Network Working Group R. Droms
Request for Comments: 1541 Bucknell University
Obsoletes: 1531 October 1993
Category: Standards Track
Dynamic Host Configuration Protocol
Status of this memo
This RFC 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" 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]. Due to some errors introduced into RFC
1531 in the editorial process, this memo is reissued as RFC 1541.
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
Droms [Page 1]RFC 1541 Dynamic Host Configuration Protocol October 1993
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
Droms [Page 2]RFC 1541 Dynamic Host Configuration Protocol October 1993
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.
Droms [Page 3]
RFC 1541 Dynamic Host Configuration Protocol October 1993
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.
Droms [Page 4]
RFC 1541 Dynamic Host Configuration Protocol October 1993
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.
Droms [Page 5]RFC 1541 Dynamic Host Configuration Protocol October 1993
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.
Droms [Page 6]RFC 1541 Dynamic Host Configuration Protocol October 1993
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 RFC 951 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.
Droms [Page 7]RFC 1541 Dynamic Host Configuration Protocol October 1993
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
'chaddr' is used both as a hardware address for transmission of BOOTP
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].
Droms [Page 8]
RFC 1541 Dynamic Host Configuration Protocol October 1993
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.
Droms [Page 9]RFC 1541 Dynamic Host Configuration Protocol October 1993
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 '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), 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
Droms [Page 10]RFC 1541 Dynamic Host Configuration Protocol October 1993
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 RFC 951 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
Droms [Page 11]
RFC 1541 Dynamic Host Configuration Protocol October 1993
is the same magic cookie as is defined in RFC 1497). The remainder
of the 'options' field consists a list of tagged parameters that are
called "options". All of the "vendor extensions" listed in RFC 1497
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
Droms [Page 12]RFC 1541 Dynamic Host Configuration Protocol October 1993
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.
Droms [Page 13]RFC 1541 Dynamic Host Configuration Protocol October 1993
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
Droms [Page 14]RFC 1541 Dynamic Host Configuration Protocol October 1993
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
Droms [Page 15]RFC 1541 Dynamic Host Configuration Protocol October 1993
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
Droms [Page 16]RFC 1541 Dynamic Host Configuration Protocol October 1993
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.
Droms [Page 17]RFC 1541 Dynamic Host Configuration Protocol October 1993
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
Droms [Page 18]RFC 1541 Dynamic Host Configuration Protocol October 1993
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
Droms [Page 19]RFC 1541 Dynamic Host Configuration Protocol October 1993
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 '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. 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.
Droms [Page 20]
RFC 1541 Dynamic Host Configuration Protocol October 1993
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 '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
Droms [Page 21]
RFC 1541 Dynamic Host Configuration Protocol October 1993
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.
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].
Droms [Page 22]
RFC 1541 Dynamic Host Configuration Protocol October 1993
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 MUST 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
Droms [Page 23]
RFC 1541 Dynamic Host Configuration Protocol October 1993
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.
Droms [Page 24]RFC 1541 Dynamic Host Configuration Protocol October 1993
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
Droms [Page 25]RFC 1541 Dynamic Host Configuration Protocol October 1993
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:
Droms [Page 26]RFC 1541 Dynamic Host Configuration Protocol October 1993
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
Droms [Page 27]RFC 1541 Dynamic Host Configuration Protocol October 1993
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
Droms [Page 28]
RFC 1541 Dynamic Host Configuration Protocol October 1993
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'
Droms [Page 29]RFC 1541 Dynamic Host Configuration Protocol October 1993
to 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 'client identifier' option, the
server will use the contents of the 'chaddr' field to identify the
client's lease.
The client generates and records a random transaction identifier and
inserts that identifier into the 'xid' field. The client records its
own local time for later use in computing the lease expiration. The
client then broadcasts the DHCPDISCOVER on the local hardware
broadcast address to 0xffffffff IP broadcast address and 'DHCP
server' UDP port.
If the 'xid' of an arriving DHCPOFFER message does not match the
'xid' of the most recent DHCPDISCOVER message, the DHCPOFFER message
must be silently discarded. Any arriving DHCPACK messages must be
silently discarded.
The client collects DHCPOFFER messages over a period of time, selects
one DHCPOFFER message from the (possibly many) incoming DHCPOFFER
messages (e.g., the first DHCPOFFER message or the DHCPOFFER message
from the previously used server) and extracts the server address from
the 'server identifier' option in the DHCPOFFER message. The time
over which the client collects messages and the mechanism used to
select one DHCPOFFER are implementation dependent. The client may
perform a check on the suggested address to ensure that the address
is not already in use. For example, if the client is on a network
that supports ARP, the client may issue an ARP request for the
suggested request. When broadcasting an ARP request for the
suggested address, the client must fill in its own hardware address
as the sender's hardware address, and 0 as the sender's IP address,
to avoid confusing ARP caches in other hosts on the same subnet. If
the network address appears to be in use, the client sends a
DHCPDECLINE message to the server and waits for another DHCPOFFER. As
the client does not have a valid network address, the client must
broadcast the DHCPDECLINE message.
Droms [Page 30]
RFC 1541 Dynamic Host Configuration Protocol October 1993
-------- -------
| | +-------------------------->| |<-------------------+
| INIT/ | | +-------------------->| INIT | |
| REBOOT |DHCPNAK/ +---------->| |<---+ |
| |Restart| | ------- | |
-------- | DHCPNAK/ | | |
| Discard offer | -/Send DHCPDISCOVER |
-/Send DHCPREQUEST | | |
| | | DHCPACK v | |
----------- | (not accept.)/ ----------- | |
| | | Send DHCPDECLINE | | | |
| REBOOTING | | | | SELECTING | | |
| | | / | | | |
----------- | / ----------- | |
| | / | | |
DHCPACK/ | / +----------------+ | |
Record lease, | | v | |
set timers ------------ | |
| +----->| | DHCPNAK, Lease expired/ |
| | | REQUESTING | Halt network |
DHCPOFFER/ | | | |
Discard ------------ | |
| | | | ----------- |
| +--------+ DHCPACK/ | | |
| Record lease, set -----| REBINDING | |
| timers T1, T2 / | | |
| | DHCPACK/ ----------- |
| v Record lease, set ^ |
+----------------> ------- /Timers T1,T2 | |
+----->| |<---+ | |
| | BOUND |<---+ | |
DHCPOFFER, DHCPACK, | | | T2 expires/ DHCPNAK/
DHCPNAK/Discard ------- | Broadcast Halt network
| | | | DHCPREQUEST |
+-------+ | DHCPACK/ | |
T1 expires/ Record lease, set | |
Send DHCPREQUEST timers T1, T2 | |
to leasing server | | |
| ---------- | |
| | |------------+ |
+->| RENEWING | |
| |----------------------------+
----------
Figure 5: State-transition diagram for DHCP clients
Droms [Page 31]RFC 1541 Dynamic Host Configuration Protocol October 1993
Field DHCPDISCOVER DHCPREQUEST DHCPDECLINE,
DHCPRELEASE
----- ------------ ----------- -----------
'op' BOOTREQUEST BOOTREQUEST BOOTREQUEST
'htype' (From "Assigned Numbers" RFC)
'hlen' (Hardware address length in octets)
'hops' 0 0 0
'xid' selected by client selected by client selected by
client
'secs' (opt.) (opt.) 0
'flags' Set 'BROADCAST' Set 'BROADCAST'
flag if client flag if client
requires broadcast requires broadcast
reply reply
0
'ciaddr' 0 previously ciaddr
allocated newtork
address
'yiaddr' 0 0 0
'siaddr' 0 0 0
'giaddr' 0 0 0
'chaddr' client's hardware client's hardware client's
hardware
address address address
'sname' options, if options, if (unused)
indicated in indicated in
'sname/file' 'sname/file'
option; otherwise option; otherwise
unused unused
'file' options, if options, if (unused)
indicated in indicated in
'sname/file' 'sname/file'
option; otherwise option; otherwise
'generic' name or 'generic' name or
null null
'options' options options (unused)
Droms [Page 32]RFC 1541 Dynamic Host Configuration Protocol October 1993
Option DHCPDISCOVER DHCPREQUEST DHCPDECLINE,
DHCPRELEASE
------ ------------ ----------- -----------
Requested IP address MAY MUST NOT MUST NOT
IP address lease time MAY MAY MUST NOT
Use 'file'/'sname' fields MAY MAY MAY
DHCP message type DHCPDISCOVER DHCPREQUEST DHCPDECLINE/
DHCPRELEASE
Client identifier MAY MAY MAY
Class identifier SHOULD SHOULD MUST NOT
Server identifier MUST NOT MUST (after MUST
DHCPDISCOVER),
MUST NOT (when
renewing)
Parameter request list MAY MAY MUST NOT
Maximum message size MAY MAY MUST NOT
Message SHOULD NOT SHOULD NOT SHOULD
Site-specific MAY MAY MUST NOT
All others MUST NOT MUST NOT MUST NOT
Table 4: Fields and options used by DHCP clients
If the parameters are acceptable, the client records the address of
the server that supplied the parameters from the 'server identifier'
field and sends that address in the 'server identifier' field of a
DHCPREQUEST broadcast message. Once the DHCPACK message from the
server arrives, the client is initialized and moves to BOUND state.
The DHCPREQUEST message contains the same 'xid' as the DHCPOFFER
message. The client records the lease expiration time as the sum of
the time at which the original request was sent and the duration of
the lease from the DHCPOFFER message. The client SHOULD broadcast an
ARP reply to announce the client's new IP address and clear any
outdated ARP cache entries in hosts on the client's subnet.
4.4.2 Initialization with known network address
The client begins in INIT-REBOOT state and sends a DHCPREQUEST message
with the 'ciaddr' field set to the client's network address. The
client may request specific configuration parameters by including
the 'parameter request list' option. The client generates and records a
random transaction identifier and inserts that identifier into the 'xid'
field. The client records its own local time for later use in
computing the lease expiration. The client MUST NOT incldue a 'server
identifier' in the DHCPREQUEST message. The client then broadcasts
the DHCPREQUEST on the local hardware broadcast address to the 'DHCP
server' UDP port.
Droms [Page 33]RFC 1541 Dynamic Host Configuration Protocol October 1993
Once a DHCPACK message with an 'xid' field matching that in the
client's DHCPREQUEST message arrives from any server, the client is
initialized and moves to BOUND state. The client records the lease
expiration time as the sum of the time at which the DHCPREQUEST
message was sent and the duration of the lease from the DHCPACK
message.
4.4.3 Initialization with a known DHCP server address
When the DHCP client knows the address of a DHCP server, in either
INIT or REBOOTING state, the client may use that address in the
DHCPDISCOVER or DHCPREQUEST rather than the IP broadcast address. If
the client receives no response to DHCP messages sent to the IP
address of a known DHCP server, the DHCP client reverts to using the
IP broadcast address.
4.4.4 Reacquisition and expiration
The client maintains two times, T1 and T2, that specify the times at
which the client tries to extend its lease on its network address. T1
is the time at which the client enters the RENEWING state and attempts
to contact the server that originally issued the client's network
address. T2 is the time at which the client enters the REBINDING
state and attempts to contact any server.
At time T1 after the client accepts the lease on its network address,
the client moves to RENEWING state and sends (via unicast) a
DHCPREQUEST message to the server to extend its lease. The client
generates a random transaction identifier and inserts that identifier
into the 'xid' field in the DHCPREQUEST. The client records the local
time at which the DHCPREQUEST message is sent for computation of the
lease expiration time. The client MUST NOT include a 'server
identifier' in the DHCPREQUEST message.
Any DHCPACK messages that arrive with an 'xid' that does not match
the 'xid' of the client's DHCPREQUEST message are silently discarded.
When the client receives a DHCPACK from the server, the client
computes the lease expiration time as the sum of the time at which the
client sent the DHCPREQUEST message and the duration of the lease in
the DHCPACK message. The client has successfully reacquired its
network address, returns to BOUND state and may continue network
processing.
If no DHCPACK arrives before time T2 (T2 > T1) before the expiration
of the client's lease on its network address, the client moves to
REBINDING state and sends (via broadcast) a DHCPREQUEST message to
extend its lease. The client sets the 'ciaddr' field in the
DHCPREQUEST to its current network address. The client MUST NOT
Droms [Page 34]
RFC 1541 Dynamic Host Configuration Protocol October 1993
include a 'server identifier' in the DHCPREQUEST message.
Times T1 and T2 are configurable by the server through options. T1
defaults to (0.5 * duration_of_lease). T2 defaults to (0.875 *
duration_of_lease). Times T1 and T2 should be chosen with some random
"fuzz" around a fixed value, to avoid synchronization of client
reacquisition.
In both RENEWING and REBINDING state, if the client receives no
response to its DHCPREQUEST message, the client should wait one-half
the remaining time until the expiration of T1 (in RENEWING state) and
T2 (in REBINDING state) down to a minimum of 60 seconds, before
retransmitting the DHCPREQUEST message.
If the lease expires before the client receives a DHCPACK, the client
moves to INIT state, MUST immediately stop any other network
processing and requests network initialization parameters as if the
client were uninitialized. If the client then receives a DHCPACK
allocating that client its previous network address, the client SHOULD
continue network processing. If the client is given a new network
address, it MUST NOT continue using the previous network address and
SHOULD notify the local users of the problem.
4.4.5 DHCPRELEASE
If the client no longer requires use of its assigned network address
(e.g., the client is gracefully shut down), the client sends a
DHCPRELEASE message to the server. Note that the correct operation of
DHCP does not depend on the transmission of DHCPRELEASE messages.
5. Acknowledgments
Greg Minshall, Leo McLaughlin and John Veizades have patiently
contributed to the the design of DHCP through innumerable discussions,
meetings and mail conversations. Jeff Mogul first proposed the
client-server based model for DHCP. Steve Deering searched the
various IP RFCs to put together the list of network parameters
supplied by DHCP. Walt Wimer contributed a wealth of practical
experience with BOOTP and wrote a document clarifying the behavior of
BOOTP/DHCP relay agents. Jesse Walker analyzed DHCP in detail,
pointing out several inconsistencies in earlier specifications of the
protocol. Steve Alexander reviewed Walker's analysis and the fixes to
the protocol based on Walker's work. And, of course, all the members
of the Dynamic Host Configuration Working Group of the IETF have
contributed to the design of the protocol through discussion and
review of the protocol design.
Droms [Page 35]
RFC 1541 Dynamic Host Configuration Protocol October 1993
6. References
[1] Acetta, M., "Resource Location Protocol", RFC 887, CMU, December
1983.
[2] Alexander, S., and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 1533, Lachman Technology, Inc., Bucknell
University, October 1993.
[3] Braden, R., Editor, "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC 1122, USC/Information Sciences
Institute, October 1989.
[4] Braden, R., Editor, "Requirements for Internet Hosts --
Application and Support, STD 3, RFC 1123, USC/Information
Sciences Institute, October 1989.
[5] Brownell, D, "Dynamic Reverse Address Resolution Protocol
(DRARP)", Work in Progress.
[6] Comer, D., and R. Droms, "Uniform Access to Internet Directory
Services", Proc. of ACM SIGCOMM '90 (Special issue of Computer
Communications Review), 20(4):50--59, 1990.
[7] Croft, B., and J. Gilmore, "Bootstrap Protocol (BOOTP)", RFC 951,
Stanford and SUN Microsystems, September 1985.
[8] Deering, S., "ICMP Router Discovery Messages", RFC 1256, Xerox
PARC, September 1991.
[9] Droms, D., "Interoperation between DHCP an BOOTP" RFC 1534,
Bucknell University, October 1993.
[10] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A Reverse
Address Resolution Protocol", RFC 903, Stanford, June 1984.
[11] Gray C., and D. Cheriton, "Leases: An Efficient Fault-Tolerant
Mechanism for Distributed File Cache Consistency", In Proc. of
the Twelfth ACM Symposium on Operating Systems Design, 1989.
[12] Mockapetris, P., "Domain Names -- Concepts and Facilities", STD
13, RFC 1034, USC/Information Sciences Institute, November 1987.
[13] Mockapetris, P., "Domain Names -- Implementation and
Specification", STD 13, RFC 1035, USC/Information Sciences
Institute, November 1987.
Droms [Page 36]RFC 1541 Dynamic Host Configuration Protocol October 1993
[14] Mogul J., and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
[15] Morgan, R., "Dynamic IP Address Assignment for Ethernet Attached
Hosts", Work in Progress.
[16] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
USC/Information Sciences Institute, September 1981.
[17] Reynolds, J., "BOOTP Vendor Information Extensions", RFC 1497,
USC/Information Sciences Institute, August 1993.
[18] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
USC/Information Sciences Institute, July 1992.
[19] Jeffrey Schiller and Mark Rosenstein. A Protocol for the Dynamic
Assignment of IP Addresses for use on an Ethernet. (Available
from the Athena Project, MIT), 1989.
[20] Sollins, K., "The TFTP Protocol (Revision 2)", RFC 783, NIC,
June 1981.
[21] Wimer, W., "Clarifications and Extensions for the Bootstrap
Protocol", RFC 1542, Carnegie Mellon University, October 1993.
7. Security Considerations
DHCP is built directly on UDP and IP which are as yet inherently
insecure. Furthermore, DHCP is generally intended to make
maintenance of remote and/or diskless hosts easier. While perhaps
not impossible, configuring such hosts with passwords or keys may be
difficult and inconvenient. Therefore, DHCP in its current form is
quite insecure.
Unauthorized DHCP servers may be easily set up. Such servers can
then send false and potentially disruptive information to clients
such as incorrect or duplicate IP addresses, incorrect routing
information (including spoof routers, etc.), incorrect domain
nameserver addresses (such as spoof nameservers), and so on.
Clearly, once this seed information is in place, an attacker can
further compromise affected systems.
Malicious DHCP clients could masquerade as legitimate clients and
retrieve information intended for those legitimate clients. Where
dynamic allocation of resources is used, a malicious client could
claim all resources for itself, thereby denying resources to
legitimate clients.
Droms [Page 37]RFC 1541 Dynamic Host Configuration Protocol October 1993
8. Author's Address
Ralph Droms
Computer Science Department
323 Dana Engineering
Bucknell University
Lewisburg, PA 17837
Phone: (717) 524-1145
EMail: droms@bucknell.edu
Droms [Page 38]
RFC 1541 Dynamic Host Configuration Protocol October 1993
A. Host Configuration Parameters
IP-layer_parameters,_per_host:_
Be a router on/off HRC 3.1
Non-local source routing on/off HRC 3.3.5
Policy filters for
non-local source routing (list) HRC 3.3.5
Maximum reassembly size integer HRC 3.3.2
Default TTL integer HRC 3.2.1.7
PMTU aging timeout integer MTU 6.6
MTU plateau table (list) MTU 7
IP-layer_parameters,_per_interface:_
IP address (address) HRC 3.3.1.6
Subnet mask (address mask) HRC 3.3.1.6
MTU integer HRC 3.3.3
All-subnets-MTU on/off HRC 3.3.3
Broadcast address flavor 0x00000000/0xffffffff HRC 3.3.6
Perform mask discovery on/off HRC 3.2.2.9
Be a mask supplier on/off HRC 3.2.2.9
Perform router discovery on/off RD 5.1
Router solicitation address (address) RD 5.1
Default routers, list of:
router address (address) HRC 3.3.1.6
preference level integer HRC 3.3.1.6
Static routes, list of:
destination (host/subnet/net) HRC 3.3.1.2
destination mask (address mask) HRC 3.3.1.2
type-of-service integer HRC 3.3.1.2
first-hop router (address) HRC 3.3.1.2
ignore redirects on/off HRC 3.3.1.2
PMTU integer MTU 6.6
perform PMTU discovery on/off MTU 6.6
Link-layer_parameters,_per_interface:_
Trailers on/off HRC 2.3.1
ARP cache timeout integer HRC 2.3.2.1
Ethernet encapsulation (RFC 894/RFC 1042) HRC 2.3.3
TCP_parameters,_per_host:_
TTL integer HRC 4.2.2.19
Keep-alive interval integer HRC 4.2.3.6
Keep-alive data size 0/1 HRC 4.2.3.6
Key:
MTU = Path MTU Discovery (RFC 1191, Proposed Standard)
RD = Router Discovery (RFC 1256, Proposed Standard)
Droms [Page 39]
