Internet Engineering Task Force J. Bound
INTERNET DRAFT Digital Equipment Corp.
DHC Working Group C. Perkins
Obsoletes: draft-ietf-dhc-dhcpv6-11.txt Sun Microsystems
21 November 1997
Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
draft-ietf-dhc-dhcpv6-11.txt
Status of This Memo
This document is a submission by the Dynamic Host Configuration
Working Group of the Internet Engineering Task Force (IETF).
Comments should be submitted to the dhcp-v6@bucknell.edu mailing
list.
Distribution of this memo is unlimited.
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Abstract
The Dynamic Host Configuration Protocol (DHCPv6) provides a framework
for passing configuration information, via extensions, to IPv6 nodes.
It offers the capability of automatic allocation of reusable network
addresses and additional configuration flexibility. This protocol
should be considered a stateful counterpart to the IPv6 Stateless
Address Autoconfiguration protocol specification, and can be used
separately or together with the latter to obtain configuration
information.
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Contents
Status of This Memo i
Abstract i
1. Introduction 1
2. Terminology and Definitions 2
2.1. IPv6 Terminology . . . . . . . . . . . . . . . . . . . . 2
2.2. DHCPv6 Terminology . . . . . . . . . . . . . . . . . . . 3
2.3. Specification Language . . . . . . . . . . . . . . . . . 4
3. Protocol Design Model 5
3.1. Design Goals . . . . . . . . . . . . . . . . . . . . . . 5
3.2. DHCP Messages . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Request/Response Processing Model . . . . . . . . . . . . 7
4. DHCP Message Formats and Field Definitions 8
4.1. DHCP Solicit Message Format . . . . . . . . . . . . . . . 8
4.2. DHCP Advertise Message Format . . . . . . . . . . . . . . 9
4.3. DHCP Request Message Format . . . . . . . . . . . . . . . 11
4.4. DHCP Reply Message Format . . . . . . . . . . . . . . . . 12
4.5. DHCP Release Message Format . . . . . . . . . . . . . . . 13
4.6. DHCP Reconfigure Message Format . . . . . . . . . . . . . 15
5. DHCP Client Considerations 16
5.1. Verifying Resource Allocations After Restarts . . . . . . 16
5.2. Sending DHCP Solicit Messages . . . . . . . . . . . . . . 16
5.3. Receiving DHCP Advertise Messages . . . . . . . . . . . . 17
5.4. Sending DHCP Request Messages . . . . . . . . . . . . . . 18
5.5. Receiving DHCP Reply Messages . . . . . . . . . . . . . . 20
5.6. Sending DHCP Release Messages . . . . . . . . . . . . . . 20
5.7. Receiving DHCP Reconfigure Messages . . . . . . . . . . . 21
5.8. Interaction with Stateless Address Autoconfiguration . . 21
6. DHCP Server Considerations 22
6.1. Receiving DHCP Solicit Messages . . . . . . . . . . . . . 22
6.2. Sending DHCP Advertise Messages . . . . . . . . . . . . . 23
6.3. DHCP Request and Reply Message Processing . . . . . . . . 23
6.3.1. Processing for Extensions to DHCP Request and Reply
Messages . . . . . . . . . . . . . . . . . 24
6.3.2. Client Requests to Deallocate Unknown Resources . 25
6.4. Receiving DHCP Release Messages . . . . . . . . . . . . . 25
6.5. Sending DHCP Reconfigure Messages . . . . . . . . . . . . 26
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7. DHCP Relay Considerations 26
7.1. DHCP Solicit and DHCP Advertise Message Processing . . . 27
7.2. DHCP Request Message Processing . . . . . . . . . . . . . 27
7.3. DHCP Reply Message Processing . . . . . . . . . . . . . . 28
8. Retransmission and Configuration Variables 28
9. Security Considerations 31
10. Year 2000 considerations 32
11. Acknowledgements 32
A. Related Work in IPv6 32
B. Comparison between DHCPv4 and DHCPv6 33
Chair's Address 39
Author's Address 39
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1. Introduction
The Dynamic Host Configuration Protocol (DHCPv6, or in this
document usually DHCP) provides configuration parameters to Internet
nodes. DHCP consists of a protocol for delivering node-specific
configuration parameters from a DHCP server to a client, and a
mechanism for allocation of network addresses and other related
parameters to IPv6 [6] nodes.
DHCP is built on a client-server model, where designated DHCP servers
allocate network addresses and automatically deliver configuration
parameters to dynamically configurable clients. Throughout the
remainder of this document, the term "server" refers to a node
providing initialization parameters by way of the DHCP protocol,
and the term "client" refers to a node requesting initialization
parameters from a DHCP server.
DHCPv6 uses Request and Reply messages to support a client/server
processing model whereby both client and server are assured that
requested configuration parameters have been received and accepted
by the client. DHCP supports optional configuration parameters and
processing for nodes through extensions described in its companion
document ``Extensions for the Dynamic Host Configuration Protocol for
IPv6'' [12].
The IPv6 Addressing Architecture [8] and IPv6 Stateless Address
Autoconfiguration [16] specifications provide new features not
available in IP version 4 (IPv4) [15], which are used to simplify
and generalize the operation of DHCP clients. This document is
intended to complement those specifications for clients attached to
the kinds of Internet media for which those specifications apply. In
particular, the specification in this document does not necessarily
apply to nodes which do not enjoy a full duplex link to the Internet.
Section 2 provides definitions for terminology used throughout
this document. Section 3 provides an overview of the protocol
design model that guided the design choices in the specification;
section 3.2 briefly describes the protocol messages and their
semantics. Section 4 provides the message formats and field
definitions used for each message. Sections 5, 6, and 7 specify
how clients, servers, and relays interact. The timeout and
retransmission guidelines and configuration variables are discussed
in Section 8. Appendix A summarizes related work in IPv6 that will
provide helpful context; it is not part of this specification, but
included for informational purposes. Appendix B discusses the
differences between DHCPv4 and DHCPv6.
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2. Terminology and Definitions
Relevant terminology from the IPv6 Protocol [6], IPv6 Addressing
Architecture [8], and IPv6 Stateless Address Autoconfiguration [16]
will be provided, and then the DHCPv6 terminology.
2.1. IPv6 Terminology
address An IP layer identifier for an interface or a set of
interfaces.
unicast address
An identifier for a single interface. A packet sent
to a unicast address is delivered to the interface
identified by that 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.
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 IP address having link-only scope, indicated by
having the routing prefix FE80::0000/64), that can be
used to reach neighboring nodes attached to the same
link. Every interface has a link-local address.
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message A unit of data carried in a packet, exchanged between
DHCP agents and clients.
neighbor A node attached to the same link.
node A device that implements IP.
packet An IP header plus payload.
prefix A bit string that consists of some number of initial
bits of an address.
router A node that forwards IP packets not explicitly
addressed to itself.
2.2. DHCPv6 Terminology
Agent Address
The address of a neighboring DHCP Agent on the same
link as the DHCP client.
binding A binding (or, client binding) in DHCP contains the
data which a DHCP server maintains for each of its
clients (see Section 6).
resource-server association
An association between a resource and a DHCP server
maintained by the client which received that resource
from that DHCP server.
configuration parameter
Any parameter that can be used by a node to configure
its network subsystem and enable communication on a
link or internetwork.
DHCP agent (or agent)
Either a DHCP server or a DHCP relay.
DHCP client (or client)
A node that initiates requests on a link to obtain
configuration parameters.
DHCP relay (or relay)
A node that acts as an intermediary to deliver DHCP
messages between clients and servers.
DHCP server (or server)
A server is a node that responds to requests from
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clients to provide: addresses, prefix lengths, or
other configuration parameters.
transaction-ID
The transaction-ID is a monotonically increasing
unsigned integer identifier specified by the client
or server, and used to match a response to a pending
message.
2.3. Specification Language
In this document, several words are used to signify the requirements
of the specification, in accordance with RFC 2119 [2]. These words
are often capitalized.
MUST This word, or the adjective "required", means that
the definition is an absolute requirement of the
specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means
that there may exist valid reasons in particular
circumstances to ignore this item, but the full
implications must be understood and carefully
weighed before choosing a different course.
Unexpected results may result otherwise.
MAY This word, or the adjective "optional", means that
this item is one of an allowed set of alternatives.
An implementation which does not include this option
MUST be prepared to interoperate with another
implementation which does include the option.
silently discard
The implementation discards the packet without
further processing, and without indicating an error
to the sender. The implementation SHOULD provide
the capability of logging the error, including the
contents of the discarded packet, and SHOULD record
the event in a statistics counter.
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3. Protocol Design Model
This section is provided for implementors to understand the DHCPv6
protocol design model from an architectural perspective. Goals and
conceptual models are presented in this section.
3.1. Design Goals
The following list gives general design goals for this DHCP
specification.
- DHCP should be a mechanism rather than a policy. DHCP MUST allow
local system administrators control over configuration parameters
where desired; e.g., local system administrators should be able
to enforce local policies concerning allocation and access to
local resources where desired.
- DHCP MUST NOT introduce any requirement for manual configuration
of DHCP clients, except when security requirements need
authentication or encryption keys. Each node should be able to
obtain appropriate local configuration parameters without user
intervention, and incorporate those parameters into its own
configuration.
- DHCP MUST NOT require a server on each link. To allow for scale
and economy, DHCP MUST work across DHCP relays.
- A DHCP client MUST be prepared to receive multiple (possibly
different) responses to solicitations for DHCP servers. Some
installations may include multiple, overlapping DHCP servers to
enhance reliability and/or to increase performance.
- DHCP MUST coexist with statically configured, non-participating
nodes and with existing network protocol implementations.
- DHCPv6 MUST be compatible with IPv6 Stateless Address
Autoconfiguration [16].
- A DHCPv6 Client implementation MAY be started in the absence of
any IPv6 routers on the client's link.
- DHCP architecture MUST support the requirements of automated
renumbering of IP addresses [3].
- DHCP servers SHOULD be able to support Dynamic Updates to
DNS [18].
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- DHCP servers MUST be able to support multiple IPv6 addresses for
each client.
- DHCP MUST work on network links that do not have routers, as long
as a DHCP server is present on the link.
- A DHCP server to server protocol is NOT part of this
specification.
- It is NOT a design goal of DHCP to specify how a server
configuration parameter database is maintained or determined.
Methods for configuring DHCP servers are outside the scope of
this document.
3.2. DHCP Messages
Each DHCP message contains a type, which defines its function
within the protocol. Processing details for these DHCP messages are
specified in Sections 5, 6, and 7. The specified message types are
as follows:
01 DHCP Solicit
The DHCP Solicit message is an IP multicast message sent by a
DHCP client to one or more DHCP agents.
02 DHCP Advertise
The DHCP Advertise is an IP unicast message sent by a DHCP
Agent in response to a DHCP client's DHCP Solicit message.
03 DHCP Request
The DHCP Request is an IP unicast message sent by a DHCP client
to a DHCP server to request configuration parameters on a
network.
04 DHCP Reply
The DHCP Reply is an IP unicast message sent by a DHCP server
in response to a client's DHCP Request, or by the DHCP relay
that relayed that client's DHCP Request. Extensions [12] to
the DHCP Reply describe the resources that the DHCP server has
committed and allocated to this client, and may contain other
information for use by this client.
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05 DHCP Release
The DHCP Release is an IP unicast message sent by a DHCP
client to inform the DHCP server that the client is releasing
resources.
06 DHCP Reconfigure
The DHCP Reconfigure is an IP unicast or multicast message sent
by a DHCP server to inform one or more clients that the server
has new configuration information of importance. Each client
is expected to initiate a new Request/Reply transaction.
DHCP message types not defined here (msg-types 0 and 7-255) are
reserved and SHOULD be silently ignored.
3.3. Request/Response Processing Model
The request/response processing for DHCPv6 is transaction based and
uses a set of best-effort messages to complete the transaction.
To find a server, a client sends a DHCP Solicit message from the
interface which it wishes to configure. The client then awaits
a DHCP Advertise message, which will provide an IP address of a
DHCP server. Transactions are started by a client with a DHCP
Request, which may be issued after the client knows the server's
address. The response (DHCP Reply) is sent from the server (possibly
via a DHCP Relay). At this point in the flow all data has been
transmitted and is presumed to have been received. To provide a
method of recovery if either the client or server do not receive the
messages to complete the transaction, the client retransmits each
DHCP Request message until it elicits the corresponding DHCP Reply,
or until it can be reasonably certain that the desired DHCP server
is unavailable, or it determines that it does not want a response
(i.e., it MAY abort the transaction). The timeout and retransmission
guidelines and configuration variables are discussed in Section 8.
DHCP uses the UDP [14] protocol to communicate between clients and
servers. UDP is not reliable, but the DHCP retransmission scheme
in the referenced section provides reliability between clients and
servers. The following well-known multicast addresses are used by
DHCP agents and clients:
FF02:0:0:0:0:0:1:2
All DHCP Agents (Servers and Relays) MUST join the
link-local All-DHCP-Agents multicast group at the address
FF02:0:0:0:0:0:1:2.
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FF05:0:0:0:0:0:1:3
All DHCP servers MUST join the site-local
All-DHCP-Servers multicast group at the address
FF05:0:0:0:0:0:1:3.
FF05:0:0:0:0:0:1:4
All DHCP Relays MUST join the site-local All-DHCP-Relays
multicast group at the address FF05:0:0:0:0:0:1:4.
A DHCP server or agent MUST transmit all messages to DHCP clients on
UDP port 546. A DHCP client MUST transmit all messages to a DHCP
agent over UDP using port 547. A DHCP server MUST transmit all
messages to DHCP Relays over UDP on port 546. The source port for
DHCP messages is arbitrary.
For the proper operation of the DHCP protocol to operate within a
network where one or more firewalls [4] are used, DHCP transactions
using UDP destination ports 546 and 547 will need to be permitted.
4. DHCP Message Formats and Field Definitions
All fields in DHCP messages MUST be initialized to binary zeroes by
both the client and server unless otherwise noted. All reserved
fields in a message MUST be ignored by the receiver of the message.
4.1. DHCP Solicit Message Format
A DHCP client transmits a DHCP Solicit message over the interface it
is trying to configure, to obtain one or more DHCP server addresses.
In the event that there is no DHCP server on this link, such a
request MAY be forwarded by a DHCP relay attached to this link (if
such a relay exists) on behalf of a client to a DHCP server.
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 |C| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| relay address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 1
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C If set, the client requests that all servers receiving
the message deallocate the resources associated with
the client.
reserved 0
client's link-local address
The IP link-local address of the client interface from
which the client issued the DHCP Request message
relay address
If nonzero, the IP address of the interface on which
the relay received the client's DHCP Solicit message
To obtain a DHCP Agent address a DHCP client SHOULD send a DHCP
Solicit message to the All-DHCP-Agents multicast address (see
section 3.3). Any DHCP Relay receiving the solicitation MUST forward
it to the All-DHCP-Servers multicast address, to instruct DHCP
servers to send their advertisements to the prospective client.
In that case, the relay MUST copy a non-link-local address of its
interface from which the client's solicitation was received into the
relay address field.
4.2. DHCP Advertise Message Format
A DHCP agent sends a DHCP Advertise message to inform a prospective
client about the IP address of a DHCP Agent to which a DHCP Request
message may be sent. When the client and server are on different
links, the server sends the advertisement back through the DHCP Relay
whence the solicitation came.
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 |S|P| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server preference (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| agent address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 2
S If set, the server address is present.
P If set, the server preference is present.
reserved 0
server preference
A 32-bit unsigned integer indicating a server's
willingness to provide service to the client.
client's link-local address
The IP link-local address of the client interface
from which the client issued the DHCP Request message
agent address
The IP address of a DHCP Agent interface on the same
link as the client.
server address
If present, the IP address of the DHCP server
extensions See [12].
Suppose that a DHCP server on the same link as a client issues the
DHCP Advertise in response to a DHCP Solicit message sent to the
All-DHCP-Agents multicast address. Then the agent address will be an
IP address of one of the server's interfaces on the same link as the
client, and the 'S' bit will be set to zero. No server address will
be present in the DHCP Advertise message.
If the 'P' bit is set, the server preference field is present. If
the 'P' bit is not set, the server preference field is not present,
but implicitly has the value of 0xffffffff (in other words, the
highest possible value).
The DHCP server MUST copy the client's link-local address into the
advertisement which is sent in response to a DHCP Solicit. Both
agent address and server address (if present) of the DHCP Advertise
message MUST have sufficient scope to be reachable by the DHCP
client. Moreover, the Agent address of any DHCP Advertise message
sent by a DHCP relay MUST NOT be a link-local address. In situations
where there are no routers sending Router Advertisements, then a DHCP
server MUST be configured on the same link as prospective clients.
The DHCPv6 protocol design does not apply to situations where the
client has no way to route messages to a server not on the same link.
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See section 5.3 for information about how clients handle the server
preference field.
4.3. DHCP Request Message Format
In order to request configuration parameters from a DHCP server, a
client sends a DHCP Request message, and MAY append extensions [12].
If the client does not know any DHCP server address, it MUST first
obtain a server address by multicasting a DHCP Solicit message (see
Section 4.1). If the client does not have a valid IP address of
sufficient scope for the DHCP server to communicate with the client,
the client MUST send the message to the local DHCP relay and insert
the DHCP relay address as the agent address in the message header.
In this case, the client cannot send the message directly to the
DHCP server because the server could not return any response to the
client. Otherwise, the client MAY omit the server address in the
DHCP Request message; in this case, the client MUST clear the S-bit
in the DHCP Request message and send it directly to to the server,
using the server address as the IP destination address in the IP
header.
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 |C|S|R|N| rsvd | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| agent address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 3
C If set, the client requests the server to remove all
resources associated with the client binding, except
those resources provided as extensions.
S If set, the server address is present
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R If set, the client has rebooted and requests that all
of its previous transaction IDs be expunged and made
available for re-use.
N If set, the client has begun operating in stateless
address autoconfiguration.
rsvd 0
transaction-ID
A monotonically increasing unsigned integer used to
identify this Request, and copied into the Reply.
client's link-local address
The IP link-local address of the client interface from
which the client issued the DHCP Request message
agent address
The IP address of an agent's interface, copied from a
DHCP Advertisement message.
server address
If present, the IP address of the DHCP server which
should receive the client's DHCP Request message.
extensions See [12].
4.4. DHCP Reply Message Format
The server sends one DHCP Reply message in response to every DHCP
Request or DHCP Release received. If the request comes with the 'S'
bit set, the client could not directly send the Request to the server
and had to use a neighboring relay agent. In that case, the server
sends back the DHCP Reply with the 'L' bit set, and the DHCP Reply is
addressed to the agent address found in the DHCP Request message. If
the 'L' bit is set, then the client's link-local address will also be
present.
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 |L| status | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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msg-type 4
L If set, the client's link-local address is present
status One of the following decimal values:
0 Success
16 Failure, reason unspecified
17 Authentication failed or nonexistent
18 Poorly formed Request or Release
19 Resources unavailable
20 Client record unavailable
21 Invalid client IP address in Release
23 Relay cannot find Server Address
24 Cannot understand selected Character Set
64 Server unreachable (ICMP error)
transaction-ID
A monotonically increasing unsigned integer used to
identify this Reply, and copied from the client's
Request.
client's link-local address
If present, the IP address of the client interface
which issued the corresponding DHCP Request message.
extensions
See [12].
If the 'L' bit is set, and thus the link-local address is present in
the Reply message, the Reply is sent by the server to the relay's
address which was specified as the agent address in the DHCP Request
message, and the relay uses the link-local address to deliver the
Reply message to the client.
4.5. DHCP Release Message Format
The DHCP Release message is sent without the assistance of any DHCP
relay. When a client sends a Release message, it is assumed to have
a valid IP address with sufficient scope to allow access to the
target server. If parameters are specified in the extensions, only
those parameters are released. The DHCP server acknowledges the
Release message by sending a DHCP Reply (Sections 4.4, 6.3).
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 |D| reserved | transaction-ID |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client's link-local address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| agent address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client address |
| (16 octets, if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 5
D If the 'D' ("Direct") flag is set, the client instructs
the server to send the DHCP Reply directly back to the
client, instead of using the given agent address and
link-local address to relay the Reply message.
reserved 0
transaction-ID
A monotonically increasing unsigned integer used to
identify this Release, and copied into the Reply.
client's link-local address
The IP link-local address of the client interface from
which the the client issued the DHCP Release message
agent address
The IP address of the agent interface to which the
client issued a previous DHCP Request message
client address
The IP address of the client interface from which the
the client issued the DHCP Release message. The client
address field is present whenever the 'D' bit is set,
even if it is equal to the link-local address.
extensions See [12]
Suppose that the client has an IP address that will still be valid
after the server performs the operations requested in the extensions
to the DHCP Release message, and which has sufficient scope to be
reachable from the server. In that case, and only then, the client
SHOULD set the 'D' flag. When the 'D' flag is set, the server MUST
send the DHCP Reply back to the client using the client address field
of the Release message. Otherwise, when the 'D' bit is not set, the
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server MUST send its DHCP Reply message to the agent address in the
Release message, so that the relay agent can subsequently forward the
Reply back to the releasing client at the client's link-local address
indicated in the Reply message. Note that it is an error (status
code 21) to include within the DHCP Release message both the 'D' bit
and an IP address extension which has the IP address used as the
client IP address field of the DHCP Release message header. If the
clients link-local address and agent address do not match a client
binding (see section 6) an error (status code 20) will be returned to
the client.
4.6. DHCP Reconfigure Message Format
Reconfigure messages can only be sent to clients which have
established an IP address which routes to the link at which they are
reachable, hence the DHCP Reconfigure message is sent without the
assistance of any DHCP relay. When a server sends a Reconfigure
message, the client to which it is sent is assumed to have a valid
IP address with sufficient scope to be accessible by the server.
Reconfigure messages can only be sent to clients which are reachable
by the DHCP server. Hence, the DHCP Reconfigure message is sent
without the assistance of any DHCP relay. Only the parameters which
are specified in the extensions to the Reconfigure message need be
requested again by the client. A Reconfigure message can either be
unicast or multicast by the server.
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 |N| reserved | transaction-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server address |
| (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extensions (variable number and length) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
msg-type 6
N The 'N' flag indicates that the client should not
expect a DHCP Reply in response to the DHCP Request
it sends as a result of the DHCP Reconfigure message.
reserved 0
transaction-ID
A monotonically increasing unsigned integer used to
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identify this Reconfigure message, and copied into
the client's Request.
server address
The IP address of the DHCP server issuing the DHCP
Reconfigure message.
extensions See [12]
5. DHCP Client Considerations
A node which is not a DHCP agent MUST silently discard any DHCP
Solicit, DHCP Request, or DHCP Release message it receives.
5.1. Verifying Resource Allocations After Restarts
A DHCP client MAY retain its configured parameters and resources
across client system reboots and DHCP client program restarts.
However, in these circumstances a DHCP client MUST also formulate a
DHCP Request message to verify that its configured parameters and
resources are still valid. This Request message MUST have the 'C'
bit set, to clean up stale client binding information at the server
which may no longer be in use by the client; stale information is
that which the client does not include in extensions to such request
messages.
If the server does not respond to the DHCP Request message after
REPLY_MSG_MIN_RETRANS (see section 8), the client may still use any
resources whose lifetimes have not yet expired. In such cases,
however, the client MUST begin to search for another server by
multicasting a new DHCP Solicit message, again with the 'C' bit set.
This also handles the case wherein a client restarts on a new
network, where its IP address is no longer valid. In this situation,
when the client receives a new IP address and the old IP address
is no longer needed, the client MUST release its old IP address by
issuing a DHCP Release message with the appropriate extension if it
can communicate with its previous server.
5.2. Sending DHCP Solicit Messages
A DHCP client MUST have the address of a DHCP server to send
a Request message. The client SHOULD locate a DHCP server by
multicasting a DHCP Solicit message to the All-DHCP-Agents link-local
multicast address, setting the Hop Limit == 1 (see Section 3.3).
If there are no DHCP servers on the same link as the node, then a
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DHCP relay MUST be present if solicitations sent from a client's
link-local address are to be handled. The prospective client SHOULD
wait for ADV_CLIENT_WAIT to get all the DHCP Advertisement messages
which may be sent in response to the solicitation.
When sending a DHCP Solicit message, a client MUST set the Relay
Address field to 16 octets of zeros.
If a DHCP client reboots and does not have a valid IP address,
it MUST set the 'C' bit in the DHCP Solicit message it sends
when restarting. By setting the 'C' bit in the solicitation, a
DHCP client requests that all the DHCP servers that receive the
solicitation should clean up their client records that match its
link-local address. The server MUST take the Relay Address Field and
use it as the agent address prefix to locate the client binding (see
section 6.0 for server bindings).
If a client sends a DHCP Solicit message after it reboots, the
solicitation SHOULD be delayed after reception of the first Router
Advertisement [11] message, by at least some random amount of time
between MIN_SOLICIT_DELAY and MAX_SOLICIT_DELAY (see section 8
seconds. This delay is intended to help stagger requests to DHCP
servers (and avoid link-layer collisions) after a power outage
causes many nodes to reboot all at once. Each subsequent DHCP
Solicit message that is issued before receiving an advertisement
MUST be delayed by twice the amount by which the previous DHCP
Solicit message was delayed, plus a small random delay between
MIN_SOLICIT_DELAY and MAX_SOLICIT_DELAY seconds.
5.3. Receiving DHCP Advertise Messages
After a DHCP client has received a DHCP Advertise message, it has the
address of a DHCP server for subsequent DHCP Request messages. If
the 'S' bit is zero, the DHCP Advertise message was transmitted by
a DHCP server on the same link as the client, and the client uses
the Agent address as the address of a DHCP server; otherwise, the
DHCP server address is located in the server address field. If the
server's address is shown as a Multicast address, the advertisement
MUST be silently discarded.
A DHCP server MAY append extensions to its Advertisements; this might
allow the DHCP client to select the configuration that best meets its
needs from among several prospective servers.
Unless a DHCP Advertisement is received with a "server preference"
field of 0xffffffff, a DHCP client must wait ADV_CLIENT_WAIT seconds
after issuing the DHCP Solicit message in order to receive the
Advertisement with the highest preference. After waiting for that
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period of time, a client MUST select the highest preference DHCP
server as the target of its DHCP request. If a DHCP client receives
an advertised preference of 0xffffffff, it does not have to wait for
any more advertisements.
If a DHCP client sends a DHCP Request to a highly preferred DHCP
server but fails to receive a DHCP reply from that server after
following the retransmission algorithm in section 8, the client may
subsequently attempt to send a DHCP Request to a less preferred
server.
A DHCP client is free to cache the result of any DHCP Advertisement
it hears. However, it should be noted that this is purely a
potential performance enhancement as the results need not be constant
over time, hence it may not get a response if it uses the address
obtained from this message and may have to emit its own DHCP Solicit
message subsequently.
5.4. Sending DHCP Request Messages
A DHCP client obtains configuration information from a DHCP server by
sending a DHCP Request message. The client MUST know the server's
address before sending the Request message, and the client MUST
have acquired a (possibly identical) DHCP agent address. If the
client and server are on the same link, the agent address used by the
client MUST be the same as the DHCP server's address. A DHCP Request
message MUST NOT be sent to any multicast address, since otherwise
multiple DHCP agents would possibly allocate resources to the client
in response to the same Request, and the client would have no way to
know which servers had made the allocations, if any packets were lost
due to collisions, etc.
If the DHCP server is off-link, and the client has no valid IP
address of sufficient scope, then the client MUST include the server
address in the appropriate field and set the 'S' bit in the DHCP
Request message. In this case, the IP destination address in the IP
header will be a DHCP relay address.
Otherwise, if the client already has a valid IP address of sufficient
scope and knows the IP address of a candidate DHCP server, it
MUST send the Request message directly to the DHCP server without
requiring the services of the local DHCP relay.
If a client wishes to instruct a DHCP server to deallocate all
unknown previous resources, configuration information, and bindings
associated with its agent address and link-local address, it sets the
'C' bit in the DHCP Request. A client MAY send in such a Request
even when it is no longer attached to the link on which the relay
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address is attached. The 'C' bit allows better reclamation of
available resources, since otherwise a client might not be able to
release resources that it has no record of using.
In any case, after choosing a transaction-ID which is numerically
greater than its previous transaction-ID, and filling in the
appropriate fields of the DHCP Request message, the client MAY append
various DHCP Extensions to the message. These extensions denote
specific requests by the client; for example, a client may request
a particular IP address, or request that the server send an update
containing the client's new IP address to a Domain Name Server. When
all desired extensions have been applied, the DHCP client sends the
DHCP Request to the appropriate DHCP Agent.
For each pending DHCP Request message, a client MUST maintain the
following information:
- The transaction-ID of the request message,
- The server address,
- The agent address (which can be the same as the server address),
- The time at which the next retransmission will be attempted, and
- All extensions appended to the request message.
If a client does not receive a DHCP Reply message (Section 5.5) with
the same transaction-ID as a pending DHCP Request message within
REPLY_MSG_TIMEOUT (see section 8) seconds, or if the received DHCP
Reply message contains a DHCP Authentication extension which fails
to provide the correct authentication information, the client MUST
retransmit the Request with the same transaction-ID and continue to
retransmit according to the rules in Section 8. If (after following
those rules) the client never receives a Reply message, it naturally
SHOULD start over again by sending a new DHCP Solicit message to find
a different server.
If the client receives an ICMP error message in response to such
a DHCP Request, it likewise naturally SHOULD start over again by
sending a new DHCP Solicit message, to find a different server.
If the client transmits a DHCP Request in response to a DHCP
Reconfigure message (see Section 5.7), the client can continue to
operate with its existing configuration information and resources
until it receives the corresponding DHCP Reply from the server. The
same retransmission rules apply as for any other DHCP Request message
from the client. When the 'N' bit is set, a DHCP Request sent in
response to a DHCP Reconfigure message will not elicit a DHCP Reply
message from the server.
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5.5. Receiving DHCP Reply Messages
When a client receives a DHCP Reply message, it MUST check whether
the transaction-ID in the Reply message matches the transaction-ID
of a pending DHCP Request message. If no match is found, the Reply
message MUST be silently discarded.
If the Reply message is acceptable, the client processes each
Extension [12], extracting the relevant configuration information
and parameters for its network operation. The client can determine
when all extensions in the Reply have been processed by using the UDP
Length field of the Reply. Some extensions in the Reply may have
status codes, which indicate to the client the reason for failure
when the server was unable to honor the request. If the server is
unable to honor the request in an extension included by the client,
that extension may simply be omitted from the Reply. The server MAY
also provide the client with configuration parameters the client did
not specifically request.
Some configuration information extracted from the extensions to the
DHCP Reply message MUST remain associated with the DHCP server that
sent the message. The particular extensions that require this extra
measure of association with the server are indicated in the DHCP
Extensions document [12]. These "resource-server" associations are
used when sending DHCP Release messages.
5.6. Sending DHCP Release Messages
If a client wishes to ask the server to release all information and
resources relevant to the client, the client SHOULD send a DHCP
Release message without any extensions; this is preferable to sending
a DHCP Request message with the 'C' bit set and no extensions. If
a DHCP client wishes to retain some of its network configuration
parameters, but determines that others are no longer needed, it
SHOULD enable the DHCP server to release allocated resources which
are no longer in use by sending a DHCP Release message to the
server, and including extensions to identify the unneeded items. The
client consults its list of resource-server associations in order to
determine which server should receive the desired Release message.
Suppose a client wishes to release resources which were granted to
it on another link, and the client has an IP address with enough
scope so that the DHCP server can reach it. In that case, the client
MUST instruct the server to send the DHCP Reply directly back to the
client at that address, instead of performing the default processing
of sending the DHCP Reply back through the agent-address included in
the DHCP Release. This is done by setting the 'D' bit in the DHCP
Release message (see section 4.5).
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5.7. Receiving DHCP Reconfigure Messages
Each DHCP client implementation MUST support listening at UDP port
546 to receive possible DHCP Reconfigure messages; in cases where the
client knows that no Reconfigure message will ever be issued, the
client MAY be configured to avoid executing this supported feature.
In some cases, the IP address at which the client listens will be
a multicast address sent to the client by the DHCP server in an
extension to an earlier DHCP Reply message. If the client does not
listen for DHCP Reconfigure messages, it is possible that the client
will not receive notification that its DHCP server has deallocated
the client's IP address and/or other resources allocated to the
client. See discussion in 6.5. The client MAY receive a prefix
update for one or more of their addresses and then MUST use that
prefix for those addresses.
If a DHCP client receives a DHCP Reconfigure message, it is a request
for the client to initiate a new DHCP Request/Reply transaction with
the server which sent the Reconfigure message. The server sending
the Reconfigure message MAY be different than the server which sent a
DHCP Reply message containing the original configuration information.
For each Extension which is present in the Reconfigure message, the
client MUST append a matching Extension to its Request message, which
it formulates to send to the server specified in the server address
field of the message. The client also copies a transaction-ID from
the Reconfigure message into the Request message. If the 'N' bit is
not set, processing from then on is the same as specified above in
Section 5.4.
Resources held by the client which are not identified by Extensions
in the server's Reconfigure message are not affected.
Note that a server may ask its client to join a multicast group
for the purpose of receiving DHCP Reconfigure messages. When a
Reconfigure message is delivered to the client by way of the selected
multicast address, the client MUST delay its further response for
a random amount of time uniformly distributed within the interval
between RECONF_MSG_MIN_RESP and RECONF_MSG_MAX_RESP seconds (see
section 8). This will minimize the likelihood that the server will
be flooded with DHCP Request messages.
5.8. Interaction with Stateless Address Autoconfiguration
If a client begins to receive Router Advertisements [11] with the 'M'
bit no longer set (so that the link is no longer a "managed" link),
then the client SHOULD send one or more DHCP Release messages to
deallocate the IP addresses it has received. The client sends the
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DHCP Release messages to the servers recorded in the resource-server
associations maintained for the IP addresses.
If a client is receiving Router Advertisements with the 'M' bit equal
to 0, then it is using IP addresses obtained by way of Stateless
Address Autoconfiguration [16]. In this case, if the client receives
a DHCP Reconfigure message, the client MUST return a DHCP Request
message to the DHCP server with the 'N' bit set.
Suppose a client, after using Stateless Address Autoconfiguration
to configure its stateless IP address, begins to receive Router
Advertisements with the 'M' bit set. Then the client MAY continue
to use its existing IP addresses, but it MUST send a DHCP Solicit
message to discover a DHCP server. If the solicitation results in
the receipt of of a DHCP Advertisement message from a DHCP server,
the client MUST subsequently transact a DHCP Request message with at
least one such DHCP server.
6. DHCP Server Considerations
A node which is not a DHCP client or DHCP relay MUST ignore any DHCP
Advertise, DHCP Reply, or DHCP Reconfigure message it receives.
A server maintains a collection of client records, called
``bindings''. Each binding is uniquely identifiable by the ordered
pair <link-local address, agent address prefix>, since the link-local
address is guaranteed to be unique [16] on the link identified by the
agent address. An implementation MUST support bindings consisting
of at least a client's link-local address, agent address prefix,
preferred lifetime and valid lifetime [16] for each client address,
and the transaction-ID. A client binding may be used to store any
other information, resources, and configuration data which will be
associated with the client. A DHCP server MUST retain its clients'
bindings across server reboots, and, whenever possible, a DHCP client
should be assigned the same configuration parameters despite server
system reboots and DHCP server program restarts. A DHCP server MUST
support fixed or permanent allocation of configuration parameters to
specific clients.
6.1. Receiving DHCP Solicit Messages
If the DHCP Solicit message was received at the All-DHCP-Servers
multicast address, the DHCP server MUST check to make sure that the
relay address is present, and not a link-local address. If the
relay address is not present, or if it is a link-local address,
the server MUST silently discard the packet. Note that if the
client sends a DHCP Solicit message from a link-local address, the
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multicast destination will be the All-DHCP-Agents address, not the
All-DHCP-Servers address.
As an optimization, a server processing a Solicit message from relays
MAY check the prefix of the IP source address in the IP header to
determine whether the server has received the Solicit from multiple
relays on the same link.
6.2. Sending DHCP Advertise Messages
Upon receiving and verifying the correctness of a DHCP Solicit
message, a server constructs a DHCP Advertise message and transmits
it on the same link as the solicitation was received from. When
the solicitation is received at the DHCP Servers multicast address,
the server SHOULD delay the transmission of its advertisement
for a random amount of time between SERVER_MIN_ADV_DELAY and
SERVER_MAX_ADV_DELAY (see section 8).
If the relay address is nonzero, the server MUST put the relay
address in the agent address field of the advertisement message, and
MUST send the advertisement message to the relay address; otherwise,
the server MUST send the advertisement to the client's link-local
address. An IP address of the interface on which the server received
the Solicit message MUST appear in the server address field of the
corresponding advertisement.
The DHCP server MAY append extensions to the Advertisement, in order
to offer the soliciting node the best possible information about
the services and resources which the server may be able to make
available.
6.3. DHCP Request and Reply Message Processing
The DHCP server MUST check to ensure that the client's link-local
address field of the Request message contains a link-local address.
If not, the message MUST be silently discarded. If the 'S' bit
is set, the server MUST check that the server address matches the
destination IP address at which the Request message was received
by the server. If the server address does not match, the Request
message MUST be silently discarded.
If the received agent address and link-local address do not
correspond to any binding known to the server, then the server
MAY create a new binding for the previously unknown client, and
send a DHCP Reply with any resources allocated to the new binding.
Otherwise, if the server cannot create a new binding, it SHOULD
return a DHCP Reply with a status of 20. If the client is updating
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its resources but the database is temporarily unavailable, the server
SHOULD return a DHCP Reply with a status of 19.
While processing the Request, the server MUST first determine whether
or not the Request is a retransmission of an earlier DHCP Request
from the same client. This is done by comparing the transaction-ID
to all those transaction-IDs received from the same client during the
previous XID_TIMEOUT seconds. If the transaction-ID is the same as
one received during that time, the server MUST take the same action
(e.g., retransmit the same DHCP Reply to the client) as it did after
processing the previous DHCP Request with the same transaction-ID.
Otherwise, if the server has no record of a message from the client
with the same transaction-ID, the server identifies and allocates
all the relevant information, resources, and configuration data that
is associated with the client. Then it sends that information to
its DHCP client by constructing a DHCP Reply message and including
the client's information in DHCP Extensions to the Reply message.
The DHCP Reply message uses the same transaction-ID as found in the
received DHCP Request message. Note that the reply message MAY
contain information not specifically requested by the client.
If the DHCP Request message has the 'S' bit set in the message
header, the DHCP server MUST send the corresponding DHCP Reply
message to the agent address found in the Request (see section 7.2).
Otherwise, the server SHOULD send the corresponding DHCP Reply
message to the IP source address in the IP header received from the
client Request message.
6.3.1. Processing for Extensions to DHCP Request and Reply Messages
The DHCP Request may contain extensions [12], which are interpreted
(by default) as advisory information from the client about its
configuration preferences. For instance, if the IP Address Extension
is present, the DHCP server SHOULD attempt to allocate or extend the
lifetime of the address indicated by the extension. Some extensions
may be marked by the client as required.
The DHCP server may accept some extensions for successful processing
and allocation, while still rejecting others, or the server may
reject various extensions for different reasons. The server sets the
status appropriately for those extensions which return status to the
client. The DHCP server sends a single Reply message in response to
each DHCP Request, with the same transaction-ID as the Request.
Whenever it is able to, the server includes an extension in the
Reply message for every extension sent by the client in the Request
message. If the client requests some extensions that cannot be
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supplied by the server, the server can simply fail to provide them,
not including them in the Reply. Other extensions can be rejected
by including them in the Reply with an appropriate status indicating
failure. The server can include extensions in the reply that were
not requested by the client.
6.3.2. Client Requests to Deallocate Unknown Resources
When a client DHCP Request is received that has the 'C' bit set, the
server should check to find out whether the extensions listed in the
Request message match those which it has associated with the client's
binding. Any resources which are not indicated by the client are
presumed to be unknown by the client, and thus possible candidates
for reallocation to satisfy requests from other clients. The DHCP
server MUST deallocate all resources associated with the client
upon reception of a DHCP Request with the 'C' bit set, except for
those which the server is willing to reallocate in response to the
client's request. It may be more efficient to avoid deallocating any
resources until after the list of extensions to the request have been
inspected.
6.4. Receiving DHCP Release Messages
If the server receives a DHCP Release Message, it MUST verify that
the link-local address field of the message contains an address which
could be a valid link-local address (see Section 2.1). If not, the
message MUST be silently discarded.
In response to a DHCP Release Message with a valid client's
link-local address and agent address, the DHCP server formulates a
DHCP Reply message that will be sent back to the releasing client by
way of the client's link-local address. A DHCP Reply message sent
in response to a DHCP Release message MUST be sent to the client's
link-local address via the agent address in the Release message
with the 'L' bit set in the Reply, unless the 'D' bit is set in the
Release message.
If the received agent address and link-local address do not
correspond to any binding known to the server, then the server SHOULD
return a DHCP Reply, indicating the error by setting the status code
to 20.
Otherwise, if the agent address and link-local address indicate a
binding known to the server, then the server continues processing the
Release message. If there are any extensions, the server releases
the particular configuration items specified in the extensions.
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If there are no extensions, the server releases all configuration
information in the client's binding.
After performing the operations indicated in the DHCP Release message
and its extensions, the DHCP server formulates a DHCP Reply message,
copying the transaction-ID from the DHCP Release message. For
each Extension in the DHCP Release message successfully processed
by the server, a matching Extension is appended to the DHCP Reply
message. For extensions in the DHCP Release message which cannot be
successfully processed by the server, a DHCP Reply message containing
extensions with the appropriate status MUST be returned by the
server. If the Release message contains no extensions, the server
does not include any extensions in the corresponding DHCP Reply
message to the client.
6.5. Sending DHCP Reconfigure Messages
If a DHCP server needs to change the configuration associated with
any of its clients, it constructs a DHCP Reconfigure message and
sends it to each such client. The Reconfigure MAY be sent to a
multicast address chosen by the server and previously sent to each of
its clients in an extension to a previous DHCP Reply message.
Often, it happens that a client does not send DHCP Request messages
after the DHCP Reconfigure message has been issued and retransmitted
according to the algorithm specified in Section 8. This can happen
when the cleint is not listening for the Reconfigure message,
possibly because the client is a mobile node disconnected from the
network, or because the client node has sustained a power outage
or operating system crash. In such cases, the DHCP server SHOULD
reserve any resources issued to the client until the client responds
at some future time, or until administrative intervention causes the
resources to be manually returned to use.
7. DHCP Relay Considerations
The DHCP protocol is constructed so that a relay does not have
to maintain any state in order to mediate DHCP client/server
interactions.
All relays MUST send DHCP Request messages using the source IP
address from the interface where the DHCP request was received.
The purpose of the DHCP relay is to enable clients and servers to
carry out DHCP protocol transactions. DHCP Solicit messages are
issued by the relay when initiated by prospective DHCP clients. By
default, the relay locates DHCP servers by use of multicasting DHCP
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solicitations to the All-DHCP-Servers multicast address, but relays
SHOULD allow this behavior to be configurable. The relay SHOULD NOT
send such a multicast solicitation on the interface from which it
received the solicitation from the client. The source address must
be a site-local or global-scope address belonging to the relay's
interface on which the client's original solicitation was received.
7.1. DHCP Solicit and DHCP Advertise Message Processing
Upon receiving a DHCP Solicit message from a prospective client, a
relay, by default, forwards the message to all DHCP servers at a site
according to the following procedure:
- copying the prospective client's solicitation message fields into
the appropriate fields of the outgoing solicitation,
- copying a non-link-local address of its interface from which the
solicitation was received from the client into the DHCP relay
address field, and
- by default, setting the TTL field in the solicitation to the
value DEFAULT_SOLICIT_TTL (see section 8).
- finally, sending the resulting message to the All-DHCP-Servers
multicast address, FF05:0:0:0:0:0:1:3.
When the relay receives a DHCP advertisement, it relays the
advertisement to the client at the client's link-local address by way
of the interface indicated in the agent's address field.
7.2. DHCP Request Message Processing
When a relay receives a DHCP Request message, it SHOULD check that
the IP source address in the IP header is a link-local address,
that the link-local address matches the link-local address field in
the Request message header, and that the agent address field of the
message matches an IP address associated with the interface from
which the DHCP Request message was received. If any of these checks
fail, the relay MUST silently discard the Request message.
The relay MUST check whether the 'S' bit is set in the message
header. If not, the packet is discarded, and the relay SHOULD
return a DHCP Reply message to the address contained in the client's
link-local address field of the Request message, with status 18.
If the received request message is acceptable, the relay then
transmits the DHCP Request message to the address of the DHCP server
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found in the Server IP Address field of the received DHCP Request
message. All of the fields of DHCP Request message transmitted by
the relay are copied over unchanged from the DHCP Request received
from the client. Only the fields in the IP header will differ from
the packet received from the client. If the Relay receives an ICMP
error, the Relay SHOULD return a DHCP Reply message to the client
address (which can be found in the payload of the ICMP message [5]),
with status 64.
7.3. DHCP Reply Message Processing
When the relay receives a DHCP Reply, it MUST check that the message
has the 'L' bit set. It MUST check that the link-local address field
contains a link-local address. If either check fails, the packet
MUST be silently discarded. If both checks are satisfied, the relay
MUST send a DHCP Reply message to the link-local address listed in
the received Reply message. Only the fields in the IP header will
differ from the packet received from the server, not the payload.
8. Retransmission and Configuration Variables
When a DHCP client does not receive a DHCP Reply in response to a
pending DHCP Request, the client MUST retransmit the identical DHCP
Request, with the same transaction-ID, to the same server again
until it can be reasonably sure that the DHCP server is unavailable
and an alternative can be chosen. The DHCP server assumes that the
client has received the configuration information included with the
extensions to the DHCP Reply message, and it is up to the client
to continue to try for a reasonable amount of time to complete the
transaction. All the actions specified for DHCP Request in this
section hold also for DHCP Release messages, that do not have the 'N'
bit set, sent by the DHCP client.
Similarly, when a client sends a DHCP Request message in response to
a Reconfigure message from the server, the client assumes that the
DHCP server has received the Request. The server MUST retransmit
the identical DHCP Reconfigure to the client for a reasonable amount
of time, to try to elicit the Request message from the client. If
no corresponding DHCP Request is received by the server within this
time, the server MAY erase or deallocate information as needed from
the client's binding.
When a client reboots and loses its previous state, the server
should no longer keep track of the transaction IDs associated with
that previous state. In order to inform the server that the client
no longer wishes any association to be maintained between used
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transaction IDs and previous transactions, the client should set the
'R' bit in its DHCP Request.
Retransmissions occur using the following configuration variables
for a DHCP implementation. These configuration variables MUST be
configurable by a client or server:
ADV_CLIENT_WAIT
The minimum amount of time a client waits to receive DHCP
Advertisements after transmitting a DHCP Solicit to the
All-DHCP Agents multicast address.
Default: 2 seconds
DEFAULT_SOLICIT_TTL
The default TTL value used by DHCP relays when sending DHCP
Solicit messages on behalf of a client.
Default: 4
SERVER_MIN_ADV_DELAY
The minimum amount of time a server waits to transmit a DHCP
Advertisement after receiving a DHCP Solicit at the All-DHCP
Servers or All-DHCP Agents multicast address.
Default: 100 milliseconds
SERVER_MAX_ADV_DELAY
The maximum amount of time a server waits to transmit a DHCP
Advertisement after receiving a DHCP Solicit at the All-DHCP
Agents multicast address.
Default: 1 second
REPLY_MSG_TIMEOUT
The time in seconds that a DHCP client waits to receive a
server's DHCP Reply before retransmitting a DHCP Request.
Default: 2 seconds.
REPLY_MSG_MIN_RETRANS
The minimum number of DHCP Request transmissions that a DHCP
client should retransmit, before aborting the request, possibly
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retrying the Request with another Server, and logging a DHCP
System Error.
Default: 10 retransmissions.
REPLY_MSG_RETRANS_INTERVAL
The time between successive retransmissions of DHCP Request
messages.
Default: 2 seconds.
RECONF_MSG_TIMEOUT
The time in seconds that a DHCP server waits to receive
a client's DHCP Request before retransmitting its DHCP
Reconfigure.
Default: 12 seconds.
RECONF_MSG_MIN_RETRANS
The minimum number of DHCP Reconfigure messages that a DHCP
server should retransmit, before assuming the the client is
unavailable and that the server can proceed with the needed
reconfiguration of that client's resources, and logging a DHCP
System Error.
Default: 10 retransmissions.
RECONF_MSG_RETRANS_INTERVAL
The least time between successive retransmissions of DHCP
Reconfigure messages.
Default: RECONF_MSG_TIMEOUT
RECONF_MSG_MIN_RESP
The minimum amount of time before a client can respond to a
DHCP Reconfigure message sent to a multicast address.
Default: 2 seconds.
RECONF_MSG_MAX_RESP
The maximum amount of time before a client MUST respond to a
DHCP Reconfigure message sent to a multicast address.
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Default: 10 seconds.
MIN_SOLICIT_DELAY
The maximum amount of time a prospective client is required to
wait, after determining from a Router Advertisement message
that the client should perform stateful address configuration,
before sending a DHCP Solicit to a DHCP server.
Default: 1 second
MAX_SOLICIT_DELAY
The maximum amount of time a prospective client is required to
wait, after determining from a Router Advertisement message
that the client should perform stateful address configuration,
before sending a DHCP Solicit to a DHCP server.
Default: 5 seconds
XID_TIMEOUT
The amount of time a DHCP server has to keep track of
client transaction-IDs in order to make sure that client
retransmissions using the same transaction-ID are idempotent.
Default: 600 seconds
Note that, if a client receives a DHCP message which fails
authentication, it should continue to wait for another message which
might be correctly authenticated just as if the failed message had
never arrived; however, receiving such failed messages SHOULD be
logged.
9. Security Considerations
DHCP clients and servers often have to authenticate the messages they
exchange. For instance, a DHCP server may wish to be certain that a
DHCP Request originated from the client identified by the <link-local
address, agent address> fields included within the Request message
header. Conversely, it is quite often essential for a DHCP client
to be certain that the configuration parameters and addresses it has
received were sent to it by an authoritative DHCP server. Similarly,
a DHCP server should only accept a DHCP Release message which seems
to be from one of its clients, if it has some assurance that the
client actually did transmit the Release message. Again, a client
might wish to only accept DHCP Reconfigure messages that are certain
to have originated from a server with authority to issue them.
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The IPv6 Authentication Header can provide security for DHCPv6
messages when both endpoints have a suitable IP address. However,
a client often has only a link-local address, and such an address
is not sufficient for a DHCP server which is off-link. In those
circumstances the DHCP relay is involved, so that the DHCP message
MUST have the relay's address in the IP destination address field,
even though the client aims to deliver the message to the DHCP
server. The DHCP Client-Server Authentication Extension [12] is
intended to be used in these circumstances.
10. Year 2000 considerations
Since all times are relative to the current time of the transaction,
there is no problem within the DHCPv6 protocol related to any
hardcoded dates or two-digit representation of the current year.
11. Acknowledgements
Thanks to the DHC Working Group for their time and input into the
specification. Ralph Droms and Thomas Narten have had a major role
in shaping the continued improvement of the protocol by their careful
reviews. Many thanks to Matt Crawford and Erik Nordmark for their
studied review as part of the Last Call process. Thanks also for the
consistent input, ideas, and review by (in alphabetical order) Mike
Carney, Brian Carpenter, Gerald Maguire, Jack McCann, Yakov Rekhter,
Matt Thomas, Sue Thomson, 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.
A. Related Work in IPv6
The related work in IPv6 that would best serve an implementor
to study is the IPv6 Specification [6], the IPv6 Addressing
Architecture [8], IPv6 Stateless Address Autoconfiguration [16], IPv6
Neighbor Discovery Processing [11], and Dynamic Updates to DNS [18].
These specifications enable DHCP to build upon the IPv6 work to
provide both robust stateful autoconfiguration and autoregistration
of DNS Host Names.
The IPv6 Specification provides the base architecture and design of
IPv6. A key point for DHCP implementors to understand is that IPv6
requires that every link in the internet have an MTU of 1500 octets
or greater (in IPv4 the requirement is 68 octets). This means that
a UDP packet of 536 octets will always pass through an internet
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(less 40 octets for the IPv6 header), as long as there are no IP
options prior to the UDP header in the packet. But, IPv6 does not
support fragmentation at routers, so that fragmentation takes place
end-to-end between hosts. If a DHCP implementation needs to send a
packet greater than 1500 octets it can either fragment the UDP packet
into fragments of 1500 octets or less, or use Path MTU Discovery [10]
to determine the size of the packet that will traverse a network
path. It is implementation dependent how this is accomplished in
DHCP. Path MTU Discovery for IPv6 is supported for both UDP and TCP
and can cause end-to-end fragmentation when the PMTU changes for a
destination.
The IPv6 Addressing Architecture specification [8] 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. This means that a client can immediately use
its link-local address and a well-known multicast address to begin
communications to discover neighbors on the link, or to send a DHCP
Solicit and locate a DHCP server or relay.
IPv6 Stateless Address Autoconfiguration [16] (addrconf) specifies
procedures by which a node may autoconfigure addresses based on
router advertisements [11], 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 addrconf is a design
requirement of DHCP (see Section 3.1).
IPv6 Neighbor Discovery [11] is the node discovery protocol in IPv6
which replaces and enhances functions of ARP [13]. To understand
IPv6 and addrconf it is strongly recommended that implementors
understand IPv6 Neighbor Discovery.
Dynamic Updates to DNS [18] 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. The security model to be used with DHCPv6 should conform as
closely as possible to the authentication model outlined in [9].
B. Comparison between DHCPv4 and DHCPv6
This appendix is provided for readers who will find it useful to see
a model and architecture comparison between DHCPv4 [7, 1] and DHCPv6.
There are three key reasons for the differences:
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o IPv6 inherently supports a new model and architecture for
communications and autoconfiguration of addresses.
o DHCPv6 in its design was able to take advantage of the inherent
benefits of IPv6.
o New features were added to support the expected evolution and
the existence of more complicated Internet network service
requirements.
IPv6 Architecture/Model Changes:
o The link-local address permits a node to have an address
immediately when the node boots, which means all clients have a
source IP address at all times to locate a server or relay agent
on the local link.
o The need for bootp compatibility and broadcast flags are removed,
which permitted a great deal of freedom in designing the new
packet formats for the client and server interaction.
o Multicast and the scoping methods in IPv6 permitted the design of
discovery packets that would inherently define their range by the
multicast address for the function required.
o Stateful autoconfiguration has to coexist and integrate with
stateless autoconfiguration supporting Duplicate Address
Detection and the two IPv6 lifetimes, to facilitate the dynamic
renumbering of addresses and the management of those addresses.
o Multiple addresses per interface are inherently supported in
IPv6.
o Most DHCPv4 options are unnecessary now because the configuration
parameters are either obtained through IPv6 Neighbor Discovery or
the Service Location protocol [17].
DHCPv6 Architecture/Model Changes:
o The message type is the first byte in the packet.
o IPv6 Address allocations are now handled in a message extension
as opposed to the main header.
o Client/Server bindings are now mandatory and take advantage of
the client's link-local address to always permit communications
either directly from an on-link server, or from a remote server
through an on-link relay-agent.
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o Servers are now discovered by a client solicit and server or
relay-agent advertisement model.
o The client will know if the server is on-link or off-link.
o The client after a solicit will be returned the addresses of
available servers either from an on-link server or from an
on-link relay-agent as agents providing the advertisements.
o The on-link relay-agent will obtain the location of remote server
addresses from system configuration or by the use of a site wide
DHCPv6 Multicast packet.
o The protocol is optimized and removes the use of ACKs and NAKs
once the client and server set-up is complete.
o The server assumes the client receives its responses unless it
receives a retransmission of the same client request. This
permits recovery in the case where the network has faulted.
o The function of DHCPINFORM is inherent in the new packet design;
a client can request configuration parameters other than IPv6
addresses in the optional extension headers.
o Clients MUST listen to their UDP port for the new Reconfigure
message from servers.
o Dynamic Updates to DNS are supported in the IPv6 Address
extension.
o New extensions have been defined.
New Internet User Features:
o Configuration of Dynamic Updates to DNS to support different
requirements.
o Configuration of what policy is enforced when addresses are
deprecated for dynamic renumbering can be implemented.
o Configuration of how relay-agents locate remote servers for a
link can be implemented.
o An Authentication extension has been added.
o Configuration of additional addresses for server applications can
be requested by a client in an implementation.
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o Reclaiming addresses allocated with very long lifetimes can be
implemented using the Reconfigure message type.
o Configuration of tightly coupled integration between stateless
and stateful address autoconfiguration can be implemented.
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References
[1] S. Alexander and R. Droms. DHCP Options and BOOTP Vendor
Extensions. RFC 2132, March 1997.
[2] S. Bradner. Key words for use in RFCs to Indicate Requirement
Levels. RFC 2119, March 1997.
[3] S. Bradner and A. Mankin. The Recommendation for the IP Next
Generation Protocol. RFC 1752, January 1995.
[4] William R. Cheswick and Steven Bellovin. Firewalls and Internet
Security. Addison-Wesley, Reading, Massachusetts, 1994. (ISBN:
0-201-63357-4).
[5] A. Conta and S. Deering. Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6). RFC 1885,
December 1995.
[6] S. Deering and R. Hinden. Internet Protocol, Version 6 (IPv6)
Specification. RFC 1883, December 1995.
[7] R. Droms. Dynamic Host Configuration Protocol. RFC 2131, March
1997.
[8] R. Hinden and S. Deering. IP Version 6 Addressing Architecture.
RFC 1884, December 1995.
[9] Stephen Kent and Randall Atkinson. IP Authentication Header.
draft-ietf-ipsec-auth-header-01.txt, July 1997. (work in
progress).
[10] J. McCann, S. Deering, and J. Mogul. Path MTU Discovery for IP
version 6. RFC 1981, August 1996.
[11] T. Narten, E. Nordmark, and W. Simpson. Neighbor Discovery for
IP version 6 (IPv6). RFC 1970, August 1996.
[12] C. Perkins. Extensions for the Dynamic Host Configuration
Protocol for IPv6. draft-ietf-dhc-dhcpv6ext-09.txt, October
1997. (work in progress).
[13] David C. Plummer. An Ethernet Address Resolution Protocol:
Or Converting Network Protocol Addresses to 48.bit Ethernet
Addresses for Transmission on Ethernet Hardware. RFC 826,
November 1982.
[14] J. B. Postel. User Datagram Protocol. RFC 768, August 1980.
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[15] J. B. Postel, Editor. Internet Protocol. RFC 791, September
1981.
[16] S. Thomson and T. Narten. IPv6 stateless address
autoconfiguration. RFC 1971, August 1996.
[17] J. Veizades, E. Guttman, C. Perkins, and S. Kaplan. Service
Location Protocol. RFC 2165, July 1997.
[18] P. Vixie, S. Thomson, Y. Rekhter, and J. Bound. Dynamic Updates
in the Domain Name System (DNS). RFC 2136, April 1997.
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Chair's Address
The working group can be contacted via the current chair:
Ralph Droms
Computer Science Department
323 Dana Engineering
Bucknell University
Lewisburg, PA 17837
Phone: (717) 524-1145
E-mail: droms@bucknell.edu
Author's Address
Questions about this memo can be directed to:
Jim Bound Charles Perkins
Digital Equipment Corporation Technology Development
110 Spitbrook Road, ZKO3-3/U14 Sun Microsystems, Inc.
Nashua, NH 03062 901 San Antonio Rd.
Palo Alto, CA 94303
Phone: +1-603-884-0400 +1-650-786-6464
Fax: +1-650-786-6445
E-mail: bound@zk3.dec.com charles.perkins@sun.com
Bound, Perkins Expires 21 May 1998 [Page 39]
