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
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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.
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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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