Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6
RFC 3736

Network Working Group R. Droms
Request for Comments: 3736 Cisco Systems
Category: Standards Track April 2004
 Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6
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 (2004). All Rights Reserved.
Abstract
 Stateless Dynamic Host Configuration Protocol service for IPv6
 (DHCPv6) 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 explains which parts of RFC 3315
 must be implemented in each of the different kinds of DHCP agents so
 that agent can support stateless DHCP.
1. Introduction
 Nodes that have obtained IPv6 addresses through some other mechanism,
 such as stateless address autoconfiguration [6] or manual
 configuration, can use stateless DHCP to obtain other configuration
 information such as a list of DNS recursive name servers or SIP
 servers. A stateless DHCP 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 DHCP specification [1] defines more than 10 protocol
 messages and 20 options, only a subset of those messages and options
 are required for stateless DHCP service. This document explains
 which messages and options defined in RFC 3315 are required for
 stateless DHCP service. The intended use of the document is to guide
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RFC 3736 Stateless DHCP Service for IPv6 April 2004
 the interoperable implementation of clients and servers that use
 stateless DHCP service.
 The operation of relay agents is the same for stateless and stateful
 DHCP service. The operation of relay agents is described in the DHCP
 specification.
 Section 4 of this document lists the sections of the DHCP document
 that an implementor should read for an overview of the DHCP
 specification and the basic requirements of a DHCP service. Section
 5 lists the specific messages and options that are specifically
 required for stateless DHCP service. Section 6 describes how
 stateless and stateful DHCP 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 RFC 2460 [2], the DHCP
 specification [1], and the DHCP DNS configuration options
 specification [3].
 "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 node has
 determined at least a link-local address as described in section 5.3
 of RFC 2461 [4].
 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 configuration
 parameters for the node. The Reply message from the server can carry
 configuration information, such as a list of DNS recursive name
 servers [3] and SIP servers [5].
 This document does not apply to the function of DHCP relay agents as
 described in RFC 3315. A network element can provide both DHCP
 server and DHCP relay service. For example, a network element can
 provide stateless DHCP service to hosts requesting stateless DHCP
 service, while relaying messages from hosts requesting address
 assignment through DHCP to another DHCP server.
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RFC 3736 Stateless DHCP Service for IPv6 April 2004
4. Basic Requirements for Implementation of DHCP
 Several sections of the DHCP specification provide background
 information or define parts of the specification that are common to
 all implementations:
 1-4: give an introduction to DHCP and an overview of DHCP message
 flows
 5: defines constants used throughout the protocol specification
 6, 7: illustrate the format of DHCP messages
 8: describes the representation of Domain Names
 9: defines the "DHCP unique identifier" (DUID)
 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
 a list of DNS recursive name servers, it identifies the DNS Recursive
 Name Server option 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.
 As described in section 18.1.5 of RFC 3315, a node may include a
 Client Identifier option in the Information-request message 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.
 RFC 3315 does not define any mechanisms through which the time at
 which a host uses an Information-request message to obtain updated
 configuration parameters can be controlled. The DHC WG has
 undertaken the development of such a mechanism or mechanisms which
 will be published as Standards-track RFC(s).
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RFC 3736 Stateless DHCP Service for IPv6 April 2004
 RFC 3315 also does not provide any guidance about when a host might
 use an Information-request message to obtain updated configuration
 parameters when the host has moved to a new link. The DHC WG is
 reviewing a related document, "Detection of Network Attachment (DNA)
 in IPv4" [8], which describes how a host using IPv4 can determine
 when to use DHCPv4. Either the DHC WG or a WG formed from the DNA
 BOF will undertake development of a similar document for IPv6.
5.1. Messages Required for Stateless DHCP Service
 Clients and servers implement the following messages for stateless
 DHCP service; the section numbers in this list refer to the DHCP
 specification:
 Information-request: sent by a DHCP client to a server to request
 configuration parameters (sections 18.1.5 and
 18.2.5)
 Reply: sent by a DHCP server to a client containing
 configuration parameters (sections 18.2.6 and
 18.2.8)
 In addition, servers and relay agents implement the following
 messages for stateless DHCP service; the section numbers in this list
 refer to the DHCP specification:
 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 DHCP
 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)
 Server Identifier: used to identify the server responding to a client
 request (section 22.3)
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RFC 3736 Stateless DHCP Service for IPv6 April 2004
 Servers and relay agents implement the following options for
 stateless DHCP service; the section numbers in this list refer to the
 DHCP 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)
 Interface-ID: sent by the DHCP relay agent and returned by the
 server to identify the interface to be used when
 forwarding 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; note that other options for configuration
 information may be specified in future Internet Standards:
 DNS Recursive Name Servers: specifies the DNS recursive name servers
 [7] the client uses for name resolution;
 see "DNS Configuration options for
 DHCPv6" [3]
 DNS search list: specifies the domain names to be searched
 during name resolution; see "DNS
 Configuration options for DHCPv6" [3]
 SIP Servers: specifies the SIP servers the client uses
 to obtain a list of domain names of IPv6
 addresses that can be mapped to one or
 more SIP outbound proxy servers [5]
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 DHCP
 specification:
 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)
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RFC 3736 Stateless DHCP Service for IPv6 April 2004
 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: used to pass information to clients in
 options defined by vendors (section
 22.17)
 Client Identifier: sent by a DHCP client to identify itself (section
 22.2). Clients are not required to send this
 option; servers send the option back if included
 in a message from a client
 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 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 message 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 DHCP service is a proper subset of the DHCP service
 described in the DHCP specification, RFC 3315 [1]. Therefore,
 stateless DHCP service introduces no additional security
 considerations beyond those discussed in sections 21, 22.11, and 23
 of the DHCP specification [1].
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RFC 3736 Stateless DHCP Service for IPv6 April 2004
 Configuration information provided to a node through stateless DHCP
 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 DHCP, as described in
 sections 21 and 22.11 of the DHCP specification [1], can be used to
 avoid attacks mounted through the stateless DHCP service.
8. Acknowledgments
 Jim Bound, Ted Lemon, and Bernie Volz reviewed this document and
 contributed editorial suggestions. Thanks to Peter Barany, Tim
 Chown, Christian Huitema, Tatuya Jinmei, Pekka Savola, and Juha
 Wiljakka for their review and comments.
9. References
9.1. Normative References
 [1] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C. and
 M. Carney, "Dynamic Host Configuration Protocol for IPv6
 (DHCPv6)", RFC 3315, July 2003.
 [2] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
 Specification", RFC 2460, December 1998.
9.2. Informative References
 [3] Droms, R., Ed., "DNS Configuration options for Dynamic Host
 Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December
 2003.
 [4] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
 IP Version 6 (IPv6)", RFC 2461, December 1998.
 [5] Schulzrinne, H. and B. Volz, "Dynamic Host Configuration Protocol
 (DHCPv6) Options for Session Initiation Protocol (SIP) Servers",
 RFC 3319, July 2003.
 [6] Thomson, S. and T. Narten, "IPv6 Stateless Address
 Autoconfiguration", RFC 2462, December 1998.
 [7] Mockapetris, P., "Domain names - concepts and facilities", STD
 13, RFC 1034, November 1987.
 [8] Aboba, B., "Detection of Network Attachment (DNA) in IPv4", Work
 in Progress.
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RFC 3736 Stateless DHCP Service for IPv6 April 2004
10. Author's Address
 Ralph Droms
 Cisco Systems
 1414 Massachusetts Avenue
 Boxborough, MA 01719
 USA
 Phone: +1 978 497 4733
 EMail: rdroms@cisco.com
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RFC 3736 Stateless DHCP Service for IPv6 April 2004
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 to the rights, licenses and restrictions contained in BCP 78 and
 except as set forth therein, the authors retain all their rights.
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 Funding for the RFC Editor function is currently provided by the
 Internet Society.
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