Network Working Group R. Droms

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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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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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 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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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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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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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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10. Author's Address

Ralph Droms
Cisco Systems
1414 Massachusetts Avenue
Boxborough, MA 01719
USA

Phone: +1 978 497 4733
EMail: [email protected]

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RFC 3736 Stateless DHCP Service for IPv6 April 2004

11. Full Copyright Statement

Copyright (C) The Internet Society (2004). This document is subject
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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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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