

Network Working Group                                           R. Droms
Internet-Draft                                             Cisco Systems
Expires: October 5, 2003                                   April 6, 2003


            A Guide to Implementing Stateless DHCPv6 Service
                 draft-ietf-dhc-dhcpv6-stateless-00.txt

Status of this Memo

      This document is an Internet-Draft and is in full conformance with
      all provisions of Section 10 of RFC2026.

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      This Internet-Draft will expire on October 5, 2003.

Copyright Notice

      Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

      Stateless DHCPv6 service is used by nodes to obtain configuration
      information such as the addresses of DNS recursive name servers
      that does not require the maintenance of any dynamic state for
      individual clients.  A node that uses stateless DHCP must have
      obtained its IPv6 addresses through some other mechanism,
      typically stateless address autoconfiguration.  This document is a
      guide to the protocol messages and options that must be
      implemented to provide stateless DHCPv6 service.






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   1. Introduction

      Nodes that have obtained IPv6 addresses through some other
      mechanism can use stateless DHCPv6 to obtain other configuration
      information such as a list of DNS recursive name servers or NTP
      servers.  A stateless DHCPv6 server provides only configuration
      information to nodes and does not perform any address assignment.
      Such a server is called "stateless" because it need not maintain
      any dynamic state for individual clients.

      While the DHCPv6 specification [1] defines more than 10 protocol
      messages and 20 options, only a subset of those messages and
      options are required for stateless DHCPv6 service.  This document
      gives guidelines about which messages and options are required for
      stateless DHCPv6 service.  The intended use of the document is to
      guide the efficient and complete implementation of clients and
      servers that use stateless DHCPv6 service.

      The operation of relay agents is the same for stateless and
      stateful DHCPv6 service.  The operation of relay agents is
      described in the DHCPv6 specification.

      Section 4 of this document lists the sections of the DHCPv6
      document that an implementor should read for an overview of the
      DHCPv6 specification and the basic requirements of a DHCPv6
      service.  Section 5 lists the specific messages and options that
      are specifically required for stateless DHCPv6 service.  Section 6
      describes how stateless and stateful DHCPv6 servers interact to
      provide service to clients that require address assignment and
      clients that require only stateless service.

   2. Terminology

      Throughout this document, "DHCP" refers to DHCP for IPv6.

      This document uses the terminology defined in RFC2460 [2], the
      DHCP specification, the DHCP DNS configuration options
      specification [3] and the DHCP NTP configuration options
      specification [4].

      "Stateless DHCP" refers to the use of DHCP to provide
      configuration information to clients that does not require the
      server to maintain dynamic state about the DHCP clients.

   3. Overview

      This document assumes that a node using stateless DHCP
      configuration is not using DHCP for address assignment, and that a



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      node has determined at least a link-local address as described in
      section 5.3 of RFC2461 [5]

      To obtain configuration parameters through stateless DHCP, a node
      uses the DHCP Information-request message.  DHCP servers respond
      to the node's message with a Reply message that carries the DNS
      configuration parameters.  The Reply message from the server can
      carry configuration information such as a list of DNS recursive
      name servers and NTP servers.

   4. Basic Requirements for Implementation of DHCP

      Several sections of the DHCP specification [1] provide background
      information or define parts of the specification that are common
      to all implementations:

      1-4:   give an introduction to DHCPv6 and an overview of DHCP
         message flows

      5:     defines constants used throughout the protocol
         specification

      6, 7:  illustrates the format of DHCP messages

      8:     describes the representation of Domain Names

      9:     defines the "DHCP unique identifier" (DUID) optionally used
         to identify DHCP participants

      13-16: describe DHCP message transmission, retransmission and
         validation

      21:    describes authentication for DHCP


   5. Implementation of stateless DHCP

      The client indicates that it is requesting configuration
      information by sending an Information-request message that
      includes an Option Request option specifying the options that it
      wishes to receive from the DHCP server.  For example, if the
      client is attempting to obtain DNS configuration information, it
      includes either or both of the DNS configuration options in the
      Information-request message.  The server determines the
      appropriate configuration parameters for the client based on its
      configuration policies and responds with a Reply message
      containing the requested parameters.  In this example, the server
      would respond with DNS configuration parameters.



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      A node uses the DUID option to identify itself to a server,
      because the server administrator may want to customize the
      server's response to each node, based on the node's identity.

   5.1 Messages required for stateless DHCP

      Clients and servers implement the following messages for stateless
      DHCP service; the section numbers in this list refer to the DHCPv6
      specification:

      Information-request: sent by a DHCP client to a server to request
         DNS configuration parameters (sections 18.1.5 and 18.2.5)

      Reply:               sent by a DHCP server to a client containing
         the DNS configuration parameters (sections 18.2.6 and 18.2.8)

      In addition, servers and relay agents implement the following
      messages for stateless DHCP service:

      Relay-forward: Sent by a DHCP relay agent to carry the client
         message to a server (section 15.13)

      Relay-reply:   Sent by a DHCP server to carry a response message
         to the relay agent (section 15.14)


   5.2 Options required for stateless DHCP service

      Clients and servers implement the following options for stateless
      DHCP service; the section numbers in this list refer to the DHCPv6
      specification:

      Option Request: specifies the configuration information that the
         client is requesting from the server (section 22.7)

      Status Code:    used to indicate completion status or other status
         information (section 22.13)

      Servers and relay agents implement the following options for
      stateless DHCP service; the section numbers in this list refer to
      the DHCPv6 specification:

      Client message: Sent by a DHCP relay agent in a Relay-forward
         message to carry the client message to a server (section 20)

      Server message: Sent by a DHCP server in a Relay-reply message to
         carry a response message to the relay agent (section 20)




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      Interface-ID:   Sent by the DHCP relay agent and returned by the
         server to identify the interface to use to forward a message to
         the client (section 22.18)


   5.3 Options used for configuration information

      Clients and servers use the following options to pass
      configuration information to clients:

      DNS Recursive Name Servers:  specifies the DNS recursive name
         servers [6] the client uses for name resolution; see "DNS
         Configuration options for DHCPv6"

      DNS search list:             specifies the domain names to be
         searched during name resolution; see "DNS Configuration options
         for DHCPv6"

      NTP Servers:                 specifies the NTP servers the client
         uses for synchronizing its clock; see "Time Configuration
         Options for DHCPv6"


   5.4 Other options used in stateless DHCP

      Clients and servers may implement the following options for
      stateless DHCP service; the section numbers in this list refer to
      the DHCPv6 specification [1]:

      Preference:     Sent by a DHCP server to indicate the preference
         level for the server (section 22.8)

      Elapsed time:   Sent by a DHCP client to indicate the time since
         the client began the DHCP configuration process (section 22.9)

      User Class:     Sent by a DHCP client to give additional
         information to the server for selecting configuration
         parameters for the client (section 22.15)

      Vendor Class:   Sent by a DHCP client to give additional
         information about the client vendor and hardware to the server
         for selecting configuration parameters for the client (section
         22.16)

      Vendor-specific Information: Sent by a DHCP server to pass
         information to clients in options defined by vendors (section
         22.17)




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      Client DUID:    Sent by a DHCP client to identify itself (section
         22.2).  Clients are not required to send this option; servers
         never send this option

      Authentication: Used to provide authentication of DHCP messages
         (section 21)


   6. Interaction with DHCP for Address Assignment

      In some networks, there may be both clients that are using
      stateless address autoconfiguration [7] and DHCP for DNS
      configuration and clients that are using DHCP for stateful address
      configuration.  Depending on the deployment and configuration of
      relay agents, DHCP servers that are intended only for stateless
      configuration may receive messages from clients that are
      performing stateful address configuration.

      A DHCP server that is only able to provide stateless configuration
      information through an Information-request/Reply message exchange
      discards any other DHCP messages it receives.  Specifically, the
      server discards any messages other than Information-Request or
      Relay-forward it receives, and the server does not participate in
      any stateful address configuration messages exchanges.  If there
      are other DHCP servers that are configured to provide stateful
      address assignment, one of those servers will provide the address
      assignment.

   7. Security Considerations

      Stateless DHCPv6 service is a proper subset of the DHCPv6 service
      described in the DHCPv6 specification [1].  Therefore, stateless
      DHCPv6 service introduces no additional security considerations
      beyond those discussed in sections 21, 22.11 and 23 of the DHCPv6
      specification.

      Configuration information provided to a node through stateless
      DHCPv6 service may be used to mount spoofing, man-in-the-middle,
      denial-of-service and other attacks.  These attacks are described
      in more detail in the specifications for each of the options that
      carry configuration information.  Authenticated DHCPv6, as
      described in sections 21 and 22.11 of the DHCPv6 specification,
      can be used to avoid attacks mounted through the stateless DHCPv6
      service.

      Usually, a node using stateless DHCPv6 service will have
      configured its interfaces with IPv6 addresses through stateless
      address autoconfiguration.  A node that has configured an



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      appropriate IPv6 address can use IPsec [8] to authenticate and
      secure DHCPv6 messages exchanged between the node and the DHCPv6
      server.

   8. Acknowledgments

      Jim Bound, Ted Lemon and Bernie Volz reviewed this document and
      contributed editorial suggestions.  Thanks to Pekka Savola and
      Christian Huitema for their review and comments.

Normative References

      [1]  Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and
           R. Droms (ed.), "Dynamic Host Configuration Protocol for IPv6
           (DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress),
           October 2002.

      [2]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
           (IPv6) Specification", RFC 2460, December 1998.

      [3]  Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and
           R. Droms, "DNS Configuration options for DHCPv6", draft-ietf-
           dhc-dhcpv6-opt-dnsconfig-01 (work in progress), October 2002.

      [4]  Vijayabhaskar, A., "Time Configuration Options for DHCPv6",
           draft-ietf-dhc-dhcpv6-opt-timeconfig-00 (work in progress),
           February 2002.

Informative References

      [5]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
           for IP Version 6 (IPv6)", RFC 2461, December 1998.

      [6]  Mockapetris, P., "Domain names - concepts and facilities",
           STD 13, RFC 1034, November 1987.

      [7]  Thomson, S. and T. Narten, "IPv6 Stateless Address
           Autoconfiguration", RFC 2462, December 1998.

      [8]  Kent, S. and R. Atkinson, "Security Architecture for the
           Internet Protocol", RFC 2401, November 1998.










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Author's Address

   Ralph Droms
   Cisco Systems
   300 Apollo Drive
   Chelmsford, MA  01824
   USA

   Phone: +1 978 497 4733
   EMail: rdroms@cisco.com









































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Acknowledgement

      Funding for the RFC Editor function is currently provided by the
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