Network Working Group R. Droms, Ed.
Request for Comments: 3315 Cisco
Category: Standards Track J. Bound
Hewlett Packard
B. Volz
EriCsson
T. Lemon
Nominum
C. Perkins
Nokia Research Center
M. Carney
Sun Microsystems
July 2003
Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
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 (2003). All Rights Reserved.
Abstract
The Dynamic Host Configuration Protocol for IPv6 (DHCP) enables DHCP
servers to pass configuration parameters sUCh as IPv6 network
addresses to IPv6 nodes. It offers the capability of automatic
allocation of reusable network addresses and additional configuration
flexibility. This protocol is a stateful counterpart to "IPv6
Stateless Address Autoconfiguration" (RFC2462), and can be used
separately or concurrently with the latter to oBTain configuration
parameters.
Table of Contents
1. Introduction and Overview . . . . . . . . . . . . . . . . . . 5
1.1. Protocols and Addressing . . . . . . . . . . . . . . . 6
1.2. Client-server Exchanges Involving Two Messages . . . . 6
1.3. Client-server Exchanges Involving Four Messages. . . . 7
2. Requirements. . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Background. . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. IPv6 Terminology . . . . . . . . . . . . . . . . . . . 9
4.2. DHCP Terminology . . . . . . . . . . . . . . . . . . . 10
5. DHCP Constants. . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Multicast Addresses. . . . . . . . . . . . . . . . . . 13
5.2. UDP Ports. . . . . . . . . . . . . . . . . . . . . . . 13
5.3. DHCP Message Types . . . . . . . . . . . . . . . . . . 13
5.4. Status Codes . . . . . . . . . . . . . . . . . . . . . 15
5.5. Transmission and Retransmission Parameters . . . . . . 16
5.6 Representation of time values and "Infinity" as a time
value. . . . . . . . . . . . . . . . . . . . . . . . . 16
6. Client/Server Message Formats . . . . . . . . . . . . . . . . 16
7. Relay Agent/Server Message Formats. . . . . . . . . . . . . . 17
7.1. Relay-forward Message. . . . . . . . . . . . . . . . . 18
7.2. Relay-reply Message. . . . . . . . . . . . . . . . . . 19
8. Representation and Use of Domain Names. . . . . . . . . . . . 19
9. DHCP Unique Identifier (DUID) . . . . . . . . . . . . . . . . 19
9.1. DUID Contents. . . . . . . . . . . . . . . . . . . . . 20
9.2. DUID Based on Link-layer Address Plus Time [DUID-LLT]. 20
9.3. DUID Assigned by Vendor Based on Enterprise Number
[DUID-EN]. . . . . . . . . . . . . . . . . . . . . . . 22
9.4. DUID Based on Link-layer Address [DUID-LL] . . . . . . 22
10. Identity Association. . . . . . . . . . . . . . . . . . . . . 23
11. Selecting Addresses for Assignment to an IA . . . . . . . . . 24
12. Management of Temporary Addresses . . . . . . . . . . . . . . 25
13. Transmission of Messages by a Client. . . . . . . . . . . . . 25
14. Reliability of Client Initiated Message Exchanges . . . . . . 26
15. Message Validation. . . . . . . . . . . . . . . . . . . . . . 27
15.1. Use of Transaction IDs . . . . . . . . . . . . . . . . 28
15.2. Solicit Message. . . . . . . . . . . . . . . . . . . . 28
15.3. Advertise Message. . . . . . . . . . . . . . . . . . . 28
15.4. Request Message. . . . . . . . . . . . . . . . . . . . 29
15.5. Confirm Message. . . . . . . . . . . . . . . . . . . . 29
15.6. Renew Message. . . . . . . . . . . . . . . . . . . . . 29
15.7. Rebind Message . . . . . . . . . . . . . . . . . . . . 29
15.8. Decline Messages . . . . . . . . . . . . . . . . . . . 30
15.9. Release Message. . . . . . . . . . . . . . . . . . . . 30
15.10. Reply Message. . . . . . . . . . . . . . . . . . . . . 30
15.11. Reconfigure Message. . . . . . . . . . . . . . . . . . 31
15.12. Information-request Message. . . . . . . . . . . . . . 31
15.13. Relay-forward Message. . . . . . . . . . . . . . . . . 31
15.14. Relay-reply Message. . . . . . . . . . . . . . . . . . 31
16. Client Source Address and Interface Selection . . . . . . . . 32
17. DHCP Server Solicitation. . . . . . . . . . . . . . . . . . . 32
17.1. Client Behavior. . . . . . . . . . . . . . . . . . . . 32
17.1.1. Creation of Solicit Messages . . . . . . . . . 32
17.1.2. Transmission of Solicit Messages . . . . . . . 33
17.1.3. Receipt of Advertise Messages. . . . . . . . . 35
17.1.4. Receipt of Reply Message . . . . . . . . . . . 35
17.2. Server Behavior. . . . . . . . . . . . . . . . . . . . 36
17.2.1. Receipt of Solicit Messages . . . . . . . . . 36
17.2.2. Creation and Transmission of Advertise Messages 36
17.2.3. Creation and Transmission of Reply Messages. . 38
18. DHCP Client-Initiated Configuration Exchange. . . . . . . . . 38
18.1. Client Behavior. . . . . . . . . . . . . . . . . . . . 39
18.1.1. Creation and Transmission of Request Messages. 39
18.1.2. Creation and Transmission of Confirm Messages. 40
18.1.3. Creation and Transmission of Renew Messages. . 41
18.1.4. Creation and Transmission of Rebind Messages . 43
18.1.5. Creation and Transmission of Information-
request Messages . . .. . . . . . . . . . . . 44
18.1.6. Creation and Transmission of Release Messages. 44
18.1.7. Creation and Transmission of Decline Messages. 46
18.1.8. Receipt of Reply Messages. . . . . . . . . . . 46
18.2. Server Behavior. . . . . . . . . . . . . . . . . . . . 48
18.2.1. Receipt of Request Messages. . . . . . . . . . 49
18.2.2. Receipt of Confirm Messages. . . . . . . . . . 50
18.2.3. Receipt of Renew Messages. . . . . . . . . . . 51
18.2.4. Receipt of Rebind Messages . . . . . . . . . . 51
18.2.5. Receipt of Information-request Messages. . . . 52
18.2.6. Receipt of Release Messages. . . . . . . . . . 53
18.2.7. Receipt of Decline Messages. . . . . . . . . . 53
18.2.8. Transmission of Reply Messages . . . . . . . . 54
19. DHCP Server-Initiated Configuration Exchange. . . . . . . . . 54
19.1. Server Behavior. . . . . . . . . . . . . . . . . . . . 55
19.1.1. Creation and Transmission of Reconfigure
Messages . . . . . . . . . . . . . . . . . . . 55
19.1.2. Time Out and Retransmission of Reconfigure
Messages . . . . . . . . . . . . . . . . . . . 56
19.2. Receipt of Renew Messages. . . . . . . . . . . . . . . 56
19.3. Receipt of Information-request Messages. . . . . . . . 56
19.4. Client Behavior. . . . . . . . . . . . . . . . . . . . 57
19.4.1. Receipt of Reconfigure Messages. . . . . . . . 57
19.4.2. Creation and Transmission of Renew Messages. . 58
19.4.3. Creation and Transmission of Information-
request Messages . . . . . . . . . . . . . . . 58
19.4.4. Time Out and Retransmission of Renew or
Information-request Messages . . . . . . . . . 58
19.4.5. Receipt of Reply Messages. . . . . . . . . . . 58
20. Relay Agent Behavior. . . . . . . . . . . . . . . . . . . . . 58
20.1. Relaying a Client Message or a Relay-forward Message . 59
20.1.1. Relaying a Message from a Client . . . . . . . 59
20.1.2. Relaying a Message from a Relay Agent. . . . . 59
20.2. Relaying a Relay-reply Message . . . . . . . . . . . . 60
20.3. Construction of Relay-reply Messages . . . . . . . . . 60
21. Authentication of DHCP Messages . . . . . . . . . . . . . . . 61
21.1. Security of Messages Sent Between Servers and Relay
Agents . . . . . . . . . . . . . . . . . . . . . . . 61
21.2. Summary of DHCP Authentication . . . . . . . . . . . . 63
21.3. Replay Detection . . . . . . . . . . . . . . . . . . . 63
21.4. Delayed Authentication Protocol. . . . . . . . . . . . 63
21.4.1. Use of the Authentication Option in the Delayed
Authentication Protocol. . . . . . . . . . . . 64
21.4.2. Message Validation . . . . . . . . . . . . . . 65
21.4.3. Key Utilization . . . . . . . . . . . . . . . 65
21.4.4. Client Considerations for Delayed Authentication
Protocol . . . . . . . . . . . . . . . . . . . 66
21.4.5. Server Considerations for Delayed Authentication
Protocol . . . . . . . . . . . . . . . . . . . 67
21.5. Reconfigure Key Authentication Protocol. . . . . . . . 68
21.5.1. Use of the Authentication Option in the
Reconfigure Key Authentication Protocol. . . . 69
21.5.2. Server considerations for Reconfigure Key
protocol . . . . . . . . . . . . . . . . . . . 69
21.5.3. Client considerations for Reconfigure Key
protocol . . . . . . . . . . . . . . . . . . . 70
22. DHCP Options. . . . . . . . . . . . . . . . . . . . . . . . . 70
22.1. Format of DHCP Options . . . . . . . . . . . . . . . . 71
22.2. Client Identifier Option . . . . . . . . . . . . . . . 71
22.3. Server Identifier Option . . . . . . . . . . . . . . . 72
22.4. Identity Association for Non-temporary Addresses Option 72
22.5. Identity Association for Temporary Addresses Option. . 75
22.6. IA Address Option. . . . . . . . . . . . . . . . . . . 76
22.7. Option Request Option. . . . . . . . . . . . . . . . . 78
22.8. Preference Option. . . . . . . . . . . . . . . . . . . 79
22.9. Elapsed Time Option. . . . . . . . . . . . . . . . . . 79
22.10. Relay Message Option . . . . . . . . . . . . . . . . . 80
22.11. Authentication Option. . . . . . . . . . . . . . . . . 81
22.12. Server Unicast Option. . . . . . . . . . . . . . . . . 82
22.13. Status Code Option . . . . . . . . . . . . . . . . . . 82
22.14. Rapid Commit Option. . . . . . . . . . . . . . . . . . 83
22.15. User Class Option. . . . . . . . . . . . . . . . . . . 84
22.16. Vendor Class Option. . . . . . . . . . . . . . . . . . 85
22.17. Vendor-specific Information Option . . . . . . . . . . 86
22.18. Interface-Id Option. . . . . . . . . . . . . . . . . . 87
22.19. Reconfigure Message Option . . . . . . . . . . . . . . 88
22.20. Reconfigure Accept Option. . . . . . . . . . . . . . . 89
23. Security Considerations . . . . . . . . . . . . . . . . . . . 89
24. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 91
24.1. Multicast Addresses. . . . . . . . . . . . . . . . . . 92
24.2. DHCP Message Types . . . . . . . . . . . . . . . . . . 93
24.3. DHCP Options . . . . . . . . . . . . . . . . . . . . . 94
24.4. Status Codes . . . . . . . . . . . . . . . . . . . . . 95
24.5. DUID . . . . . . . . . . . . . . . . . . . . . . . . . 95
25. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 95
26. References. . . . . . . . . . . . . . . . . . . . . . . . . . 96
26.1. Normative References . . . . . . . . . . . . . . . . . 96
26.2. Informative References . . . . . . . . . . . . . . . . 97
A. Appearance of Options in Message Types . . . . . . . . . . . . 98
B. Appearance of Options in the Options Field of DHCP Options . . 99
Chair's Address . . . . . . . . . . . . . . . . . . . . . . . . . 99
Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . . 100
Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 101
1. Introduction and Overview
This document describes DHCP for IPv6 (DHCP), a client/server
protocol that provides managed configuration of devices.
DHCP can provide a device with addresses assigned by a DHCP server
and other configuration information, which are carried in options.
DHCP can be extended through the definition of new options to carry
configuration information not specified in this document.
DHCP is the "stateful address autoconfiguration protocol" and the
"stateful autoconfiguration protocol" referred to in "IPv6 Stateless
Address Autoconfiguration" [17].
The operational models and relevant configuration information for
DHCPv4 [18][19] and DHCPv6 are sufficiently different that
integration between the two services is not included in this
document. If there is sufficient interest and demand, integration
can be specified in a document that extends DHCPv6 to carry IPv4
addresses and configuration information.
The remainder of this introduction summarizes DHCP, eXPlaining the
message exchange mechanisms and example message flows. The message
flows in sections 1.2 and 1.3 are intended as illustrations of DHCP
operation rather than an exhaustive list of all possible
client-server interactions. Sections 17, 18, and 19 explain client
and server operation in detail.
1.1. Protocols and Addressing
Clients and servers exchange DHCP messages using UDP [15]. The
client uses a link-local address or addresses determined through
other mechanisms for transmitting and receiving DHCP messages.
DHCP servers receive messages from clients using a reserved,
link-scoped multicast address. A DHCP client transmits most messages
to this reserved multicast address, so that the client need not be
configured with the address or addresses of DHCP servers.
To allow a DHCP client to send a message to a DHCP server that is not
attached to the same link, a DHCP relay agent on the client's link
will relay messages between the client and server. The operation of
the relay agent is transparent to the client and the discussion of
message exchanges in the remainder of this section will omit the
description of message relaying by relay agents.
Once the client has determined the address of a server, it may under
some circumstances send messages directly to the server using
unicast.
1.2. Client-server Exchanges Involving Two Messages
When a DHCP client does not need to have a DHCP server assign it IP
addresses, the client can obtain configuration information such as a
list of available DNS servers [20] or NTP servers [21] through a
single message and reply exchanged with a DHCP server. To obtain
configuration information the client first sends an
Information-Request message to the All_DHCP_Relay_Agents_and_Servers
multicast address. Servers respond with a Reply message containing
the configuration information for the client.
This message exchange assumes that the client requires only
configuration information and does not require the assignment of any
IPv6 addresses.
When a server has IPv6 addresses and other configuration information
committed to a client, the client and server may be able to complete
the exchange using only two messages, instead of four messages as
described in the next section. In this case, the client sends a
Solicit message to the All_DHCP_Relay_Agents_and_Servers requesting
the assignment of addresses and other configuration information.
This message includes an indication that the client is willing to
accept an immediate Reply message from the server. The server that
is willing to commit the assignment of addresses to the client
immediately responds with a Reply message. The configuration
information and the addresses in the Reply message are then
immediately available for use by the client.
Each address assigned to the client has associated preferred and
valid lifetimes specified by the server. To request an extension of
the lifetimes assigned to an address, the client sends a Renew
message to the server. The server sends a Reply message to the
client with the new lifetimes, allowing the client to continue to use
the address without interruption.
1.3. Client-server Exchanges Involving Four Messages
To request the assignment of one or more IPv6 addresses, a client
first locates a DHCP server and then requests the assignment of
addresses and other configuration information from the server. The
client sends a Solicit message to the
All_DHCP_Relay_Agents_and_Servers address to find available DHCP
servers. Any server that can meet the client's requirements responds
with an Advertise message. The client then chooses one of the
servers and sends a Request message to the server aSKINg for
confirmed assignment of addresses and other configuration
information. The server responds with a Reply message that contains
the confirmed addresses and configuration.
As described in the previous section, the client sends a Renew
message to the server to extend the lifetimes associated with its
addresses, allowing the client to continue to use those addresses
without interruption.
2. Requirements
The keyWords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [1].
This document also makes use of internal conceptual variables to
describe protocol behavior and external variables that an
implementation must allow system administrators to change. The
specific variable names, how their values change, and how their
settings influence protocol behavior are provided to demonstrate
protocol behavior. An implementation is not required to have them in
the exact form described here, so long as its external behavior is
consistent with that described in this document.
3. Background
The IPv6 Specification provides the base architecture and design of
IPv6. Related work in IPv6 that would best serve an implementor to
study includes the IPv6 Specification [3], the IPv6 Addressing
Architecture [5], IPv6 Stateless Address Autoconfiguration [17], IPv6
Neighbor Discovery Processing [13], and Dynamic Updates to DNS [22].
These specifications enable DHCP to build upon the IPv6 work to
provide both robust stateful autoconfiguration and autoregistration
of DNS Host Names.
The IPv6 Addressing Architecture specification [5] defines the
address scope that can be used in an IPv6 implementation, and the
various configuration architecture guidelines for network designers
of the IPv6 address space. Two advantages of IPv6 are that support
for multicast is required and nodes can create link-local addresses
during initialization. The availability of these features means that
a client can use its link-local address and a well-known multicast
address to discover and communicate with DHCP servers or relay agents
on its link.
IPv6 Stateless Address Autoconfiguration [17] specifies procedures by
which a node may autoconfigure addresses based on router
advertisements [13], and the use of a valid lifetime to support
renumbering of addresses on the Internet. In addition, the protocol
interaction by which a node begins stateless or stateful
autoconfiguration is specified. DHCP is one vehicle to perform
stateful autoconfiguration. Compatibility with stateless address
autoconfiguration is a design requirement of DHCP.
IPv6 Neighbor Discovery [13] is the node discovery protocol in IPv6
which replaces and enhances functions of ARP [14]. To understand
IPv6 and stateless address autoconfiguration, it is strongly
recommended that implementors understand IPv6 Neighbor Discovery.
Dynamic Updates to DNS [22] is a specification that supports the
dynamic update of DNS records for both IPv4 and IPv6. DHCP can use
the dynamic updates to DNS to integrate addresses and name space to
not only support autoconfiguration, but also autoregistration in
IPv6.
4. Terminology
This sections defines terminology specific to IPv6 and DHCP used in
this document.
4.1. IPv6 Terminology
IPv6 terminology relevant to this specification from the IPv6
Protocol [3], IPv6 Addressing Architecture [5], and IPv6 Stateless
Address Autoconfiguration [17] is included below.
address An IP layer identifier for an interface
or a set of interfaces.
host Any node that is not a router.
IP Internet Protocol Version 6 (IPv6). The
terms IPv4 and IPv6 are used only in
contexts where it is necessary to avoid
ambiguity.
interface A node's attachment to a link.
link A communication facility or medium over
which nodes can communicate at the link
layer, i.e., the layer immediately
below IP. Examples are Ethernet (simple
or bridged); Token Ring; PPP links,
X.25, Frame Relay, or ATM networks; and
Internet (or higher) layer "tunnels",
such as tunnels over IPv4 or IPv6
itself.
link-layer identifier A link-layer identifier for an
interface. Examples include IEEE 802
addresses for Ethernet or Token Ring
network interfaces, and E.164 addresses
for ISDN links.
link-local address An IPv6 address having a link-only
scope, indicated by having the prefix
(FE80::/10), that can be used to reach
neighboring nodes attached to the same
link. Every interface has a link-local
address.
multicast address An identifier for a set of interfaces
(typically belonging to different
nodes). A packet sent to a multicast
address is delivered to all interfaces
identified by that address.
neighbor A node attached to the same link.
node A device that implements IP.
packet An IP header plus payload.
prefix The initial bits of an address, or a
set of IP addresses that share the same
initial bits.
prefix length The number of bits in a prefix.
router A node that forwards IP packets not
explicitly addressed to itself.
unicast address An identifier for a single interface.
A packet sent to a unicast address is
delivered to the interface identified by
that address.
4.2. DHCP Terminology
Terminology specific to DHCP can be found below.
appropriate to the link An address is "appropriate to the link"
when the address is consistent with the
DHCP server's knowledge of the network
topology, prefix assignment and address
assignment policies.
binding A binding (or, client binding) is a
group of server data records containing
the information the server has about
the addresses in an IA or configuration
information explicitly assigned to the
client. Configuration information that
has been returned to a client through a
policy - for example, the information
returned to all clients on the same
link - does not require a binding. A
binding containing information about
an IA is indexed by the tuple <DUID,
IA-type, IAID> (where IA-type is the
type of address in the IA; for example,
temporary). A binding containing
configuration information for a client
is indexed by <DUID>.
configuration parameter An element of the configuration
information set on the server and
delivered to the client using DHCP.
Such parameters may be used to carry
information to be used by a node to
configure its network subsystem and
enable communication on a link or
internetwork, for example.
DHCP Dynamic Host Configuration Protocol
for IPv6. The terms DHCPv4 and DHCPv6
are used only in contexts where it is
necessary to avoid ambiguity.
DHCP client (or client) A node that initiates requests on a link
to obtain configuration parameters from
one or more DHCP servers.
DHCP domain A set of links managed by DHCP and
operated by a single administrative
entity.
DHCP realm A name used to identify the DHCP
administrative domain from which a DHCP
authentication key was selected.
DHCP relay agent (or relay agent) A node that acts as an
intermediary to deliver DHCP messages
between clients and servers, and is on
the same link as the client.
DHCP server (or server) A node that responds to requests from
clients, and may or may not be on the
same link as the client(s).
DUID A DHCP Unique IDentifier for a DHCP
participant; each DHCP client and server
has exactly one DUID. See section 9 for
details of the ways in which a DUID may
be constructed.
Identity association (IA) A collection of addresses assigned to
a client. Each IA has an associated
IAID. A client may have more than one
IA assigned to it; for example, one for
each of its interfaces.
Each IA holds one type of address;
for example, an identity association
for temporary addresses (IA_TA) holds
temporary addresses (see "identity
association for temporary addresses").
Throughout this document, "IA" is used
to refer to an identity association
without identifying the type of
addresses in the IA.
Identity association identifier (IAID) An identifier for an IA,
chosen by the client. Each IA has an
IAID, which is chosen to be unique among
all IAIDs for IAs belonging to that
client.
Identity association for non-temporary addresses (IA_NA) An IA
that carries assigned addresses that are
not temporary addresses (see "identity
association for temporary addresses")
Identity association for temporary addresses (IA_TA) An IA that
carries temporary addresses (see RFC
3041 [12]).
message A unit of data carried as the payload
of a UDP datagram, exchanged among DHCP
servers, relay agents and clients.
Reconfigure key A key supplied to a client by a server
used to provide security for Reconfigure
messages.
relaying A DHCP relay agent relays DHCP messages
between DHCP participants.
transaction ID An opaque value used to match responses
with replies initiated either by a
client or server.
5. DHCP Constants
This section describes various program and networking constants used
by DHCP.
5.1. Multicast Addresses
DHCP makes use of the following multicast addresses:
All_DHCP_Relay_Agents_and_Servers (FF02::1:2) A link-scoped
multicast address used by a client to communicate with
neighboring (i.e., on-link) relay agents and servers.
All servers and relay agents are members of this
multicast group.
All_DHCP_Servers (FF05::1:3) A site-scoped multicast address used
by a relay agent to communicate with servers, either
because the relay agent wants to send messages to
all servers or because it does not know the unicast
addresses of the servers. Note that in order for
a relay agent to use this address, it must have an
address of sufficient scope to be reachable by the
servers. All servers within the site are members of
this multicast group.
5.2. UDP Ports
Clients listen for DHCP messages on UDP port 546. Servers and relay
agents listen for DHCP messages on UDP port 547.
5.3. DHCP Message Types
DHCP defines the following message types. More detail on these
message types can be found in sections 6 and 7. Message types not
listed here are reserved for future use. The numeric encoding for
each message type is shown in parentheses.
SOLICIT (1) A client sends a Solicit message to locate
servers.
ADVERTISE (2) A server sends an Advertise message to indicate
that it is available for DHCP service, in
response to a Solicit message received from a
client.
REQUEST (3) A client sends a Request message to request
configuration parameters, including IP
addresses, from a specific server.
CONFIRM (4) A client sends a Confirm message to any
available server to determine whether the
addresses it was assigned are still appropriate
to the link to which the client is connected.
RENEW (5) A client sends a Renew message to the server
that originally provided the client's addresses
and configuration parameters to extend the
lifetimes on the addresses assigned to the
client and to update other configuration
parameters.
REBIND (6) A client sends a Rebind message to any
available server to extend the lifetimes on the
addresses assigned to the client and to update
other configuration parameters; this message is
sent after a client receives no response to a
Renew message.
REPLY (7) A server sends a Reply message containing
assigned addresses and configuration parameters
in response to a Solicit, Request, Renew,
Rebind message received from a client. A
server sends a Reply message containing
configuration parameters in response to an
Information-request message. A server sends a
Reply message in response to a Confirm message
confirming or denying that the addresses
assigned to the client are appropriate to the
link to which the client is connected. A
server sends a Reply message to acknowledge
receipt of a Release or Decline message.
RELEASE (8) A client sends a Release message to the server
that assigned addresses to the client to
indicate that the client will no longer use one
or more of the assigned addresses.
DECLINE (9) A client sends a Decline message to a server to
indicate that the client has determined that
one or more addresses assigned by the server
are already in use on the link to which the
client is connected.
RECONFIGURE (10) A server sends a Reconfigure message to a
client to inform the client that the server has
new or updated configuration parameters, and
that the client is to initiate a Renew/Reply
or Information-request/Reply transaction with
the server in order to receive the updated
information.
INFORMATION-REQUEST (11) A client sends an Information-request
message to a server to request configuration
parameters without the assignment of any IP
addresses to the client.
RELAY-FORW (12) A relay agent sends a Relay-forward message
to relay messages to servers, either directly
or through another relay agent. The received
message, either a client message or a
Relay-forward message from another relay
agent, is encapsulated in an option in the
Relay-forward message.
RELAY-REPL (13) A server sends a Relay-reply message to a relay
agent containing a message that the relay
agent delivers to a client. The Relay-reply
message may be relayed by other relay agents
for delivery to the destination relay agent.
The server encapsulates the client message as
an option in the Relay-reply message, which the
relay agent extracts and relays to the client.
5.4. Status Codes
DHCPv6 uses status codes to communicate the success or failure of
operations requested in messages from clients and servers, and to
provide additional information about the specific cause of the
failure of a message. The specific status codes are defined in
section 24.4.
5.5. Transmission and Retransmission Parameters
This section presents a table of values used to describe the message
transmission behavior of clients and servers.
Parameter Default Description
-------------------------------------
SOL_MAX_DELAY 1 sec Max delay of first Solicit
SOL_TIMEOUT 1 sec Initial Solicit timeout
SOL_MAX_RT 120 secs Max Solicit timeout value
REQ_TIMEOUT 1 sec Initial Request timeout
REQ_MAX_RT 30 secs Max Request timeout value
REQ_MAX_RC 10 Max Request retry attempts
CNF_MAX_DELAY 1 sec Max delay of first Confirm
CNF_TIMEOUT 1 sec Initial Confirm timeout
CNF_MAX_RT 4 secs Max Confirm timeout
CNF_MAX_RD 10 secs Max Confirm duration
REN_TIMEOUT 10 secs Initial Renew timeout
REN_MAX_RT 600 secs Max Renew timeout value
REB_TIMEOUT 10 secs Initial Rebind timeout
REB_MAX_RT 600 secs Max Rebind timeout value
INF_MAX_DELAY 1 sec Max delay of first Information-request
INF_TIMEOUT 1 sec Initial Information-request timeout
INF_MAX_RT 120 secs Max Information-request timeout value
REL_TIMEOUT 1 sec Initial Release timeout
REL_MAX_RC 5 MAX Release attempts
DEC_TIMEOUT 1 sec Initial Decline timeout
DEC_MAX_RC 5 Max Decline attempts
REC_TIMEOUT 2 secs Initial Reconfigure timeout
REC_MAX_RC 8 Max Reconfigure attempts
HOP_COUNT_LIMIT 32 Max hop count in a Relay-forward message
5.6 Representation of time values and "Infinity" as a time value
All time values for lifetimes, T1 and T2 are unsigned integers. The
value 0xffffffff is taken to mean "infinity" when used as a lifetime
(as in RFC2461 [17]) or a value for T1 or T2.
6. Client/Server Message Formats
All DHCP messages sent between clients and servers share an identical
fixed format header and a variable format area for options.
All values in the message header and in options are in network byte
order.
Options are stored serially in the options field, with no padding
between the options. Options are byte-aligned but are not aligned in
any other way such as on 2 or 4 byte boundaries.
The following diagram illustrates the format of DHCP messages sent
between clients and servers:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type transaction-id
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. options .
. (variable) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type Identifies the DHCP message type; the
available message types are listed in
section 5.3.
transaction-id The transaction ID for this message exchange.
options Options carried in this message; options are
described in section 22.
7. Relay Agent/Server Message Formats
Relay agents exchange messages with servers to relay messages between
clients and servers that are not connected to the same link.
All values in the message header and in options are in network byte
order.
Options are stored serially in the options field, with no padding
between the options. Options are byte-aligned but are not aligned in
any other way such as on 2 or 4 byte boundaries.
There are two relay agent messages, which share the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type hop-count
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
link-address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
peer-address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. options (variable number and length) .... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following sections describe the use of the Relay Agent message
header.
7.1. Relay-forward Message
The following table defines the use of message fields in a Relay-
forward message.
msg-type RELAY-FORW
hop-count Number of relay agents that have relayed this
message.
link-address A global or site-local address that will be used by
the server to identify the link on which the client
is located.
peer-address The address of the client or relay agent from which
the message to be relayed was received.
options MUST include a "Relay Message option" (see
section 22.10); MAY include other options added by
the relay agent.
7.2. Relay-reply Message
The following table defines the use of message fields in a
Relay-reply message.
msg-type RELAY-REPL
hop-count Copied from the Relay-forward message
link-address Copied from the Relay-forward message
peer-address Copied from the Relay-forward message
options MUST include a "Relay Message option"; see
section 22.10; MAY include other options
8. Representation and Use of Domain Names
So that domain names may be encoded uniformly, a domain name or a
list of domain names is encoded using the technique described in
section 3.1 of RFC1035 [10]. A domain name, or list of domain
names, in DHCP MUST NOT be stored in compressed form, as described in
section 4.1.4 of RFC1035.
9. DHCP Unique Identifier (DUID)
Each DHCP client and server has a DUID. DHCP servers use DUIDs to
identify clients for the selection of configuration parameters and in
the association of IAs with clients. DHCP clients use DUIDs to
identify a server in messages where a server needs to be identified.
See sections 22.2 and 22.3 for the representation of a DUID in a DHCP
message.
Clients and servers MUST treat DUIDs as opaque values and MUST only
compare DUIDs for equality. Clients and servers MUST NOT in any
other way interpret DUIDs. Clients and servers MUST NOT restrict
DUIDs to the types defined in this document, as additional DUID types
may be defined in the future.
The DUID is carried in an option because it may be variable length
and because it is not required in all DHCP messages. The DUID is
designed to be unique across all DHCP clients and servers, and stable
for any specific client or server - that is, the DUID used by a
client or server SHOULD NOT change over time if at all possible; for
example, a device's DUID should not change as a result of a change in
the device's network hardware.
The motivation for having more than one type of DUID is that the DUID
must be globally unique, and must also be easy to generate. The sort
of globally-unique identifier that is easy to generate for any given
device can differ quite widely. Also, some devices may not contain
any persistent storage. Retaining a generated DUID in such a device
is not possible, so the DUID scheme must accommodate such devices.
9.1. DUID Contents
A DUID consists of a two-octet type code represented in network byte
order, followed by a variable number of octets that make up the
actual identifier. A DUID can be no more than 128 octets long (not
including the type code). The following types are currently defined:
1 Link-layer address plus time
2 Vendor-assigned unique ID based on Enterprise Number
3 Link-layer address
Formats for the variable field of the DUID for each of the above
types are shown below.
9.2. DUID Based on Link-layer Address Plus Time [DUID-LLT]
This type of DUID consists of a two octet type field containing the
value 1, a two octet hardware type code, four octets containing a
time value, followed by link-layer address of any one network
interface that is connected to the DHCP device at the time that the
DUID is generated. The time value is the time that the DUID is
generated represented in seconds since midnight (UTC), January 1,
2000, modulo 2^32. The hardware type MUST be a valid hardware type
assigned by the IANA as described in RFC826 [14]. Both the time and
the hardware type are stored in network byte order. The link-layer
address is stored in canonical form, as described in RFC2464 [2].
The following diagram illustrates the format of a DUID-LLT:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 hardware type (16 bits)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
time (32 bits)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. link-layer address (variable length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The choice of network interface can be completely arbitrary, as long
as that interface provides a globally unique link-layer address for
the link type, and the same DUID-LLT SHOULD be used in configuring
all network interfaces connected to the device, regardless of which
interface's link-layer address was used to generate the DUID-LLT.
Clients and servers using this type of DUID MUST store the DUID-LLT
in stable storage, and MUST continue to use this DUID-LLT even if the
network interface used to generate the DUID-LLT is removed. Clients
and servers that do not have any stable storage MUST NOT use this
type of DUID.
Clients and servers that use this DUID SHOULD attempt to configure
the time prior to generating the DUID, if that is possible, and MUST
use some sort of time source (for example, a real-time clock) in
generating the DUID, even if that time source could not be configured
prior to generating the DUID. The use of a time source makes it
unlikely that two identical DUID-LLTs will be generated if the
network interface is removed from the client and another client then
uses the same network interface to generate a DUID-LLT. A collision
between two DUID-LLTs is very unlikely even if the clocks have not
been configured prior to generating the DUID.
This method of DUID generation is recommended for all general purpose
computing devices such as desktop computers and laptop computers, and
also for devices such as printers, routers, and so on, that contain
some form of writable non-volatile storage.
Despite our best efforts, it is possible that this algorithm for
generating a DUID could result in a client identifier collision. A
DHCP client that generates a DUID-LLT using this mechanism MUST
provide an administrative interface that replaces the existing DUID
with a newly-generated DUID-LLT.
9.3. DUID Assigned by Vendor Based on Enterprise Number [DUID-EN]
This form of DUID is assigned by the vendor to the device. It
consists of the vendor's registered Private Enterprise Number as
maintained by IANA [6] followed by a unique identifier assigned by
the vendor. The following diagram summarizes the structure of a
DUID-EN:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2 enterprise-number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
enterprise-number (contd)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. identifier .
. (variable length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The source of the identifier is left up to the vendor defining it,
but each identifier part of each DUID-EN MUST be unique to the device
that is using it, and MUST be assigned to the device at the time it
is manufactured and stored in some form of non-volatile storage. The
generated DUID SHOULD be recorded in non-erasable storage. The
enterprise-number is the vendor's registered Private Enterprise
Number as maintained by IANA [6]. The enterprise-number is stored as
an unsigned 32 bit number.
An example DUID of this type might look like this:
+---+---+---+---+---+---+---+---+
0 2 0 0 0 9 12192
+---+---+---+---+---+---+---+---+
132221 3 0 9 18
+---+---+---+---+---+---+
This example includes the two-octet type of 2, the Enterprise Number
(9), followed by eight octets of identifier data
(0x0CC084D303000912).
9.4. DUID Based on Link-layer Address [DUID-LL]
This type of DUID consists of two octets containing the DUID type 3,
a two octet network hardware type code, followed by the link-layer
address of any one network interface that is permanently connected to
the client or server device. For example, a host that has a network
interface implemented in a chip that is unlikely to be removed and
used elsewhere could use a DUID-LL. The hardware type MUST be a
valid hardware type assigned by the IANA, as described in RFC826
[14]. The hardware type is stored in network byte order. The
link-layer address is stored in canonical form, as described in RFC
2464 [2]. The following diagram illustrates the format of a DUID-LL:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3 hardware type (16 bits)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. link-layer address (variable length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The choice of network interface can be completely arbitrary, as long
as that interface provides a unique link-layer address and is
permanently attached to the device on which the DUID-LL is being
generated. The same DUID-LL SHOULD be used in configuring all
network interfaces connected to the device, regardless of which
interface's link-layer address was used to generate the DUID.
DUID-LL is recommended for devices that have a permanently-connected
network interface with a link-layer address, and do not have
nonvolatile, writable stable storage. DUID-LL MUST NOT be used by
DHCP clients or servers that cannot tell whether or not a network
interface is permanently attached to the device on which the DHCP
client is running.
10. Identity Association
An "identity-association" (IA) is a construct through which a server
and a client can identify, group, and manage a set of related IPv6
addresses. Each IA consists of an IAID and associated configuration
information.
A client must associate at least one distinct IA with each of its
network interfaces for which it is to request the assignment of IPv6
addresses from a DHCP server. The client uses the IAs assigned to an
interface to obtain configuration information from a server for that
interface. Each IA must be associated with exactly one interface.
The IAID uniquely identifies the IA and must be chosen to be unique
among the IAIDs on the client. The IAID is chosen by the client.
For any given use of an IA by the client, the IAID for that IA MUST
be consistent across restarts of the DHCP client. The client may
maintain consistency either by storing the IAID in non-volatile
storage or by using an algorithm that will consistently produce the
same IAID as long as the configuration of the client has not changed.
There may be no way for a client to maintain consistency of the IAIDs
if it does not have non-volatile storage and the client's hardware
configuration changes.
The configuration information in an IA consists of one or more IPv6
addresses along with the times T1 and T2 for the IA. See section
22.4 for the representation of an IA in a DHCP message.
Each address in an IA has a preferred lifetime and a valid lifetime,
as defined in RFC2462 [17]. The lifetimes are transmitted from the
DHCP server to the client in the IA option. The lifetimes apply to
the use of IPv6 addresses, as described in section 5.5.4 of RFC2462.
11. Selecting Addresses for Assignment to an IA
A server selects addresses to be assigned to an IA according to the
address assignment policies determined by the server administrator
and the specific information the server determines about the client
from some combination of the following sources:
- The link to which the client is attached. The server determines
the link as follows:
* If the server receives the message directly from the client and
the source address in the IP datagram in which the message was
received is a link-local address, then the client is on the
same link to which the interface over which the message was
received is attached.
* If the server receives the message from a forwarding relay
agent, then the client is on the same link as the one to which
the interface, identified by the link-address field in the
message from the relay agent, is attached.
* If the server receives the message directly from the client and
the source address in the IP datagram in which the message was
received is not a link-local address, then the client is on the
link identified by the source address in the IP datagram (note
that this situation can occur only if the server has enabled
the use of unicast message delivery by the client and the
client has sent a message for which unicast delivery is
allowed).
- The DUID supplied by the client.
- Other information in options supplied by the client.
- Other information in options supplied by the relay agent.
Any address assigned by a server that is based on an EUI-64
identifier MUST include an interface identifier with the "u"
(universal/local) and "g" (individual/group) bits of the interface
identifier set appropriately, as indicated in section 2.5.1 of RFC
2373 [5].
A server MUST NOT assign an address that is otherwise reserved for
some other purpose. For example, a server MUST NOT assign reserved
anycast addresses, as defined in RFC2526, from any subnet.
12. Management of Temporary Addresses
A client may request the assignment of temporary addresses (see RFC
3041 [12] for the definition of temporary addresses). DHCPv6
handling of address assignment is no different for temporary
addresses. DHCPv6 says nothing about details of temporary addresses
like lifetimes, how clients use temporary addresses, rules for
generating successive temporary addresses, etc.
Clients ask for temporary addresses and servers assign them.
Temporary addresses are carried in the Identity Association for
Temporary Addresses (IA_TA) option (see section 22.5). Each IA_TA
option contains at most one temporary address for each of the
prefixes on the link to which the client is attached.
The IAID number space for the IA_TA option IAID number space is
separate from the IA_NA option IAID number space.
The server MAY update the DNS for a temporary address, as described
in section 4 of RFC3041.
13. Transmission of Messages by a Client
Unless otherwise specified in this document, or in a document that
describes how IPv6 is carried over a specific type of link (for link
types that do not support multicast), a client sends DHCP messages to
the All_DHCP_Relay_Agents_and_Servers.
A client uses multicast to reach all servers or an individual server.
An individual server is indicated by specifying that server's DUID in
a Server Identifier option (see section 22.3) in the client's message
(all servers will receive this message but only the indicated server
will respond). All servers are indicated by not supplying this
option.
A client may send some messages directly to a server using unicast,
as described in section 22.12.
14. Reliability of Client Initiated Message Exchanges
DHCP clients are responsible for reliable delivery of messages in the
client-initiated message exchanges described in sections 17 and 18.
If a DHCP client fails to receive an expected response from a server,
the client must retransmit its message. This section describes the
retransmission strategy to be used by clients in client-initiated
message exchanges.
Note that the procedure described in this section is slightly
modified when used with the Solicit message. The modified procedure
is described in section 17.1.2.
The client begins the message exchange by transmitting a message to
the server. The message exchange terminates when either the client
successfully receives the appropriate response or responses from a
server or servers, or when the message exchange is considered to have
failed according to the retransmission mechanism described below.
The client retransmission behavior is controlled and described by the
following variables:
RT Retransmission timeout
IRT Initial retransmission time
MRC Maximum retransmission count
MRT Maximum retransmission time
MRD Maximum retransmission duration
RAND Randomization factor
With each message transmission or retransmission, the client sets RT
according to the rules given below. If RT expires before the message
exchange terminates, the client recomputes RT and retransmits the
message.
Each of the computations of a new RT include a randomization factor
(RAND), which is a random number chosen with a uniform distribution
between -0.1 and +0.1. The randomization factor is included to
minimize synchronization of messages transmitted by DHCP clients.
The algorithm for choosing a random number does not need to be
cryptographically sound. The algorithm SHOULD produce a different
sequence of random numbers from each invocation of the DHCP client.
RT for the first message transmission is based on IRT:
RT = IRT + RAND*IRT
RT for each subsequent message transmission is based on the previous
value of RT:
RT = 2*RTprev + RAND*RTprev
MRT specifies an upper bound on the value of RT (disregarding the
randomization added by the use of RAND). If MRT has a value of 0,
there is no upper limit on the value of RT. Otherwise:
if (RT > MRT)
RT = MRT + RAND*MRT
MRC specifies an upper bound on the number of times a client may
retransmit a message. Unless MRC is zero, the message exchange fails
once the client has transmitted the message MRC times.
MRD specifies an upper bound on the length of time a client may
retransmit a message. Unless MRD is zero, the message exchange fails
once MRD seconds have elapsed since the client first transmitted the
message.
If both MRC and MRD are non-zero, the message exchange fails whenever
either of the conditions specified in the previous two paragraphs are
met.
If both MRC and MRD are zero, the client continues to transmit the
message until it receives a response.
15. Message Validation
Clients and servers SHOULD discard any messages that contain options
that are not allowed to appear in the received message. For example,
an IA option is not allowed to appear in an Information-request
message. Clients and servers MAY choose to extract information from
such a message if the information is of use to the recipient.
A server MUST discard any Solicit, Confirm, Rebind or
Information-request messages it receives with a unicast destination
address.
Message validation based on DHCP authentication is discussed in
section 21.4.2.
If a server receives a message that contains options it should not
contain (such as an Information-request message with an IA option),
is missing options that it should contain, or is otherwise not valid,
it MAY send a Reply (or Advertise as appropriate) with a Server
Identifier option, a Client Identifier option if one was included in
the message and a Status Code option with status UnSpecFail.
15.1. Use of Transaction IDs
The "transaction-id" field holds a value used by clients and servers
to synchronize server responses to client messages. A client SHOULD
generate a random number that cannot easily be guessed or predicted
to use as the transaction ID for each new message it sends. Note
that if a client generates easily predictable transaction
identifiers, it may become more vulnerable to certain kinds of
attacks from off-path intruders. A client MUST leave the transaction
ID unchanged in retransmissions of a message.
15.2. Solicit Message
Clients MUST discard any received Solicit messages.
Servers MUST discard any Solicit messages that do not include a
Client Identifier option or that do include a Server Identifier
option.
15.3. Advertise Message
Clients MUST discard any received Advertise messages that meet any of
the following conditions:
- the message does not include a Server Identifier option.
- the message does not include a Client Identifier option.
- the contents of the Client Identifier option does not match the
client's DUID.
- the "transaction-id" field value does not match the value the
client used in its Solicit message.
Servers and relay agents MUST discard any received Advertise
messages.
15.4. Request Message
Clients MUST discard any received Request messages.
Servers MUST discard any received Request message that meet any of
the following conditions:
- the message does not include a Server Identifier option.
- the contents of the Server Identifier option do not match the
server's DUID.
- the message does not include a Client Identifier option.
15.5. Confirm Message
Clients MUST discard any received Confirm messages.
Servers MUST discard any received Confirm messages that do not
include a Client Identifier option or that do include a Server
Identifier option.
15.6. Renew Message
Clients MUST discard any received Renew messages.
Servers MUST discard any received Renew message that meets any of the
following conditions:
- the message does not include a Server Identifier option.
- the contents of the Server Identifier option does not match the
server's identifier.
- the message does not include a Client Identifier option.
15.7. Rebind Message
Clients MUST discard any received Rebind messages.
Servers MUST discard any received Rebind messages that do not include
a Client Identifier option or that do include a Server Identifier
option.
15.8. Decline Messages
Clients MUST discard any received Decline messages.
Servers MUST discard any received Decline message that meets any of
the following conditions:
- the message does not include a Server Identifier option.
- the contents of the Server Identifier option does not match the
server's identifier.
- the message does not include a Client Identifier option.
15.9. Release Message
Clients MUST discard any received Release messages.
Servers MUST discard any received Release message that meets any of
the following conditions:
- the message does not include a Server Identifier option.
- the contents of the Server Identifier option does not match the
server's identifier.
- the message does not include a Client Identifier option.
15.10. Reply Message
Clients MUST discard any received Reply message that meets any of the
following conditions:
- the message does not include a Server Identifier option.
- the "transaction-id" field in the message does not match the value
used in the original message.
If the client included a Client Identifier option in the original
message, the Reply message MUST include a Client Identifier option
and the contents of the Client Identifier option MUST match the DUID
of the client; OR, if the client did not include a Client Identifier
option in the original message, the Reply message MUST NOT include a
Client Identifier option.
Servers and relay agents MUST discard any received Reply messages.
15.11. Reconfigure Message
Servers and relay agents MUST discard any received Reconfigure
messages.
Clients MUST discard any Reconfigure messages that meets any of the
following conditions:
- the message was not unicast to the client.
- the message does not include a Server Identifier option.
- the message does not include a Client Identifier option that
contains the client's DUID.
- the message does not contain a Reconfigure Message option and the
msg-type must be a valid value.
- the message includes any IA options and the msg-type in the
Reconfigure Message option is INFORMATION-REQUEST.
- the message does not include DHCP authentication:
* the message does not contain an authentication option.
* the message does not pass the authentication validation
performed by the client.
15.12. Information-request Message
Clients MUST discard any received Information-request messages.
Servers MUST discard any received Information-request message that
meets any of the following conditions:
- The message includes a Server Identifier option and the DUID in
the option does not match the server's DUID.
- The message includes an IA option.
15.13. Relay-forward Message
Clients MUST discard any received Relay-forward messages.
15.14. Relay-reply Message
Clients and servers MUST discard any received Relay-reply messages.
16. Client Source Address and Interface Selection
When a client sends a DHCP message to the
All_DHCP_Relay_Agents_and_Servers address, it SHOULD send the message
through the interface for which configuration information is being
requested. However, the client MAY send the message through another
interface attached to the same link, if and only if the client is
certain the two interfaces are attached to the same link. The client
MUST use a link-local address assigned to the interface for which it
is requesting configuration information as the source address in the
header of the IP datagram.
When a client sends a DHCP message directly to a server using unicast
(after receiving the Server Unicast option from that server), the
source address in the header of the IP datagram MUST be an address
assigned to the interface for which the client is interested in
obtaining configuration and which is suitable for use by the server
in responding to the client.
17. DHCP Server Solicitation
This section describes how a client locates servers that will assign
addresses to IAs belonging to the client.
The client is responsible for creating IAs and requesting that a
server assign IPv6 addresses to the IA. The client first creates an
IA and assigns it an IAID. The client then transmits a Solicit
message containing an IA option describing the IA. Servers that can
assign addresses to the IA respond to the client with an Advertise
message. The client then initiates a configuration exchange as
described in section 18.
If the client will accept a Reply message with committed address
assignments and other resources in response to the Solicit message,
the client includes a Rapid Commit option (see section 22.14) in the
Solicit message.
17.1. Client Behavior
A client uses the Solicit message to discover DHCP servers configured
to assign addresses or return other configuration parameters on the
link to which the client is attached.
17.1.1. Creation of Solicit Messages
The client sets the "msg-type" field to SOLICIT. The client
generates a transaction ID and inserts this value in the
"transaction-id" field.
The client MUST include a Client Identifier option to identify itself
to the server. The client includes IA options for any IAs to which
it wants the server to assign addresses. The client MAY include
addresses in the IAs as a hint to the server about addresses for
which the client has a preference. The client MUST NOT include any
other options in the Solicit message, except as specifically allowed
in the definition of individual options.
The client uses IA_NA options to request the assignment of non-
temporary addresses and uses IA_TA options to request the assignment
of temporary addresses. Either IA_NA or IA_TA options, or a
combination of both, can be included in DHCP messages.
The client SHOULD include an Option Request option (see section 22.7)
to indicate the options the client is interested in receiving. The
client MAY additionally include instances of those options that are
identified in the Option Request option, with data values as hints to
the server about parameter values the client would like to have
returned.
The client includes a Reconfigure Accept option (see section 22.20)
if the client is willing to accept Reconfigure messages from the
server.
17.1.2. Transmission of Solicit Messages
The first Solicit message from the client on the interface MUST be
delayed by a random amount of time between 0 and SOL_MAX_DELAY. In
the case of a Solicit message transmitted when DHCP is initiated by
IPv6 Neighbor Discovery, the delay gives the amount of time to wait
after IPv6 Neighbor Discovery causes the client to invoke the
stateful address autoconfiguration protocol (see section 5.5.3 of RFC
2462). This random delay desynchronizes clients which start at the
same time (for example, after a power outage).
The client transmits the message according to section 14, using the
following parameters:
IRT SOL_TIMEOUT
MRT SOL_MAX_RT
MRC 0
MRD 0
If the client has included a Rapid Commit option in its Solicit
message, the client terminates the waiting process as soon as a Reply
message with a Rapid Commit option is received.
If the client is waiting for an Advertise message, the mechanism in
section 14 is modified as follows for use in the transmission of
Solicit messages. The message exchange is not terminated by the
receipt of an Advertise before the first RT has elapsed. Rather, the
client collects Advertise messages until the first RT has elapsed.
Also, the first RT MUST be selected to be strictly greater than IRT
by choosing RAND to be strictly greater than 0.
A client MUST collect Advertise messages for the first RT seconds,
unless it receives an Advertise message with a preference value of
255. The preference value is carried in the Preference option
(section 22.8). Any Advertise that does not include a Preference
option is considered to have a preference value of 0. If the client
receives an Advertise message that includes a Preference option with
a preference value of 255, the client immediately begins a client-
initiated message exchange (as described in section 18) by sending a
Request message to the server from which the Advertise message was
received. If the client receives an Advertise message that does not
include a Preference option with a preference value of 255, the
client continues to wait until the first RT elapses. If the first RT
elapses and the client has received an Advertise message, the client
SHOULD continue with a client-initiated message exchange by sending a
Request message.
If the client does not receive any Advertise messages before the
first RT has elapsed, it begins the retransmission mechanism
described in section 14. The client terminates the retransmission
process as soon as it receives any Advertise message, and the client
acts on the received Advertise message without waiting for any
additional Advertise messages.
A DHCP client SHOULD choose MRC and MRD to be 0. If the DHCP client
is configured with either MRC or MRD set to a value other than 0, it
MUST stop trying to configure the interface if the message exchange
fails. After the DHCP client stops trying to configure the
interface, it SHOULD restart the reconfiguration process after some
external event, such as user input, system restart, or when the
client is attached to a new link.
17.1.3. Receipt of Advertise Messages
The client MUST ignore any Advertise message that includes a Status
Code option containing the value NoAddrsAvail, with the exception
that the client MAY display the associated status message to the
user.
Upon receipt of one or more valid Advertise messages, the client
selects one or more Advertise messages based upon the following
criteria.
- Those Advertise messages with the highest server preference value
are preferred over all other Advertise messages.
- Within a group of Advertise messages with the same server
preference value, a client MAY select those servers whose
Advertise messages advertise information of interest to the
client. For example, the client may choose a server that returned
an advertisement with configuration options of interest to the
client.
- The client MAY choose a less-preferred server if that server has a
better set of advertised parameters, such as the available
addresses advertised in IAs.
Once a client has selected Advertise message(s), the client will
typically store information about each server, such as server
preference value, addresses advertised, when the advertisement was
received, and so on.
If the client needs to select an alternate server in the case that a
chosen server does not respond, the client chooses the next server
according to the criteria given above.
17.1.4. Receipt of Reply Message
If the client includes a Rapid Commit option in the Solicit message,
it will expect a Reply message that includes a Rapid Commit option in
response. The client discards any Reply messages it receives that do
not include a Rapid Commit option. If the client receives a valid
Reply message that includes a Rapid Commit option, it processes the
message as described in section 18.1.8. If it does not receive such
a Reply message and does receive a valid Advertise message, the
client processes the Advertise message as described in section
17.1.3.
If the client subsequently receives a valid Reply message that
includes a Rapid Commit option, it either:
processes the Reply message as described in section 18.1.8, and
discards any Reply messages received in response to the Request
message, or
processes any Reply messages received in response to the Request
message and discards the Reply message that includes the Rapid
Commit option.
17.2. Server Behavior
A server sends an Advertise message in response to valid Solicit
messages it receives to announce the availability of the server to
the client.
17.2.1. Receipt of Solicit Messages
The server determines the information about the client and its
location as described in section 11 and checks its administrative
policy about responding to the client. If the server is not
permitted to respond to the client, the server discards the Solicit
message. For example, if the administrative policy for the server is
that it may only respond to a client that is willing to accept a
Reconfigure message, if the client indicates with a Reconfigure
Accept option in the Solicit message that it will not accept a
Reconfigure message, the servers discard the Solicit message.
If the client has included a Rapid Commit option in the Solicit
message and the server has been configured to respond with committed
address assignments and other resources, the server responds to the
Solicit with a Reply message as described in section 17.2.3.
Otherwise, the server ignores the Rapid Commit option and processes
the remainder of the message as if no Rapid Commit option were
present.
17.2.2. Creation and Transmission of Advertise Messages
The server sets the "msg-type" field to ADVERTISE and copies the
contents of the transaction-id field from the Solicit message
received from the client to the Advertise message. The server
includes its server identifier in a Server Identifier option and
copies the Client Identifier from the Solicit message into the
Advertise message.
The server MAY add a Preference option to carry the preference value
for the Advertise message. The server implementation SHOULD allow
the setting of a server preference value by the administrator. The
server preference value MUST default to zero unless otherwise
configured by the server administrator.
The server includes a Reconfigure Accept option if the server wants
to require that the client accept Reconfigure messages.
The server includes options the server will return to the client in a
subsequent Reply message. The information in these options may be
used by the client in the selection of a server if the client
receives more than one Advertise message. If the client has included
an Option Request option in the Solicit message, the server includes
options in the Advertise message containing configuration parameters
for all of the options identified in the Option Request option that
the server has been configured to return to the client. The server
MAY return additional options to the client if it has been configured
to do so. The server must be aware of the recommendations on packet
sizes and the use of fragmentation in section 5 of RFC2460.
If the Solicit message from the client included one or more IA
options, the server MUST include IA options in the Advertise message
containing any addresses that would be assigned to IAs contained in
the Solicit message from the client. If the client has included
addresses in the IAs in the Solicit message, the server uses those
addresses as hints about the addresses the client would like to
receive.
If the server will not assign any addresses to any IAs in a
subsequent Request from the client, the server MUST send an Advertise
message to the client that includes only a Status Code option with
code NoAddrsAvail and a status message for the user, a Server
Identifier option with the server's DUID, and a Client Identifier
option with the client's DUID.
If the Solicit message was received directly by the server, the
server unicasts the Advertise message directly to the client using
the address in the source address field from the IP datagram in which
the Solicit message was received. The Advertise message MUST be
unicast on the link from which the Solicit message was received.
If the Solicit message was received in a Relay-forward message, the
server constructs a Relay-reply message with the Advertise message in
the payload of a "relay-message" option. If the Relay-forward
messages included an Interface-id option, the server copies that
option to the Relay-reply message. The server unicasts the
Relay-reply message directly to the relay agent using the address in
the source address field from the IP datagram in which the Relay-
forward message was received.
17.2.3. Creation and Transmission of Reply Messages
The server MUST commit the assignment of any addresses or other
configuration information message before sending a Reply message to a
client in response to a Solicit message.
DISCUSSION:
When using the Solicit-Reply message exchange, the server commits
the assignment of any addresses before sending the Reply message.
The client can assume it has been assigned the addresses in the
Reply message and does not need to send a Request message for
those addresses.
Typically, servers that are configured to use the Solicit-Reply
message exchange will be deployed so that only one server will
respond to a Solicit message. If more than one server responds,
the client will only use the addresses from one of the servers,
while the addresses from the other servers will be committed to
the client but not used by the client.
The server includes a Rapid Commit option in the Reply message to
indicate that the Reply is in response to a Solicit message.
The server includes a Reconfigure Accept option if the server wants
to require that the client accept Reconfigure messages.
The server produces the Reply message as though it had received a
Request message, as described in section 18.2.1. The server
transmits the Reply message as described in section 18.2.8.
18. DHCP Client-Initiated Configuration Exchange
A client initiates a message exchange with a server or servers to
acquire or update configuration information of interest. The client
may initiate the configuration exchange as part of the operating
system configuration process, when requested to do so by the
application layer, when required by Stateless Address
Autoconfiguration or as required to extend the lifetime of an address
(Renew and Rebind messages).
18.1. Client Behavior
A client uses Request, Renew, Rebind, Release and Decline messages
during the normal life cycle of addresses. It uses Confirm to
validate addresses when it may have moved to a new link. It uses
Information-Request messages when it needs configuration information
but no addresses.
If the client has a source address of sufficient scope that can be
used by the server as a return address, and the client has received a
Server Unicast option (section 22.12) from the server, the client
SHOULD unicast any Request, Renew, Release and Decline messages to
the server.
DISCUSSION:
Use of unicast may avoid delays due to the relaying of messages by
relay agents, as well as avoid overhead and duplicate responses by
servers due to the delivery of client messages to multiple
servers. Requiring the client to relay all DHCP messages through
a relay agent enables the inclusion of relay agent options in all
messages sent by the client. The server should enable the use of
unicast only when relay agent options will not be used.
18.1.1. Creation and Transmission of Request Messages
The client uses a Request message to populate IAs with addresses and
obtain other configuration information. The client includes one or
more IA options in the Request message. The server then returns
addresses and other information about the IAs to the client in IA
options in a Reply message.
The client generates a transaction ID and inserts this value in the
"transaction-id" field.
The client places the identifier of the destination server in a
Server Identifier option.
The client MUST include a Client Identifier option to identify itself
to the server. The client adds any other appropriate options,
including one or more IA options (if the client is requesting that
the server assign it some network addresses).
The client MUST include an Option Request option (see section 22.7)
to indicate the options the client is interested in receiving. The
client MAY include options with data values as hints to the server
about parameter values the client would like to have returned.
The client includes a Reconfigure Accept option (see section 22.20)
indicating whether or not the client is willing to accept Reconfigure
messages from the server.
The client transmits the message according to section 14, using the
following parameters:
IRT REQ_TIMEOUT
MRT REQ_MAX_RT
MRC REQ_MAX_RC
MRD 0
If the message exchange fails, the client takes an action based on
the client's local policy. Examples of actions the client might take
include:
- Select another server from a list of servers known to the client;
for example, servers that responded with an Advertise message.
- Initiate the server discovery process described in section 17.
- Terminate the configuration process and report failure.
18.1.2. Creation and Transmission of Confirm Messages
Whenever a client may have moved to a new link, the prefixes from the
addresses assigned to 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 MUST initiate a Confirm/Reply message exchange. The client
includes any IAs assigned to the interface that may have moved to a
new link, along with the addresses associated with those IAs, in its
Confirm message. Any responding servers will indicate whether those
addresses are appropriate for the link to which the client is
attached with the status in the Reply message it returns to the
client.
The client sets the "msg-type" field to CONFIRM. The client
generates a transaction ID and inserts this value in the
"transaction-id" field.
The client MUST include a Client Identifier option to identify itself
to the server. The client includes IA options for all of the IAs
assigned to the interface for which the Confirm message is being
sent. The IA options include all of the addresses the client
currently has associated with those IAs. The client SHOULD set the
T1 and T2 fields in any IA_NA options, and the preferred-lifetime and
valid-lifetime fields in the IA Address options to 0, as the server
will ignore these fields.
The first Confirm message from the client on the interface MUST be
delayed by a random amount of time between 0 and CNF_MAX_DELAY. The
client transmits the message according to section 14, using the
following parameters:
IRT CNF_TIMEOUT
MRT CNF_MAX_RT
MRC 0
MRD CNF_MAX_RD
If the client receives no responses before the message transmission
process terminates, as described in section 14, the client SHOULD
continue to use any IP addresses, using the last known lifetimes for
those addresses, and SHOULD continue to use any other previously
obtained configuration parameters.
18.1.3. Creation and Transmission of Renew Messages
To extend the valid and preferred lifetimes for the addresses
associated with an IA, the client sends a Renew message to the server
from which the client obtained the addresses in the IA containing an
IA option for the IA. The client includes IA Address options in the
IA option for the addresses associated with the IA. The server
determines new lifetimes for the addresses in the IA according to the
administrative configuration of the server. The server may also add
new addresses to the IA. The server may remove addresses from the IA
by setting the preferred and valid lifetimes of those addresses to
zero.
The server controls the time at which the client contacts the server
to extend the lifetimes on assigned addresses through the T1 and T2
parameters assigned to an IA.
At time T1 for an IA, the client initiates a Renew/Reply message
exchange to extend the lifetimes on any addresses in the IA. The
client includes an IA option with all addresses currently assigned to
the IA in its Renew message.
If T1 or T2 is set to 0 by the server (for an IA_NA) or there are no
T1 or T2 times (for an IA_TA), the client may send a Renew or Rebind
message, respectively, at the client's discretion.
The client sets the "msg-type" field to RENEW. The client generates
a transaction ID and inserts this value in the "transaction-id"
field.
The client places the identifier of the destination server in a
Server Identifier option.
The client MUST include a Client Identifier option to identify itself
to the server. The client adds any appropriate options, including
one or more IA options. The client MUST include the list of
addresses the client currently has associated with the IAs in the
Renew message.
The client MUST include an Option Request option (see section 22.7)
to indicate the options the client is interested in receiving. The
client MAY include options with data values as hints to the server
about parameter values the client would like to have returned.
The client transmits the message according to section 14, using the
following parameters:
IRT REN_TIMEOUT
MRT REN_MAX_RT
MRC 0
MRD Remaining time until T2
The message exchange is terminated when time T2 is reached (see
section 18.1.4), at which time the client begins a Rebind message
exchange.
18.1.4. Creation and Transmission of Rebind Messages
At time T2 for an IA (which will only be reached if the server to
which the Renew message was sent at time T1 has not responded), the
client initiates a Rebind/Reply message exchange with any available
server. The client includes an IA option with all addresses
currently assigned to the IA in its Rebind message.
The client sets the "msg-type" field to REBIND. The client generates
a transaction ID and inserts this value in the "transaction-id"
field.
The client MUST include a Client Identifier option to identify itself
to the server. The client adds any appropriate options, including
one or more IA options. The client MUST include the list of
addresses the client currently has associated with the IAs in the
Rebind message.
The client MUST include an Option Request option (see section 22.7)
to indicate the options the client is interested in receiving. The
client MAY include options with data values as hints to the server
about parameter values the client would like to have returned.
The client transmits the message according to section 14, using the
following parameters:
IRT REB_TIMEOUT
MRT REB_MAX_RT
MRC 0
MRD Remaining time until valid lifetimes of all addresses have
expired
The message exchange is terminated when the valid lifetimes of all
the addresses assigned to the IA expire (see section 10), at which
time the client has several alternative actions to choose from; for
example:
- The client may choose to use a Solicit message to locate a new
DHCP server and send a Request for the expired IA to the new
server.
- The client may have other addresses in other IAs, so the client
may choose to discard the expired IA and use the addresses in the
other IAs.
18.1.5. Creation and Transmission of Information-request Messages
The client uses an Information-request message to obtain
configuration information without having addresses assigned to it.
The client sets the "msg-type" field to INFORMATION-REQUEST. The
client generates a transaction ID and inserts this value in the
"transaction-id" field.
The client SHOULD include a Client Identifier option to identify
itself to the server. If the client does not include a Client
Identifier option, the server will not be able to return any client-
specific options to the client, or the server may choose not to
respond to the message at all. The client MUST include a Client
Identifier option if the Information-Request message will be
authenticated.
The client MUST include an Option Request option (see section 22.7)
to indicate the options the client is interested in receiving. The
client MAY include options with data values as hints to the server
about parameter values the client would like to have returned.
The first Information-request message from the client on the
interface MUST be delayed by a random amount of time between 0 and
INF_MAX_DELAY. The client transmits the message according to section
14, using the following parameters:
IRT INF_TIMEOUT
MRT INF_MAX_RT
MRC 0
MRD 0
18.1.6. Creation and Transmission of Release Messages
To release one or more addresses, a client sends a Release message to
the server.
The client sets the "msg-type" field to RELEASE. The client
generates a transaction ID and places this value in the
"transaction-id" field.
The client places the identifier of the server that allocated the
address(es) in a Server Identifier option.
The client MUST include a Client Identifier option to identify itself
to the server. The client includes options containing the IAs for
the addresses it is releasing in the "options" field. The addresses
to be released MUST be included in the IAs. Any addresses for the
IAs the client wishes to continue to use MUST NOT be added to the
IAs.
The client MUST NOT use any of the addresses it is releasing as the
source address in the Release message or in any subsequently
transmitted message.
Because Release messages may be lost, the client should retransmit
the Release if no Reply is received. However, there are scenarios
where the client may not wish to wait for the normal retransmission
timeout before giving up (e.g., on power down). Implementations
SHOULD retransmit one or more times, but MAY choose to terminate the
retransmission procedure early.
The client transmits the message according to section 14, using the
following parameters:
IRT REL_TIMEOUT
MRT 0
MRC REL_MAX_RC
MRD 0
The client MUST stop using all of the addresses being released as
soon as the client begins the Release message exchange process. If
addresses are released but the Reply from a DHCP server is lost, the
client will retransmit the Release message, and the server may
respond with a Reply indicating a status of NoBinding. Therefore,
the client does not treat a Reply message with a status of NoBinding
in a Release message exchange as if it indicates an error.
Note that if the client fails to release the addresses, each address
assigned to the IA will be reclaimed by the server when the valid
lifetime of that address expires.
18.1.7. Creation and Transmission of Decline Messages
If a client detects that one or more addresses assigned to it by a
server are already in use by another node, the client sends a Decline
message to the server to inform it that the address is suspect.
The client sets the "msg-type" field to DECLINE. The client
generates a transaction ID and places this value in the
"transaction-id" field.
The client places the identifier of the server that allocated the
address(es) in a Server Identifier option.
The client MUST include a Client Identifier option to identify itself
to the server. The client includes options containing the IAs for
the addresses it is declining in the "options" field. The addresses
to be declined MUST be included in the IAs. Any addresses for the
IAs the client wishes to continue to use should not be in added to
the IAs.
The client MUST NOT use any of the addresses it is declining as the
source address in the Decline message or in any subsequently
transmitted message.
The client transmits the message according to section 14, using the
following parameters:
IRT DEC_TIMEOUT
MRT 0
MRC DEC_MAX_RC
MRD 0
If addresses are declined but the Reply from a DHCP server is lost,
the client will retransmit the Decline message, and the server may
respond with a Reply indicating a status of NoBinding. Therefore,
the client does not treat a Reply message with a status of NoBinding
in a Decline message exchange as if it indicates an error.
18.1.8. Receipt of Reply Messages
Upon the receipt of a valid Reply message in response to a Solicit
(with a Rapid Commit option), Request, Confirm, Renew, Rebind or
Information-request message, the client extracts the configuration
information contained in the Reply. The client MAY choose to report
any status code or message from the status code option in the Reply
message.
The client SHOULD perform duplicate address detection [17] on each of
the addresses in any IAs it receives in the Reply message before
using that address for traffic. If any of the addresses are found to
be in use on the link, the client sends a Decline message to the
server as described in section 18.1.7.
If the Reply was received in response to a Solicit (with a Rapid
Commit option), Request, Renew or Rebind message, the client updates
the information it has recorded about IAs from the IA options
contained in the Reply message:
- Record T1 and T2 times.
- Add any new addresses in the IA option to the IA as recorded by
the client.
- Update lifetimes for any addresses in the IA option that the
client already has recorded in the IA.
- Discard any addresses from the IA, as recorded by the client, that
have a valid lifetime of 0 in the IA Address option.
- Leave unchanged any information about addresses the client has
recorded in the IA but that were not included in the IA from the
server.
Management of the specific configuration information is detailed in
the definition of each option in section 22.
If the client receives a Reply message with a Status Code containing
UnspecFail, the server is indicating that it was unable to process
the message due to an unspecified failure condition. If the client
retransmits the original message to the same server to retry the
desired operation, the client MUST limit the rate at which it
retransmits the message and limit the duration of the time during
which it retransmits the message.
When the client receives a Reply message with a Status Code option
with the value UseMulticast, the client records the receipt of the
message and sends subsequent messages to the server through the
interface on which the message was received using multicast. The
client resends the original message using multicast.
When the client receives a NotOnLink status from the server in
response to a Confirm message, the client performs DHCP server
solicitation, as described in section 17, and client-initiated
configuration as described in section 18. If the client receives any
Reply messages that do not indicate a NotOnLink status, the client
can use the addresses in the IA and ignore any messages that indicate
a NotOnLink status.
When the client receives a NotOnLink status from the server in
response to a Request, the client can either re-issue the Request
without specifying any addresses or restart the DHCP server discovery
process (see section 17).
The client examines the status code in each IA individually. If the
status code is NoAddrsAvail, the client has received no usable
addresses in the IA and may choose to try obtaining addresses for the
IA from another server. The client uses addresses and other
information from any IAs that do not contain a Status Code option
with the NoAddrsAvail code. If the client receives no addresses in
any of the IAs, it may either try another server (perhaps restarting
the DHCP server discovery process) or use the Information-request
message to obtain other configuration information only.
When the client receives a Reply message in response to a Renew or
Rebind message, the client examines each IA independently. For each
IA in the original Renew or Rebind message, the client:
- sends a Request message if the IA contained a Status Code option
with the NoBinding status (and does not send any additional
Renew/Rebind messages)
- sends a Renew/Rebind if the IA is not in the Reply message
- otherwise accepts the information in the IA
When the client receives a valid Reply message in response to a
Release message, the client considers the Release event completed,
regardless of the Status Code option(s) returned by the server.
When the client receives a valid Reply message in response to a
Decline message, the client considers the Decline event completed,
regardless of the Status Code option(s) returned by the server.
18.2. Server Behavior
For this discussion, the Server is assumed to have been configured in
an implementation specific manner with configuration of interest to
clients.
In most instances, the server will send a Reply in response to a
client message. This Reply message MUST always contain the Server
Identifier option containing the server's DUID and the Client
Identifier option from the client message if one was present.
In most Reply messages, the server includes options containing
configuration information for the client. The server must be aware
of the recommendations on packet sizes and the use of fragmentation
in section 5 of RFC2460. If the client included an Option Request
option in its message, the server includes options in the Reply
message containing configuration parameters for all of the options
identified in the Option Request option that the server has been
configured to return to the client. The server MAY return additional
options to the client if it has been configured to do so.
18.2.1. Receipt of Request Messages
When the server receives a Request message via unicast from a client
to which the server has not sent a unicast option, the server
discards the Request message and responds with a Reply message
containing a Status Code option with the value UseMulticast, a Server
Identifier option containing the server's DUID, the Client Identifier
option from the client message, and no other options.
When the server receives a valid Request message, the server creates
the bindings for that client according to the server's policy and
configuration information and records the IAs and other information
requested by the client.
The server constructs a Reply message by setting the "msg-type" field
to REPLY, and copying the transaction ID from the Request message
into the transaction-id field.
The server MUST include a Server Identifier option containing the
server's DUID and the Client Identifier option from the Request
message in the Reply message.
If the server finds that the prefix on one or more IP addresses in
any IA in the message from the client is not appropriate for the link
to which the client is connected, the server MUST return the IA to
the client with a Status Code option with the value NotOnLink.
If the server cannot assign any addresses to an IA in the message
from the client, the server MUST include the IA in the Reply message
with no addresses in the IA and a Status Code option in the IA
containing status code NoAddrsAvail.
For any IAs to which the server can assign addresses, the server
includes the IA with addresses and other configuration parameters,
and records the IA as a new client binding.
The server includes a Reconfigure Accept option if the server wants
to require that the client accept Reconfigure messages.
The server includes other options containing configuration
information to be returned to the client as described in section
18.2.
If the server finds that the client has included an IA in the Request
message for which the server already has a binding that associates
the IA with the client, the client has resent a Request message for
which it did not receive a Reply message. The server either resends
a previously cached Reply message or sends a new Reply message.
18.2.2. Receipt of Confirm Messages
When the server receives a Confirm message, the server determines
whether the addresses in the Confirm message are appropriate for the
link to which the client is attached. If all of the addresses in the
Confirm message pass this test, the server returns a status of
Success. If any of the addresses do not pass this test, the server
returns a status of NotOnLink. If the server is unable to perform
this test (for example, the server does not have information about
prefixes on the link to which the client is connected), or there were
no addresses in any of the IAs sent by the client, the server MUST
NOT send a reply to the client.
The server ignores the T1 and T2 fields in the IA options and the
preferred-lifetime and valid-lifetime fields in the IA Address
options.
The server constructs a Reply message by setting the "msg-type" field
to REPLY, and copying the transaction ID from the Confirm message
into the transaction-id field.
The server MUST include a Server Identifier option containing the
server's DUID and the Client Identifier option from the Confirm
message in the Reply message. The server includes a Status Code
option indicating the status of the Confirm message.
18.2.3. Receipt of Renew Messages
When the server receives a Renew message via unicast from a client to
which the server has not sent a unicast option, the server discards
the Renew message and responds with a Reply message containing a
Status Code option with the value UseMulticast, a Server Identifier
option containing the server's DUID, the Client Identifier option
from the client message, and no other options.
When the server receives a Renew message that contains an IA option
from a client, it locates the client's binding and verifies that the
information in the IA from the client matches the information stored
for that client.
If the server cannot find a client entry for the IA the server
returns the IA containing no addresses with a Status Code option set
to NoBinding in the Reply message.
If the server finds that any of the addresses are not appropriate for
the link to which the client is attached, the server returns the
address to the client with lifetimes of 0.
If the server finds the addresses in the IA for the client then the
server sends back the IA to the client with new lifetimes and T1/T2
times. The server may choose to change the list of addresses and the
lifetimes of addresses in IAs that are returned to the client.
The server constructs a Reply message by setting the "msg-type" field
to REPLY, and copying the transaction ID from the Renew message into
the transaction-id field.
The server MUST include a Server Identifier option containing the
server's DUID and the Client Identifier option from the Renew message
in the Reply message.
The server includes other options containing configuration
information to be returned to the client as described in section
18.2.
18.2.4. Receipt of Rebind Messages
When the server receives a Rebind message that contains an IA option
from a client, it locates the client's binding and verifies that the
information in the IA from the client matches the information stored
for that client.
If the server cannot find a client entry for the IA and the server
determines that the addresses in the IA are not appropriate for the
link to which the client's interface is attached according to the
server's explicit configuration information, the server MAY send a
Reply message to the client containing the client's IA, with the
lifetimes for the addresses in the IA set to zero. This Reply
constitutes an explicit notification to the client that the addresses
in the IA are no longer valid. In this situation, if the server does
not send a Reply message it silently discards the Rebind message.
If the server finds that any of the addresses are no longer
appropriate for the link to which the client is attached, the server
returns the address to the client with lifetimes of 0.
If the server finds the addresses in the IA for the client then the
server SHOULD send back the IA to the client with new lifetimes and
T1/T2 times.
The server constructs a Reply message by setting the "msg-type" field
to REPLY, and copying the transaction ID from the Rebind message into
the transaction-id field.
The server MUST include a Server Identifier option containing the
server's DUID and the Client Identifier option from the Rebind
message in the Reply message.
The server includes other options containing configuration
information to be returned to the client as described in section
18.2.
18.2.5. Receipt of Information-request Messages
When the server receives an Information-request message, the client
is requesting configuration information that does not include the
assignment of any addresses. The server determines all configuration
parameters appropriate to the client, based on the server
configuration policies known to the server.
The server constructs a Reply message by setting the "msg-type" field
to REPLY, and copying the transaction ID from the Information-request
message into the transaction-id field.
The server MUST include a Server Identifier option containing the
server's DUID in the Reply message. If the client included a Client
Identification option in the Information-request message, the server
copies that option to the Reply message.
The server includes options containing configuration information to
be returned to the client as described in section 18.2.
If the Information-request message received from the client did not
include a Client Identifier option, the server SHOULD respond with a
Reply message containing any configuration parameters that are not
determined by the client's identity. If the server chooses not to
respond, the client may continue to retransmit the
Information-request message indefinitely.
18.2.6. Receipt of Release Messages
When the server receives a Release message via unicast from a client
to which the server has not sent a unicast option, the server
discards the Release message and responds with a Reply message
containing a Status Code option with value UseMulticast, a Server
Identifier option containing the server's DUID, the Client Identifier
option from the client message, and no other options.
Upon the receipt of a valid Release message, the server examines the
IAs and the addresses in the IAs for validity. If the IAs in the
message are in a binding for the client, and the addresses in the IAs
have been assigned by the server to those IAs, the server deletes the
addresses from the IAs and makes the addresses available for
assignment to other clients. The server ignores addresses not
assigned to the IA, although it may choose to log an error.
After all the addresses have been processed, the server generates a
Reply message and includes a Status Code option with value Success, a
Server Identifier option with the server's DUID, and a Client
Identifier option with the client's DUID. For each IA in the Release
message for which the server has no binding information, the server
adds an IA option using the IAID from the Release message, and
includes a Status Code option with the value NoBinding in the IA
option. No other options are included in the IA option.
A server may choose to retain a record of assigned addresses and IAs
after the lifetimes on the addresses have expired to allow the server
to reassign the previously assigned addresses to a client.
18.2.7. Receipt of Decline Messages
When the server receives a Decline message via unicast from a client
to which the server has not sent a unicast option, the server
discards the Decline message and responds with a Reply message
containing a Status Code option with the value UseMulticast, a Server
Identifier option containing the server's DUID, the Client Identifier
option from the client message, and no other options.
Upon the receipt of a valid Decline message, the server examines the
IAs and the addresses in the IAs for validity. If the IAs in the
message are in a binding for the client, and the addresses in the IAs
have been assigned by the server to those IAs, the server deletes the
addresses from the IAs. The server ignores addresses not assigned to
the IA (though it may choose to log an error if it finds such an
address).
The client has found any addresses in the Decline messages to be
already in use on its link. Therefore, the server SHOULD mark the
addresses declined by the client so that those addresses are not
assigned to other clients, and MAY choose to make a notification that
addresses were declined. Local policy on the server determines when
the addresses identified in a Decline message may be made available
for assignment.
After all the addresses have been processed, the server generates a
Reply message and includes a Status Code option with the value
Success, a Server Identifier option with the server's DUID, and a
Client Identifier option with the client's DUID. For each IA in the
Decline message for which the server has no binding information, the
server adds an IA option using the IAID from the Release message and
includes a Status Code option with the value NoBinding in the IA
option. No other options are included in the IA option.
18.2.8. Transmission of Reply Messages
If the original message was received directly by the server, the
server unicasts the Reply message directly to the client using the
address in the source address field from the IP datagram in which the
original message was received. The Reply message MUST be unicast
through the interface on which the original message was received.
If the original message was received in a Relay-forward message, the
server constructs a Relay-reply message with the Reply message in the
payload of a Relay Message option (see section 22.10). If the
Relay-forward messages included an Interface-id option, the server
copies that option to the Relay-reply message. The server unicasts
the Relay-reply message directly to the relay agent using the address
in the source address field from the IP datagram in which the
Relay-forward message was received.
19. DHCP Server-Initiated Configuration Exchange
A server initiates a configuration exchange to cause DHCP clients to
obtain new addresses and other configuration information. For
example, an administrator may use a server-initiated configuration
exchange when links in the DHCP domain are to be renumbered. Other
examples include changes in the location of Directory servers,
addition of new services such as printing, and availability of new
software.
19.1. Server Behavior
A server sends a Reconfigure message to cause a client to initiate
immediately a Renew/Reply or Information-request/Reply message
exchange with the server.
19.1.1. Creation and Transmission of Reconfigure Messages
The server sets the "msg-type" field to RECONFIGURE. The server sets
the transaction-id field to 0. The server includes a Server
Identifier option containing its DUID and a Client Identifier option
containing the client's DUID in the Reconfigure message.
The server MAY include an Option Request option to inform the client
of what information has been changed or new information that has been
added. In particular, the server specifies the IA option in the
Option Request option if the server wants the client to obtain new
address information. If the server identifies the IA option in the
Option Request option, the server MUST include an IA option that
contains no other sub-options to identify each IA that is to be
reconfigured on the client.
Because of the risk of denial of service attacks against DHCP
clients, the use of a security mechanism is mandated in Reconfigure
messages. The server MUST use DHCP authentication in the Reconfigure
message.
The server MUST include a Reconfigure Message option (defined in
section 22.19) to select whether the client responds with a Renew
message or an Information-Request message.
The server MUST NOT include any other options in the Reconfigure
except as specifically allowed in the definition of individual
options.
A server sends each Reconfigure message to a single DHCP client,
using an IPv6 unicast address of sufficient scope belonging to the
DHCP client. If the server does not have an address to which it can
send the Reconfigure message directly to the client, the server uses
a Relay-reply message (as described in section 20.3) to send the
Reconfigure message to a relay agent that will relay the message to
the client. The server may obtain the address of the client (and the
appropriate relay agent, if required) through the information the
server has about clients that have been in contact with the server,
or through some external agent.
To reconfigure more than one client, the server unicasts a separate
message to each client. The server may initiate the reconfiguration
of multiple clients concurrently; for example, a server may send a
Reconfigure message to additional clients while previous
reconfiguration message exchanges are still in progress.
The Reconfigure message causes the client to initiate a Renew/Reply
or Information-request/Reply message exchange with the server. The
server interprets the receipt of a Renew or Information-request
message (whichever was specified in the original Reconfigure message)
from the client as satisfying the Reconfigure message request.
19.1.2. Time Out and Retransmission of Reconfigure Messages
If the server does not receive a Renew or Information-request message
from the client in REC_TIMEOUT milliseconds, the server retransmits
the Reconfigure message, doubles the REC_TIMEOUT value and waits
again. The server continues this process until REC_MAX_RC
unsuccessful attempts have been made, at which point the server
SHOULD abort the reconfigure process for that client.
Default and initial values for REC_TIMEOUT and REC_MAX_RC are
documented in section 5.5.
19.2. Receipt of Renew Messages
The server generates and sends a Reply message to the client as
described in sections 18.2.3 and 18.2.8, including options for
configuration parameters.
The server MAY include options containing the IAs and new values for
other configuration parameters in the Reply message, even if those
IAs and parameters were not requested in the Renew message from the
client.
19.3. Receipt of Information-request Messages
The server generates and sends a Reply message to the client as
described in sections 18.2.5 and 18.2.8, including options for
configuration parameters.
The server MAY include options containing new values for other
configuration parameters in the Reply message, even if those
parameters were not requested in the Information-request message from
the client.
19.4. Client Behavior
A client receives Reconfigure messages sent to the UDP port 546 on
interfaces for which it has acquired configuration information
through DHCP. These messages may be sent at any time. Since the
results of a reconfiguration event may affect application layer
programs, the client SHOULD log these events, and MAY notify these
programs of the change through an implementation-specific interface.
19.4.1. Receipt of Reconfigure Messages
Upon receipt of a valid Reconfigure message, the client responds with
either a Renew message or an Information-request message as indicated
by the Reconfigure Message option (as defined in section 22.19). The
client ignores the transaction-id field in the received Reconfigure
message. While the transaction is in progress, the client silently
discards any Reconfigure messages it receives.
DISCUSSION:
The Reconfigure message acts as a trigger that signals the client
to complete a successful message exchange. Once the client has
received a Reconfigure, the client proceeds with the message
exchange (retransmitting the Renew or Information-request message
if necessary); the client ignores any additional Reconfigure
messages until the exchange is complete. Subsequent Reconfigure
messages cause the client to initiate a new exchange.
How does this mechanism work in the face of duplicated or
retransmitted Reconfigure messages? Duplicate messages will be
ignored because the client will begin the exchange after the
receipt of the first Reconfigure. Retransmitted messages will
either trigger the exchange (if the first Reconfigure was not
received by the client) or will be ignored. The server can
discontinue retransmission of Reconfigure messages to the client
once the server receives the Renew or Information-request message
from the client.
It might be possible for a duplicate or retransmitted Reconfigure
to be sufficiently delayed (and delivered out of order) to arrive
at the client after the exchange (initiated by the original
Reconfigure) has been completed. In this case, the client would
initiate a redundant exchange. The likelihood of delayed and out
of order delivery is small enough to be ignored. The consequence
of the redundant exchange is inefficiency rather than incorrect
operation.
19.4.2. Creation and Transmission of Renew Messages
When responding to a Reconfigure, the client creates and sends the
Renew message in exactly the same manner as outlined in section
18.1.3, with the exception that the client copies the Option Request
option and any IA options from the Reconfigure message into the Renew
message.
19.4.3. Creation and Transmission of Information-request Messages
When responding to a Reconfigure, the client creates and sends the
Information-request message in exactly the same manner as outlined in
section 18.1.5, with the exception that the client includes a Server
Identifier option with the identifier from the Reconfigure message to
which the client is responding.
19.4.4. Time Out and Retransmission of Renew or Information-request
Messages
The client uses the same variables and retransmission algorithm as it
does with Renew or Information-request messages generated as part of
a client-initiated configuration exchange. See sections 18.1.3 and
18.1.5 for details. If the client does not receive a response from
the server by the end of the retransmission process, the client
ignores and discards the Reconfigure message.
19.4.5. Receipt of Reply Messages
Upon the receipt of a valid Reply message, the client processes the
options and sets (or resets) configuration parameters appropriately.
The client records and updates the lifetimes for any addresses
specified in IAs in the Reply message.
20. Relay Agent Behavior
The relay agent MAY be configured to use a list of destination
addresses, which MAY include unicast addresses, the All_DHCP_Servers
multicast address, or other addresses selected by the network
administrator. If the relay agent has not been explicitly
configured, it MUST use the All_DHCP_Servers multicast address as the
default.
If the relay agent relays messages to the All_DHCP_Servers multicast
address or other multicast addresses, it sets the Hop Limit field to
32.
20.1. Relaying a Client Message or a Relay-forward Message
A relay agent relays both messages from clients and Relay-forward
messages from other relay agents. When a relay agent receives a
valid message to be relayed, it constructs a new Relay-forward
message. The relay agent copies the source address from the header
of the IP datagram in which the message was received to the
peer-address field of the Relay-forward message. The relay agent
copies the received DHCP message (excluding any IP or UDP headers)
into a Relay Message option in the new message. The relay agent adds
to the Relay-forward message any other options it is configured to
include.
20.1.1. Relaying a Message from a Client
If the relay agent received the message to be relayed from a client,
the relay agent places a global or site-scoped address with a prefix
assigned to the link on which the client should be assigned an
address in the link-address field. This address will be used by the
server to determine the link from which the client should be assigned
an address and other configuration information. The hop-count in the
Relay-forward message is set to 0.
If the relay agent cannot use the address in the link-address field
to identify the interface through which the response to the client
will be relayed, the relay agent MUST include an Interface-id option
(see section 22.18) in the Relay-forward message. The server will
include the Interface-id option in its Relay-reply message. The
relay agent fills in the link-address field as described in the
previous paragraph regardless of whether the relay agent includes an
Interface-id option in the Relay-forward message.
20.1.2. Relaying a Message from a Relay Agent
If the message received by the relay agent is a Relay-forward message
and the hop-count in the message is greater than or equal to
HOP_COUNT_LIMIT, the relay agent discards the received message.
The relay agent copies the source address from the IP datagram in
which the message was received from the client into the peer-address
field in the Relay-forward message and sets the hop-count field to
the value of the hop-count field in the received message incremented
by 1.
If the source address from the IP datagram header of the received
message is a global or site-local address (and the device on which
the relay agent is running belongs to only one site), the relay agent
sets the link-address field to 0; otherwise the relay agent sets the
link-address field to a global or site-local address assigned to the
interface on which the message was received, or includes an
Interface-ID option to identify the interface on which the message
was received.
20.2. Relaying a Relay-reply Message
The relay agent processes any options included in the Relay-reply
message in addition to the Relay Message option, and then discards
those options.
The relay agent extracts the message from the Relay Message option
and relays it to the address contained in the peer-address field of
the Relay-reply message.
If the Relay-reply message includes an Interface-id option, the relay
agent relays the message from the server to the client on the link
identified by the Interface-id option. Otherwise, if the
link-address field is not set to zero, the relay agent relays the
message on the link identified by the link-address field.
20.3. Construction of Relay-reply Messages
A server uses a Relay-reply message to return a response to a client
if the original message from the client was relayed to the server in
a Relay-forward message or to send a Reconfigure message to a client
if the server does not have an address it can use to send the message
directly to the client.
A response to the client MUST be relayed through the same relay
agents as the original client message. The server causes this to
happen by creating a Relay-reply message that includes a Relay
Message option containing the message for the next relay agent in the
return path to the client. The contained Relay-reply message
contains another Relay Message option to be sent to the next relay
agent, and so on. The server must record the contents of the
peer-address fields in the received message so it can construct the
appropriate Relay-reply message carrying the response from the
server.
For example, if client C sent a message that was relayed by relay
agent A to relay agent B and then to the server, the server would
send the following Relay-Reply message to relay agent B:
msg-type: RELAY-REPLY
hop-count: 1
link-address: 0
peer-address: A
Relay Message option, containing:
msg-type: RELAY-REPLY
hop-count: 0
link-address: address from link to which C is attached
peer-address: C
Relay Message option: <response from server>
When sending a Reconfigure message to a client through a relay agent,
the server creates a Relay-reply message that includes a Relay
Message option containing the Reconfigure message for the next relay
agent in the return path to the client. The server sets the
peer-address field in the Relay-reply message header to the address
of the client, and sets the link-address field as required by the
relay agent to relay the Reconfigure message to the client. The
server obtains the addresses of the client and the relay agent
through prior interaction with the client or through some external
mechanism.
21. Authentication of DHCP Messages
Some network administrators may wish to provide authentication of the
source and contents of DHCP messages. For example, clients may be
subject to denial of service attacks through the use of bogus DHCP
servers, or may simply be misconfigured due to unintentionally
instantiated DHCP servers. Network administrators may wish to
constrain the allocation of addresses to authorized hosts to avoid
denial of service attacks in "hostile" environments where the network
medium is not physically secured, such as wireless networks or
college residence halls.
The DHCP authentication mechanism is based on the design of
authentication for DHCPv4 [4].
21.1. Security of Messages Sent Between Servers and Relay Agents
Relay agents and servers that exchange messages securely use the
IPsec mechanisms for IPv6 [7]. If a client message is relayed
through multiple relay agents, each of the relay agents must have
established independent, pairwise trust relationships. That is, if
messages from client C will be relayed by relay agent A to relay
agent B and then to the server, relay agents A and B must be
configured to use IPSec for the messages they exchange, and relay
agent B and the server must be configured to use IPSec for the
messages they exchange.
Relay agents and servers that support secure relay agent to server or
relay agent to relay agent communication use IPsec under the
following conditions:
Selectors Relay agents are manually configured with the
addresses of the relay agent or server to which
DHCP messages are to be forwarded. Each relay
agent and server that will be using IPsec for
securing DHCP messages must also be configured
with a list of the relay agents to which messages
will be returned. The selectors for the relay
agents and servers will be the pairs of addresses
defining relay agents and servers that exchange
DHCP messages on the DHCPv6 UDP ports 546 and
547.
Mode Relay agents and servers use transport mode and
ESP. The information in DHCP messages is not
generally considered confidential, so encryption
need not be used (i.e., NULL encryption can be
used).
Key management Because the relay agents and servers are used
within an organization, public key schemes are
not necessary. Because the relay agents and
servers must be manually configured, manually
configured key management may suffice, but does
not provide defense against replayed messages.
Accordingly, IKE with preshared secrets SHOULD be
supported. IKE with public keys MAY be
supported.
Security policy DHCP messages between relay agents and servers
should only be accepted from DHCP peers as
identified in the local configuration.
Authentication Shared keys, indexed to the source IP address of
the received DHCP message, are adequate in this
application.
Availability Appropriate IPsec implementations are likely to
be available for servers and for relay agents in
more featureful devices used in enterprise and
core ISP networks. IPsec is less likely to be
available for relay agents in low end devices
primarily used in the home or small Office
markets.
21.2. Summary of DHCP Authentication
Authentication of DHCP messages is accomplished through the use of
the Authentication option (see section 22.11). The authentication
information carried in the Authentication option can be used to
reliably identify the source of a DHCP message and to confirm that
the contents of the DHCP message have not been tampered with.
The Authentication option provides a framework for multiple
authentication protocols. Two such protocols are defined here.
Other protocols defined in the future will be specified in separate
documents.
Any DHCP message MUST NOT include more than one Authentication
option.
The protocol field in the Authentication option identifies the
specific protocol used to generate the authentication information
carried in the option. The algorithm field identifies a specific
algorithm within the authentication protocol; for example, the
algorithm field specifies the hash algorithm used to generate the
message authentication code (MAC) in the authentication option. The
replay detection method (RDM) field specifies the type of replay
detection used in the replay detection field.
21.3. Replay Detection
The Replay Detection Method (RDM) field determines the type of replay
detection used in the Replay Detection field.
If the RDM field contains 0x00, the replay detection field MUST be
set to the value of a monotonically increasing counter. Using a
counter value, such as the current time of day (for example, an NTP-
format timestamp [9]), can reduce the danger of replay attacks. This
method MUST be supported by all protocols.
21.4. Delayed Authentication Protocol
If the protocol field is 2, the message is using the "delayed
authentication" mechanism. In delayed authentication, the client
requests authentication in its Solicit message, and the server
replies with an Advertise message that includes authentication
information. This authentication information contains a nonce value
generated by the source as a message authentication code (MAC) to
provide message authentication and entity authentication.
The use of a particular technique based on the HMAC protocol [8]
using the MD5 hash [16] is defined here.
21.4.1. Use of the Authentication Option in the Delayed Authentication
Protocol
In a Solicit message, the client fills in the protocol, algorithm and
RDM fields in the Authentication option with the client's
preferences. The client sets the replay detection field to zero and
omits the authentication information field. The client sets the
option-len field to 11.
In all other messages, the protocol and algorithm fields identify the
method used to construct the contents of the authentication
information field. The RDM field identifies the method used to
construct the contents of the replay detection field.
The format of the Authentication information is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DHCP realm
(variable length)
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
key ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
HMAC-MD5
(128 bits)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DHCP realm The DHCP realm that identifies the key used to
generate the HMAC-MD5 value.
key ID The key identifier that identified the key used to
generate the HMAC-MD5 value.
HMAC-MD5 The message authentication code generated by applying
MD5 to the DHCP message using the key identified by
the DHCP realm, client DUID, and key ID.
The sender computes the MAC using the HMAC generation algorithm [8]
and the MD5 hash function [16]. The entire DHCP message (setting the
MAC field of the authentication option to zero), including the DHCP
message header and the options field, is used as input to the HMAC-
MD5 computation function.
DISCUSSION:
Algorithm 1 specifies the use of HMAC-MD5. Use of a different
technique, such as HMAC-SHA, will be specified as a separate
protocol.
The DHCP realm used to identify authentication keys is chosen to
be unique among administrative domains. Use of the DHCP realm
allows DHCP administrators to avoid conflict in the use of key
identifiers, and allows a host using DHCP to use authenticated
DHCP while roaming among DHCP administrative domains.
21.4.2. Message Validation
Any DHCP message that includes more than one authentication option
MUST be discarded.
To validate an incoming message, the receiver first checks that the
value in the replay detection field is acceptable according to the
replay detection method specified by the RDM field. Next, the
receiver computes the MAC as described in [8]. The entire DHCP
message (setting the MAC field of the authentication option to 0) is
used as input to the HMAC-MD5 computation function. If the MAC
computed by the receiver does not match the MAC contained in the
authentication option, the receiver MUST discard the DHCP message.
21.4.3. Key Utilization
Each DHCP client has a set of keys. Each key is identified by <DHCP
realm, client DUID, key id>. Each key also has a lifetime. The key
may not be used past the end of its lifetime. The client's keys are
initially distributed to the client through some out-of-band
mechanism. The lifetime for each key is distributed with the key.
Mechanisms for key distribution and lifetime specification are beyond
the scope of this document.
The client and server use one of the client's keys to authenticate
DHCP messages during a session (until the next Solicit message sent
by the client).
21.4.4. Client Considerations for Delayed Authentication Protocol
The client announces its intention to use DHCP authentication by
including an Authentication option in its Solicit message. The
server selects a key for the client based on the client's DUID. The
client and server use that key to authenticate all DHCP messages
exchanged during the session.
21.4.4.1. Sending Solicit Messages
When the client sends a Solicit message and wishes to use
authentication, it includes an Authentication option with the desired
protocol, algorithm and RDM as described in section 21.4. The client
does not include any replay detection or authentication information
in the Authentication option.
21.4.4.2. Receiving Advertise Messages
The client validates any Advertise messages containing an
Authentication option specifying the delayed authentication protocol
using the validation test described in section 21.4.2.
Client behavior, if no Advertise messages include authentication
information or pass the validation test, is controlled by local
policy on the client. According to client policy, the client MAY
choose to respond to an Advertise message that has not been
authenticated.
The decision to set local policy to accept unauthenticated messages
should be made with care. Accepting an unauthenticated Advertise
message can make the client vulnerable to spoofing and other attacks.
If local users are not explicitly informed that the client has
accepted an unauthenticated Advertise message, the users may
incorrectly assume that the client has received an authenticated
address and is not subject to DHCP attacks through unauthenticated
messages.
A client MUST be configurable to discard unauthenticated messages,
and SHOULD be configured by default to discard unauthenticated
messages if the client has been configured with an authentication key
or other authentication information. A client MAY choose to
differentiate between Advertise messages with no authentication
information and Advertise messages that do not pass the validation
test; for example, a client might accept the former and discard the
latter. If a client does accept an unauthenticated message, the
client SHOULD inform any local users and SHOULD log the event.
21.4.4.3. Sending Request, Confirm, Renew, Rebind, Decline or Release
Messages
If the client authenticated the Advertise message through which the
client selected the server, the client MUST generate authentication
information for subsequent Request, Confirm, Renew, Rebind or Release
messages sent to the server, as described in section 21.4. When the
client sends a subsequent message, it MUST use the same key used by
the server to generate the authentication information.
21.4.4.4. Sending Information-request Messages
If the server has selected a key for the client in a previous message
exchange (see section 21.4.5.1), the client MUST use the same key to
generate the authentication information throughout the session.
21.4.4.5. Receiving Reply Messages
If the client authenticated the Advertise it accepted, the client
MUST validate the associated Reply message from the server. The
client MUST discard the Reply if the message fails to pass the
validation test and MAY log the validation failure. If the Reply
fails to pass the validation test, the client MUST restart the DHCP
configuration process by sending a Solicit message.
If the client accepted an Advertise message that did not include
authentication information or did not pass the validation test, the
client MAY accept an unauthenticated Reply message from the server.
21.4.4.6. Receiving Reconfigure Messages
The client MUST discard the Reconfigure if the message fails to pass
the validation test and MAY log the validation failure.
21.4.5. Server Considerations for Delayed Authentication Protocol
After receiving a Solicit message that contains an Authentication
option, the server selects a key for the client, based on the
client's DUID and key selection policies with which the server has
been configured. The server identifies the selected key in the
Advertise message and uses the key to validate subsequent messages
between the client and the server.
21.4.5.1. Receiving Solicit Messages and Sending Advertise Messages
The server selects a key for the client and includes authentication
information in the Advertise message returned to the client as
specified in section 21.4. The server MUST record the identifier of
the key selected for the client and use that same key for validating
subsequent messages with the client.
21.4.5.2. Receiving Request, Confirm, Renew, Rebind or Release Messages
and Sending Reply Messages
The server uses the key identified in the message and validates the
message as specified in section 21.4.2. If the message fails to pass
the validation test or the server does not know the key identified by
the 'key ID' field, the server MUST discard the message and MAY
choose to log the validation failure.
If the message passes the validation test, the server responds to the
specific message as described in section 18.2. The server MUST
include authentication information generated using the key identified
in the received message, as specified in section 21.4.
21.5. Reconfigure Key Authentication Protocol
The Reconfigure key authentication protocol provides protection
against misconfiguration of a client caused by a Reconfigure message
sent by a malicious DHCP server. In this protocol, a DHCP server
sends a Reconfigure Key to the client in the initial exchange of DHCP
messages. The client records the Reconfigure Key for use in
authenticating subsequent Reconfigure messages from that server. The
server then includes an HMAC computed from the Reconfigure Key in
subsequent Reconfigure messages.
Both the Reconfigure Key sent from the server to the client and the
HMAC in subsequent Reconfigure messages are carried as the
Authentication information in an Authentication option. The format
of the Authentication information is defined in the following
section.
The Reconfigure Key protocol is used (initiated by the server) only
if the client and server are not using any other authentication
protocol and the client and server have negotiated to use Reconfigure
messages.
21.5.1. Use of the Authentication Option in the Reconfigure Key
Authentication Protocol
The following fields are set in an Authentication option for the
Reconfigure Key Authentication Protocol:
protocol 3
algorithm 1
RDM 0
The format of the Authentication information for the Reconfigure Key
Authentication Protocol is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Value (128 bits)
+-+-+-+-+-+-+-+-+
. .
. .
. +-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Type of data in Value field carried in this option:
1 Reconfigure Key value (used in Reply message).
2 HMAC-MD5 digest of the message (used in Reconfigure
message).
Value Data as defined by field.
21.5.2. Server considerations for Reconfigure Key protocol
The server selects a Reconfigure Key for a client during the
Request/Reply, Solicit/Reply or Information-request/Reply message
exchange. The server records the Reconfigure Key and transmits that
key to the client in an Authentication option in the Reply message.
The Reconfigure Key is 128 bits long, and MUST be a cryptographically
strong random or pseudo-random number that cannot easily be
predicted.
To provide authentication for a Reconfigure message, the server
selects a replay detection value according to the RDM selected by the
server, and computes an HMAC-MD5 of the Reconfigure message using the
Reconfigure Key for the client. The server computes the HMAC-MD5
over the entire DHCP Reconfigure message, including the
Authentication option; the HMAC-MD5 field in the Authentication
option is set to zero for the HMAC-MD5 computation. The server
includes the HMAC-MD5 in the authentication information field in an
Authentication option included in the Reconfigure message sent to the
client.
21.5.3. Client considerations for Reconfigure Key protocol
The client will receive a Reconfigure Key from the server in the
initial Reply message from the server. The client records the
Reconfigure Key for use in authenticating subsequent Reconfigure
messages.
To authenticate a Reconfigure message, the client computes an
HMAC-MD5 over the DHCP Reconfigure message, using the Reconfigure Key
received from the server. If this computed HMAC-MD5 matches the
value in the Authentication option, the client accepts the
Reconfigure message.
22. DHCP Options
Options are used to carry additional information and parameters in
DHCP messages. Every option shares a common base format, as
described in section 22.1. All values in options are represented in
network byte order.
This document describes the DHCP options defined as part of the base
DHCP specification. Other options may be defined in the future in
separate documents.
Unless otherwise noted, each option may appear only in the options
area of a DHCP message and may appear only once. If an option does
appear multiple times, each instance is considered separate and the
data areas of the options MUST NOT be concatenated or otherwise
combined.
22.1. Format of DHCP Options
The format of DHCP options is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-data
(option-len octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code An unsigned integer identifying the specific option
type carried in this option.
option-len An unsigned integer giving the length of the
option-data field in this option in octets.
option-data The data for the option; the format of this data
depends on the definition of the option.
DHCPv6 options are scoped by using encapsulation. Some options apply
generally to the client, some are specific to an IA, and some are
specific to the addresses within an IA. These latter two cases are
discussed in sections 22.4 and 22.6.
22.2. Client Identifier Option
The Client Identifier option is used to carry a DUID (see section 9)
identifying a client between a client and a server. The format of
the Client Identifier option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_CLIENTID option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. DUID .
. (variable length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_CLIENTID (1).
option-len Length of DUID in octets.
DUID The DUID for the client.
22.3. Server Identifier Option
The Server Identifier option is used to carry a DUID (see section 9)
identifying a server between a client and a server. The format of
the Server Identifier option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_SERVERID option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. DUID .
. (variable length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_SERVERID (2).
option-len Length of DUID in octets.
DUID The DUID for the server.
22.4. Identity Association for Non-temporary Addresses Option
The Identity Association for Non-temporary Addresses option (IA_NA
option) is used to carry an IA_NA, the parameters associated with the
IA_NA, and the non-temporary addresses associated with the IA_NA.
Addresses appearing in an IA_NA option are not temporary addresses
(see section 22.5).
The format of the IA_NA option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_IA_NA option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IAID (4 octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
T1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
T2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. IA_NA-options .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_IA_NA (3).
option-len 12 + length of IA_NA-options field.
IAID The unique identifier for this IA_NA; the
IAID must be unique among the identifiers for
all of this client's IA_NAs. The number
space for IA_NA IAIDs is separate from the
number space for IA_TA IAIDs.
T1 The time at which the client contacts the
server from which the addresses in the IA_NA
were obtained to extend the lifetimes of the
addresses assigned to the IA_NA; T1 is a
time duration relative to the current time
expressed in units of seconds.
T2 The time at which the client contacts any
available server to extend the lifetimes of
the addresses assigned to the IA_NA; T2 is a
time duration relative to the current time
expressed in units of seconds.
IA_NA-options Options associated with this IA_NA.
The IA_NA-options field encapsulates those options that are specific
to this IA_NA. For example, all of the IA Address Options carrying
the addresses associated with this IA_NA are in the IA_NA-options
field.
An IA_NA option may only appear in the options area of a DHCP
message. A DHCP message may contain multiple IA_NA options.
The status of any operations involving this IA_NA is indicated in a
Status Code option in the IA_NA-options field.
Note that an IA_NA has no explicit "lifetime" or "lease length" of
its own. When the valid lifetimes of all of the addresses in an
IA_NA have expired, the IA_NA can be considered as having expired.
T1 and T2 are included to give servers explicit control over when a
client recontacts the server about a specific IA_NA.
In a message sent by a client to a server, values in the T1 and T2
fields indicate the client's preference for those parameters. The
client sets T1 and T2 to 0 if it has no preference for those values.
In a message sent by a server to a client, the client MUST use the
values in the T1 and T2 fields for the T1 and T2 parameters, unless
those values in those fields are 0. The values in the T1 and T2
fields are the number of seconds until T1 and T2.
The server selects the T1 and T2 times to allow the client to extend
the lifetimes of any addresses in the IA_NA before the lifetimes
expire, even if the server is unavailable for some short period of
time. Recommended values for T1 and T2 are .5 and .8 times the
shortest preferred lifetime of the addresses in the IA that the
server is willing to extend, respectively. If the "shortest"
preferred lifetime is 0xffffffff ("infinity"), the recommended T1 and
T2 values are also 0xffffffff. If the time at which the addresses in
an IA_NA are to be renewed is to be left to the discretion of the
client, the server sets T1 and T2 to 0.
If a server receives an IA_NA with T1 greater than T2, and both T1
and T2 are greater than 0, the server ignores the invalid values of
T1 and T2 and processes the IA_NA as though the client had set T1 and
T2 to 0.
If a client receives an IA_NA with T1 greater than T2, and both T1
and T2 are greater than 0, the client discards the IA_NA option and
processes the remainder of the message as though the server had not
included the invalid IA_NA option.
Care should be taken in setting T1 or T2 to 0xffffffff ("infinity").
A client will never attempt to extend the lifetimes of any addresses
in an IA with T1 set to 0xffffffff. A client will never attempt to
use a Rebind message to locate a different server to extend the
lifetimes of any addresses in an IA with T2 set to 0xffffffff.
22.5. Identity Association for Temporary Addresses Option
The Identity Association for the Temporary Addresses (IA_TA) option
is used to carry an IA_TA, the parameters associated with the IA_TA
and the addresses associated with the IA_TA. All of the addresses in
this option are used by the client as temporary addresses, as defined
in RFC3041 [12]. The format of the IA_TA option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_IA_TA option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IAID (4 octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. IA_TA-options .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_IA_TA (4).
option-len 4 + length of IA_TA-options field.
IAID The unique identifier for this IA_TA; the
IAID must be unique among the identifiers
for all of this client's IA_TAs. The number
space for IA_TA IAIDs is separate from the
number space for IA_NA IAIDs.
IA_TA-options Options associated with this IA_TA.
The IA_TA-Options field encapsulates those options that are specific
to this IA_TA. For example, all of the IA Address Options carrying
the addresses associated with this IA_TA are in the IA_TA-options
field.
Each IA_TA carries one "set" of temporary addresses; that is, at most
one address from each prefix assigned to the link to which the client
is attached.
An IA_TA option may only appear in the options area of a DHCP
message. A DHCP message may contain multiple IA_TA options.
The status of any operations involving this IA_TA is indicated in a
Status Code option in the IA_TA-options field.
Note that an IA has no explicit "lifetime" or "lease length" of its
own. When the valid lifetimes of all of the addresses in an IA_TA
have expired, the IA can be considered as having expired.
An IA_TA option does not include values for T1 and T2. A client MAY
request that the lifetimes on temporary addresses be extended by
including the addresses in a IA_TA option sent in a Renew or Rebind
message to a server. For example, a client would request an
extension on the lifetime of a temporary address to allow an
application to continue to use an established TCP connection.
The client obtains new temporary addresses by sending an IA_TA option
with a new IAID to a server. Requesting new temporary addresses from
the server is the equivalent of generating new temporary addresses as
described in RFC3041. The server will generate new temporary
addresses and return them to the client. The client should request
new temporary addresses before the lifetimes on the previously
assigned addresses expire.
A server MUST return the same set of temporary address for the same
IA_TA (as identified by the IAID) as long as those addresses are
still valid. After the lifetimes of the addresses in an IA_TA have
expired, the IAID may be reused to identify a new IA_TA with new
temporary addresses.
This option MAY appear in a Confirm message if the lifetimes on the
temporary addresses in the associated IA have not expired.
22.6. IA Address Option
The IA Address option is used to specify IPv6 addresses associated
with an IA_NA or an IA_TA. The IA Address option must be
encapsulated in the Options field of an IA_NA or IA_TA option. The
Options field encapsulates those options that are specific to this
address.
The format of the IA Address option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_IAADDR option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IPv6 address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
preferred-lifetime
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
valid-lifetime
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. IAaddr-options .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_IAADDR (5).
option-len 24 + length of IAaddr-options field.
IPv6 address An IPv6 address.
preferred-lifetime The preferred lifetime for the IPv6 address in
the option, expressed in units of seconds.
valid-lifetime The valid lifetime for the IPv6 address in the
option, expressed in units of seconds.
IAaddr-options Options associated with this address.
In a message sent by a client to a server, values in the preferred
and valid lifetime fields indicate the client's preference for those
parameters. The client may send 0 if it has no preference for the
preferred and valid lifetimes. In a message sent by a server to a
client, the client MUST use the values in the preferred and valid
lifetime fields for the preferred and valid lifetimes. The values in
the preferred and valid lifetimes are the number of seconds remaining
in each lifetime.
A client discards any addresses for which the preferred lifetime is
greater than the valid lifetime. A server ignores the lifetimes set
by the client if the preferred lifetime is greater than the valid
lifetime and ignores the values for T1 and T2 set by the client if
those values are greater than the preferred lifetime.
Care should be taken in setting the valid lifetime of an address to
0xffffffff ("infinity"), which amounts to a permanent assignment of
an address to a client.
An IA Address option may appear only in an IA_NA option or an IA_TA
option. More than one IA Address Option can appear in an IA_NA
option or an IA_TA option.
The status of any operations involving this IA Address is indicated
in a Status Code option in the IAaddr-options field.
22.7. Option Request Option
The Option Request option is used to identify a list of options in a
message between a client and a server. The format of the Option
Request option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_ORO option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
requested-option-code-1 requested-option-code-2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_ORO (6).
option-len 2 * number of requested options.
requested-option-code-n The option code for an option requested by
the client.
A client MAY include an Option Request option in a Solicit, Request,
Renew, Rebind, Confirm or Information-request message to inform the
server about options the client wants the server to send to the
client. A server MAY include an Option Request option in a
Reconfigure option to indicate which options the client should
request from the server.
22.8. Preference Option
The Preference option is sent by a server to a client to affect the
selection of a server by the client.
The format of the Preference option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_PREFERENCE option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
pref-value
+-+-+-+-+-+-+-+-+
option-code OPTION_PREFERENCE (7).
option-len 1.
pref-value The preference value for the server in this message.
A server MAY include a Preference option in an Advertise message to
control the selection of a server by the client. See section 17.1.3
for the use of the Preference option by the client and the
interpretation of Preference option data value.
22.9. Elapsed Time Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_ELAPSED_TIME option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
elapsed-time
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_ELAPSED_TIME (8).
option-len 2.
elapsed-time The amount of time since the client began its
current DHCP transaction. This time is expressed in
hundredths of a second (10^-2 seconds).
A client MUST include an Elapsed Time option in messages to indicate
how long the client has been trying to complete a DHCP message
exchange. The elapsed time is measured from the time at which the
client sent the first message in the message exchange, and the
elapsed-time field is set to 0 in the first message in the message
exchange. Servers and Relay Agents use the data value in this option
as input to policy controlling how a server responds to a client
message. For example, the elapsed time option allows a secondary
DHCP server to respond to a request when a primary server has not
answered in a reasonable time. The elapsed time value is an
unsigned, 16 bit integer. The client uses the value 0xffff to
represent any elapsed time values greater than the largest time value
that can be represented in the Elapsed Time option.
22.10. Relay Message Option
The Relay Message option carries a DHCP message in a Relay-forward or
Relay-reply message.
The format of the Relay Message option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_RELAY_MSG option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. DHCP-relay-message .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_RELAY_MSG (9)
option-len Length of DHCP-relay-message
DHCP-relay-message In a Relay-forward message, the received
message, relayed verbatim to the next relay agent
or server; in a Relay-reply message, the message to
be copied and relayed to the relay agent or client
whose address is in the peer-address field of the
Relay-reply message
22.11. Authentication Option
The Authentication option carries authentication information to
authenticate the identity and contents of DHCP messages. The use of
the Authentication option is described in section 21. The format of
the Authentication option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_AUTH option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
protocol algorithm RDM
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
replay detection (64 bits) +-+-+-+-+-+-+-+-+
auth-info
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. authentication information .
. (variable length) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_AUTH (11)
option-len 11 + length of authentication
information field
protocol The authentication protocol used in
this authentication option
algorithm The algorithm used in the
authentication protocol
RDM The replay detection method used in
this authentication option
Replay detection The replay detection information for
the RDM
authentication information The authentication information,
as specified by the protocol and
algorithm used in this authentication
option
22.12. Server Unicast Option
The server sends this option to a client to indicate to the client
that it is allowed to unicast messages to the server. The format of
the Server Unicast option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_UNICAST option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
server-address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_UNICAST (12).
option-len 16.
server-address The IP address to which the client should send
messages delivered using unicast.
The server specifies the IPv6 address to which the client is to send
unicast messages in the server-address field. When a client receives
this option, where permissible and appropriate, the client sends
messages directly to the server using the IPv6 address specified in
the server-address field of the option.
When the server sends a Unicast option to the client, some messages
from the client will not be relayed by Relay Agents, and will not
include Relay Agent options from the Relay Agents. Therefore, a
server should only send a Unicast option to a client when Relay
Agents are not sending Relay Agent options. A DHCP server rejects
any messages sent inappropriately using unicast to ensure that
messages are relayed by Relay Agents when Relay Agent options are in
use.
Details about when the client may send messages to the server using
unicast are in section 18.
22.13. Status Code Option
This option returns a status indication related to the DHCP message
or option in which it appears. The format of the Status Code option
is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_STATUS_CODE option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
status-code
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. status-message .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_STATUS_CODE (13).
option-len 2 + length of status-message.
status-code The numeric code for the status encoded in
this option. The status codes are defined in
section 24.4.
status-message A UTF-8 encoded text string suitable for
display to an end user, which MUST NOT be
null-terminated.
A Status Code option may appear in the options field of a DHCP
message and/or in the options field of another option. If the Status
Code option does not appear in a message in which the option could
appear, the status of the message is assumed to be Success.
22.14. Rapid Commit Option
The Rapid Commit option is used to signal the use of the two message
exchange for address assignment. The format of the Rapid Commit
option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_RAPID_COMMIT 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_RAPID_COMMIT (14).
option-len 0.
A client MAY include this option in a Solicit message if the client
is prepared to perform the Solicit-Reply message exchange described
in section 17.1.1.
A server MUST include this option in a Reply message sent in response
to a Solicit message when completing the Solicit-Reply message
exchange.
DISCUSSION:
Each server that responds with a Reply to a Solicit that includes
a Rapid Commit option will commit the assigned addresses in the
Reply message to the client, and will not receive any confirmation
that the client has received the Reply message. Therefore, if
more than one server responds to a Solicit that includes a Rapid
Commit option, some servers will commit addresses that are not
actually used by the client.
The problem of unused addresses can be minimized, for example, by
designing the DHCP service so that only one server responds to the
Solicit or by using relatively short lifetimes for assigned
addresses.
22.15. User Class Option
The User Class option is used by a client to identify the type or
category of user or applications it represents.
The format of the User Class option is:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_USER_CLASS option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. user-class-data .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_USER_CLASS (15).
option-len Length of user class data field.
user-class-data The user classes carried by the client.
The information contained in the data area of this option is
contained in one or more opaque fields that represent the user class
or classes of which the client is a member. A server selects
configuration information for the client based on the classes
identified in this option. For example, the User Class option can be
used to configure all clients of people in the accounting department
with a different printer than clients of people in the marketing
department. The user class information carried in this option MUST
be configurable on the client.
The data area of the user class option MUST contain one or more
instances of user class data. Each instance of the user class data
is formatted as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
user-class-len opaque-data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
The user-class-len is two octets long and specifies the length of the
opaque user class data in network byte order.
A server interprets the classes identified in this option according
to its configuration to select the appropriate configuration
information for the client. A server may use only those user classes
that it is configured to interpret in selecting configuration
information for a client and ignore any other user classes. In
response to a message containing a User Class option, a server
includes a User Class option containing those classes that were
successfully interpreted by the server, so that the client can be
informed of the classes interpreted by the server.
22.16. Vendor Class Option
This option is used by a client to identify the vendor that
manufactured the hardware on which the client is running. The
information contained in the data area of this option is contained in
one or more opaque fields that identify details of the hardware
configuration. The format of the Vendor Class option is:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_VENDOR_CLASS option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
enterprise-number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. vendor-class-data .
. . . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_VENDOR_CLASS (16).
option-len 4 + length of vendor class data field.
enterprise-number The vendor's registered Enterprise Number as
registered with IANA [6].
vendor-class-data The hardware configuration of the host on
which the client is running.
The vendor-class-data is composed of a series of separate items, each
of which describes some characteristic of the client's hardware
configuration. Examples of vendor-class-data instances might include
the version of the operating system the client is running or the
amount of memory installed on the client.
Each instance of the vendor-class-data is formatted as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
vendor-class-len opaque-data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
The vendor-class-len is two octets long and specifies the length of
the opaque vendor class data in network byte order.
22.17. Vendor-specific Information Option
This option is used by clients and servers to exchange
vendor-specific information.
The format of the Vendor-specific Information option is:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_VENDOR_OPTS option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
enterprise-number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. option-data .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_VENDOR_OPTS (17)
option-len 4 + length of option-data field
enterprise-number The vendor's registered Enterprise Number as
registered with IANA [6].
option-data An opaque object of option-len octets,
interpreted by vendor-specific code on the
clients and servers
The definition of the information carried in this option is vendor
specific. The vendor is indicated in the enterprise-number field.
Use of vendor-specific information allows enhanced operation,
utilizing additional features in a vendor's DHCP implementation. A
DHCP client that does not receive requested vendor-specific
information will still configure the host device's IPv6 stack to be
functional.
The encapsulated vendor-specific options field MUST be encoded as a
sequence of code/length/value fields of identical format to the DHCP
options field. The option codes are defined by the vendor identified
in the enterprise-number field and are not managed by IANA. Each of
the encapsulated options is formatted as follows:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt-code option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. option-data .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
opt-code The code for the encapsulated option.
option-len An unsigned integer giving the length of the
option-data field in this encapsulated option
in octets.
option-data The data area for the encapsulated option.
Multiple instances of the Vendor-specific Information option may
appear in a DHCP message. Each instance of the option is interpreted
according to the option codes defined by the vendor identified by the
Enterprise Number in that option.
22.18. Interface-Id Option
The relay agent MAY send the Interface-id option to identify the
interface on which the client message was received. If a relay agent
receives a Relay-reply message with an Interface-id option, the relay
agent relays the message to the client through the interface
identified by the option.
The format of the Interface ID option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_INTERFACE_ID option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. interface-id .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_INTERFACE_ID (18).
option-len Length of interface-id field.
interface-id An opaque value of arbitrary length generated
by the relay agent to identify one of the
relay agent's interfaces.
The server MUST copy the Interface-Id option from the Relay-Forward
message into the Relay-Reply message the server sends to the relay
agent in response to the Relay-Forward message. This option MUST NOT
appear in any message except a Relay-Forward or Relay-Reply message.
Servers MAY use the Interface-ID for parameter assignment policies.
The Interface-ID SHOULD be considered an opaque value, with policies
based on exact match only; that is, the Interface-ID SHOULD NOT be
internally parsed by the server. The Interface-ID value for an
interface SHOULD be stable and remain unchanged, for example, after
the relay agent is restarted; if the Interface-ID changes, a server
will not be able to use it reliably in parameter assignment policies.
22.19. Reconfigure Message Option
A server includes a Reconfigure Message option in a Reconfigure
message to indicate to the client whether the client responds with a
Renew message or an Information-request message. The format of this
option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_RECONF_MSG option-len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type
+-+-+-+-+-+-+-+-+
option-code OPTION_RECONF_MSG (19).
option-len 1.
msg-type 5 for Renew message, 11 for
Information-request message.
The Reconfigure Message option can only appear in a Reconfigure
message.
22.20. Reconfigure Accept Option
A client uses the Reconfigure Accept option to announce to the server
whether the client is willing to accept Reconfigure messages, and a
server uses this option to tell the client whether or not to accept
Reconfigure messages. The default behavior, in the absence of this
option, means unwillingness to accept Reconfigure messages, or
instruction not to accept Reconfigure messages, for the client and
server messages, respectively. The following figure gives the format
of the Reconfigure Accept option:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OPTION_RECONF_ACCEPT 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_RECONF_ACCEPT (20).
option-len 0.
23. Security Considerations
The threat to DHCP is inherently an insider threat (assuming a
properly configured network where DHCPv6 ports are blocked on the
perimeter gateways of the enterprise). Regardless of the gateway
configuration, however, the potential attacks by insiders and
outsiders are the same.
Use of manually configured preshared keys for IPsec between relay
agents and servers does not defend against replayed DHCP messages.
Replayed messages can represent a DOS attack through exhaustion of
processing resources, but not through mis-configuration or exhaustion
of other resources such as assignable addresses.
One attack specific to a DHCP client is the establishment of a
malicious server with the intent of providing incorrect configuration
information to the client. The motivation for doing so may be to
mount a "man in the middle" attack that causes the client to
communicate with a malicious server instead of a valid server for
some service such as DNS or NTP. The malicious server may also mount
a denial of service attack through misconfiguration of the client
that causes all network communication from the client to fail.
There is another threat to DHCP clients from mistakenly or
accidentally configured DHCP servers that answer DHCP client requests
with unintentionally incorrect configuration parameters.
A DHCP client may also be subject to attack through the receipt of a
Reconfigure message from a malicious server that causes the client to
obtain incorrect configuration information from that server. Note
that although a client sends its response (Renew or
Information-request message) through a relay agent and, therefore,
that response will only be received by servers to which DHCP messages
are relayed, a malicious server could send a Reconfigure message to a
client, followed (after an appropriate delay) by a Reply message that
would be accepted by the client. Thus, a malicious server that is
not on the network path between the client and the server may still
be able to mount a Reconfigure attack on a client. The use of
transaction IDs that are cryptographically sound and cannot easily be
predicted will also reduce the probability that such an attack will
be successful.
The threat specific to a DHCP server is an invalid client
masquerading as a valid client. The motivation for this may be for
theft of service, or to circumvent auditing for any number of
nefarious purposes.
The threat common to both the client and the server is the resource
"denial of service" (DoS) attack. These attacks typically involve
the exhaustion of available addresses, or the exhaustion of CPU or
network bandwidth, and are present anytime there is a shared
resource.
In the case where relay agents add additional options to Relay
Forward messages, the messages exchanged between relay agents and
servers may be used to mount a "man in the middle" or denial of
service attack.
This threat model does not consider the privacy of the contents of
DHCP messages to be important. DHCP is not used to exchange
authentication or configuration information that must be kept secret
from other networks nodes.
DHCP authentication provides for authentication of the identity of
DHCP clients and servers, and for the integrity of messages delivered
between DHCP clients and servers. DHCP authentication does not
provide any privacy for the contents of DHCP messages.
The Delayed Authentication protocol described in section 21.4 uses a
secret key that is shared between a client and a server. The use of
a "DHCP realm" in the shared key allows identification of
administrative domains so that a client can select the appropriate
key or keys when roaming between administrative domains. However,
the Delayed Authentication protocol does not define any mechanism for
sharing of keys, so a client may require separate keys for each
administrative domain it encounters. The use of shared keys may not
scale well and does not provide for repudiation of compromised keys.
This protocol is focused on solving the intradomain problem where the
out-of-band exchange of a shared key is feasible.
Because of the opportunity for attack through the Reconfigure
message, a DHCP client MUST discard any Reconfigure message that does
not include authentication or that does not pass the validation
process for the authentication protocol.
The Reconfigure Key protocol described in section 21.5 provides
protection against the use of a Reconfigure message by a malicious
DHCP server to mount a denial of service or man-in-the-middle attack
on a client. This protocol can be compromised by an attacker that
can intercept the initial message in which the DHCP server sends the
key to the client.
Communication between a server and a relay agent, and communication
between relay agents, can be secured through the use of IPSec, as
described in section 21.1. The use of manual configuration and
installation of static keys are acceptable in this instance because
relay agents and the server will belong to the same administrative
domain and the relay agents will require other specific configuration
(for example, configuration of the DHCP server address) as well as
the IPSec configuration.
24. IANA Considerations
This document defines several new name spaces associated with DHCPv6
and DHCPv6 options:
- Message types
- Status codes
- DUID
- Option codes
IANA has established a registry of values for each of these name
spaces, which are described in the remainder of this section. These
name spaces will be managed by the IANA and all will be managed
separately from the name spaces defined for DHCPv4.
New multicast addresses, message types, status codes, and DUID types
are assigned via Standards Action [11].
New DHCP option codes are tentatively assigned after the
specification for the associated option, published as an Internet
Draft, has received expert review by a designated expert [11]. The
final assignment of DHCP option codes is through Standards Action, as
defined in RFC2434 [11].
This document also references three name spaces in section 21 that
are associated with the Authentication Option (section 22.11). These
name spaces are defined by the authentication mechanism for DHCPv4 in
RFC3118 [4].
The authentication name spaces currently registered by IANA will
apply to both DHCPv6 and DHCPv4. In the future, specifications that
define new Protocol, Algorithm and RDM mechanisms will explicitly
define whether the new mechanisms are used with DHCPv4, DHCPv6 or
both.
24.1. Multicast Addresses
Section 5.1 defines the following multicast addresses, which have
been assigned by IANA for use by DHCPv6:
All_DHCP_Relay_Agents_and_Servers address: FF02::1:2
All_DHCP_Servers address: FF05::1:3
24.2. DHCP Message Types
IANA has recorded the following message types (defined in section
5.3). IANA will maintain the registry of DHCP message types.
SOLICIT 1
ADVERTISE 2
REQUEST 3
CONFIRM 4
RENEW 5
REBIND 6
REPLY 7
RELEASE 8
DECLINE 9
RECONFIGURE 10
INFORMATION-REQUEST 11
RELAY-FORW 12
RELAY-REPL 13
24.3. DHCP Options
IANA has recorded the following option-codes (as defined in section
22). IANA will maintain the registry of DHCP option codes.
OPTION_CLIENTID 1
OPTION_SERVERID 2
OPTION_IA_NA 3
OPTION_IA_TA 4
OPTION_IAADDR 5
OPTION_ORO 6
OPTION_PREFERENCE 7
OPTION_ELAPSED_TIME 8
OPTION_RELAY_MSG 9
OPTION_AUTH 11
OPTION_UNICAST 12
OPTION_STATUS_CODE 13
OPTION_RAPID_COMMIT 14
OPTION_USER_CLASS 15
OPTION_VENDOR_CLASS 16
OPTION_VENDOR_OPTS 17
OPTION_INTERFACE_ID 18
OPTION_RECONF_MSG 19
OPTION_RECONF_ACCEPT 20
24.4. Status Codes
IANA has recorded the status codes defined in the following table.
IANA will manage the definition of additional status codes in the
future.
Name Code Description
---------- ---- -----------
Success 0 Success.
UnspecFail 1 Failure, reason unspecified; this
status code is sent by either a client
or a server to indicate a failure
not explicitly specified in this
document.
NoAddrsAvail 2 Server has no addresses available to assign to
the IA(s).
NoBinding 3 Client record (binding) unavailable.
NotOnLink 4 The prefix for the address is not appropriate for
the link to which the client is attached.
UseMulticast 5 Sent by a server to a client to force the
client to send messages to the server.
using the All_DHCP_Relay_Agents_and_Servers
address.
24.5. DUID
IANA has recorded the following DUID types (as defined in section
9.1). IANA will manage the definition of additional DUID types in
the future.
DUID-LLT 1
DUID-EN 2
DUID-LL 3
25. Acknowledgments
Thanks to the DHC Working Group and the members of the IETF for their
time and input into the specification. In particular, thanks also
for the consistent input, ideas, and review by (in alphabetical
order) Bernard Aboba, Bill Arbaugh, Thirumalesh Bhat, Steve Bellovin,
A. K. Vijayabhaskar, Brian Carpenter, Matt Crawford, Francis Dupont,
Richard Hussong, Kim Kinnear, Fredrik Lindholm, Tony Lindstrom, Josh
Littlefield, Gerald Maguire, Jack McCann, Shin Miyakawa, Thomas
Narten, Erik Nordmark, Jarno Rajahalme, Yakov Rekhter, Mark Stapp,
Matt Thomas, Sue Thomson, Tatuya Jinmei and Phil Wells.
Thanks to Steve Deering and Bob Hinden, who have consistently taken
the time to discuss the more complex parts of the IPv6
specifications.
And, thanks to Steve Deering for pointing out at IETF 51 in London
that the DHCPv6 specification has the highest revision number of any
Internet Draft.
26. References
26.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[2] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC2464, December 1998.
[3] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC2460, December 1998.
[4] Droms, R., Ed. and W. Arbaugh, Ed., "Authentication for DHCP
Messages", RFC3118, June 2001.
[5] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC2373, July 1998.
[6] IANA. Private Enterprise Numbers.
http://www.iana.org/assignments/enterprise-numbers.Html.
[7] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC2401, November 1998.
[8] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC2104, February 1997.
[9] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation", RFC1305, March 1992.
[10] Mockapetris, P., "Domain names - implementation and
specification", RFC1035, November 1987.
[11] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC2434, October 1998.
[12] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC3041, January 2001.
[13] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC2461, December 1998.
[14] Plummer, D.C., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet address
for transmission on Ethernet hardware", STD 37, RFC826,
November 1982.
[15] Postel, J., "User Datagram Protocol", STD 6, RFC768, August
1980.
[16] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321, April
1992.
[17] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC2462, December 1998.
26.2. Informative References
[18] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC2132, March 1997.
[19] Droms, R., "Dynamic Host Configuration Protocol", RFC2131,
March 1997.
[20] R. Droms, Ed. DNS Configuration options for DHCPv6. April
2002. Work in Progress.
[21] A. K. Vijayabhaskar. Time Configuration Options for DHCPv6.
May 2002. Work in Progress.
[22] Vixie, P., Ed., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC2136, April
1997.
A. Appearance of Options in Message Types
The following table indicates with a "*" the options are allowed in
each DHCP message type:
Client Server IA_NA Option Pref Time Relay Auth. Server
ID ID IA_TA Request Msg. Unica.
Solicit * * * * *
Advert. * * * * *
Request * * * * * *
Confirm * * * * *
Renew * * * * * *
Rebind * * * * *
Decline * * * * * *
Release * * * * * *
Reply * * * * * *
Reconf. * * * *
Inform. * (see note) * * *
R-forw. * *
R-repl. * *
NOTE:
Only included in Information-Request messages that are sent
in response to a Reconfigure (see section 19.4.3).
Status Rap. User Vendor Vendor Inter. Recon. Recon.
Code Comm. Class Class Spec. ID Msg. Accept
Solicit * * * * *
Advert. * * * * *
Request * * * *
Confirm * * *
Renew * * * *
Rebind * * * *
Decline * * *
Release * * *
Reply * * * * * *
Reconf. *
Inform. * * * *
R-forw. * * * *
R-repl. * * * *
B. Appearance of Options in the Options Field of DHCP Options
The following table indicates with a "*" where options can appear in
the options field of other options:
Option IA_NA/ IAADDR Relay Relay
Field IA_TA Forw. Reply
Client ID *
Server ID *
IA_NA/IA_TA *
IAADDR *
ORO *
Preference *
Elapsed Time *
Relay Message * *
Authentic. *
Server Uni. *
Status Code * * *
Rapid Comm. *
User Class *
Vendor Class *
Vendor Info. *
Interf. ID * *
Reconf. MSG. *
Reconf. Accept *
Note: "Relay Forw" / "Relay Reply" options appear in the options
field of the message but may only appear in these messages.
Chair's Address
The working group can be contacted via the current chair:
Ralph Droms
Cisco Systems
1414 Massachusetts Avenue
Boxborough, MA 01719
Phone: (978) 936-1674
EMail: rdroms@cisco.com
Authors' Addresses
Jim Bound
Hewlett Packard Corporation
ZK3-3/W20
110 Spit Brook Road
Nashua, NH 03062-2698
USA
Phone: +1 603 884 0062
EMail: Jim.Bound@hp.com
Bernie Volz
116 Hawkins Pond Road
Center Harbor, NH 03226-3103
USA
Phone: +1-508-259-3734
EMail: volz@metrocast.net
Ted Lemon
Nominum, Inc.
950 Charter Street
Redwood City, CA 94043
USA
EMail: Ted.Lemon@nominum.com
Charles E. Perkins
Communications Systems Lab
Nokia Research Center
313 Fairchild Drive
Mountain View, California 94043
USA
Phone: +1-650 625-2986
EMail: charles.perkins@nokia.com
Mike Carney
Sun Microsystems, Inc
17 Network Circle
Menlo Park, CA 94025
USA
Phone: +1-650-786-4171
EMail: michael.carney@sun.com
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