic Host Configuration Protocol Version Four (DHCPv4) client
   identifiers, so that those identifiers will be interchangeable with
   identifiers used in the DHCPv6 protocol.  This document also
   addresses and corrects some problems in RFC 2131 and RFC 2132 with
   respect to the handling of DHCP client identifiers.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................2
   3. Applicability ...................................................2
   4. Problem Statement ...............................................3
      4.1. Client identities are ephemeral. ...........................3
      4.2. Clients can accidentally present multiple identities. ......3
      4.3. RFC 2131/2132 and RFC 3315 identifiers are incompatible. ...4
      4.4. RFC 2131 does not require the use of a client identifier. ..4
   5. Requirements ....................................................4
   6. Implementation ..................................................6
      6.1. DHCPv4 Client Behavior .....................................6
      6.2. DHCPv6 Client Behavior .....................................7
      6.3. DHCPv4 Server Behavior .....................................7
      6.4. Changes from RFC 2131 ......................................8
      6.5. Changes from RFC 2132 ......................................9
   7. Notes on DHCP Clients in Multi-stage Network Booting ............9
   8. Security Considerations ........................................10
   9. References .....................................................10
      9.1. Normative References ......................................10
      9.2. Informative References ....................................10

1.  Introduction

   This document specifies the way in which Dynamic Host Configuration
   Protocol Version 4 [RFC2131] clients should identify themselves.
   DHCPv4 client implementations that conform to this specification use
   a DHCP Unique Identifier (DUID) as specified in Dynamic Host
   Configuration Protocol for IPv6 (DHCPv6) [RFC3315].  The DUID is
   encapsulated in a DHCPv4 client identifier option, as described in
   "DHCP Options and BOOTP Vendor Extensions" [RFC2132].  The behaviour
   described here supersedes the behavior specified in RFC2131 and
   RFC2132.

   The reason for making this change is that as we make the transition
   from IPv4 to IPv6, there will be network devices that must use both
   DHCPv4 and DHCPv6.  Users of these devices will have a smoother
   network experience if the devices identify themselves consistently,
   regardless of the version of DHCP they are using at any given moment.
   Most obviously, DNS updates made by the DHCP server on behalf of the
   client will be handled more correctly.  This change also addresses
   certain limitations in the functioning of RFC 2131/2132-style DHCP
   client identifiers.

   This document first describes the problem to be solved.  It then
   states the new technique that is to be used to solve the problem.
   Finally, it describes the specific changes that one would have to
   make to RFC 2131 and RFC 2132 in order for those documents not to
   contradict what is described in this document.

2.  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 RFC 2119 [RFC2119].

3.  Applicability

   This document updates RFC 2131 and RFC 2132.  This document also
   specifies behavior that is required of DHCPv4 and DHCPv6 clients that
   are intended to operate in a dual-stack configuration.  Finally, this
   document recommends behavior for host configurations where more than
   one DHCP client must operate in sequence in order to fully configure
   the client (e.g., a network boot loader and the operating system it
   loads).

   DHCPv4 clients and servers that are implemented according to this
   document should be implemented as if the changes specified in
   sections 6.3 and 6.4 have been made to RFC 2131 and RFC 2132.  DHCPv4
   clients should, in addition, follow the behavior specified in section
   6.1.  DHCPv6 clients should follow the behavior specified in section
   6.2.  DHCPv4 servers should additionally follow the behavior
   specified in section 6.3.

4.  Problem Statement

4.1.  Client identities are ephemeral.

   RFC 2132 recommends that client identifiers be generated by using the
   permanent link-layer address of the network interface that the client
   is trying to configure.  One result of this recommendation is that
   when the network interface hardware on a client computer is replaced,
   the identity of the client changes.  The client loses its IP address
   and any other resources associated with its old identifier (for
   example, its domain name as published through the DHCPv4 server).

4.2.  Clients can accidentally present multiple identities.

   Consider a DHCPv4 client that has two network interfaces, one of
   which is wired and one of which is wireless.  The DHCPv4 client will
   succeed in configuring either zero, one, or two network interfaces.
   Under the current specification, each network interface will receive
   a different IP address.  The DHCPv4 server will treat each network
   interface as a completely independent DHCPv4 client, on a completely
   independent host.

   Thus, when the client presents some information to be updated in a
   network directory service, such as the DNS, the name that is
   presented will be the same on both interfaces, but the identity that
   is presented will be different.  What will happen is that one of the
   two interfaces will get the name, and will retain that name as long
   as it has a valid lease, even if it loses its connection to the
   network, while the other network interface will never get the name.
   In some cases, this will achieve the desired result; when only one
   network interface is connected, sometimes its IP address will be
   published.  In some cases, the one connected interface's IP address
   will not be the one that is published.  When there are two
   interfaces, sometimes the correct one will be published, and
   sometimes not.

   This is likely to be a particular problem with modern laptops, which
   usually have built-in wireless ethernet and wired ethernet.  When the
   user is near a wired outlet, he or she may want the additional speed
   and privacy provided by a wired connection, but that same user may
   unplug from the wired network and wander around, still connected to
   the wireless network.  When a transition like this happens, under the
   current scheme, if the address of the wired interface is the one that
   gets published, this client will be seen by hosts attempting to
   connect to it as if it has intermittent connectivity, even though it
   actually has continuous network connectivity through the wireless
   port.

   Another common case of a duplicate identity being presented occurs
   when a boot monitor such as a Pre-Boot Execution Environment (PXE)
   loader specifies one DHCP client identifier, and then the operating
   system loaded by the boot loader specifies a different identity.

4.3.  RFC 2131/2132 and RFC 3315 identifiers are incompatible.

   The 'client identifier' option is used by DHCPv4 clients and servers
   to identify clients.  In some cases, the value of the 'client
   identifier' option is used to mediate access to resources (for
   example, the client's domain name, as published through the DHCPv4
   server).  RFC 2132 and RFC 3315 specify different methods for
   deriving client identifiers.  These methods guarantee that the DHCPv4
   and DHCPv6 identifiers will be different.  This means that mediation
   of access to resources using these identifiers will not work
   correctly in cases where a node may be configured using DHCPv4 in
   some cases and DHCPv6 in other cases.

4.4.  RFC 2131 does not require the use of a client identifier.

   RFC 2131 allows the DHCPv4 server to identify clients either by using
   the client identifier option sent by the client or, if the client did
   not send one, the client's link-layer address.  Like the client
   identifier format recommended by RFC 2131, this suffers from the
   problems previously described in sections 4.2 and 4.3.

5.  Requirements

   In order to address the problems stated in section 4, DHCPv4 client
   identifiers must have the following characteristics:

   - They must be persistent, in the sense that a particular host's
     client identifier must not change simply because a piece of network
     hardware is added or removed.

   - It must be possible for the client to represent itself as having
     more than one network identity, for example, so that a client with
     two network interfaces can express to the DHCPv4 server that these
     two network interfaces are to receive different IP addresses, even
     if they happen to be connected to the same link.

   - In cases where the DHCPv4 client is expressing more than one
     network identity at the same time, it must nevertheless be possible
     for the DHCPv4 server to determine that the two network identities
     belong to the same host.

   - In some cases it may be desirable for a DHCP client to present the
     same identity on two interfaces, so that if they both happen to be
     connected to the same network, they will both receive the same IP
     address.  In such cases, it must be possible for the client to use
     exactly the same identifier for each interface.

   - DHCPv4 servers that do not conform to this specification, but that
     are compliant with the older client identifier specification, must
     correctly handle client identifiers sent by clients that conform to
     this specification.

   - DHCPv4 servers that do conform to this specification must
     interoperate correctly with DHCPv4 clients that do not conform to
     this specification, except that when configuring such clients,
     behaviors such as those described in section 2 may occur.

   - The use by DHCPv4 clients of the chaddr field of the DHCPv4 packet
     as an identifier must be deprecated.

   - DHCPv4 client identifiers used by dual-stack hosts that also use
     DHCPv6 must use the same host identification string for both DHCPv4
     and DHCPv6.  For example, a DHCPv4 server that uses the client's
     identity to update the DNS on behalf of a DHCPv4 client must
     register the same client identity in the DNS that would be
     registered by the DHCPv6 server on behalf of the DHCPv6 client
     running on that host, and vice versa.

   In order to satisfy all but the last of these requirements, we need
   to construct a DHCPv4 client identifier out of two parts.  One part
   must be unique to the host on which the client is running.  The other
   must be unique to the network identity being presented.  The DHCP
   Unique Identifier (DUID) and Identity Association Identifier (IAID)
   specified in RFC 3315 satisfy these requirements.

   In order to satisfy the last requirement, we must use the DUID to
   identify the DHCPv4 client.  So, taking all the requirements
   together, the DUID and IAID described in RFC 3315 are the only
   possible solution.

   By following these rules, a compliant DHCPv4 client will interoperate
   correctly with both compliant and non-compliant DHCPv4 servers.  A
   non-compliant DHCPv4 client will also interoperate correctly with a
   compliant DHCPv4 server.  If either server or client is not
   compliant, the goals stated in the document are not met, but there is
   no loss of functionality.

6.  Implementation

   Here we specify changes to the behavior of DHCPv4 clients and
   servers.  We also specify changes to the wording in RFC 2131 and RFC
   2132.  DHCPv4 clients, servers, and relay agents that conform to this
   specification must implement RFC 2131 and RFC 2132 with the wording
   changes specified in sections 6.3 and 6.4.

6.1.  DHCPv4 Client Behavior

   DHCPv4 clients conforming to this specification MUST use stable
   DHCPv4 node identifiers in the dhcp-client-identifier option.  DHCPv4
   clients MUST NOT use client identifiers based solely on layer two
   addresses that are hard-wired to the layer two device (e.g., the
   ethernet MAC address) as suggested in RFC 2131, except as allowed in
   section 9.2 of RFC 3315.  DHCPv4 clients MUST send a 'client
   identifier' option containing an Identity Association Unique
   Identifier, as defined in section 10 of RFC 3315, and a DHCP Unique
   Identifier, as defined in section 9 of RFC 3315.  These together
   constitute an RFC 3315-style binding identifier.

   The general format of the DHCPv4 'client identifier' option is
   defined in section 9.14 of RFC 2132.

   To send an RFC 3315-style binding identifier in a DHCPv4 'client
   identifier' option, the type of the 'client identifier' option is set
   to 255.  The type field is immediately followed by the IAID, which is
   an opaque 32-bit quantity.  The IAID is immediately followed by the
   DUID, which consumes the remaining contents of the 'client
   identifier' option.  The format of the 'client identifier' option is
   as follows:

       Code  Len  Type  IAID                DUID
       +----+----+-----+----+----+----+----+----+----+---
       | 61 | n  | 255 | i1 | i2 | i3 | i4 | d1 | d2 |...
       +----+----+-----+----+----+----+----+----+----+---

   Any DHCPv4 client that conforms to this specification SHOULD provide
   a means by which an operator can learn what DUID the client has
   chosen.  Such clients SHOULD also provide a means by which the
   operator can configure the DUID.  A device that is normally
   configured by both a DHCPv4 and DHCPv6 client SHOULD automatically
   use the same DUID for DHCPv4 and DHCPv6 without any operator
   intervention.

   DHCPv4 clients that support more than one network interface SHOULD
   use the same DUID on every interface.  DHCPv4 clients that support
   more than one network interface SHOULD use a different IAID on each
   interface.

   A DHCPv4 client that generates a DUID and that has stable storage
   MUST retain this DUID for use in subsequent DHCPv4 messages, even
   after an operating system reboot.

6.2.  DHCPv6 Client Behavior

   Any DHCPv6 client that conforms to this specification SHOULD provide
   a means by which an operator can learn what DUID the client has
   chosen.  Such clients SHOULD also provide a means by which the
   operator can configure the DUID.  A device that is normally
   configured by both a DHCPv4 and DHCPv6 client SHOULD automatically
   use the same DUID for DHCPv4 and DHCPv6 without any operator
   intervention.

6.3.  DHCPv4 Server Behavior

   This document does not require any change to DHCPv4 or DHCPv6 servers
   that follow RFC 2131 and RFC 2132.  However, some DHCPv4 servers can
   be configured not to conform to RFC 2131 and RFC 2132, in the sense
   that they ignore the 'client identifier' option and use the client's
   hardware address instead.

   DHCPv4 servers that conform to this specification MUST use the
   'client identifier' option to identify the client if the client sends
   it.

   DHCPv4 servers MAY use administrator-supplied values for chaddr and
   htype to identify the client in the case where the administrator is
   assigning a fixed IP address to the client, even if the client sends
   a client identifier option.  This is ONLY permitted in the case where
   the DHCPv4 server administrator has provided the values for chaddr
   and htype, because in this case if it causes a problem, the
   administrator can correct the problem by removing the offending
   configuration information.

6.4.  Changes from RFC 2131

   In section 2 of RFC 2131, on page 9, the text that reads "; for
   example, the 'client identifier' may contain a hardware address,
   identical to the contents of the 'chaddr' field, or it may contain
   another type of identifier, such as a DNS name" is deleted.

   In section 4.2 of RFC 2131, the text "The client MAY choose to
   explicitly provide the identifier through the 'client identifier'
   option.  If the client supplies a 'client identifier', the client
   MUST use the same 'client identifier' in all subsequent messages, and
   the server MUST use that identifier to identify the client.  If the
   client does not provide a 'client identifier' option, the server MUST
   use the contents of the 'chaddr' field to identify the client." is
   replaced by the text "The client MUST explicitly provide a client
   identifier through the 'client identifier' option.  The client MUST
   use the same 'client identifier' option for all messages."

   In the same section, the text "Use of 'chaddr' as the client's unique
   identifier may cause unexpected results, as that identifier may be
   associated with a hardware interface that could be moved to a new
   client.  Some sites may choose to use a manufacturer's serial number
   as the 'client identifier', to avoid unexpected changes in a client's
   network address due to transfer of hardware interfaces among
   computers.  Sites may also choose to use a DNS name as the 'client
   identifier', causing address leases to be associated with the DNS
   name rather than a specific hardware box." is replaced by the text
   "The DHCP client MUST NOT rely on the 'chaddr' field to identify it."

   In section 4.4.1 of RFC 2131, the text "The client MAY include a
   different unique identifier" is replaced with "The client MUST
   include a unique identifier".

   In section 3.1, items 4 and 6; section 3.2 item 3 and 4; and section
   4.3.1, where RFC 2131 says that 'chaddr' may be used instead of the
   'client identifier' option, the text "or 'chaddr'" and "'chaddr' or"
   is deleted.

   Note that these changes do not relieve the DHCPv4 server of the
   obligation to use 'chaddr' as an identifier if the client does not
   send a 'client identifier' option.  Rather, they oblige clients that
   conform with this document to send a 'client identifier' option, and
   not rely on 'chaddr' for identification.  DHCPv4 servers MUST use
   'chaddr' as an identifier in cases where 'client identifier' is not
   sent, in order to support old clients that do not conform with this
   document.

6.5.  Changes from RFC 2132

   The text in section 9.14, beginning with "The client identifier MAY
   consist of" through "that meet this requirement for uniqueness." is
   replaced with "the client identifier 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."  The
   text "its minimum length is 2" in the following paragraph is deleted.

7.  Notes on DHCP Clients in Multi-stage Network Booting

   In some cases a single device may actually run more than one DHCP
   client in sequence, in the process of loading an operating system
   over the network.  In such cases, it may be that the first-stage boot
   uses a different client identifier, or no client identifier, than the
   subsequent stage or stages.

   The effect of this, under the DHCPv4 protocol, is that the two (in
   some cases more than two!) boot stages will present different
   identities.  A DHCPv4 server will therefore allocate two different IP
   addresses to the two different boot stages.

   Some DHCP servers work around this problem for the common case where
   the boot Programmable Read Only Memory (PROM) presents no client
   identifier, and the operating system DHCP client presents a client
   identifier constructed from the Message Authentication Code (MAC)
   address of the network interface -- both are treated as the same
   identifier.  This prevents the consumption of an extra IP address.

   A compliant DHCPv4 client does not use a client identifier
   constructed from the MAC address of the network interface, because
   network interfaces are not stable.  So a compliant DHCPv4 client
   cannot be supported by a simple hack like the one described
   previously; this may have some significant impact at some sites.

   We cannot state the solution to this problem as a set of
   requirements, because the circumstances in which this occurs vary too
   widely.  However, we can make some suggestions.

   First, we suggest that DHCP clients in network boot loaders request
   short lease times, so that their IP addresses are not retained.  Such
   clients should send a DHCPRELEASE message to the DHCP server before
   moving on to the next stage of the boot process.  Such clients should
   provide a way for the operating system DHCP client to configure a
   DUID to use in subsequent boots.  DHCP clients in the final stage
   should, where possible, configure the DUID used by the boot PROM to
   be the same as the DUID used by the operating system.

   Second, implementors of DHCPv4 clients that are expected to only be
   used in a multi-stage network boot configuration, that are not
   expected ever to network boot using DHCPv6, and that have a MAC
   address that cannot be easily changed may not need to implement the
   changes described in this specification.  There is some danger in
   making this assumption--the first solution suggested is definitely
   better.  A compromise might be to have the final-stage DHCP client
   detect whether it is running on legacy hardware; if it is, it uses
   the old identifier; if it is not, it follows the scheme described in
   the previous paragraph.

8.  Security Considerations

   This document raises no new security issues.  Potential exposure to
   attack in the DHCPv4 protocol is discussed in section 7 of the DHCP
   protocol specification [RFC2131] and in Authentication for DHCP
   messages [RFC3118].  Potential exposure to attack in the DHCPv6
   protocol is discussed in section 23 of RFC 3315.

9.  References

9.1.  Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
             March 1997.

   [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
             Extensions", RFC 2132, March 1997.

   [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and
             M. Carney, "Dynamic Host Configuration Protocol for IPv6
             (DHCPv6)", RFC 3315, July 2003.

9.2.  Informative References

   [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
             Messages", RFC 3118, June 2001.

Authors' Addresses

   Ted Lemon
   Nominum
   2385 Bay Road
   Redwood City, CA 94063 USA

   Phone: +1 650 381 6000
   EMail: mellon@nominum.com


   Bill Sommerfeld
   Sun Microsystems
   1 Network Drive
   Burlington, MA 01824

   Phone: +1 781 442 3458
   EMail: sommerfeld@sun.com

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