Network Working Group R. Droms
Request for Comments: 2131 BUCknell University
Obsoletes: 1541 March 1997
Category: Standards Track
Dynamic Host Configuration Protocol
Status of this memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Dynamic Host Configuration Protocol (DHCP) provides a framework
for passing configuration information to hosts on a TCPIP 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, 21], and DHCP participants can interoperate
with BOOTP participants [9].
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Changes to RFC1541. . . . . . . . . . . . . . . . . . . . . . 3
1.2 Related Work. . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Problem definition and issues . . . . . . . . . . . . . . . . 4
1.4 Requirements. . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.6 Design goals. . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Protocol Summary. . . . . . . . . . . . . . . . . . . . . . . 8
2.1 Configuration parameters repository . . . . . . . . . . . . . 11
2.2 Dynamic allocation of network addresses . . . . . . . . . . . 12
3. The Client-Server Protocol. . . . . . . . . . . . . . . . . . 13
3.1 Client-server interaction - allocating a network address. . . 13
3.2 Client-server interaction - reusing a previously allocated
network address . . . . . . . . . . . . . . . . . . . . . . . 17
3.3 Interpretation and representation of time values. . . . . . . 20
3.4 OBTaining parameters with externally configured network
address . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.5 Client parameters in DHCP . . . . . . . . . . . . . . . . . . 21
3.6 Use of DHCP in clients with multiple interfaces . . . . . . . 22
3.7 When clients should use DHCP. . . . . . . . . . . . . . . . . 22
4. Specification of the DHCP client-server protocol. . . . . . . 22
4.1 Constructing and sending DHCP messages. . . . . . . . . . . . 22
4.2 DHCP server administrative controls . . . . . . . . . . . . . 25
4.3 DHCP server behavior. . . . . . . . . . . . . . . . . . . . . 26
4.4 DHCP client behavior. . . . . . . . . . . . . . . . . . . . . 34
5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .42
6. References . . . . . . . . . . . . . . . . . . . . . . . . . .42
7. Security Considerations. . . . . . . . . . . . . . . . . . . .43
8. Author"s Address . . . . . . . . . . . . . . . . . . . . . . .44
A. Host Configuration Parameters . . . . . . . . . . . . . . . .45
List of Figures
1. Format of a DHCP message . . . . . . . . . . . . . . . . . . . 9
2. Format of the "flags" field. . . . . . . . . . . . . . . . . . 11
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. . . . . . . . . . . 34
List of Tables
1. Description of fields in a DHCP message. . . . . . . . . . . . 10
2. DHCP messages. . . . . . . . . . . . . . . . . . . . . . . . . 14
3. Fields and options used by DHCP servers. . . . . . . . . . . . 28
4. Client messages from various states. . . . . . . . . . . . . . 33
5. Fields and options used by DHCP clients. . . . . . . . . . . . 37
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
client. In "dynamic allocation", DHCP assigns an IP address to a
client for a limited period of time (or until the client explicitly
relinquishes the address). In "manual allocation", a client"s IP
address is assigned by the network administrator, and DHCP is used
simply to convey the assigned address to the client. 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
client to which it was assigned. Thus, dynamic allocation is
particularly useful for assigning an address to a client that will be
connected to the network only temporarily or for sharing a limited
pool of IP addresses among a group of clients that do not need
permanent IP addresses. Dynamic allocation may also be a good choice
for assigning an IP address to a new client being permanently
connected to a network where IP addresses are sufficiently scarce
that it is important to reclaim them when old clients 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, 21] and to allow interoperability of existing
BOOTP clients with DHCP servers. Using BOOTP relay agents eliminates
the necessity of having a DHCP server on each physical network
segment.
1.1 Changes to RFC1541
This document updates the DHCP protocol specification that appears in
RFC1541. A new DHCP message type, DHCPINFORM, has been added; see
section 3.4, 4.3 and 4.4 for details. The classing mechanism for
identifying DHCP clients to DHCP servers has been extended to include
"vendor" classes as defined in sections 4.2 and 4.3. The minimum
lease time restriction has been removed. Finally, many editorial
changes have been made to clarify the text as a result of experience
gained in DHCP interoperability tests.
1.2 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]. The Address Resolution Protocol (ARP) has been proposed
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.
1.3 Problem definition and issues
DHCP is designed to supply DHCP clients with the configuration
parameters defined in the Host Requirements RFCs. After obtaining
parameters via DHCP, a DHCP client should be able to exchange packets
with any other host in the Internet. The TCP/IP stack parameters
supplied by DHCP are listed in Appendix A.
Not all of these parameters are required for a newly initialized
client. 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 client
parameters not directly related to the IP protocol. DHCP also does
not address registration of newly configured clients with the Domain
Name System (DNS) [12, 13].
DHCP is not intended for use in configuring routers.
1.4 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.
1.5 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 or 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.
1.6 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 Clients should require no manual configuration. Each client
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 manual configuration for individual
clients. Under normal circumstances, the network manager
should not have to enter any per-client 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 relay agents.
o A DHCP client 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 RFC1542 [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 DHCP client at a time,
o Retain DHCP client configuration across DHCP client reboot. A
DHCP client should, whenever possible, be assigned the same
configuration parameters (e.g., network address) in response
to each request,
o Retain DHCP client configuration across server reboots, and,
whenever possible, a DHCP client should be assigned the same
configuration parameters despite restarts of the DHCP mechanism,
o Allow automated assignment of configuration parameters to new
clients to avoid hand configuration for new clients,
o Support fixed or permanent allocation of configuration
parameters to specific clients.
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. RFC1542 [2] 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 finite 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."
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 (variable)
+---------------------------------------------------------------+
Figure 1: Format of a DHCP message
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
"chaddr" is used both as a hardware address for transmission of BOOTP
reply messages and as a client identifier. The "client identifier"
is an opaque key, not to be interpreted by the server; for example,
the "client identifier" 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. The "client identifier" chosen by a
DHCP client MUST be unique to that client within the subnet to which
the client is attached. If the client uses a "client identifier" in
one message, it MUST use that same identifier in all subsequent
messages, to ensure that all servers correctly identify the client.
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.
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
began address acquisition or renewal process.
flags 2 Flags (see figure 2).
ciaddr 4 Client IP address; only filled in if client is in
BOUND, RENEW or REBINDING state and can respond
to ARP requests.
yiaddr 4 "your" (client) IP address.
siaddr 4 IP address of next server to use in bootstrap;
returned in DHCPOFFER, DHCPACK 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 var Optional parameters field. See the options
documents for a list of defined options.
Table 1: Description of fields in a DHCP message
The "options" field is now variable length. A DHCP client must be
prepared to receive DHCP messages with an "options" field of at least
length 312 octets. This requirement implies that a DHCP client must
be prepared to receive a message of up to 576 octets, the minimum IP
datagram size an IP 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.
In the case of a client using DHCP for initial configuration (before
the client"s TCP/IP software has been completely configured), DHCP
requires creative use of the client"s TCP/IP software and liberal
interpretation of RFC1122. The TCP/IP software SHOULD accept and
forward to the IP layer any IP packets delivered to the client"s
hardware address before the IP address is configured; DHCP servers
and BOOTP relay agents may not be able to deliver DHCP messages to
clients that cannot accept hardware unicast datagrams before the
TCP/IP software is configured.
To work around some clients that cannot accept IP unicast datagrams
before the TCP/IP software is configured as discussed in the previous
paragraph, 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 "flags" field.
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
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) (note that the "hardware-address" should be typed by the
type of hardware to accommodate possible duplication of hardware
addresses resulting from bit-ordering problems in a mixed-media,
bridged network) 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 DHCP client that has been moved to a
different subnet or has changed hardware addresses (perhaps because
the network interface failed and was replaced). The protocol defines
that the key will be (IP-subnet-number, hardware-address) unless the
client explicitly supplies an identifier using the "client
identifier" option. 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 clients. 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 client 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 allocating
server SHOULD probe the reused address before allocating the address,
e.g., with an ICMP echo request, and the client SHOULD 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 RFC1497 [17]). The
remainder of the "options" field consists of a list of tagged
parameters that are called "options". All of the "vendor extensions"
listed in RFC1497 are also DHCP options. RFC1533 gives the
complete set of options defined for use with DHCP.
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. When allocating a new address,
servers SHOULD check that the offered network address is not
already in use; e.g., the server may probe the offered address
with an ICMP Echo Request. Servers SHOULD be implemented so that
network administrators MAY choose to disable probes of newly
allocated addresses. The server transmits the DHCPOFFER message
to the client, using the BOOTP relay agent if necessary.
Message Use
------- ---
DHCPDISCOVER - Client broadcast to locate available servers.
DHCPOFFER - Server to client in response to DHCPDISCOVER with
offer of configuration parameters.
DHCPREQUEST - Client message to servers either (a) requesting
offered parameters from one server and implicitly
declining offers from all others, (b) confirming
correctness of previously allocated address after,
e.g., system reboot, or (c) extending the lease on a
particular network address.
DHCPACK - Server to client with configuration parameters,
including committed network address.
DHCPNAK - Server to client indicating client"s notion of network
address is incorrect (e.g., client has moved to new
subnet) or client"s lease as expired
DHCPDECLINE - Client to server indicating network address is already
in use.
DHCPRELEASE - Client to server relinquishing network address and
cancelling remaining lease.
DHCPINFORM - Client to server, asking only for local configuration
parameters; client already has externally configured
network address.
Table 2: DHCP messages
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
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 that MAY include other options
specifying desired configuration values. The "requested IP
address" option MUST be set to the value of "yiaddr" in the
DHCPOFFER message from the server. This DHCPREQUEST message is
broadcast and relayed through DHCP/BOOTP relay agents. To help
ensure that any 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 "client identifier" or
"chaddr" and assigned network address constitute a 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.
Any configuration parameters in the DHCPACK message SHOULD NOT
conflict with those in the earlier DHCPOFFER message to which the
client is responding. The server SHOULD NOT check the offered
network address at this point. 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.
5. The client receives the DHCPACK message with configuration
parameters. The client SHOULD perform a final check on the
parameters (e.g., ARP for allocated network address), and notes the
duration of the lease specified in the DHCPACK message. At this
point, the client is configured. If the client detects that the
address is already in use (e.g., through the use of ARP), the
client MUST send 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. The client should choose to retransmit
the DHCPREQUEST enough times to give adequate probability of
contacting the server without causing the client (and the user of
that client) to wait overly long before giving up; e.g., a client
retransmitting as described in section 4.1 might retransmit the
DHCPREQUEST message four times, for a total delay of 60 seconds,
before restarting the initialization procedure. If the client
receives neither a DHCPACK or a DHCPNAK message after employing the
retransmission algorithm, 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 with its "client identifier",
or "chaddr" and network address in the DHCPRELEASE message. If the
client used a "client identifier" when it obtained the lease, it
MUST use the same "client identifier" in the DHCPRELEASE message.
3.2 Client-server interaction - reusing a previously allocated network
address
If a client remembers and wishes to reuse a previously allocated
network address, 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 message includes the client"s network address in the
"requested IP address" option. As the client has not received its
network address, it MUST NOT fill in the "ciaddr" field. BOOTP
relay agents pass the message on to DHCP servers not on the same
subnet. If the client used a "client identifier" to obtain its
address, the client MUST use the same "client identifier" in the
DHCPREQUEST message.
2. Servers with knowledge of the client"s configuration parameters
respond with a DHCPACK message to the client. Servers SHOULD NOT
check that the client"s network address is already in use; the
client may respond to ICMP Echo Request messages at this point.
Server Client Server
v v v

Begins
initialization

/
_________ __/ __________
/DHCPREQU EST 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"s request is invalid (e.g., the client has moved
to a new subnet), servers SHOULD respond with a DHCPNAK message to
the client. Servers SHOULD NOT respond if their information is not
guaranteed to be accurate. For example, a server that identifies a
request for an expired binding that is owned by another server SHOULD
NOT respond with a DHCPNAK unless the servers are using an explicit
mechanism to maintain coherency among the servers.
If "giaddr" is 0x0 in the DHCPREQUEST message, the client is on
the same subnet as the server. The server MUST
broadcast the DHCPNAK message to the 0xffffffff broadcast address
because the client may not have a correct network address or subnet
mask, and the client may not be answering ARP requests.
Otherwise, the server MUST send the DHCPNAK message to the IP
address of the BOOTP relay agent, as recorded in "giaddr". The
relay agent will, in turn, forward the message directly to the
client"s hardware address, so that the DHCPNAK can be delivered even
if the client has moved to a new network.
3. The client receives the DHCPACK message with configuration
parameters. The client performs a final check on the parameters
(as in section 3.1), and notes the duration of the lease specified
in the DHCPACK message. The specific lease is implicitly identified
by the "client identifier" or "chaddr" and the network address. At
this point, the client is configured.
If the client detects that the IP address in the DHCPACK message
is already in use, the client MUST send 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.
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
client retransmits the DHCPREQUEST according to the retransmission
algorithm in section 4.1. The client should choose to retransmit
the DHCPREQUEST enough times to give adequate probability of
contacting the server without causing the client (and the user of
that client) to wait overly long before giving up; e.g., a client
retransmitting as described in section 4.1 might retransmit the
DHCPREQUEST message four times, for a total delay of 60 seconds,
before restarting the initialization procedure. If the client
receives neither a DHCPACK or a DHCPNAK message after employing
the retransmission algorithm, 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 its
"client identifier", or "chaddr" and network address in the
DHCPRELEASE message.
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".
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 Obtaining parameters with externally configured network address
If a client has obtained a network address through some other means
(e.g., manual configuration), it may use a DHCPINFORM request message
to obtain other local configuration parameters. Servers receiving a
DHCPINFORM message construct a DHCPACK message with any local
configuration parameters appropriate for the client without:
allocating a new address, checking for an existing binding, filling
in "yiaddr" or including lease time parameters. The servers SHOULD
unicast the DHCPACK reply to the address given in the "ciaddr" field
of the DHCPINFORM message.
The server SHOULD check the network address in a DHCPINFORM message
for consistency, but MUST NOT check for an existing lease. The
server forms a DHCPACK message containing the configuration
parameters for the requesting client and sends the DHCPACK message
directly to the client.
3.5 Client 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. If the client includes a list of parameters in a DHCPDISCOVER
message, it MUST include that list in any subsequent DHCPREQUEST
messages.
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 "requested IP address" option to suggest that a particular IP
address be assigned, and may include the "IP address lease time"
option to suggest the lease time it would like. Other options
representing "hints" at configuration parameters are allowed in a
DHCPDISCOVER or DHCPREQUEST message. However, additional options may
be ignored by servers, and multiple servers may, therefore, not
return identical values for some options. The "requested IP address"
option is to be filled in only in a DHCPREQUEST message when the
client is verifying network parameters obtained previously. The
client fills in the "ciaddr" field only when correctly configured
with an IP address in BOUND, RENEWING or REBINDING state.
If a server receives a DHCPREQUEST message with an invalid "requested
IP address", 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.
3.6 Use of DHCP in clients with multiple interfaces
A client with multiple network interfaces must use DHCP through each
interface independently to obtain configuration information
parameters for those separate interfaces.
3.7 When clients should use DHCP
A client 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 client"s or user"s knowledge.
If a client has knowledge of a previous network address and is unable
to contact a local DHCP server, the client may continue to use the
previous network address until the lease for that address expires.
If the lease expires before the client can contact a DHCP server, the
client 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). A server with multiple network address (e.g., a multi-homed
host) MAY use any of its network addresses in outgoing DHCP messages.
The "server identifier" field is used both to identify a DHCP server
in a DHCP message and as a destination address from clients to
servers. A server with multiple network addresses MUST be prepared
to to accept any of its network addresses as identifying that server
in a DHCP message. To accommodate potentially incomplete network
connectivity, a server MUST choose an address as a "server
identifier" that, to the best of the server"s knowledge, is reachable
from the client. For example, if the DHCP server and the DHCP client
are connected to the same subnet (i.e., the "giaddr" field in the
message from the client is zero), the server SHOULD select the IP
address the server is using for communication on that subnet as the
"server identifier". If the server is using multiple IP addresses on
that subnet, any such address may be used. If the server has
received a message through a DHCP relay agent, the server SHOULD
choose an address from the interface on which the message was
recieved as the "server identifier" (unless the server has other,
better information on which to make its choice). DHCP clients MUST
use the IP address provided in the "server identifier" option for any
unicast requests to the DHCP server.
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
BOOTP relay agent whose address appears in "giaddr". If the "giaddr"
field is zero and the "ciaddr" field is nonzero, then the server
unicasts DHCPOFFER and DHCPACK messages to the address in "ciaddr".
If "giaddr" is zero and "ciaddr" is zero, and the broadcast bit is
set, then the server broadcasts DHCPOFFER and DHCPACK messages to
0xffffffff. If the broadcast bit is not set and "giaddr" is zero and
"ciaddr" is zero, then the server unicasts DHCPOFFER and DHCPACK
messages to the client"s hardware address and "yiaddr" address. In
all cases, when "giaddr" is zero, the server broadcasts any DHCPNAK
messages to 0xffffffff.
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 RFC1533. If the
"option overload" option is present in the "options" field, the
options in the "options" field MUST be terminated by an "end" option,
and MAY contain one or more "pad" options to fill the 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
"option overload" option indicates that the "file" field contains
DHCP options), followed by the "sname" field.
The values to be passed in an "option" tag may be too long to fit in
the 255 octets available to a single option (e.g., a list of routers
in a "router" option [21]). Options may appear only once, unless
otherwise specified in the options document. The client concatenates
the values of multiple instances of the same option into a single
parameter list for configuration.
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 between retransmissions SHOULD be
chosen to allow sufficient time for replies from the server to be
delivered based on the characteristics of the internetwork between
the client and the server. For example, in a 10Mb/sec Ethernet
internetwork, the delay before the first retransmission SHOULD 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 SHOULD
be 8 seconds randomized by the value of a uniform number chosen from
the range -1 to +1. The retransmission delay SHOULD 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 "xid" field is used by the client to match incoming DHCP messages
with pending requests. A DHCP client MUST choose "xid"s in such a
way as to minimize the chance of using an "xid" identical to one used
by another client. For example, a client may choose a different,
random initial "xid" each time the client is rebooted, and
subsequently use sequential "xid"s until the next reboot. Selecting
a new "xid" for each retransmission is an implementation decision. A
client may choose to reuse the same "xid" or select a new "xid" for
each retransmitted message.
Normally, DHCP servers and BOOTP relay agents attempt to deliver
DHCPOFFER, DHCPACK and DHCPNAK messages directly to the client using
uicast 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 0xffffffff)
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 0xffffffff) 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 clients attached
to the network, may choose to configure DHCP servers to respond only
to clients 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 vendor class options included in DHCPDISCOVER or DHCPREQUEST
messages when determining the correct parameters for a particular
client.
A DHCP server needs to 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
supplies a "client identifier", the client MUST use the same "client
identifier" in all subsequent messages, and the server MUST use that
identifier to identify the client. If the client does not provide a
"client identifier" option, the server MUST use the contents of the
"chaddr" field to identify the client. It is crucial for a DHCP
client to use an identifier unique within the subnet to which the
client is attached in the "client identifier" option. Use of
"chaddr" as the client"s unique identifier may cause unexpected
results, as that identifier may be associated with a hardware
interface that could be moved to a new client. Some sites may choose
to use a manufacturer"s serial number as the "client identifier", to
avoid unexpected changes in a clients network address due to transfer
of hardware interfaces among computers. Sites may also choose to use
a DNS name as the "client identifier", causing address leases to be
associated with the DNS name rather than a specific hardware box.
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 "vendor class identifier" values.
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
o DHCPINFORM
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. If an address is available, the new address
SHOULD be chosen as follows:
o The client"s current address as recorded in the client"s current
binding, ELSE
o The client"s previous address as recorded in the client"s (now
expired or released) 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; the address is selected based on the subnet from which
the message was received (if "giaddr" is 0) or on the address of
the relay agent that forwarded the message ("giaddr" when not 0).
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.
Note that, in some network architectures (e.g., internets with more
than one IP subnet assigned to a physical network segment), it may be
the case that the DHCP client should be assigned an address from a
different subnet than the address recorded in "giaddr". Thus, DHCP
does not require that the client be assigned as address from the
subnet in "giaddr". A server is free to choose some other subnet,
and it is beyond the scope of the DHCP specification to describe ways
in which the assigned IP address might be chosen.
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.
Field DHCPOFFER DHCPACK DHCPN