RFC1531 - Dynamic Host Configuration Protocol

Network Working Group R. Droms
Request for Comments: 1531 BUCknell University
Category: Standards Track October 1993
Dynamic Host Configuration Protocol
Status of this memo
This RFCspecifies 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" for the standardization state and status
of this protocol. Distribution of this memo is unlimited.
Abstract
The Dynamic Host Configuration Protocol (DHCP) provides a framework
for passing configuration information to hosts on a TCP/IP network.
DHCP is based on the Bootstrap Protocol (BOOTP) [7], adding the
capability of automatic allocation of reusable network addresses and
additional configuration options [19]. DHCP captures the behavior of
BOOTP relay agents [7, 23], and DHCP participants can interoperate
with BOOTP participants [9].
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Related Work. . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Problem definition and issues . . . . . . . . . . . . . . . . 4
1.3 Requirements. . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5 Design goals. . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Protocol Summary . . . . . . . . . . . . . . . . . . . . . . . 8
2.1 Configuration parameters repository . . . . . . . . . . . . . 10
2.2 Dynamic allocation of network addresses . . . . . . . . . . . 11
3. The Client-Server Protocol . . . . . . . . . . . . . . . . . . 11
3.1 Client-server interaction - allocating a network address. . . 12
3.2 Client-server interaction - reusing a previously allocated
network address . . . . . . . . . . . . . . . . . . . . . . . 17
3.3 Interpretation and representation of time values. . . . . . . 19
3.4 Host parameters in DHCP . . . . . . . . . . . . . . . . . . . 19
3.5 Use of DHCP in clients with multiple interfaces . . . . . . . 20
3.6 When clients should use DHCP. . . . . . . . . . . . . . . . . 20
4. Specification of the DHCP client-server protocol . . . . . . . 21
4.1 Constructing and sending DHCP messages. . . . . . . . . . . . 21
4.2 DHCP server administrative controls . . . . . . . . . . . . . 23
4.3 DHCP server behavior. . . . . . . . . . . . . . . . . . . . . 24
4.3.1 DHCPDISCOVER message. . . . . . . . . . . . . . . . . . . . 24
4.3.2 DHCPREQUEST message . . . . . . . . . . . . . . . . . . . . 27
4.3.3 DHCPDECLINE message . . . . . . . . . . . . . . . . . . . . 29
4.3.4 DHCPRELEASE message . . . . . . . . . . . . . . . . . . . . 29
4.4 DHCP client behavior. . . . . . . . . . . . . . . . . . . . . 29
4.4.1 Initialization and allocation of network address. . . . . . 29
4.4.2 Initialization with known network address . . . . . . . . . 33
4.4.3 Initialization with a known DHCP server address . . . . . . 34
4.4.4 Reacquisition and eXPiration. . . . . . . . . . . . . . . . 34
4.4.5 DHCPRELEASE . . . . . . . . . . . . . . . . . . . . . . . . 35
5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 35
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
7. Security Considerations. . . . . . . . . . . . . . . . . . . . 37
8. Author"s Address . . . . . . . . . . . . . . . . . . . . . . . 38
A. Host Configuration Parameters . . . . . . . . . . . . . . . . 39
List of Figures
1. Format of a DHCP message . . . . . . . . . . . . . . . . . . . 9
2. Format of the "flags" field. . . . . . . . . . . . . . . . . . 10
3. Timeline diagram of messages exchanged between DHCP client and
servers when allocating a new network address. . . . . . . . . 15
4. Timeline diagram of messages exchanged between DHCP client and
servers when reusing a previously allocated network address. . 18
5. State-transition diagram for DHCP clients. . . . . . . . . . . 31
List of Tables
1. Description of fields in a DHCP message. . . . . . . . . . . . 14
2. DHCP messages. . . . . . . . . . . . . . . . . . . . . . . . . 16
3. Fields and options used by DHCP servers. . . . . . . . . . . . 25
4. Fields and options used by DHCP clients. . . . . . . . . . . . 32
1. Introduction
The Dynamic Host Configuration Protocol (DHCP) provides configuration
parameters to Internet hosts. DHCP consists of two components: a
protocol for delivering host-specific configuration parameters from a
DHCP server to a host and a mechanism for allocation of network
addresses to hosts.
DHCP is built on a client-server model, where designated DHCP server
hosts allocate network addresses and deliver configuration parameters
to dynamically configured hosts. Throughout the remainder of this
document, the term "server" refers to a host providing initialization
parameters through DHCP, and the term "client" refers to a host
requesting initialization parameters from a DHCP server.
A host should not act as a DHCP server unless explicitly configured
to do so by a system administrator. The diversity of hardware and
protocol implementations in the Internet would preclude reliable
operation if random hosts were allowed to respond to DHCP requests.
For example, IP requires the setting of many parameters within the
protocol implementation software. Because IP can be used on many
dissimilar kinds of network hardware, values for those parameters
cannot be guessed or assumed to have correct defaults. Also,
distributed address allocation schemes depend on a polling/defense
mechanism for discovery of addresses that are already in use. IP
hosts may not always be able to defend their network addresses, so
that such a distributed address allocation scheme cannot be
guaranteed to avoid allocation of duplicate network addresses.
DHCP supports three mechanisms for IP address allocation. In
"automatic allocation", DHCP assigns a permanent IP address to a
host. In "dynamic allocation", DHCP assigns an IP address to a host
for a limited period of time (or until the host explicitly
relinquishes the address). In "manual allocation", a host"s IP
address is assigned by the network administrator, and DHCP is used
simply to convey the assigned address to the host. A particular
network will use one or more of these mechanisms, depending on the
policies of the network administrator.
Dynamic allocation is the only one of the three mechanisms that
allows automatic reuse of an address that is no longer needed by the
host to which it was assigned. Thus, dynamic allocation is
particularly useful for assigning an address to a host that will be
connected to the network only temporarily or for sharing a limited
pool of IP addresses among a group of hosts that do not need
permanent IP addresses. Dynamic allocation may also be a good choice
for assigning an IP address to a new host being permanently connected
to a network where IP addresses are sufficiently scarce that it is
important to reclaim them when old hosts are retired. Manual
allocation allows DHCP to be used to eliminate the error-prone
process of manually configuring hosts with IP addresses in
environments where (for whatever reasons) it is desirable to manage
IP address assignment outside of the DHCP mechanisms.
The format of DHCP messages is based on the format of BOOTP messages,
to capture the BOOTP relay agent behavior described as part of the
BOOTP specification [7, 23] and to allow interoperability of existing
BOOTP clients with DHCP servers. Using BOOTP relaying agents
eliminates the necessity of having a DHCP server on each physical
network segment.
1.1 Related Work
There are several Internet protocols and related mechanisms that
address some parts of the dynamic host configuration problem. The
Reverse Address Resolution Protocol (RARP) [10] (through the
extensions defined in the Dynamic RARP (DRARP) [5]) explicitly
addresses the problem of network address discovery, and includes an
automatic IP address assignment mechanism. The Trivial File Transfer
Protocol (TFTP) [20] provides for transport of a boot image from a
boot server. The Internet Control Message Protocol (ICMP) [16]
provides for informing hosts of additional routers via "ICMP
redirect" messages. ICMP also can provide subnet mask information
through the "ICMP mask request" message and other information through
the (obsolete) "ICMP information request" message. Hosts can locate
routers through the ICMP router discovery mechanism [8].
BOOTP is a transport mechanism for a collection of configuration
information. BOOTP is also extensible, and official extensions [17]
have been defined for several configuration parameters. Morgan has
proposed extensions to BOOTP for dynamic IP address assignment [15].
The Network Information Protocol (NIP), used by the Athena project at
MIT, is a distributed mechanism for dynamic IP address assignment
[19]. The Resource Location Protocol RLP [1] provides for location
of higher level services. Sun Microsystems diskless workstations use
a boot procedure that employs RARP, TFTP and an RPC mechanism called
"bootparams" to deliver configuration information and operating
system code to diskless hosts. (Sun Microsystems, Sun Workstation
and SunOS are trademarks of Sun Microsystems, Inc.) Some Sun
networks also use DRARP and an auto-installation mechanism to
automate the configuration of new hosts in an existing network.
In other related work, the path minimum transmission unit (MTU)
discovery algorithm can determine the MTU of an arbitrary internet
path [14]. Comer and Droms have proposed the use of the Address
Resolution Protocol (ARP) as a transport protocol for resource
location and selection [6]. Finally, the Host Requirements RFCs [3,
4] mention specific requirements for host reconfiguration and suggest
a scenario for initial configuration of diskless hosts.
1.2 Problem definition and issues
DHCP is designed to supply hosts with the configuration parameters
defined in the Host Requirements RFCs. After oBTaining parameters
via DHCP, a host should be able to exchange packets with any other
host in the Internet. The parameters supplied by DHCP are listed in
Appendix A.
Not all of these parameters are required for a newly initialized
host. A client and server may negotiate for the transmission of only
those parameters required by the client or specific to a particular
subnet.
DHCP allows but does not require the configuration of host parameters
not directly related to the IP protocol. DHCP also does not address
registration of newly configured hosts with the Domain Name System
(DNS) [12, 13].
DHCP is not intended for use in configuring routers.
1.3 Requirements
Throughout this document, the Words that are used to define the
significance of particular requirements are capitalized. These words
are:
o "MUST"
This word or the adjective "REQUIRED" means that the
item is an absolute requirement of this specification.
o "MUST NOT"
This phrase means that the item is an absolute prohibition
of this specification.
o "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 should be understood and
the case carefully weighed before choosing a different course.
o "SHOULD NOT"
This phrase means that there may exist valid reasons in
particular circumstances when the listed behavior is acceptable
or even useful, but the full implications should be understood
and the case carefully weighed before implementing any behavior
described with this label.
o "MAY"
This word or the adjective "OPTIONAL" means that this item is
truly optional. One vendor may choose to include the item
because a particular marketplace requires it or because it
enhances the product, for example; another vendor may omit the
same item.
1.4 Terminology
This document uses the following terms:
o "DHCP client"
A DHCP client is an Internet host using DHCP to obtain
configuration parameters such as a network address.
o "DHCP server"
A DHCP server is an Internet host that returns configuration
parameters to DHCP clients.
o "BOOTP relay agent"
A BOOTP relay agent is an Internet host or router that passes
DHCP messages between DHCP clients and DHCP servers. DHCP is
designed to use the same relay agent behavior as specified in
the BOOTP protocol specification.
o "binding"
A binding is a collection of configuration parameters, including
at least an IP address, associated with or "bound to" a DHCP
client. Bindings are managed by DHCP servers.
1.5 Design goals
The following list gives general design goals for DHCP.
o 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.
o Hosts should require no manual configuration. Each host should
be able to discover appropriate local configuration parameters
without user intervention and incorporate those parameters into
its own configuration.
o Networks should require no hand configuration for individual
hosts. Under normal circumstances, the network manager should
not have to enter any per-host configuration parameters.
o DHCP should not require a server on each subnet. To allow for
scale and economy, DHCP must work across routers or through the
intervention of BOOTP/DHCP relay agents.
o A DHCP host must be prepared to receive multiple responses to a
request for configuration parameters. Some installations may
include multiple, overlapping DHCP servers to enhance
reliability and increase performance.
o DHCP must coexist with statically configured, non-participating
hosts and with existing network protocol implementations.
o DHCP must interoperate with the BOOTP relay agent behavior as
described by RFC951 and by Wimer [21].
o DHCP must provide service to existing BOOTP clients.
The following list gives design goals specific to the transmission of
the network layer parameters. DHCP must:
o Guarantee that any specific network address will not be in
use by more than one host at a time,
o Retain host configuration across host reboot. A host should,
whenever possible, be assigned the same configuration parameters
(e.g., network address) in response to each request,
o Retain host configuration across server reboots, and, whenever
possible, a host should be assigned the same configuration
parameters despite restarts of the DHCP mechanism,
o Allow automatic assignment of configuration parameters to new
hosts to avoid hand configuration for new hosts,
o Support fixed or permanent allocation of configuration
parameters to specific hosts.
2. Protocol Summary
From the client"s point of view, DHCP is an extension of the BOOTP
mechanism. This behavior allows existing BOOTP clients to
interoperate with DHCP servers without requiring any change to the
clients" initialization software. A separate document details the
interactions between BOOTP and DHCP clients and servers [9]. There
are some new, optional transactions that optimize the interaction
between DHCP clients and servers that are described in sections 3 and
4.
Figure 1 gives the format of a DHCP message and table 1 describes
each of the fields in the DHCP message. The numbers in parentheses
indicate the size of each field in octets. The names for the fields
given in the figure will be used throughout this document to refer to
the fields in DHCP messages.
There are two primary differences between DHCP and BOOTP. First,
DHCP defines mechanisms through which clients can be assigned a
network address for a fixed lease, allowing for serial reassignment
of network addresses to different clients. Second, DHCP provides the
mechanism for a client to acquire all of the IP configuration
parameters that it needs in order to operate.
DHCP introduces a small change in terminology intended to clarify the
meaning of one of the fields. What was the "vendor extensions" field
in BOOTP has been re-named the "options" field in DHCP. Similarly,
the tagged data items that were used inside the BOOTP "vendor
extensions" field, which were formerly referred to as "vendor
extensions," are now termed simply "options."
DHCP defines a new "client identifier" option that is used to pass an
explicit client identifier to a DHCP server. This change eliminates
the overloading of the "chaddr" field in BOOTP messages, where reply
messages and as a client identifier. The "client identifier" option
may contain a hardware address, identical to the contents of the
"chaddr" field, or it may contain another type of identifier, such as
a DNS name. Other client identifier types may be defined as needed
for use with DHCP. New client identifier types will be registered
with the IANA [18] and will be included in new revisions of the
Assigned Numbers document, as well as described in detail in future
revisions of the DHCP Options [2].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
op (1) htype (1) hlen (1) hops (1)
+---------------+---------------+---------------+---------------+
xid (4)
+-------------------------------+-------------------------------+
secs (2) flags (2)
+-------------------------------+-------------------------------+
ciaddr (4)
+---------------------------------------------------------------+
yiaddr (4)
+---------------------------------------------------------------+
siaddr (4)
+---------------------------------------------------------------+
giaddr (4)
+---------------------------------------------------------------+

chaddr (16)


+---------------------------------------------------------------+

sname (64)
+---------------------------------------------------------------+

file (128)
+---------------------------------------------------------------+

options (312)
+---------------------------------------------------------------+
Figure 1: Format of a DHCP message
DHCP clarifies the interpretation of the "siaddr" field as the
address of the server to use in the next step of the client"s
bootstrap process. A DHCP server may return its own address in the
"siaddr" field, if the server is prepared to supply the next
bootstrap service (e.g., delivery of an operating system executable
image). A DHCP server always returns its own address in the "server
identifier" option.
The options field is now variable length, with the minimum extended
to 312 octets. This brings the minimum size of a DHCP message up to
576 octets, the minimum IP datagram size a host must be prepared to
accept [3]. DHCP clients may negotiate the use of larger DHCP
messages through the "Maximum DHCP message size" option. The options
field may be further extended into the "file" and "sname" fields.
A new option, called "vendor specific information", has been added to
allow for expansion of the number of options that can be supported
[2]. Options encapsulated as "vendor specific information" must be
carefully defined and documented so as to allow for interoperability
between clients and servers from diferent vendors. In particular,
vendors defining "vendor specific information" MUST document those
options in the form of the DHCP Options document, MUST choose to
represent those options either in data types already defined for DHCP
options or in other well-defined data types, and MUST choose options
that can be readily encoded in configuration files for exchange with
servers provided by other vendors. Options included as "vendor
specific options" MUST be readily supportable by all servers.
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
B MBZ
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
B: BROADCAST flag
MBZ: MUST BE ZERO (reserved for future use)
Figure 2: Format of the "flags" field
DHCP uses the "flags" field [21]. The leftmost bit is defined as the
BROADCAST (B) flag. The semantics of this flag are discussed in
section 4.1 of this document. The remaining bits of the flags field
are reserved for future use. They MUST be set to zero by clients and
ignored by servers and relay agents. Figure 2 gives the format of
the
2.1 Configuration parameters repository
The first service provided by DHCP is to provide persistent storage
of network parameters for network clients. The model of DHCP
persistent storage is that the DHCP service stores a key-value entry
for each client, where the key is some unique identifier (for
example, an IP subnet number and a unique identifier within the
subnet) and the value contains the configuration parameters for the
client.
For example, the key might be the pair (IP-subnet-number, hardware-
address), allowing for serial or concurrent reuse of a hardware
address on different subnets, and for hardware addresses that may not
be globally unique. Alternately, the key might be the pair (IP-
subnet-number, hostname), allowing the server to assign parameters
intelligently to a host that has been moved to a different subnet or
has changed hardware addresses (perhaps because the network interface
failed and was replaced).
A client can query the DHCP service to retrieve its configuration
parameters. The client interface to the configuration parameters
repository consists of protocol messages to request configuration
parameters and responses from the server carrying the configuration
parameters.
2.2 Dynamic allocation of network addresses
The second service provided by DHCP is the allocation of temporary or
permanent network (IP) addresses to hosts. The basic mechanism for
the dynamic allocation of network addresses is simple: a client
requests the use of an address for some period of time. The
allocation mechanism (the collection of DHCP servers) guarantees not
to reallocate that address within the requested time and attempts to
return the same network address each time the client requests an
address. In this document, the period over which a network address
is allocated to a client is referred to as a "lease" [11]. The
client may extend its lease with subsequent requests. The client may
issue a message to release the address back to the server when the
client no longer needs the address. The client may ask for a
permanent assignment by aSKINg for an infinite lease. Even when
assigning "permanent" addresses, a server may choose to give out
lengthy but non-infinite leases to allow detection of the fact that
the host has been retired.
In some environments it will be necessary to reassign network
addresses due to exhaustion of available addresses. In such
environments, the allocation mechanism will reuse addresses whose
lease has expired. The server should use whatever information is
available in the configuration information repository to choose an
address to reuse. For example, the server may choose the least
recently assigned address. As a consistency check, the allocation
mechanism may probe the reused address, e.g., with an ICMP echo
request, before allocating the address, and the client will probe the
newly received address, e.g., with ARP.
3. The Client-Server Protocol
DHCP uses the BOOTP message format defined in RFC951 and given in
table 1 and figure 1. The "op" field of each DHCP message sent from
a client to a server contains BOOTREQUEST. BOOTREPLY is used in the
"op" field of each DHCP message sent from a server to a client.
The first four octets of the "options" field of the DHCP message
contain the (decimal) values 99, 130, 83 and 99, respectively (this
is the same magic cookie as is defined in RFC1395). The remainder
of the "options" field consists a list of tagged parameters that are
called "options". All of the "vendor extensions" listed in RFC1395
are also DHCP options. A separate document gives the complete set of
options defined for use with DHCP [2].
Several options have been defined so far. One particular option -
the "DHCP message type" option - must be included in every DHCP
message. This option defines the "type" of the DHCP message.
Additional options may be allowed, required, or not allowed,
depending on the DHCP message type.
Throughout this document, DHCP messages that include a "DHCP message
type" option will be referred to by the type of the message; e.g., a
DHCP message with "DHCP message type" option type 1 will be referred
to as a "DHCPDISCOVER" message.
3.1 Client-server interaction - allocating a network address
The following summary of the protocol exchanges between clients and
servers refers to the DHCP messages described in table 2. The
timeline diagram in figure 3 shows the timing relationships in a
typical client-server interaction. If the client already knows its
address, some steps may be omitted; this abbreviated interaction is
described in section 3.2.
1. The client broadcasts a DHCPDISCOVER message on its local physical
subnet. The DHCPDISCOVER message may include options that suggest
values for the network address and lease duration. BOOTP relay
agents may pass the message on to DHCP servers not on the same
physical subnet.
2. Each server may respond with a DHCPOFFER message that includes an
available network address in the "yiaddr" field (and other
configuration parameters in DHCP options). Servers need not
reserve the offered network address, although the protocol will
work more efficiently if the server avoids allocating the offered
network address to another client. The server unicasts the
DHCPOFFER message to the client (using the DHCP/BOOTP relay agent
if necessary) if possible, or may broadcast the message to a
broadcast address (preferably 255.255.255.255) on the client"s
subnet.
3. The client receives one or more DHCPOFFER messages from one or
more servers. The client may choose to wait for multiple
responses. The client chooses one server from which to request
configuration parameters, based on the configuration parameters
offered in the DHCPOFFER messages. The client broadcasts a
DHCPREQUEST message that MUST include the "server identifier"
option to indicate which server it has selected, and may include
other options specifying desired configuration values. This
DHCPREQUEST message is broadcast and relayed through DHCP/BOOTP
relay agents. To help ensure that any DHCP/BOOTP relay agents
forward the DHCPREQUEST message to the same set of DHCP servers
that received the original DHCPDISCOVER message, the DHCPREQUEST
message must use the same value in the DHCP message header"s
"secs" field and be sent to the same IP broadcast address as the
original DHCPDISCOVER message. The client times out and
retransmits the DHCPDISCOVER message if the client receives no
DHCPOFFER messages.
4. The servers receive the DHCPREQUEST broadcast from the client.
Those servers not selected by the DHCPREQUEST message use the
message as notification that the client has declined that server"s
offer. The server selected in the DHCPREQUEST message commits the
binding for the client to persistent storage and responds with a
DHCPACK message containing the configuration parameters for the
requesting client. The combination of "chaddr" and assigned
network address constitute an unique identifier for the client"s
lease and are used by both the client and server to identify a
lease referred to in any DHCP messages. The "yiaddr" field in the
DHCPACK messages is filled in with the selected network address.
If the selected server is unable to satisfy the DHCPREQUEST message
(e.g., the requested network address has been allocated), the
server SHOULD respond with a DHCPNAK message.
A server may choose to mark addresses offered to clients in
DHCPOFFER messages as unavailable. The server should mark an
address offered to a client in a DHCPOFFER message as available if
the server receives no DHCPREQUEST message from that client.
FIELD OCTETS DESCRIPTION
----- ------ -----------
op 1 Message op code / message type.
1 = BOOTREQUEST, 2 = BOOTREPLY
htype 1 Hardware address type, see ARP section in "Assigned
Numbers" RFC; e.g., "1" = 10mb ethernet.
hlen 1 Hardware address length (e.g. "6" for 10mb
ethernet).
hops 1 Client sets to zero, optionally used by relay-agents
when booting via a relay-agent.
xid 4 Transaction ID, a random number chosen by the
client, used by the client and server to associate
messages and responses between a client and a
server.
secs 2 Filled in by client, seconds elapsed since client
started trying to boot.
flags 2 Flags (see figure 2).
ciaddr 4 Client IP address; filled in by client in
DHCPREQUEST if verifying previously allocated
configuration parameters.
yiaddr 4 "your" (client) IP address.
siaddr 4 IP address of next server to use in bootstrap;
returned in DHCPOFFER, DHCPACK and DHCPNAK by
server.
giaddr 4 Relay agent IP address, used in booting via a
relay-agent.
chaddr 16 Client hardware address.
sname 64 Optional server host name, null terminated string.
file 128 Boot file name, null terminated string; "generic"
name or null in DHCPDISCOVER, fully qualified
Directory-path name in DHCPOFFER.
options 312 Optional parameters field. See the options
documents for a list of defined options.
Table 1: Description of fields in a DHCP message
Server Client Server
(not selected) (selected)
v v v

Begins initialization

_____________/_____________
/ DHCPDISCOVER DHCPDISCOVER

Determines Determines
configuration configuration

____________/
_________ /DHCPOFFER
DHCPOFFER /

Collects replies

Selects configuration

_____________/_____________
/ DHCPREQUEST DHCPREQUEST

Commits configuration

_____________/
/ DHCPACK

Initialization complete

. . .
. . .

Graceful shutdown

_____________
DHCPRELEASE

Discards lease

v v v
Figure 3: Timeline diagram of messages exchanged between DHCP
client and servers when allocating a new network address
Message Use
------- ---
DHCPDISCOVER - Client broadcast to locate available servers.
DHCPOFFER - Server to client in response to DHCPDISCOVER with
offer of configuration parameters.
DHCPREQUEST - Client broadcast to servers requesting offered
parameters from one server and implicitly declining
offers from all others.
DHCPACK - Server to client with configuration parameters,
including committed network address.
DHCPNAK - Server to client refusing request for configuration
parameters (e.g., requested network address already
allocated).
DHCPDECLINE - Client to server indicating configuration parameters
(e.g., network address) invalid.
DHCPRELEASE - Client to server relinquishing network address and
cancelling remaining lease.
Table 2: DHCP messages
5. The client receives the DHCPACK message with configuration
parameters. The client performs a final check on the parameters
(e.g., ARP for allocated network address), and notes the duration
of the lease and the lease identification cookie specified in the
DHCPACK message. At this point, the client is configured. If the
client detects a problem with the parameters in the DHCPACK
message, the client sends a DHCPDECLINE message to the server and
restarts the configuration process. The client should wait a
minimum of ten seconds before restarting the configuration process
to avoid excessive network traffic in case of looping.
If the client receives a DHCPNAK message, the client restarts the
configuration process.
The client times out and retransmits the DHCPREQUEST message if the
client receives neither a DHCPACK or a DHCPNAK message. The client
retransmits the DHCPREQUEST according to the retransmission
algorithm in section 4.1. If the client receives neither a DHCPACK
or a DHCPNAK message after ten retransmissions of the DHCPREQUEST
message, the client reverts to INIT state and restarts the
initialization process. The client SHOULD notify the user that the
initialization process has failed and is restarting.
6. The client may choose to relinquish its lease on a network address
by sending a DHCPRELEASE message to the server. The client
identifies the lease to be released by including its network
address in the "ciaddr" field and its hardware address in the
"chaddr" field.
3.2 Client-server interaction - reusing a previously allocated network
address
If a client remembers and wishes to reuse a previously allocated
network address (allocated either by DHCP or some means outside the
protocol), a client may choose to omit some of the steps described in
the previous section. The timeline diagram in figure 4 shows the
timing relationships in a typical client-server interaction for a
client reusing a previously allocated network address.
1. The client broadcasts a DHCPREQUEST message on its local subnet.
The DHCPREQUEST message includes the client"s network address in
the "ciaddr" field. DHCP/BOOTP relay agents pass the message on
to DHCP servers not on the same subnet.
2. Servers with knowledge of the client"s configuration parameters
respond with a DHCPACK message to the client.
If the client"s request is invalid (e.g., the client has moved
to a new subnet), servers may respond with a DHCPNAK message to
the client.
3. The client receives the DHCPACK message with configuration
prameters. The client performs a final check on the parameters
(as in section 3.1), and notes the duration of the lease and
the lease identification cookie specified in the DHCPACK
message. At this point, the client is configured.
If the client detects a problem with the parameters in the
DHCPACK message, the client sends a DHCPDECLINE message to the
server and restarts the configuration process by requesting a
new network address. This action corresponds to the client
moving to the INIT state in the DHCP state diagram, which is
described in section 4.4.
Server Client Server
v v v

Begins
initialization

/
___________/ ___________
/DHCPREQUEST DHCPREQUEST
/

Locates Locates
configuration configuration

/
___________/
/ DHCPACK
_______ /
DHCPACK
Initialization
complete


(Subsequent
DHCPACKS
ignored)


v v v
Figure 4: Timeline diagram of messages exchanged between DHCP
client and servers when reusing a previously allocated
network address
If the client receives a DHCPNAK message, it cannot reuse its
remembered network address. It must instead request a new
address by restarting the configuration process, this time
using the (non-abbreviated) procedure described in section
3.1. This action also corresponds to the client moving to
the INIT state in the DHCP state diagram.
The client times out and retransmits the DHCPREQUEST message if
the client receives neither a DHCPACK nor a DHCPNAK message.
The time between retransmission MUST be chosen according to
the algorithm given in section 4.1. If the client receives no
answer after transmitting 4 DHCPREQUEST messages, the client
MAY choose to use the previously allocated network address and
configuration parameters for the remainder of the unexpired
lease. This corresponds to moving to BOUND state in the client
state transition diagram shown in figure 5.
4. The client may choose to relinquish its lease on a network
address by sending a DHCPRELEASE message to the server. The
client identifies the lease to be released with the lease
identification cookie.
Note that in this case, where the client retains its network
address locally, the client will not normally relinquish its
lease during a graceful shutdown. Only in the case where the
client explicitly needs to relinquish its lease, e.g., the client
is about to be moved to a different subnet, will the client send
a DHCPRELEASE message.
3.3 Interpretation and representation of time values
A client acquires a lease for a network address for a fixed period of
time (which may be infinite). Throughout the protocol, times are to
be represented in units of seconds. The time value of 0xffffffff is
reserved to represent "infinity". The minimum lease duration is one
hour.
As clients and servers may not have synchronized clocks, times are
represented in DHCP messages as relative times, to be interpreted
with respect to the client"s local clock. Representing relative
times in units of seconds in an unsigned 32 bit word gives a range of
relative times from 0 to approximately 100 years, which is sufficient
for the relative times to be measured using DHCP.
The algorithm for lease duration interpretation given in the previous
paragraph assumes that client and server clocks are stable relative
to each other. If there is drift between the two clocks, the server
may consider the lease expired before the client does. To
compensate, the server may return a shorter lease duration to the
client than the server commits to its local database of client
information.
3.4 Host parameters in DHCP
Not all clients require initialization of all parameters listed in
Appendix A. Two techniques are used to reduce the number of
parameters transmitted from the server to the client. First, most of
the parameters have defaults defined in the Host Requirements RFCs;
if the client receives no parameters from the server that override
the defaults, a client uses those default values. Second, in its
initial DHCPDISCOVER or DHCPREQUEST message, a client may provide the
server with a list of specific parameters the client is interested
in.
The client SHOULD include the "maximum DHCP message size" option to
let the server know how large the server may make its DHCP messages.
The parameters returned to a client may still exceed the space
allocated to options in a DHCP message. In this case, two additional
options flags (which must appear in the "options" field of the
message) indicate that the "file" and "sname" fields are to be used
for options.
The client can inform the server which configuration parameters the
client is interested in by including the "parameter request list"
option. The data portion of this option explicitly lists the options
requested by tag number.
In addition, the client may suggest values for the network address
and lease time in the DHCPDISCOVER message. The client may include
the be assigned, and may include the "IP address lease time" option
to suggest the lease time it would like. No other options
representing "hints" at configuration parameters are allowed in a
DHCPDISCOVER or DHCPREQUEST message. The "ciaddr" field is to be
filled in only in a DHCPREQUEST message when the client is requesting
use of a previously allocated IP address.
If a server receives a DHCPREQUEST message with an invalid "ciaddr",
the server SHOULD respond to the client with a DHCPNAK message and
may choose to report the problem to the system administrator. The
server may include an error message in the "message" option.
3.5 Use of DHCP in clients with multiple interfaces
A host with multiple network interfaces must use DHCP through each
interface independently to obtain configuration information
parameters for those separate interfaces.
3.6 When clients should use DHCP
A host should use DHCP to reacquire or verify its IP address and
network parameters whenever the local network parameters may have
changed; e.g., at system boot time or after a disconnection from the
local network, as the local network configuration may change without
the host"s or user"s knowledge.
If a host has knowledge of a previous network address and is unable
to contact a local DHCP server, the host may continue to use the
previous network address until the lease for that address expires.
If the lease expires before the host can contact a DHCP server, the
host must immediately discontinue use of the previous network address
and may inform local users of the problem.
4. Specification of the DHCP client-server protocol
In this section, we assume that a DHCP server has a block of network
addresses from which it can satisfy requests for new addresses. Each
server also maintains a database of allocated addresses and leases in
local permanent storage.
4.1 Constructing and sending DHCP messages
DHCP clients and servers both construct DHCP messages by filling in
fields in the fixed format section of the message and appending
tagged data items in the variable length option area. The options
area includes first a four-octet "magic cookie" (which was described
in section 3), followed by the options. The last option must always
be the "end" option.
DHCP uses UDP as its transport protocol. DHCP messages from a client
to a server are sent to the "DHCP server" port (67), and DHCP
messages from a server to a client are sent to the "DHCP client" port
(68).
DHCP messages broadcast by a client prior to that client obtaining
its IP address must have the source address field in the IP header
set to 0.
If the "giaddr" field in a DHCP message from a client is non-zero,
the server sends any return messages to the "DHCP server" port on the
DHCP relaying agent whose address appears in "giaddr". If the
"giaddr" field is zero, the client is on the same subnet, and the
server sends any return messages to either the client"s network
address, if that address was supplied in the "ciaddr" field, or to
the client"s hardware address or to the local subnet broadcast
address.
If the options in a DHCP message extend into the "sname" and "file"
fields, the "option overload" option MUST appear in the "options"
field, with value 1, 2 or 3, as specified in the DHCP options
document [2]. If the "option overload" option is present in the
"options" field, the options in the "options" field MUST be
terminated by an options field. The options in the "sname" and
"file" fields (if in use as indicated by the "options overload"
option) MUST begin with the first octet of the field, MUST be
terminated by an "end" option, and MUST be followed by "pad" options
to fill the remainder of the field. Any individual option in the
"options", "sname" and "file" fields MUST be entirely contained in
that field. The options in the "options" field MUST be interpreted
first, so that any "option overload" options may be interpreted. The
"file" field MUST be interpreted next (if the options), followed by
the "sname" field.
DHCP clients are responsible for all message retransmission. The
client MUST adopt a retransmission strategy that incorporates a
randomized exponential bacKOFf algorithm to determine the delay
between retransmissions. The delay before the first retransmission
MUST be 4 seconds randomized by the value of a uniform random number
chosen from the range -1 to +1. Clients with clocks that provide
resolution granularity of less than one second may choose a non-
integer randomization value. The delay before the next
retransmission MUST be 8 seconds randomized by the value of a uniform
number chosen from the range -1 to +1. The retransmission delay MUST
be doubled with subsequent retransmissions up to a maximum of 64
seconds. The client MAY provide an indication of retransmission
attempts to the user as an indication of the progress of the
configuration process. The protocol specification in the remainder
of this section will describe, for each DHCP message, when it is
appropriate for the client to retransmit that message forever, and
when it is appropriate for a client to abandon that message and
attempt to use a different DHCP message.
Normally, DHCP servers and BOOTP relay agents attempt to deliver
DHCPOFFER, DHCPACK and DHCPNAK messages directly to the client using
unicast delivery. The IP destination address (in the IP header) is
set to the DHCP "yiaddr" address and the link-layer destination
address is set to the DHCP "chaddr" address. Unfortunately, some
client implementations are unable to receive such unicast IP
datagrams until the implementation has been configured with a valid
IP address (leading to a deadlock in which the client"s IP address
cannot be delivered until the client has been configured with an IP
address).
A client that cannot receive unicast IP datagrams until its protocol
software has been configured with an IP address SHOULD set the
BROADCAST bit in the "flags" field to 1 in any DHCPDISCOVER or
DHCPREQUEST messages that client sends. The BROADCAST bit will
provide a hint to the DHCP server and BOOTP relay agent to broadcast
any messages to the client on the client"s subnet. A client that can
receive unicast IP datagrams before its protocol software has been
configured SHOULD clear the BROADCAST bit to 0. The BOOTP
clarifications document discusses the ramifications of the use of the
BROADCAST bit [21].
A server or relay agent sending or relaying a DHCP message directly
to a DHCP client (i.e., not to a relay agent specified in the
"giaddr" field) SHOULD examine the BROADCAST bit in the "flags"
field. If this bit is set to 1, the DHCP message SHOULD be sent as
an IP broadcast using an IP broadcast address (preferably
255.255.255.255) as the IP destination address and the link-layer
broadcast address as the link-layer destination address. If the
BROADCAST bit is cleared to 0, the message SHOULD be sent as an IP
unicast to the IP address specified in the "yiaddr" field and the
link-layer address specified in the "chaddr" field. If unicasting is
not possible, the message MAY be sent as an IP broadcast using an IP
broadcast address (preferably 255.255.255.255) as the IP destination
address and the link-layer broadcast address as the link-layer
destination address.
4.2 DHCP server administrative controls
DHCP servers are not required to respond to every DHCPDISCOVER and
DHCPREQUEST message they receive. For example, a network
administrator, to retain stringent control over the hosts attached to
the network, may choose to configure DHCP servers to respond only to
hosts that have been previously registered through some external
mechanism. The DHCP specification describes only the interactions
between clients and servers when the clients and servers choose to
interact; it is beyond the scope of the DHCP specification to
describe all of the administrative controls that system
administrators might want to use. Specific DHCP server
implementations may incorporate any controls or policies desired by a
network administrator.
In some environments, a DHCP server will have to consider the values
of the "chaddr" field and/or the "class-identifier" option included
in the DHCPDISCOVER or DHCPREQUEST messages when determining the
correct parameters for a particular client. For example, an
organization might have a separate bootstrap server for each type of
client it uses, requiring the DHCP server to examine the "class-
identifier" to determine which bootstrap server address to return in
the "siaddr" field of a DHCPOFFER or DHCPACK message.
A DHCP server must use some unique identifier to associate a client
with its lease. The client may choose to explicitly provide the
identifier through the "client identifier" option. If the client
does not provide a "client identifier" option, the server MSUT use
the contents of the "chaddr" field to identify the client.
DHCP clients are free to use any strategy in selecting a DHCP server
among those from which the client receives a DHCPOFFER message. The
client implementation of DHCP should provide a mechanism for the user
to select directly the "class-identifier" value.
4.3 DHCP server behavior
A DHCP server processes incoming DHCP messages from a client based on
the current state of the binding for that client. A DHCP server can
receive the following messages from a client:
o DHCPDISCOVER
o DHCPREQUEST
o DHCPDECLINE
o DHCPRELEASE
Table 3 gives the use of the fields and options in a DHCP message by
a server. The remainder of this section describes the action of the
DHCP server for each possible incoming message.
4.3.1 DHCPDISCOVER message
When a server receives a DHCPDISCOVER message from a client, the
server chooses a network address for the requesting client. If no
address is available, the server may choose to report the problem to
the system administrator and may choose to reply to the client with a
DHCPNAK message. If the server chooses to respond to the client, it
may include an error message in the "message" option. If an address
is available, the new address should be chosen as follows:
o The client"s previous address as recorded in the client"s binding,
if that address is in the server"s pool of available addresses and
not already allocated, else
o The address requested in the "Requested IP Address" option, if that
address is valid and not already allocated, else
o A new address allocated from the server"s pool of available
addresses.
Field DHCPOFFER DHCPACK DHCPNAK
----- --------- ------- -------
"op" BOOTREPLY BOOTREPLY BOOTREPLY
"htype" (From "Assigned Numbers" RFC)
"hlen" (Hardware address length in octets)
"hops" 0 0 0
"xid" "xid" from client "xid" from client "xid" from client
DHCPDISCOVER DHCPREQUEST DHCPREQUEST
message message message
"secs" 0 0 0
"ciaddr" 0 "ciaddr" from "ciaddr" from
DHCPREQUEST or 0 DHCPREQUEST or 0
"yiaddr" IP address offered IP address 0
to client assigned to client
"siaddr" IP address of next IP address of next 0
bootstrap server bootstrap server
"flags" if "giaddr" is not 0 then "flags" from client message else 0
"giaddr" 0 0 0
"chaddr" "chaddr" from "chaddr" from "chaddr" from
client client DHCPREQUEST client DHCPREQUEST
DHCPDISCOVER message message
message
"sname" Server host name Server host name (unused)
or options or options
"file" Client boot file Client boot file (unused)
name or options name or options
"options" options options
Option DHCPOFFER DHCPACK DHCPNAK
------ --------- ------- -------
Requested IP address MUST NOT MUST NOT MUST NOT
IP address lease time MUST MUST MUST NOT
Use "file"/"sname" MAY MAY MUST NOT
fields
DHCP message type DHCPOFFER DHCPACK DHCPNAK
Parameter request list MUST NOT MUST NOT MUST NOT
Message SHOULD SHOULD SHOULD
Client identifier MUST NOT MUST NOT MUST NOT
Class identifier MUST NOT MUST NOT MUST NOT
Server identifier MUST MAY MAY
Maximum message size MUST NOT MUST NOT MUST NOT
All others MAY MAY MUST NOT
Table 3: Fields and options used by DHCP servers
As described in section 4.2, a server MAY, for administrative
reasons, assign an address other than the one requested, or may
refuse to allocate an address to a particular client even though free
addresses are available.
While not required for correct operation of DHCP, the server should
not reuse the selected network address before the client responds to
the server"s DHCPOFFER message. The server may choose to record the
address as offered to the client.
The server must also choose an expiration time for the lease, as
follows:
o IF the client has not requested a specific lease in the
DHCPDISCOVER message and the client already has an assigned network
address, the server returns the lease expiration time previously
assigned to that address (note that the client must explicitly
request a specific lease to extend the expiration time on a
previously assigned address), ELSE
o IF the client has not requested a specific lease in the
DHCPDISCOVER message and the client does not have an assigned
network address, the server assigns a locally configured default
lease time, ELSE
o IF the client has requested a specific lease in the DHCPDISCOVER
message (regardless of whether the client has an assigned network
address), the server may choose either to return the requested
lease (if the lease is acceptable to local policy) or select
another lease.
Once the network address and lease have been determined, the server
constructs a DHCPOFFER message with the offered configuration
parameters. It is important for all DHCP servers to return the same
parameters (with the possible exception of a newly allocated network
address) to ensure predictable host behavior regardless of the which
server the client selects. The configuration parameters MUST be
selected by applying the following rules in the order given below.
The network administrator is responsible for configuring multiple
DHCP servers to ensure uniform responses from those servers. The
server MUST return to the client:
o The client"s network address, as determined by the rules given
earlier in this section, and the subnet mask for the network to
which the client is connected,
o The expiration time for the client"s lease, as determined by the
rules given earlier in this section,
o Parameters requested by the client, according to the following
rules:
-- IF the server has been explicitly configured with a default
value for the parameter, the server MUST include that value
in an appropriate option in the "option" field, ELSE
-- IF the server recognizes the parameter as a parameter
defined in the Host Requirements Document, the server MUST
include the default value for that parameter as given in the
Host Requirements Document in an appropriate option in the
"option" field, ELSE
-- The server MUST NOT return a value for that parameter,
o Any parameters from the existing binding that differ from the Host
Requirements documents defaults,
o Any parameters specific to this client (as identified by
the contents of "chaddr" in the DHCPDISCOVER or DHCPREQUEST
message), e.g., as configured by the network administrator,
o Any parameters specific to this client"s class (as identified
by the contents of the "class identifier" option in the
DHCPDISCOVER or DHCPREQUEST message), e.g., as configured by
the network administrator; the parameters MUST be identified
by an exact match between the client"s "client class" and the
client class identified in the server,
o Parameters with non-default values on the client"s subnet.
The server inserts the "xid" field from the DHCPDISCOVER message into
the "xid" field of the DHCPOFFER message and sends the DHCPOFFER
message to the requesting client.
4.3.2 DHCPREQUEST message
A DHCPREQUEST message may come from a client responding to a
DHCPOFFER message from a server, or from a client verifying a
previously allocated IP address. If the DHCPREQUEST message contains
a "server identifier" option, the message is in response to a
DHCPOFFER message. Otherwise, the message is a request to renew or
extend an existing lease.
Consider first the case of a DHCPREQUEST message in response to a
DHCPOFFER message. If the server is identified in the "server
identifier" option in the DHCPREQUEST message, the server checks to
confirm that the requested parameters are acceptable. Usually, the
requested parameters will match those returned to the client in the
DHCPOFFER message; however, the client may choose to request a
different lease duration. Also, there is no requirement that the
server cache the parameters from the DHCPOFFER message. The server
must simply check that the parameters requested in the DHCPREQUEST
are acceptable. If the parameters are acceptable, the server records
the new client binding and returns a DHCPACK message to the client.
If the requested parameters are unacceptable, e.g., the requested
lease time is unacceptable to local policy, the server sends a
DHCPNAK message to the client. The server may choose to return an
error message in the "message" option.
If a different server is identified in the "server identifier" field,
the client has selected a different server from which to obtain
configuration parameters. The server may discard any information it
may have cached about the client"s request, and may free the network
address that it had offered to the client.
Note that the client may choose to collect several DHCPOFFER messages
and select the "best" offer. The client indicates its selection by
identifying the offering server in the DHCPREQUEST message. If the
client receives no acceptable offers, the client may choose to try
another DHCPDISCOVER message. Therefore, the servers may not receive
a specific DHCPREQUEST from which they can decide whether or not the
client has accepted the offer. Because the servers have not
committed any network address assignments on the basis of a
DHCPOFFER, servers are free to reuse offered network addresses in
response to subsequent requests. As an implementation detail,
servers should not reuse offered addresses and may use an
implementation-specific timeout mechanism to decide when to reuse an
offered address.
In the second case, when there is no "server identifier" option, the
client is renewing or extending a previously allocated IP address.
The server checks to confirm that the requested parameters are
acceptable. If the parameters specified in the DHCPREQUEST message
match the previous parameters, or if the request for an extension of
the lease (indicated by an extended "IP address lease time" option)
is acceptable, the server returns a DHCPACK message to the requesting
client. Otherwise, the server returns a DHCPNAK message to the
client. In particular, if the previously allocated network address
in the "ciaddr" field from the client does not match the network
address recorded by the server for that client, the server sends a
DHCPNAK to the client.
A DHCP server chooses the parameters to return in a DHCPACK message
according to the same rules as used in constructing a DHCPOFFER
message, as given in section 4.3.1.
4.3.3 DHCPDECLINE message
If the server receives a DHCPDECLINE message, the client has
discovered through some other means that the suggested network
address is already in use. The server MUST mark the network address
as not allocated and SHOULD notify the local system administrator of
a possible configuration problem.
4.3.4 DHCPRELEASE message
Upon receipt of a DHCPRELEASE message, the server marks the network
address as not allocated. The server should retain a record of the
client"s initialization parameters for possible reuse in response to
subsequent requests from the client.
4.4 DHCP client behavior
Figure 5 gives a state-transition diagram for a DHCP client. A
client can receive the following messages from a server:
o DHCPOFFER
o DHCPACK
o DHCPNAK
Table 4 gives the use of the fields and options in a DHCP message by
a client. The remainder of this section describes the action of the
DHCP client for each possible incoming message. The description in
the following section corresponds to the full configuration procedure
previously described in section 3.1, and the text in the subsequent
section corresponds to the abbreviated configuration procedure
described in section 3.2.
4.4.1 Initialization and allocation of network address
The client begins in INIT state and forms a DHCPDISCOVER message.
The client should wait a random time between one and ten seconds to
desynchronize the use of DHCP at startup. The client sets "ciaddr"
to all 0x00000000. The client MAY request specific parameters by
including the "parameter request list" option. The client MAY
suggest a network address and/or lease time by including the
"requested IP address" and "IP address lease time" options. The
client MUST include its hardware address in the "chaddr" field for
use in delivery of DHCP reply messages. The client MAY include a
different unique identifier in the "client identifier" option. If
the client does not include the
The client generates and records a rando