rfc2131

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
   Droms Standards Track [Page 1]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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

6. 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
Droms Standards Track [Page 2]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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 RFC 1541

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. Droms Standards Track [Page 3]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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. Droms Standards Track [Page 4]

RFC 2131 Dynamic Host Configuration Protocol March 1997

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. Droms Standards Track [Page 5]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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. Droms Standards Track [Page 6]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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 RFC 951 and by RFC 1542 [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. Droms Standards Track [Page 7]

RFC 2131 Dynamic Host Configuration Protocol March 1997

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. RFC 1542 [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." Droms Standards Track [Page 8]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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. Droms Standards Track [Page 9]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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 Droms Standards Track [Page 10]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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 RFC 1122. 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 Droms Standards Track [Page 11]

RFC 2131 Dynamic Host Configuration Protocol March 1997

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. Droms Standards Track [Page 12]

RFC 2131 Dynamic Host Configuration Protocol March 1997

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 is the same magic cookie as is defined in RFC 1497 [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 RFC 1497 are also DHCP options. RFC 1533 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
Droms Standards Track [Page 13]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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 Droms Standards Track [Page 14]

RFC 2131 Dynamic Host Configuration Protocol March 1997

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 Standards Track [Page 15]

RFC 2131 Dynamic Host Configuration Protocol March 1997

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 Droms Standards Track [Page 16]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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. Droms Standards Track [Page 17]

RFC 2131 Dynamic Host Configuration Protocol March 1997

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
Droms Standards Track [Page 18]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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 Droms Standards Track [Page 19]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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 Droms Standards Track [Page 20]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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 Droms Standards Track [Page 21]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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 Droms Standards Track [Page 22]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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 RFC 1533. If the Droms Standards Track [Page 23]

RFC 2131 Dynamic Host Configuration Protocol March 1997
’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. Droms Standards Track [Page 24]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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 Droms Standards Track [Page 25]

RFC 2131 Dynamic Host Configuration Protocol March 1997

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
Droms Standards Track [Page 26]

RFC 2131 Dynamic Host Configuration Protocol March 1997

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: Droms Standards Track [Page 27]

RFC 2131 Dynamic Host Configuration Protocol March 1997

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 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 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’ ’flags’ from ’flags’ from ’flags’ from

client DHCPDISCOVER client DHCPREQUEST client DHCPREQUEST

message message message
’giaddr’ ’giaddr’ from ’giaddr’ from ’giaddr’ from

client DHCPDISCOVER client DHCPREQUEST client DHCPREQUEST

message message message
’chaddr’ ’chaddr’ from ’chaddr’ from ’chaddr’ from

client DHCPDISCOVER client DHCPREQUEST client DHCPREQUEST

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 Droms Standards Track [Page 28]

RFC 2131 Dynamic Host Configuration Protocol March 1997

Option DHCPOFFER DHCPACK DHCPNAK------ --------- ------- ------

Requested IP address MUST NOT MUST NOT MUST NOT

IP address lease time MUST MUST (DHCPREQUEST) MUST NOT

MUST NOT (DHCPINFORM)
Use ’file’/’sname’ fields MAY MAY MUST NOT

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 MAY

Vendor class identifier MAY MAY MAY

Server identifier MUST MUST MUST

Maximum message size MUST NOT MUST NOT MUST NOT

All others MAY MAY MUST NOT
Table 3: Fields and options used by DHCP servers 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 client behavior regardless of which
server the client selects. The configuration parameters MUST be selected by applying the following rules in the order given below.

The network administrator is responsible for configuring multiple
DHCP servers to ensure uniform responses from those servers. The server MUST return to the client: o The client’s network address, as determined by the rules given earlier in this section, 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, Droms Standards Track [Page 29]

RFC 2131 Dynamic Host Configuration Protocol March 1997

The server MUST supply as many of the requested parameters as
possible and MUST omit any parameters it cannot provide. The

server MUST include each requested parameter only once unless

explicitly allowed in the DHCP Options and BOOTP Vendor
Extensions document.

o Any parameters from the existing binding that differ from the Host
Requirements Document defaults,

o Any parameters specific to this client (as identified by
the contents of ’chaddr’ or ’client identifier’ 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 ’vendor 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 vendor class identifiers and the client’s classes identified in the server, o Parameters with non-default values on the client’s subnet. The server MAY choose to return the ’vendor class identifier’ used to

determine the parameters in the DHCPOFFER message to assist the
client in selecting which DHCPOFFER to accept. 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, from a client verifying a previously

allocated IP address or from a client extending the lease on a
network address. If the DHCPREQUEST message contains a ’server identifier’ option, the message is in response to a DHCPOFFER message. Otherwise, the message is a request to verify or extend an existing lease. If the client uses a ’client identifier’ in a DHCPREQUEST message, it MUST use that same ’client identifier’ in all subsequent messages. If the client included a list of requested parameters in a DHCPDISCOVER message, it MUST include that list in all subsequent messages. Droms Standards Track [Page 30]

RFC 2131 Dynamic Host Configuration Protocol March 1997

Any configuration parameters in the DHCPACK message SHOULD NOT

conflict with those in the earlier DHCPOFFER message to which the
client is responding. The client SHOULD use the parameters in the DHCPACK message for configuration. Clients send DHCPREQUEST messages as follows: o DHCPREQUEST generated during SELECTING state: Client inserts the address of the selected server in ’server identifier’, ’ciaddr’ MUST be zero, ’requested IP address’ MUST be filled in with the yiaddr value from the chosen DHCPOFFER.

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. o DHCPREQUEST generated during INIT-REBOOT state: ’server identifier’ MUST NOT be filled in, ’requested IP address’ option MUST be filled in with client’s notion of its previously assigned address. ’ciaddr’ MUST be zero. The client is seeking to verify a previously allocated, cached configuration. Server SHOULD send a DHCPNAK message to the client if the ’requested IP address’ is incorrect, or is on the wrong network.

Determining whether a client in the INIT-REBOOT state is on the
correct network is done by examining the contents of ’giaddr’, the ’requested IP address’ option, and a database lookup. If the DHCP server detects that the client is on the wrong net (i.e., the

result of applying the local subnet mask or remote subnet mask (if
’giaddr’ is not zero) to ’requested IP address’ option value doesn’t match reality), then the server SHOULD send a DHCPNAK message to the client. Droms Standards Track [Page 31]

RFC 2131 Dynamic Host Configuration Protocol March 1997
If the network is correct, then the DHCP server should check if the client’s notion of its IP address is correct. If not, then the server SHOULD send a DHCPNAK message to the client. If the DHCP server has no record of this client, then it MUST remain silent, and MAY output a warning to the network administrator. This

behavior is necessary for peaceful coexistence of non
communicating DHCP servers on the same wire. 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. If ’giaddr’ is set in the DHCPREQUEST message, the client is on a different subnet. The server MUST set the broadcast bit in the DHCPNAK, so that the relay agent will broadcast the DHCPNAK to the client, because the client may not have a correct network address or subnet mask, and the client may not be answering ARP requests. o DHCPREQUEST generated during RENEWING state: ’server identifier’ MUST NOT be filled in, ’requested IP address’ option MUST NOT be filled in, ’ciaddr’ MUST be filled in with client’s IP address. In this situation, the client is completely configured, and is trying to extend its lease. This message will be unicast, so no relay agents will be involved in its transmission. Because ’giaddr’ is therefore not filled in, the DHCP server will trust the value in ’ciaddr’, and use it when replying to the client. A client MAY choose to renew or extend its lease prior to T1. The

server may choose not to extend the lease (as a policy decision by
the network administrator), but should return a DHCPACK message regardless. o DHCPREQUEST generated during REBINDING state: ’server identifier’ MUST NOT be filled in, ’requested IP address’ option MUST NOT be filled in, ’ciaddr’ MUST be filled in with client’s IP address. In this situation, the client is completely configured, and is trying to extend its lease. This message MUST be broadcast to the 0xffffffff IP broadcast address. The DHCP server SHOULD check ’ciaddr’ for correctness before replying to the DHCPREQUEST. Droms Standards Track [Page 32]

RFC 2131 Dynamic Host Configuration Protocol March 1997

The DHCPREQUEST from a REBINDING client is intended to accommodate

sites that have multiple DHCP servers and a mechanism for
maintaining consistency among leases managed by multiple servers. A DHCP server MAY extend a client’s lease only if it has local administrative authority to do so. 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 available 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. 4.3.5 DHCPINFORM message

The server responds to a DHCPINFORM message by sending a DHCPACK
message directly to the address given in the ’ciaddr’ field of the DHCPINFORM message. The server MUST NOT send a lease expiration time to the client and SHOULD NOT fill in ’yiaddr’. The server includes other parameters in the DHCPACK message as defined in section 4.3.1. 4.3.6 Client messages

Table 4 details the differences between messages from clients in
various states.

| |INIT-REBOOT |SELECTING |RENEWING |REBINDING |

|broad/unicast |broadcast |broadcast |unicast |broadcast |
|server-ip |MUST NOT |MUST |MUST NOT |MUST NOT |
|requested-ip |MUST |MUST |MUST NOT |MUST NOT |
|ciaddr |zero |zero |IP address |IP address|

Table 4: Client messages from different states Droms Standards Track [Page 33]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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
The DHCPINFORM message is not shown in figure 5. A client simply sends the DHCPINFORM and waits for DHCPACK messages. Once the client has selected its parameters, it has completed the configuration process.

Table 5 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. Droms Standards Track [Page 34]

RFC 2131 Dynamic Host Configuration Protocol March 1997

| | +-------------------------->| |<-------------------+
| INIT- | | +-------------------->| INIT | |
| REBOOT |DHCPNAK/ +---------->| |<---+ |
| |Restart| | ------- | |
-------- | DHCPNAK/ | | |
| Discard offer | -/Send DHCPDISCOVER |-/Send DHCPREQUEST | | |
| | | DHCPACK v | |
----------- | (not accept.)/ ----------- | |
| | | Send DHCPDECLINE | | |
| REBOOTING | | | | SELECTING |<----+ |
| | | / | | |DHCPOFFER/ |
----------- | / ----------- | |Collect |
| | / | | | replies |
DHCPACK/ | / +----------------+ +-------+ |
Record lease, set| | v Select offer/ | timers T1, T2 ------------ send DHCPREQUEST | |
| +----->| | 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 Standards Track [Page 35]

RFC 2131 Dynamic Host Configuration Protocol March 1997
4.4.1 Initialization and allocation of network address The client begins in INIT state and forms a DHCPDISCOVER message.

The client SHOULD wait a random time between one and ten seconds to
desynchronize the use of DHCP at startup. The client sets ’ciaddr’ to 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, if necessary for delivery of DHCP reply messages. The client MAY include a different unique identifier in the ’client identifier’ option, as discussed in section 4.2. If the client included a list of requested parameters in a DHCPDISCOVER message, it MUST include that list in all subsequent messages.

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 the 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. Droms Standards Track [Page 36]

RFC 2131 Dynamic Host Configuration Protocol March 1997
Field DHCPDISCOVER DHCPREQUEST DHCPDECLINE,

DHCPINFORM DHCPRELEASE----- ------------ ----------- ----------
’op’ BOOTREQUEST BOOTREQUEST BOOTREQUEST ’htype’ (From "Assigned Numbers" RFC) ’hlen’ (Hardware address length in octets) ’hops’ 0 0 0 ’xid’ selected by client ’xid’ from server selected by

DHCPOFFER message client
’secs’ 0 or seconds since 0 or seconds since 0

DHCP process started DHCP process started
’flags’ Set ’BROADCAST’ Set ’BROADCAST’ 0

flag if client flag if client

requires broadcast requires broadcast

reply reply
’ciaddr’ 0 (DHCPDISCOVER) 0 or client’s 0 (DHCPDECLINE) client’s network address client’s network

network address (BOUND/RENEW/REBIND) address
(DHCPINFORM) (DHCPRELEASE) ’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

unused unused
’options’ options options (unused) Droms Standards Track [Page 37]

RFC 2131 Dynamic Host Configuration Protocol March 1997
Option DHCPDISCOVER DHCPREQUEST DHCPDECLINE,

DHCPINFORM DHCPRELEASE------ ------------ ----------- ----------

Requested IP address MAY MUST (in MUST
(DISCOVER) SELECTING or (DHCPDECLINE),

MUST NOT INIT-REBOOT) MUST NOT
(INFORM) MUST NOT (in (DHCPRELEASE) BOUND or

RENEWING)

IP address lease time MAY MAY MUST NOT
(DISCOVER) MUST NOT (INFORM) Use ’file’/’sname’ fields MAY MAY MAY

DHCP message type DHCPDISCOVER/ DHCPREQUEST DHCPDECLINE/

DHCPINFORM DHCPRELEASE

Client identifier MAY MAY MAY

Vendor class identifier MAY MAY MUST NOT

Server identifier MUST NOT MUST (after MUST

SELECTING)

MUST NOT (after
INIT-REBOOT, BOUND, RENEWING

or REBINDING)

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 MAY MAY MUST NOT
Table 5: 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 DHCPACK message. The client SHOULD 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 Droms Standards Track [Page 38]

RFC 2131 Dynamic Host Configuration Protocol March 1997
network address appears to be in use, the client MUST send a DHCPDECLINE message to the server. 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. The client MUST insert its known network address as a ’requested IP address’ option in the DHCPREQUEST message. 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 include 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. 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 an externally assigned network address The client sends a DHCPINFORM message. 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 places its own network address in the ’ciaddr’ field. The client SHOULD NOT request lease time parameters.

The client then unicasts the DHCPINFORM to the DHCP server if it
knows the server’s address, otherwise it broadcasts the message to the limited (all 1s) broadcast address. DHCPINFORM messages MUST be directed to the ’DHCP server’ UDP port. Once a DHCPACK message with an ’xid’ field matching that in the client’s DHCPINFORM message arrives from any server, the client is initialized.

If the client does not receive a DHCPACK within a reasonable period

of time (60 seconds or 4 tries if using timeout suggested in section
4.1), then it SHOULD display a message informing the user of the problem, and then SHOULD begin network processing using suitable Droms Standards Track [Page 39]

RFC 2131 Dynamic Host Configuration Protocol March 1997
defaults as per Appendix A. 4.4.4 Use of broadcast and unicast The DHCP client broadcasts DHCPDISCOVER, DHCPREQUEST and DHCPINFORM messages, unless the client knows the address of a DHCP server. The client unicasts DHCPRELEASE messages to the server. Because the client is declining the use of the IP address supplied by the server, the client broadcasts DHCPDECLINE messages. 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.

The client may also use unicast to send DHCPINFORM messages to a
known DHCP server. 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. 4.4.5 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. T1 MUST be earlier than T2, which, in turn, MUST be earlier than the time at which the client’s lease will expire. To avoid the need for synchronized clocks, T1 and T2 are expressed in options as relative times [2].

At time T1 the client moves to RENEWING state and sends (via unicast)
a DHCPREQUEST message to the server to extend its lease. The client sets the ’ciaddr’ field in the DHCPREQUEST to its current network address. 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. Droms Standards Track [Page 40]

RFC 2131 Dynamic Host Configuration Protocol March 1997
If no DHCPACK arrives before time T2, 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 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. A client MAY choose to renew or extend its lease prior to T1. The server MAY choose to extend the client’s lease according to policy set by the network administrator. The server SHOULD return T1 and T2, and their values SHOULD be adjusted from their original values to take account of the time remaining on the lease. In both RENEWING and REBINDING states, if the client receives no response to its DHCPREQUEST message, the client SHOULD wait one-half

of the remaining time until T2 (in RENEWING state) and one-half of
the remaining lease time (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. 4.4.6 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. Droms Standards Track [Page 41]

RFC 2131 Dynamic Host Configuration Protocol March 1997

5. Acknowledgments
   The author thanks the many (and too numerous to mention!) members of

the DHC WG for their tireless and ongoing efforts in the development
of DHCP and this document. The efforts of J Allard, Mike Carney, Dave Lapp, Fred Lien and John

Mendonca in organizing DHCP interoperability testing sessions are
gratefully acknowledged.

The development of this document was supported in part by grants from
the Corporation for National Research Initiatives (CNRI), Bucknell University and Sun Microsystems.

6. References
   [1] Acetta, M., "Resource Location Protocol", RFC 887, CMU, December

[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 and BOOTP", RFC 1534,
Bucknell University, October 1993. Droms Standards Track [Page 42]

RFC 2131 Dynamic Host Configuration Protocol March 1997
[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.
[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 1700,
USC/Information Sciences Institute, October 1994.
[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. Droms Standards Track [Page 43]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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.

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 Standards Track [Page 44]

RFC 2131 Dynamic Host Configuration Protocol March 1997
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 Standards Track [Page 45]