Network Working Group R. Droms
Internet-Draft Cisco Systems
Expires: May 25, 2004 November 25, 2003
A Guide to Implementing Stateless DHCPv6 Service
draft-ietf-dhc-dhcpv6-stateless-02.txt
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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.
1. Introduction
Nodes that have obtained IPv6 addresses through some other mechanism
such as stateless address autoconfiguration [6] or manual
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configuration can use stateless DHCPv6 to obtain other configuration
information such as a list of DNS recursive name servers or SIP
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 [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 RFC2461 [4]
To obtain configuration parameters through stateless DHCP, a node
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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 DHCPv6 relay agents
as described in RFC 3315. A network element can provide both DHCPv6
server and DHCPv6 relay service. For example, a network element can
provide stateless DHCPv6 service to hosts requesting stateless DHCP
service, while relaying messages from hosts requesting address
assignment through DHCPv6 to another DHCPv6 server.
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 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)
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 identifier 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.
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A node uses the Client Identifier 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.
Whenever a client may have moved to a new link, the configuration
parameters obtained for the interfaces on that link may no longer be
appropriate for the link to which the client is attached. Examples
of times when a client may have moved to a new link include:
o The client reboots.
o The client is physically connected to a wired connection.
o The client returns from sleep mode.
o The client using a wireless technology changes access points.
In any situation when a client may have moved to a new link, the
client initiates an Information-request/Reply message exchange.
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 DHCPv6
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 DHCPv6 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 DHCPv6
specification:
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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)
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)
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; 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 DHCPv6
specification:
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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: 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 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, RFC 3315. Therefore, stateless
DHCPv6 service introduces no additional security considerations
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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.
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, Ted Lemon, Pekka Savola and
Juha Wiljakka for their review and comments.
Normative References
[1] Droms, R., 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.
Informative References
[3] Droms, R., "DNS Configuration Options for DHCPv6",
draft-ietf-dhc-dhcpv6-opt-dnsconfig-04 (work in progress),
August 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.
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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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