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rfc:rfc1531

Network Working Group R. Droms Request for Comments: 1531 Bucknell University Category: Standards Track October 1993

                Dynamic Host Configuration Protocol

Status of this memo

 This RFC specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" for the standardization state and status
 of this protocol.  Distribution of this memo is unlimited.

Abstract

 The Dynamic Host Configuration Protocol (DHCP) provides a framework
 for passing configuration information to hosts on a TCP/IP network.
 DHCP is based on the Bootstrap Protocol (BOOTP) [7], adding the
 capability of automatic allocation of reusable network addresses and
 additional configuration options [19].  DHCP captures the behavior of
 BOOTP relay agents [7, 23], and DHCP participants can interoperate
 with BOOTP participants [9].

Table of Contents

 1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  2
 1.1 Related Work. . . . . . . . . . . . . . . . . . . . . . . . .  4
 1.2 Problem definition and issues . . . . . . . . . . . . . . . .  4
 1.3 Requirements. . . . . . . . . . . . . . . . . . . . . . . . .  5
 1.4 Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  6
 1.5 Design goals. . . . . . . . . . . . . . . . . . . . . . . . .  6
 2. Protocol Summary . . . . . . . . . . . . . . . . . . . . . . .  8
 2.1 Configuration parameters repository . . . . . . . . . . . . . 10
 2.2 Dynamic allocation of network addresses . . . . . . . . . . . 11
 3. The Client-Server Protocol . . . . . . . . . . . . . . . . . . 11
 3.1 Client-server interaction - allocating a network address. . . 12
 3.2 Client-server interaction - reusing a  previously allocated
     network address . . . . . . . . . . . . . . . . . . . . . . . 17
 3.3 Interpretation and representation of time values. . . . . . . 19
 3.4 Host parameters in DHCP . . . . . . . . . . . . . . . . . . . 19
 3.5 Use of DHCP in clients with multiple interfaces . . . . . . . 20
 3.6 When clients should use DHCP. . . . . . . . . . . . . . . . . 20
 4. Specification of the DHCP client-server protocol . . . . . . . 21
 4.1 Constructing and sending DHCP messages. . . . . . . . . . . . 21
 4.2 DHCP server administrative controls . . . . . . . . . . . . . 23
 4.3 DHCP server behavior. . . . . . . . . . . . . . . . . . . . . 24

Droms [Page 1] RFC 1531 Dynamic Host Configuration Protocol October 1993

 4.3.1 DHCPDISCOVER message. . . . . . . . . . . . . . . . . . . . 24
 4.3.2 DHCPREQUEST message . . . . . . . . . . . . . . . . . . . . 27
 4.3.3 DHCPDECLINE message . . . . . . . . . . . . . . . . . . . . 29
 4.3.4 DHCPRELEASE message . . . . . . . . . . . . . . . . . . . . 29
 4.4 DHCP client behavior. . . . . . . . . . . . . . . . . . . . . 29
 4.4.1 Initialization and allocation of network address. . . . . . 29
 4.4.2 Initialization with known network address . . . . . . . . . 33
 4.4.3 Initialization with a known DHCP server address . . . . . . 34
 4.4.4 Reacquisition and expiration. . . . . . . . . . . . . . . . 34
 4.4.5 DHCPRELEASE . . . . . . . . . . . . . . . . . . . . . . . . 35
 5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 35
 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
 7. Security Considerations. . . . . . . . . . . . . . . . . . . . 37
 8. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 38
 A. Host Configuration Parameters  . . . . . . . . . . . . . . . . 39

List of Figures

 1. Format of a DHCP message . . . . . . . . . . . . . . . . . . .  9
 2. Format of the 'flags' field. . . . . . . . . . . . . . . . . . 10
 3. Timeline diagram of messages exchanged between DHCP client and
    servers when allocating a new network address. . . . . . . . . 15
 4. Timeline diagram of messages exchanged between DHCP client and
    servers when reusing a previously allocated network address. . 18
 5. State-transition diagram for DHCP clients. . . . . . . . . . . 31

List of Tables

 1. Description of fields in a DHCP message. . . . . . . . . . . . 14
 2. DHCP messages. . . . . . . . . . . . . . . . . . . . . . . . . 16
 3. Fields and options used by DHCP servers. . . . . . . . . . . . 25
 4. Fields and options used by DHCP clients. . . . . . . . . . . . 32

1. Introduction

 The Dynamic Host Configuration Protocol (DHCP) provides configuration
 parameters to Internet hosts.  DHCP consists of two components: a
 protocol for delivering host-specific configuration parameters from a
 DHCP server to a host and a mechanism for allocation of network
 addresses to hosts.
 DHCP is built on a client-server model, where designated DHCP server
 hosts allocate network addresses and deliver configuration parameters
 to dynamically configured hosts.  Throughout the remainder of this
 document, the term "server" refers to a host providing initialization
 parameters through DHCP, and the term "client" refers to a host
 requesting initialization parameters from a DHCP server.

Droms [Page 2] RFC 1531 Dynamic Host Configuration Protocol October 1993

 A host should not act as a DHCP server unless explicitly configured
 to do so by a system administrator.  The diversity of hardware and
 protocol implementations in the Internet would preclude reliable
 operation if random hosts were allowed to respond to DHCP requests.
 For example, IP requires the setting of many parameters within the
 protocol implementation software.  Because IP can be used on many
 dissimilar kinds of network hardware, values for those parameters
 cannot be guessed or assumed to have correct defaults.  Also,
 distributed address allocation schemes depend on a polling/defense
 mechanism for discovery of addresses that are already in use.  IP
 hosts may not always be able to defend their network addresses, so
 that such a distributed address allocation scheme cannot be
 guaranteed to avoid allocation of duplicate network addresses.
 DHCP supports three mechanisms for IP address allocation.  In
 "automatic allocation", DHCP assigns a permanent IP address to a
 host.  In "dynamic allocation", DHCP assigns an IP address to a host
 for a limited period of time (or until the host explicitly
 relinquishes the address).  In "manual allocation", a host's IP
 address is assigned by the network administrator, and DHCP is used
 simply to convey the assigned address to the host.  A particular
 network will use one or more of these mechanisms, depending on the
 policies of the network administrator.
 Dynamic allocation is the only one of the three mechanisms that
 allows automatic reuse of an address that is no longer needed by the
 host to which it was assigned.  Thus, dynamic allocation is
 particularly useful for assigning an address to a host that will be
 connected to the network only temporarily or for sharing a limited
 pool of IP addresses among a group of hosts that do not need
 permanent IP addresses.  Dynamic allocation may also be a good choice
 for assigning an IP address to a new host being permanently connected
 to a network where IP addresses are sufficiently scarce that it is
 important to reclaim them when old hosts are retired.  Manual
 allocation allows DHCP to be used to eliminate the error-prone
 process of manually configuring hosts with IP addresses in
 environments where (for whatever reasons) it is desirable to manage
 IP address assignment outside of the DHCP mechanisms.
 The format of DHCP messages is based on the format of BOOTP messages,
 to capture the BOOTP relay agent behavior described as part of the
 BOOTP specification [7, 23] and to allow interoperability of existing
 BOOTP clients with DHCP servers.  Using BOOTP relaying agents
 eliminates the necessity of having a DHCP server on each physical
 network segment.

Droms [Page 3] RFC 1531 Dynamic Host Configuration Protocol October 1993

1.1 Related Work

 There are several Internet protocols and related mechanisms that
 address some parts of the dynamic host configuration problem.  The
 Reverse Address Resolution Protocol (RARP) [10] (through the
 extensions defined in the Dynamic RARP (DRARP) [5]) explicitly
 addresses the problem of network address discovery, and includes an
 automatic IP address assignment mechanism.  The Trivial File Transfer
 Protocol (TFTP) [20] provides for transport of a boot image from a
 boot server.  The Internet Control Message Protocol (ICMP) [16]
 provides for informing hosts of additional routers via "ICMP
 redirect" messages.  ICMP also can provide subnet mask information
 through the "ICMP mask request" message and other information through
 the (obsolete) "ICMP information request" message.  Hosts can locate
 routers through the ICMP router discovery mechanism [8].
 BOOTP is a transport mechanism for a collection of configuration
 information.  BOOTP is also extensible, and official extensions [17]
 have been defined for several configuration parameters.  Morgan has
 proposed extensions to BOOTP for dynamic IP address assignment [15].
 The Network Information Protocol (NIP), used by the Athena project at
 MIT, is a distributed mechanism for dynamic IP address assignment
 [19].  The Resource Location Protocol RLP [1] provides for location
 of higher level services.  Sun Microsystems diskless workstations use
 a boot procedure that employs RARP, TFTP and an RPC mechanism called
 "bootparams" to deliver configuration information and operating
 system code to diskless hosts.  (Sun Microsystems, Sun Workstation
 and SunOS are trademarks of Sun Microsystems, Inc.)  Some Sun
 networks also use DRARP and an auto-installation mechanism to
 automate the configuration of new hosts in an existing network.
 In other related work, the path minimum transmission unit (MTU)
 discovery algorithm can determine the MTU of an arbitrary internet
 path [14].  Comer and Droms have proposed the use of the Address
 Resolution Protocol (ARP) as a transport protocol for resource
 location and selection [6].  Finally, the Host Requirements RFCs [3,
 4] mention specific requirements for host reconfiguration and suggest
 a scenario for initial configuration of diskless hosts.

1.2 Problem definition and issues

 DHCP is designed to supply hosts with the configuration parameters
 defined in the Host Requirements RFCs.  After obtaining parameters
 via DHCP, a host should be able to exchange packets with any other
 host in the Internet.  The parameters supplied by DHCP are listed in
 Appendix A.

Droms [Page 4] RFC 1531 Dynamic Host Configuration Protocol October 1993

 Not all of these parameters are required for a newly initialized
 host.  A client and server may negotiate for the transmission of only
 those parameters required by the client or specific to a particular
 subnet.
 DHCP allows but does not require the configuration of host parameters
 not directly related to the IP protocol.  DHCP also does not address
 registration of newly configured hosts with the Domain Name System
 (DNS) [12, 13].
 DHCP is not intended for use in configuring routers.

1.3 Requirements

 Throughout this document, the words that are used to define the
 significance of particular requirements are capitalized.  These words
 are:
    o "MUST"
      This word or the adjective "REQUIRED" means that the
      item is an absolute requirement of this specification.
    o "MUST NOT"
      This phrase means that the item is an absolute prohibition
      of this specification.
    o "SHOULD"
      This word or the adjective "RECOMMENDED" means that there
      may exist valid reasons in particular circumstances to ignore
      this item, but the full implications should be understood and
      the case carefully weighed before choosing a different course.
    o "SHOULD NOT"
      This phrase means that there may exist valid reasons in
      particular circumstances when the listed behavior is acceptable
      or even useful, but the full implications should be understood
      and the case carefully weighed before implementing any behavior
      described with this label.

Droms [Page 5] RFC 1531 Dynamic Host Configuration Protocol October 1993

    o "MAY"
      This word or the adjective "OPTIONAL" means that this item is
      truly optional.  One vendor may choose to include the item
      because a particular marketplace requires it or because it
      enhances the product, for example; another vendor may omit the
      same item.

1.4 Terminology

 This document uses the following terms:
    o "DHCP client"
      A DHCP client is an Internet host using DHCP to obtain
      configuration parameters such as a network address.
    o "DHCP server"
      A DHCP server is an Internet host that returns configuration
      parameters to DHCP clients.
    o "BOOTP relay agent"
      A BOOTP relay agent is an Internet host or router that passes
      DHCP messages between DHCP clients and DHCP servers.  DHCP is
      designed to use the same relay agent behavior as specified in
      the BOOTP protocol specification.
    o "binding"
      A binding is a collection of configuration parameters, including
      at least an IP address, associated with or "bound to" a DHCP
      client.  Bindings are managed by DHCP servers.

1.5 Design goals

 The following list gives general design goals for DHCP.
    o DHCP should be a mechanism rather than a policy.  DHCP must
      allow local system administrators control over configuration
      parameters where desired; e.g., local system administrators
      should be able to enforce local policies concerning allocation
      and access to local resources where desired.

Droms [Page 6] RFC 1531 Dynamic Host Configuration Protocol October 1993

    o Hosts should require no manual configuration.  Each host should
      be able to discover appropriate local configuration parameters
      without user intervention and incorporate those parameters into
      its own configuration.
    o Networks should require no hand configuration for individual
      hosts.  Under normal circumstances, the network manager should
      not have to enter any per-host configuration parameters.
    o DHCP should not require a server on each subnet.  To allow for
      scale and economy, DHCP must work across routers or through the
      intervention of BOOTP/DHCP relay agents.
    o A DHCP host must be prepared to receive multiple responses to a
      request for configuration parameters.  Some installations may
      include multiple, overlapping DHCP servers to enhance
      reliability and increase performance.
    o DHCP must coexist with statically configured, non-participating
      hosts and with existing network protocol implementations.
    o DHCP must interoperate with the BOOTP relay agent behavior as
      described by RFC 951 and by Wimer [21].
    o DHCP must provide service to existing BOOTP clients.
 The following list gives design goals specific to the transmission of
 the network layer parameters.  DHCP must:
    o Guarantee that any specific network address will not be in
      use by more than one host at a time,
    o Retain host configuration across host reboot.  A host should,
      whenever possible, be assigned the same configuration parameters
      (e.g., network address) in response to each request,
    o Retain host configuration across server reboots, and, whenever
      possible, a host should be assigned the same configuration
      parameters despite restarts of the DHCP mechanism,
    o Allow automatic assignment of configuration parameters to new
      hosts to avoid hand configuration for new hosts,
    o Support fixed or permanent allocation of configuration
      parameters to specific hosts.

Droms [Page 7] RFC 1531 Dynamic Host Configuration Protocol October 1993

2. Protocol Summary

 From the client's point of view, DHCP is an extension of the BOOTP
 mechanism.  This behavior allows existing BOOTP clients to
 interoperate with DHCP servers without requiring any change to the
 clients' initialization software.  A separate document details the
 interactions between BOOTP and DHCP clients and servers [9].  There
 are some new, optional transactions that optimize the interaction
 between DHCP clients and servers that are described in sections 3 and
 4.
 Figure 1 gives the format of a DHCP message and table 1 describes
 each of the fields in the DHCP message.  The numbers in parentheses
 indicate the size of each field in octets.  The names for the fields
 given in the figure will be used throughout this document to refer to
 the fields in DHCP messages.
 There are two primary differences between DHCP and BOOTP.  First,
 DHCP defines mechanisms through which clients can be assigned a
 network address for a fixed lease, allowing for serial reassignment
 of network addresses to different clients.  Second, DHCP provides the
 mechanism for a client to acquire all of the IP configuration
 parameters that it needs in order to operate.
 DHCP introduces a small change in terminology intended to clarify the
 meaning of one of the fields.  What was the "vendor extensions" field
 in BOOTP has been re-named the "options" field in DHCP. Similarly,
 the tagged data items that were used inside the BOOTP "vendor
 extensions" field, which were formerly referred to as "vendor
 extensions," are now termed simply "options."
 DHCP defines a new 'client identifier' option that is used to pass an
 explicit client identifier to a DHCP server.  This change eliminates
 the overloading of the 'chaddr' field in BOOTP messages, where reply
 messages and as a client identifier.  The 'client identifier' option
 may contain a hardware address, identical to the contents of the
 'chaddr' field, or it may contain another type of identifier, such as
 a DNS name.  Other client identifier types may be defined as needed
 for use with DHCP.  New client identifier types will be registered
 with the IANA [18] and will be included in new revisions of the
 Assigned Numbers document, as well as described in detail in future
 revisions of the DHCP Options [2].

Droms [Page 8] RFC 1531 Dynamic Host Configuration Protocol October 1993

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     op (1)    |   htype (1)   |   hlen (1)    |   hops (1)    |
 +---------------+---------------+---------------+---------------+
 |                            xid (4)                            |
 +-------------------------------+-------------------------------+
 |           secs (2)            |           flags (2)           |
 +-------------------------------+-------------------------------+
 |                          ciaddr  (4)                          |
 +---------------------------------------------------------------+
 |                          yiaddr  (4)                          |
 +---------------------------------------------------------------+
 |                          siaddr  (4)                          |
 +---------------------------------------------------------------+
 |                          giaddr  (4)                          |
 +---------------------------------------------------------------+
 |                                                               |
 |                          chaddr  (16)                         |
 |                                                               |
 |                                                               |
 +---------------------------------------------------------------+
 |                                                               |
 |                          sname   (64)                         |
 +---------------------------------------------------------------+
 |                                                               |
 |                          file    (128)                        |
 +---------------------------------------------------------------+
 |                                                               |
 |                          options (312)                        |
 +---------------------------------------------------------------+
                Figure 1:  Format of a DHCP message
 DHCP clarifies the interpretation of the 'siaddr' field as the
 address of the server to use in the next step of the client's
 bootstrap process.  A DHCP server may return its own address in the
 'siaddr' field, if the server is prepared to supply the next
 bootstrap service (e.g., delivery of an operating system executable
 image).  A DHCP server always returns its own address in the 'server
 identifier' option.
 The options field is now variable length, with the minimum extended
 to 312 octets.  This brings the minimum size of a DHCP message up to
 576 octets, the minimum IP datagram size a host must be prepared to
 accept [3].  DHCP clients may negotiate the use of larger DHCP
 messages through the 'Maximum DHCP message size' option.  The options
 field may be further extended into the 'file' and 'sname' fields.

Droms [Page 9] RFC 1531 Dynamic Host Configuration Protocol October 1993

 A new option, called 'vendor specific information', has been added to
 allow for expansion of the number of options that can be supported
 [2].  Options encapsulated as 'vendor specific information' must be
 carefully defined and documented so as to allow for interoperability
 between clients and servers from diferent vendors.  In particular,
 vendors defining 'vendor specific information' MUST document those
 options in the form of the DHCP Options document, MUST choose to
 represent those options either in data types already defined for DHCP
 options or in other well-defined data types, and MUST choose options
 that can be readily encoded in configuration files for exchange with
 servers provided by other vendors.  Options included as 'vendor
 specific options' MUST be readily supportable by all servers.
                                  1 1 1 1 1 1
              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
              -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              B|             MBZ             |
              -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              B:  BROADCAST flag
              MBZ:  MUST BE ZERO (reserved for future use)
              Figure 2:  Format of the 'flags' field
 DHCP uses the 'flags' field [21].  The leftmost bit is defined as the
 BROADCAST (B) flag.  The semantics of this flag are discussed in
 section 4.1 of this document.  The remaining bits of the flags field
 are reserved for future use.  They MUST be set to zero by clients and
 ignored by servers and relay agents.  Figure 2 gives the format of
 the

2.1 Configuration parameters repository

 The first service provided by DHCP is to provide persistent storage
 of network parameters for network clients.  The model of DHCP
 persistent storage is that the DHCP service stores a key-value entry
 for each client, where the key is some unique identifier (for
 example, an IP subnet number and a unique identifier within the
 subnet) and the value contains the configuration parameters for the
 client.
 For example, the key might be the pair (IP-subnet-number, hardware-
 address), allowing for serial or concurrent reuse of a hardware
 address on different subnets, and for hardware addresses that may not
 be globally unique.  Alternately, the key might be the pair (IP-
 subnet-number, hostname), allowing the server to assign parameters
 intelligently to a host that has been moved to a different subnet or

Droms [Page 10] RFC 1531 Dynamic Host Configuration Protocol October 1993

 has changed hardware addresses (perhaps because the network interface
 failed and was replaced).
 A client can query the DHCP service to retrieve its configuration
 parameters.  The client interface to the configuration parameters
 repository consists of protocol messages to request configuration
 parameters and responses from the server carrying the configuration
 parameters.

2.2 Dynamic allocation of network addresses

 The second service provided by DHCP is the allocation of temporary or
 permanent network (IP) addresses to hosts.  The basic mechanism for
 the dynamic allocation of network addresses is simple: a client
 requests the use of an address for some period of time.  The
 allocation mechanism (the collection of DHCP servers) guarantees not
 to reallocate that address within the requested time and attempts to
 return the same network address each time the client requests an
 address.  In this document, the period over which a network address
 is allocated to a client is referred to as a "lease" [11].  The
 client may extend its lease with subsequent requests.  The client may
 issue a message to release the address back to the server when the
 client no longer needs the address.  The client may ask for a
 permanent assignment by asking for an infinite lease.  Even when
 assigning "permanent" addresses, a server may choose to give out
 lengthy but non-infinite leases to allow detection of the fact that
 the host has been retired.
 In some environments it will be necessary to reassign network
 addresses due to exhaustion of available addresses.  In such
 environments, the allocation mechanism will reuse addresses whose
 lease has expired.  The server should use whatever information is
 available in the configuration information repository to choose an
 address to reuse.  For example, the server may choose the least
 recently assigned address.  As a consistency check, the allocation
 mechanism may probe the reused address, e.g., with an ICMP echo
 request, before allocating the address, and the client will probe the
 newly received address, e.g., with ARP.

3. The Client-Server Protocol

 DHCP uses the BOOTP message format defined in RFC 951 and given in
 table 1 and figure 1.  The 'op' field of each DHCP message sent from
 a client to a server contains BOOTREQUEST. BOOTREPLY is used in the
 'op' field of each DHCP message sent from a server to a client.
 The first four octets of the 'options' field of the DHCP message
 contain the (decimal) values 99, 130, 83 and 99, respectively (this

Droms [Page 11] RFC 1531 Dynamic Host Configuration Protocol October 1993

 is the same magic cookie as is defined in RFC 1395).  The remainder
 of the 'options' field consists a list of tagged parameters that are
 called "options".  All of the "vendor extensions" listed in RFC 1395
 are also DHCP options.  A separate document gives the complete set of
 options defined for use with DHCP [2].
 Several options have been defined so far.  One particular option -
 the "DHCP message type" option - must be included in every DHCP
 message.  This option defines the "type" of the DHCP message.
 Additional options may be allowed, required, or not allowed,
 depending on the DHCP message type.
 Throughout this document, DHCP messages that include a 'DHCP message
 type' option will be referred to by the type of the message; e.g., a
 DHCP message with 'DHCP message type' option type 1 will be referred
 to as a "DHCPDISCOVER" message.

3.1 Client-server interaction - allocating a network address

 The following summary of the protocol exchanges between clients and
 servers refers to the DHCP messages described in table 2.  The
 timeline diagram in figure 3 shows the timing relationships in a
 typical client-server interaction.  If the client already knows its
 address, some steps may be omitted; this abbreviated interaction is
 described in section 3.2.
 1. The client broadcasts a DHCPDISCOVER message on its local physical
    subnet.  The DHCPDISCOVER message may include options that suggest
    values for the network address and lease duration.  BOOTP relay
    agents may pass the message on to DHCP servers not on the same
    physical subnet.
 2. Each server may respond with a DHCPOFFER message that includes an
    available network address in the 'yiaddr' field (and other
    configuration parameters in DHCP options).  Servers need not
    reserve the offered network address, although the protocol will
    work more efficiently if the server avoids allocating the offered
    network address to another client.  The server unicasts the
    DHCPOFFER message to the client (using the DHCP/BOOTP relay agent
    if necessary) if possible, or may broadcast the message to a
    broadcast address (preferably 255.255.255.255) on the client's
    subnet.
 3. The client receives one or more DHCPOFFER messages from one or
    more servers.  The client may choose to wait for multiple
    responses.  The client chooses one server from which to request
    configuration parameters, based on the configuration parameters
    offered in the DHCPOFFER messages.  The client broadcasts a

Droms [Page 12] RFC 1531 Dynamic Host Configuration Protocol October 1993

    DHCPREQUEST message that MUST include the 'server identifier'
    option to indicate which server it has selected, and may include
    other options specifying desired configuration values.  This
    DHCPREQUEST message is broadcast and relayed through DHCP/BOOTP
    relay agents.  To help ensure that any DHCP/BOOTP relay agents
    forward the DHCPREQUEST message to the same set of DHCP servers
    that received the original DHCPDISCOVER message, the DHCPREQUEST
    message must use the same value in the DHCP message header's
    'secs' field and be sent to the same IP broadcast address as the
    original DHCPDISCOVER message.  The client times out and
    retransmits the DHCPDISCOVER message if the client receives no
    DHCPOFFER messages.
 4. The servers receive the DHCPREQUEST broadcast from the client.
    Those servers not selected by the DHCPREQUEST message use the
    message as notification that the client has declined that server's
    offer.  The server selected in the DHCPREQUEST message commits the
    binding for the client to persistent storage and responds with a
    DHCPACK message containing the configuration parameters for the
    requesting client.  The combination of 'chaddr' and assigned
    network address constitute an unique identifier for the client's
    lease and are used by both the client and server to identify a
    lease referred to in any DHCP messages.  The 'yiaddr' field in the
    DHCPACK messages is filled in with the selected network address.
    If the selected server is unable to satisfy the DHCPREQUEST message
    (e.g., the requested network address has been allocated), the
    server SHOULD respond with a DHCPNAK message.
    A server may choose to mark addresses offered to clients in
    DHCPOFFER messages as unavailable.  The server should mark an
    address offered to a client in a DHCPOFFER message as available if
    the server receives no DHCPREQUEST message from that client.

Droms [Page 13] RFC 1531 Dynamic Host Configuration Protocol October 1993

 FIELD      OCTETS       DESCRIPTION
 -----      ------       -----------
 op            1  Message op code / message type.
                  1 = BOOTREQUEST, 2 = BOOTREPLY
 htype         1  Hardware address type, see ARP section in "Assigned
                  Numbers" RFC; e.g., '1' = 10mb ethernet.
 hlen          1  Hardware address length (e.g.  '6' for 10mb
                  ethernet).
 hops          1  Client sets to zero, optionally used by relay-agents
                  when booting via a relay-agent.
 xid           4  Transaction ID, a random number chosen by the
                  client, used by the client and server to associate
                  messages and responses between a client and a
                  server.
 secs          2  Filled in by client, seconds elapsed since client
                  started trying to boot.
 flags         2  Flags (see figure 2).
 ciaddr        4  Client IP address; filled in by client in
                  DHCPREQUEST if verifying previously allocated
                  configuration parameters.
 yiaddr        4  'your' (client) IP address.
 siaddr        4  IP address of next server to use in bootstrap;
                  returned in DHCPOFFER, DHCPACK and DHCPNAK by
                  server.
 giaddr        4  Relay agent IP address, used in booting via a
                  relay-agent.
 chaddr       16  Client hardware address.
 sname        64  Optional server host name, null terminated string.
 file        128  Boot file name, null terminated string; "generic"
                  name or null in DHCPDISCOVER, fully qualified
                  directory-path name in DHCPOFFER.
 options     312  Optional parameters field.  See the options
                  documents for a list of defined options.
           Table 1:  Description of fields in a DHCP message

Droms [Page 14] RFC 1531 Dynamic Host Configuration Protocol October 1993

              Server          Client          Server
          (not selected)                    (selected)
                v               v               v
                |               |               |
                |     Begins initialization     |
                |               |               |
                | _____________/|\_____________ |
                |/ DHCPDISCOVER | DHCPDISCOVER \|
                |               |               |
            Determines          |          Determines
           configuration        |         configuration
                |               |               |
                |\              |  ____________/|
                | \_________    | /DHCPOFFER    |
                |  DHCPOFFER\   |/              |
                |            \  |               |
                |       Collects replies        |
                |              \|               |
                |     Selects configuration     |
                |               |               |
                | _____________/|\_____________ |
                |/ DHCPREQUEST  |  DHCPREQUEST \|
                |               |               |
                |               |     Commits configuration
                |               |               |
                |               | _____________/|
                |               |/ DHCPACK      |
                |               |               |
                |    Initialization complete    |
                |               |               |
                .               .               .
                .               .               .
                |               |               |
                |      Graceful shutdown        |
                |               |               |
                |               |\_____________ |
                |               |  DHCPRELEASE \|
                |               |               |
                |               |        Discards lease
                |               |               |
                v               v               v
   Figure 3: Timeline diagram of messages exchanged between DHCP
             client and servers when allocating a new network address

Droms [Page 15] RFC 1531 Dynamic Host Configuration Protocol October 1993

 Message         Use
 -------         ---
 DHCPDISCOVER -  Client broadcast to locate available servers.
 DHCPOFFER    -  Server to client in response to DHCPDISCOVER with
                 offer of configuration parameters.
 DHCPREQUEST  -  Client broadcast to servers requesting offered
                 parameters from one server and implicitly declining
                 offers from all others.
 DHCPACK      -  Server to client with configuration parameters,
                 including committed network address.
 DHCPNAK      -  Server to client refusing request for configuration
                 parameters (e.g., requested network address already
                 allocated).
 DHCPDECLINE  -  Client to server indicating configuration parameters
                 (e.g., network address) invalid.
 DHCPRELEASE  -  Client to server relinquishing network address and
                 cancelling remaining lease.
                        Table 2:  DHCP messages
 5. The client receives the DHCPACK message with configuration
    parameters.  The client performs a final check on the parameters
    (e.g., ARP for allocated network address), and notes the duration
    of the lease and the lease identification cookie specified in the
    DHCPACK message.  At this point, the client is configured.  If the
    client detects a problem with the parameters in the DHCPACK
    message, the client sends a DHCPDECLINE message to the server and
    restarts the configuration process.  The client should wait a
    minimum of ten seconds before restarting the configuration process
    to avoid excessive network traffic in case of looping.
    If the client receives a DHCPNAK message, the client restarts the
    configuration process.
    The client times out and retransmits the DHCPREQUEST message if the
    client receives neither a DHCPACK or a DHCPNAK message.  The client
    retransmits the DHCPREQUEST according to the retransmission
    algorithm in section 4.1.  If the client receives neither a DHCPACK
    or a DHCPNAK message after ten retransmissions of the DHCPREQUEST
    message, the client reverts to INIT state and restarts the
    initialization process.  The client SHOULD notify the user that the

Droms [Page 16] RFC 1531 Dynamic Host Configuration Protocol October 1993

    initialization process has failed and is restarting.
 6. The client may choose to relinquish its lease on a network address
    by sending a DHCPRELEASE message to the server.  The client
    identifies the lease to be released by including its network
    address in the 'ciaddr' field and its hardware address in the
    'chaddr' field.

3.2 Client-server interaction - reusing a previously allocated network

  address
 If a client remembers and wishes to reuse a previously allocated
 network address (allocated either by DHCP or some means outside the
 protocol), a client may choose to omit some of the steps described in
 the previous section.  The timeline diagram in figure 4 shows the
 timing relationships in a typical client-server interaction for a
 client reusing a previously allocated network address.
    1. The client broadcasts a DHCPREQUEST message on its local subnet.
       The DHCPREQUEST message includes the client's network address in
       the 'ciaddr' field.  DHCP/BOOTP relay agents pass the message on
       to DHCP servers not on the same subnet.
    2. Servers with knowledge of the client's configuration parameters
       respond with a DHCPACK message to the client.
       If the client's request is invalid (e.g., the client has moved
       to a new subnet), servers may respond with a DHCPNAK message to
       the client.
    3. The client receives the DHCPACK message with configuration
       prameters.  The client performs a final check on the parameters
       (as in section 3.1), and notes the duration of the lease and
       the lease identification cookie specified in the DHCPACK
       message.  At this point, the client is configured.
       If the client detects a problem with the parameters in the
       DHCPACK message, the client sends a DHCPDECLINE message to the
       server and restarts the configuration process by requesting a
       new network address.  This action corresponds to the client
       moving to the INIT state in the DHCP state diagram, which is
       described in section 4.4.

Droms [Page 17] RFC 1531 Dynamic Host Configuration Protocol October 1993

              Server          Client          Server
                v               v               v
                |               |               |
                |             Begins            |
                |         initialization        |
                |               |               |
                |              /|\              |
                |  ___________/ | \___________  |
                | /DHCPREQUEST  |  DHCPREQUEST\ |
                |/              |              \|
                |               |               |
             Locates            |            Locates
          configuration         |         configuration
                |               |               |
                |\              |              /|
                | \             |  ___________/ |
                |  \            | /  DHCPACK    |
                |   \_______    |/              |
                |    DHCPACK\   |               |
                |         Initialization        |
                |            complete           |
                |              \|               |
                |               |               |
                |          (Subsequent          |
                |            DHCPACKS           |
                |            ignored)           |
                |               |               |
                |               |               |
                v               v               v
   Figure 4: Timeline diagram of messages exchanged between DHCP
             client and servers when reusing a previously allocated
             network address
       If the client receives a DHCPNAK message, it cannot reuse its
       remembered network address.  It must instead request a new
       address by restarting the configuration process, this time
       using the (non-abbreviated) procedure described in section
       3.1.  This action also corresponds to the client moving to
       the INIT state in the DHCP state diagram.
       The client times out and retransmits the DHCPREQUEST message if
       the client receives neither a DHCPACK nor a DHCPNAK message.
       The   time between retransmission MUST be chosen according to
       the algorithm given in section 4.1.  If the client receives no
       answer after transmitting 4 DHCPREQUEST messages, the client
       MAY choose to use the previously allocated network address and

Droms [Page 18] RFC 1531 Dynamic Host Configuration Protocol October 1993

       configuration parameters for the remainder of the unexpired
       lease.  This corresponds to moving to BOUND state in the client
       state transition diagram shown in figure 5.
    4. The client may choose to relinquish its lease on a network
       address by sending a DHCPRELEASE message to the server.  The
       client identifies the lease to be released with the lease
       identification cookie.
       Note that in this case, where the client retains its network
       address locally, the client will not normally relinquish its
       lease during a graceful shutdown.  Only in the case where the
       client explicitly needs to relinquish its lease, e.g., the client
       is about to be moved to a different subnet, will the client send
       a DHCPRELEASE message.

3.3 Interpretation and representation of time values

 A client acquires a lease for a network address for a fixed period of
 time (which may be infinite).  Throughout the protocol, times are to
 be represented in units of seconds.  The time value of 0xffffffff is
 reserved to represent "infinity".  The minimum lease duration is one
 hour.
 As clients and servers may not have synchronized clocks, times are
 represented in DHCP messages as relative times, to be interpreted
 with respect to the client's local clock.  Representing relative
 times in units of seconds in an unsigned 32 bit word gives a range of
 relative times from 0 to approximately 100 years, which is sufficient
 for the relative times to be measured using DHCP.
 The algorithm for lease duration interpretation given in the previous
 paragraph assumes that client and server clocks are stable relative
 to each other.  If there is drift between the two clocks, the server
 may consider the lease expired before the client does.  To
 compensate, the server may return a shorter lease duration to the
 client than the server commits to its local database of client
 information.

3.4 Host parameters in DHCP

 Not all clients require initialization of all parameters listed in
 Appendix A.  Two techniques are used to reduce the number of
 parameters transmitted from the server to the client.  First, most of
 the parameters have defaults defined in the Host Requirements RFCs;
 if the client receives no parameters from the server that override
 the defaults, a client uses those default values.  Second, in its
 initial DHCPDISCOVER or DHCPREQUEST message, a client may provide the

Droms [Page 19] RFC 1531 Dynamic Host Configuration Protocol October 1993

 server with a list of specific parameters the client is interested
 in.
 The client SHOULD include the 'maximum DHCP message size' option to
 let the server know how large the server may make its DHCP messages.
 The parameters returned to a client may still exceed the space
 allocated to options in a DHCP message.  In this case, two additional
 options flags (which must appear in the 'options' field of the
 message) indicate that the 'file' and 'sname' fields are to be used
 for options.
 The client can inform the server which configuration parameters the
 client is interested in by including the 'parameter request list'
 option.  The data portion of this option explicitly lists the options
 requested by tag number.
 In addition, the client may suggest values for the network address
 and lease time in the DHCPDISCOVER message.  The client may include
 the be assigned, and may include the 'IP address lease time' option
 to suggest the lease time it would like.  No other options
 representing "hints" at configuration parameters are allowed in a
 DHCPDISCOVER or DHCPREQUEST message.  The 'ciaddr' field is to be
 filled in only in a DHCPREQUEST message when the client is requesting
 use of a previously allocated IP address.
 If a server receives a DHCPREQUEST message with an invalid 'ciaddr',
 the server SHOULD respond to the client with a DHCPNAK message and
 may choose to report the problem to the system administrator.  The
 server may include an error message in the 'message' option.

3.5 Use of DHCP in clients with multiple interfaces

 A host with multiple network interfaces must use DHCP through each
 interface independently to obtain configuration information
 parameters for those separate interfaces.

3.6 When clients should use DHCP

 A host should use DHCP to reacquire or verify its IP address and
 network parameters whenever the local network parameters may have
 changed; e.g., at system boot time or after a disconnection from the
 local network, as the local network configuration may change without
 the host's or user's knowledge.
 If a host has knowledge of a previous network address and is unable
 to contact a local DHCP server, the host may continue to use the
 previous network address until the lease for that address expires.
 If the lease expires before the host can contact a DHCP server, the

Droms [Page 20] RFC 1531 Dynamic Host Configuration Protocol October 1993

 host must immediately discontinue use of the previous network address
 and may inform local users of the problem.

4. Specification of the DHCP client-server protocol

 In this section, we assume that a DHCP server has a block of network
 addresses from which it can satisfy requests for new addresses.  Each
 server also maintains a database of allocated addresses and leases in
 local permanent storage.

4.1 Constructing and sending DHCP messages

 DHCP clients and servers both construct DHCP messages by filling in
 fields in the fixed format section of the message and appending
 tagged data items in the variable length option area.  The options
 area includes first a four-octet 'magic cookie' (which was described
 in section 3), followed by the options.  The last option must always
 be the 'end' option.
 DHCP uses UDP as its transport protocol.  DHCP messages from a client
 to a server are sent to the 'DHCP server' port (67), and DHCP
 messages from a server to a client are sent to the 'DHCP client' port
 (68).
 DHCP messages broadcast by a client prior to that client obtaining
 its IP address must have the source address field in the IP header
 set to 0.
 If the 'giaddr' field in a DHCP message from a client is non-zero,
 the server sends any return messages to the 'DHCP server' port on the
 DHCP relaying agent whose address appears in 'giaddr'.  If the
 'giaddr' field is zero, the client is on the same subnet, and the
 server sends any return messages to either the client's network
 address, if that address was supplied in the 'ciaddr' field, or to
 the client's hardware address or to the local subnet broadcast
 address.
 If the options in a DHCP message extend into the 'sname' and 'file'
 fields, the 'option overload' option MUST appear in the 'options'
 field, with value 1, 2 or 3, as specified in the DHCP options
 document [2].  If the 'option overload' option is present in the
 'options' field, the options in the 'options' field MUST be
 terminated by an options field.  The options in the 'sname' and
 'file' fields (if in use as indicated by the 'options overload'
 option) MUST begin with the first octet of the field, MUST be
 terminated by an 'end' option, and MUST be followed by 'pad' options
 to fill the remainder of the field.  Any individual option in the
 'options', 'sname' and 'file' fields MUST be entirely contained in

Droms [Page 21] RFC 1531 Dynamic Host Configuration Protocol October 1993

 that field.  The options in the 'options' field MUST be interpreted
 first, so that any 'option overload' options may be interpreted.  The
 'file' field MUST be interpreted next (if the options), followed by
 the 'sname' field.
 DHCP clients are responsible for all message retransmission.  The
 client MUST adopt a retransmission strategy that incorporates a
 randomized exponential backoff algorithm to determine the delay
 between retransmissions.  The delay before the first retransmission
 MUST be 4 seconds randomized by the value of a uniform random number
 chosen from the range -1 to +1.  Clients with clocks that provide
 resolution granularity of less than one second may choose a non-
 integer randomization value.  The delay before the next
 retransmission MUST be 8 seconds randomized by the value of a uniform
 number chosen from the range -1 to +1.  The retransmission delay MUST
 be doubled with subsequent retransmissions up to a maximum of 64
 seconds.  The client MAY provide an indication of retransmission
 attempts to the user as an indication of the progress of the
 configuration process.  The protocol specification in the remainder
 of this section will describe, for each DHCP message, when it is
 appropriate for the client to retransmit that message forever, and
 when it is appropriate for a client to abandon that message and
 attempt to use a different DHCP message.
 Normally, DHCP servers and BOOTP relay agents attempt to deliver
 DHCPOFFER, DHCPACK and DHCPNAK messages directly to the client using
 unicast delivery.  The IP destination address (in the IP header) is
 set to the DHCP 'yiaddr' address and the link-layer destination
 address is set to the DHCP 'chaddr' address.  Unfortunately, some
 client implementations are unable to receive such unicast IP
 datagrams until the implementation has been configured with a valid
 IP address (leading to a deadlock in which the client's IP address
 cannot be delivered until the client has been configured with an IP
 address).
 A client that cannot receive unicast IP datagrams until its protocol
 software has been configured with an IP address SHOULD set the
 BROADCAST bit in the 'flags' field to 1 in any DHCPDISCOVER or
 DHCPREQUEST messages that client sends.  The BROADCAST bit will
 provide a hint to the DHCP server and BOOTP relay agent to broadcast
 any messages to the client on the client's subnet.  A client that can
 receive unicast IP datagrams before its protocol software has been
 configured SHOULD clear the BROADCAST bit to 0.  The BOOTP
 clarifications document discusses the ramifications of the use of the
 BROADCAST bit [21].
 A server or relay agent sending or relaying a DHCP message directly
 to a DHCP client (i.e., not to a relay agent specified in the

Droms [Page 22] RFC 1531 Dynamic Host Configuration Protocol October 1993

 'giaddr' field) SHOULD examine the BROADCAST bit in the 'flags'
 field.  If this bit is set to 1, the DHCP message SHOULD be sent as
 an IP broadcast using an IP broadcast address (preferably
 255.255.255.255) as the IP destination address and the link-layer
 broadcast address as the link-layer destination address.  If the
 BROADCAST bit is cleared to 0, the message SHOULD be sent as an IP
 unicast to the IP address specified in the 'yiaddr' field and the
 link-layer address specified in the 'chaddr' field.  If unicasting is
 not possible, the message MAY be sent as an IP broadcast using an IP
 broadcast address (preferably 255.255.255.255) as the IP destination
 address and the link-layer broadcast address as the link-layer
 destination address.

4.2 DHCP server administrative controls

 DHCP servers are not required to respond to every DHCPDISCOVER and
 DHCPREQUEST message they receive.  For example, a network
 administrator, to retain stringent control over the hosts attached to
 the network, may choose to configure DHCP servers to respond only to
 hosts that have been previously registered through some external
 mechanism.  The DHCP specification describes only the interactions
 between clients and servers when the clients and servers choose to
 interact; it is beyond the scope of the DHCP specification to
 describe all of the administrative controls that system
 administrators might want to use.  Specific DHCP server
 implementations may incorporate any controls or policies desired by a
 network administrator.
 In some environments, a DHCP server will have to consider the values
 of the 'chaddr' field and/or the 'class-identifier' option included
 in the DHCPDISCOVER or DHCPREQUEST messages when determining the
 correct parameters for a particular client.  For example, an
 organization might have a separate bootstrap server for each type of
 client it uses, requiring the DHCP server to examine the 'class-
 identifier' to determine which bootstrap server address to return in
 the 'siaddr' field of a DHCPOFFER or DHCPACK message.
 A DHCP server must use some unique identifier to associate a client
 with its lease.  The client may choose to explicitly provide the
 identifier through the 'client identifier' option.  If the client
 does not provide a 'client identifier' option, the server MSUT use
 the contents of the 'chaddr' field to identify the client.
 DHCP clients are free to use any strategy in selecting a DHCP server
 among those from which the client receives a DHCPOFFER message.  The
 client implementation of DHCP should provide a mechanism for the user
 to select directly the 'class-identifier' value.

Droms [Page 23] RFC 1531 Dynamic Host Configuration Protocol October 1993

4.3 DHCP server behavior

 A DHCP server processes incoming DHCP messages from a client based on
 the current state of the binding for that client.  A DHCP server can
 receive the following messages from a client:
    o DHCPDISCOVER
    o DHCPREQUEST
    o DHCPDECLINE
    o DHCPRELEASE
 Table 3 gives the use of the fields and options in a DHCP message by
 a server.  The remainder of this section describes the action of the
 DHCP server for each possible incoming message.

4.3.1 DHCPDISCOVER message

 When a server receives a DHCPDISCOVER message from a client, the
 server chooses a network address for the requesting client.  If no
 address is available, the server may choose to report the problem to
 the system administrator and may choose to reply to the client with a
 DHCPNAK message.  If the server chooses to respond to the client, it
 may include an error message in the 'message' option.  If an address
 is available, the new address should be chosen as follows:
 o The client's previous address as recorded in the client's binding,
   if that address is in the server's pool of available addresses and
   not already allocated, else
 o The address requested in the 'Requested IP Address' option, if that
   address is valid and not already allocated, else
 o A new address allocated from the server's pool of available
   addresses.

Droms [Page 24] RFC 1531 Dynamic Host Configuration Protocol October 1993

Field      DHCPOFFER            DHCPACK             DHCPNAK
-----      ---------            -------             -------
'op'       BOOTREPLY            BOOTREPLY           BOOTREPLY
'htype'    (From "Assigned Numbers" RFC)
'hlen'     (Hardware address length in octets)
'hops'     0                    0                   0
'xid'      'xid' from client    'xid' from client   'xid' from client
           DHCPDISCOVER         DHCPREQUEST         DHCPREQUEST
           message              message             message
'secs'     0                    0                   0
'ciaddr'   0                    'ciaddr' from       'ciaddr' from
                                DHCPREQUEST or 0    DHCPREQUEST or 0
'yiaddr'   IP address offered   IP address          0
           to client            assigned to client
'siaddr'   IP address of next   IP address of next  0
           bootstrap server     bootstrap server
'flags'    if 'giaddr' is not 0 then 'flags' from client message else 0
'giaddr'   0                    0                   0
'chaddr'   'chaddr' from        'chaddr' from       'chaddr' from
           client               client DHCPREQUEST  client DHCPREQUEST
           DHCPDISCOVER         message             message
           message
'sname'    Server host name     Server host name    (unused)
           or options           or options
'file'     Client boot file     Client boot file    (unused)
           name or options      name or options
'options'  options              options
Option                   DHCPOFFER        DHCPACK          DHCPNAK
------                   ---------        -------          -------
Requested IP address     MUST NOT         MUST NOT         MUST NOT
IP address lease time    MUST             MUST             MUST NOT
Use 'file'/'sname'       MAY              MAY              MUST NOT
fields
DHCP message type        DHCPOFFER        DHCPACK          DHCPNAK
Parameter request list   MUST NOT         MUST NOT         MUST NOT
Message                  SHOULD           SHOULD           SHOULD
Client identifier        MUST NOT         MUST NOT         MUST NOT
Class identifier         MUST NOT         MUST NOT         MUST NOT
Server identifier        MUST             MAY              MAY
Maximum message size     MUST NOT         MUST NOT         MUST NOT
All others               MAY              MAY              MUST NOT
         Table 3:  Fields and options used by DHCP servers

Droms [Page 25] RFC 1531 Dynamic Host Configuration Protocol October 1993

 As described in section 4.2, a server MAY, for administrative
 reasons, assign an address other than the one requested, or may
 refuse to allocate an address to a particular client even though free
 addresses are available.
 While not required for correct operation of DHCP, the server should
 not reuse the selected network address before the client responds to
 the server's DHCPOFFER message.  The server may choose to record the
 address as offered to the client.
 The server must also choose an expiration time for the lease, as
 follows:
 o IF the client has not requested a specific lease in the
   DHCPDISCOVER message and the client already has an assigned network
   address, the server returns the lease expiration time previously
   assigned to that address (note that the client must explicitly
   request a specific lease to extend the expiration time on a
   previously assigned address), ELSE
 o IF the client has not requested a specific lease in the
   DHCPDISCOVER message and the client does not have an assigned
   network address, the server assigns a locally configured default
   lease time, ELSE
 o IF the client has requested a specific lease in the DHCPDISCOVER
   message (regardless of whether the client has an assigned network
   address), the server may choose either to return the requested
   lease (if the lease is acceptable to local policy) or select
   another lease.
 Once the network address and lease have been determined, the server
 constructs a DHCPOFFER message with the offered configuration
 parameters.  It is important for all DHCP servers to return the same
 parameters (with the possible exception of a newly allocated network
 address) to ensure predictable host behavior regardless of the which
 server the client selects.  The configuration parameters MUST be
 selected by applying the following rules in the order given below.
 The network administrator is responsible for configuring multiple
 DHCP servers to ensure uniform responses from those servers.  The
 server MUST return to the client:

Droms [Page 26] RFC 1531 Dynamic Host Configuration Protocol October 1993

 o The client's network address, as determined by the rules given
   earlier in this section, and the subnet mask for the network to
   which the client is connected,
 o The expiration time for the client's lease, as determined by the
   rules given earlier in this section,
 o Parameters requested by the client, according to the following
   rules:
  1. - 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
  1. - 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
  1. - The server MUST NOT return a value for that parameter,
 o Any parameters from the existing binding that differ from the Host
   Requirements documents defaults,
 o Any parameters specific to this client (as identified by
   the contents of 'chaddr' in the DHCPDISCOVER or DHCPREQUEST
   message), e.g., as configured by the network administrator,
 o Any parameters specific to this client's class (as identified
   by the contents of the 'class identifier' option in the
   DHCPDISCOVER or DHCPREQUEST message), e.g., as configured by
   the network administrator; the parameters MUST be identified
   by an exact match between the client's 'client class' and the
   client class identified in the server,
 o Parameters with non-default values on the client's subnet.
 The server inserts the 'xid' field from the DHCPDISCOVER message into
 the 'xid' field of the DHCPOFFER message and sends the DHCPOFFER
 message to the requesting client.

4.3.2 DHCPREQUEST message

 A DHCPREQUEST message may come from a client responding to a
 DHCPOFFER message from a server, or from a client verifying a
 previously allocated IP address.  If the DHCPREQUEST message contains
 a 'server identifier' option, the message is in response to a

Droms [Page 27] RFC 1531 Dynamic Host Configuration Protocol October 1993

 DHCPOFFER message.  Otherwise, the message is a request to renew or
 extend an existing lease.
 Consider first the case of a DHCPREQUEST message in response to a
 DHCPOFFER message.  If the server is identified in the 'server
 identifier' option in the DHCPREQUEST message, the server checks to
 confirm that the requested parameters are acceptable.  Usually, the
 requested parameters will match those returned to the client in the
 DHCPOFFER message; however, the client may choose to request a
 different lease duration.  Also, there is no requirement that the
 server cache the parameters from the DHCPOFFER message.  The server
 must simply check that the parameters requested in the DHCPREQUEST
 are acceptable.  If the parameters are acceptable, the server records
 the new client binding and returns a DHCPACK message to the client.
 If the requested parameters are unacceptable, e.g., the requested
 lease time is unacceptable to local policy, the server sends a
 DHCPNAK message to the client.  The server may choose to return an
 error message in the 'message' option.
 If a different server is identified in the 'server identifier' field,
 the client has selected a different server from which to obtain
 configuration parameters.  The server may discard any information it
 may have cached about the client's request, and may free the network
 address that it had offered to the client.
 Note that the client may choose to collect several DHCPOFFER messages
 and select the "best" offer.  The client indicates its selection by
 identifying the offering server in the DHCPREQUEST message.  If the
 client receives no acceptable offers, the client may choose to try
 another DHCPDISCOVER message.  Therefore, the servers may not receive
 a specific DHCPREQUEST from which they can decide whether or not the
 client has accepted the offer.  Because the servers have not
 committed any network address assignments on the basis of a
 DHCPOFFER, servers are free to reuse offered network addresses in
 response to subsequent requests.  As an implementation detail,
 servers should not reuse offered addresses and may use an
 implementation-specific timeout mechanism to decide when to reuse an
 offered address.
 In the second case, when there is no 'server identifier' option, the
 client is renewing or extending a previously allocated IP address.
 The server checks to confirm that the requested parameters are
 acceptable.  If the parameters specified in the DHCPREQUEST message
 match the previous parameters, or if the request for an extension of
 the lease (indicated by an extended 'IP address lease time' option)
 is acceptable, the server returns a DHCPACK message to the requesting
 client.  Otherwise, the server returns a DHCPNAK message to the

Droms [Page 28] RFC 1531 Dynamic Host Configuration Protocol October 1993

 client.  In particular, if the previously allocated network address
 in the 'ciaddr' field from the client does not match the network
 address recorded by the server for that client, the server sends a
 DHCPNAK to the client.
 A DHCP server chooses the parameters to return in a DHCPACK message
 according to the same rules as used in constructing a DHCPOFFER
 message, as given in section 4.3.1.

4.3.3 DHCPDECLINE message

 If the server receives a DHCPDECLINE message, the client has
 discovered through some other means that the suggested network
 address is already in use.  The server MUST mark the network address
 as not allocated and SHOULD notify the local system administrator of
 a possible configuration problem.

4.3.4 DHCPRELEASE message

 Upon receipt of a DHCPRELEASE message, the server marks the network
 address as not allocated.  The server should retain a record of the
 client's initialization parameters for possible reuse in response to
 subsequent requests from the client.

4.4 DHCP client behavior

 Figure 5 gives a state-transition diagram for a DHCP client.  A
 client can receive the following messages from a server:
    o DHCPOFFER
    o DHCPACK
    o DHCPNAK
 Table 4 gives the use of the fields and options in a DHCP message by
 a client.  The remainder of this section describes the action of the
 DHCP client for each possible incoming message.  The description in
 the following section corresponds to the full configuration procedure
 previously described in section 3.1, and the text in the subsequent
 section corresponds to the abbreviated configuration procedure
 described in section 3.2.

4.4.1 Initialization and allocation of network address

 The client begins in INIT state and forms a DHCPDISCOVER message.
 The client should wait a random time between one and ten seconds to
 desynchronize the use of DHCP at startup.  The client sets 'ciaddr'

Droms [Page 29] RFC 1531 Dynamic Host Configuration Protocol October 1993

 to all 0x00000000.  The client MAY request specific parameters by
 including the 'parameter request list' option.  The client MAY
 suggest a network address and/or lease time by including the
 'requested IP address' and 'IP address lease time' options.  The
 client MUST include its hardware address in the 'chaddr' field for
 use in delivery of DHCP reply messages.  The client MAY include a
 different unique identifier in the 'client identifier' option.  If
 the client does not include the
 The client generates and records a 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 all-ones IP broadcast address and 'DHCP
 server' UDP port.
 If the 'xid' of an arriving DHCPOFFER message does not match the
 'xid' of the most recent DHCPDISCOVER message, the DHCPOFFER message
 must be silently discarded.  Any arriving DHCPACK messages must be
 silently discarded.
 The client collects DHCPOFFER messages over a period of time, selects
 one DHCPOFFER message from the (possibly many) incoming DHCPOFFER
 messages (e.g., the first DHCPOFFER message or the DHCPOFFER message
 from the previously used server) and extracts the server address from
 the 'server identifier' option in the DHCPOFFER message.  The time
 over which the client collects messages and the mechanism used to
 select one DHCPOFFER are implementation dependent.  The client may
 perform a check on the suggested address to ensure that the address
 is not already in use.  For example, if the client is on a network
 that supports ARP, the client may issue an ARP request for the
 suggested request.  When broadcasting an ARP request for the
 suggested address, the client must fill in its own hardware address
 as the sender's hardware address, and 0 as the sender's IP address,
 to avoid confusing ARP caches in other hosts on the same subnet.  If
 the network address appears to be in use, the client sends a
 DHCPDECLINE message to the server and waits for another DHCPOFFER. As
 the client does not have a valid network address, the client must
 broadcast the DHCPDECLINE message.

Droms [Page 30] RFC 1531 Dynamic Host Configuration Protocol October 1993

——– ——-

+————————–>
INIT/ +——————–> INIT
REBOOT DHCPNAK/ +———→ ←–+
Restart ——-

——– | DHCPNAK/ | | |

  |      Discard offer   |      -/Send DHCPDISCOVER               |

-/Send DHCPREQUEST | | |

  |      |     |      DHCPACK            v        |               |

———– | (not accept.)/ ———– | |

Send DHCPDECLINE
REBOOTING SELECTING
/

———– | / ———– | |

  |            |      /                  |        |               |

DHCPACK/ | / +—————-+ | | Record lease, | | v | | set timers ———— | |

  |   +----->|            |             DHCPNAK, Lease expired/   |
  |   |      | REQUESTING |                  Halt network         |
  DHCPOFFER/ |            |                       |               |
  Discard     ------------                        |               |
  |   |        |        |                   -----------           |
  |   +--------+     DHCPACK/              |           |          |
  |              Record lease, set    -----| REBINDING |          |
  |                timers T1, T2     /     |           |          |
  |                     |        DHCPACK/   -----------           |
  |                     v     Record lease, set   ^               |
  +----------------> -------      /Timers T1,T2   |               |
             +----->|       |<---+                |               |
             |      | BOUND |<---+                |               |
DHCPOFFER, DHCPACK, |       |    |            T2 expires/   DHCPNAK/
 DHCPNAK/Discard     -------     |             Broadcast  Halt network
             |       | |         |            DHCPREQUEST         |
             +-------+ |        DHCPACK/          |               |
                  T1 expires/   Record lease, set |               |
               Send DHCPREQUEST timers T1, T2     |               |
               to leasing server |                |               |
                       |   ----------             |               |
                       |  |          |------------+               |
                       +->| RENEWING |                            |
                          |          |----------------------------+
                           ----------
        Figure 5:  State-transition diagram for DHCP clients

Droms [Page 31] RFC 1531 Dynamic Host Configuration Protocol October 1993

Field      DHCPDISCOVER          DHCPREQUEST           DHCPDECLINE,
                                                       DHCPRELEASE
-----      ------------          -----------           -----------
'op'       BOOTREQUEST           BOOTREQUEST           BOOTREQUEST
'htype'    (From "Assigned Numbers" RFC)
'hlen'     (Hardware address length in octets)
'hops'     0                     0                     0
'xid'      selected by client    selected by client    selected by
                                                       client
'secs'     (opt.)                (opt.)                0
'flags'    Set 'BROADCAST'       Set 'BROADCAST'
           flag if client        flag if client
           requires broadcast    requires broadcast
           reply                 reply
           0
'ciaddr'   0                     previously            ciaddr
                                 allocated newtork
                                 address
'yiaddr'   0                     0                     0
'siaddr'   0                     0                     0
'giaddr'   0                     0                     0
'chaddr'   client's hardware     client's hardware     client's
                                                       hardware
           address               address               address
'sname'    options, if           options, if           (unused)
           indicated in          indicated in
           'sname/file'          'sname/file'
           option; otherwise     option; otherwise
           unused                unused
'file'     options, if           options, if           (unused)
           indicated in          indicated in
           'sname/file'          'sname/file'
           option; otherwise     option; otherwise
           'generic' name or     'generic' name or
           null                  null
'options'  options               options               (unused)

Droms [Page 32] RFC 1531 Dynamic Host Configuration Protocol October 1993

Option                     DHCPDISCOVER  DHCPREQUEST      DHCPDECLINE,
                                                          DHCPRELEASE
------                     ------------  -----------      -----------
Requested IP address       MAY           MUST NOT         MUST NOT
IP address lease time      MAY           MAY              MUST NOT
Use 'file'/'sname' fields  MAY           MAY              MAY
DHCP message type          DHCPDISCOVER  DHCPREQUEST      DHCPDECLINE/
                                                          DHCPRELEASE
Client identifier          MAY           MAY              MAY
Class identifier           SHOULD        SHOULD           MUST NOT
Server identifier          MUST NOT      MUST (after      MUST
                                         DHCPDISCOVER),
                                         MUST NOT (when
                                         renewing)
Parameter request list     MAY           MAY              MUST NOT
Maximum message size       MAY           MAY              MUST NOT
Message                    SHOULD NOT    SHOULD NOT       SHOULD
Site-specific              MAY           MAY              MUST NOT
All others                 MUST NOT      MUST NOT         MUST NOT
         Table 4:  Fields and options used by DHCP clients
 If the parameters are acceptable, the client records the address of
 the server that supplied the parameters from the 'server identifier'
 field and sends that address in the 'server identifier' field of a
 DHCPREQUEST broadcast message.  Once the DHCPACK message from the
 server arrives, the client is initialized and moves to BOUND state.
 The DHCPREQUEST message contains the same 'xid' as the DHCPOFFER
 message.  The client records the lease expiration time as the sum of
 the time at which the original request was sent and the duration of
 the lease from the DHCPOFFER message.  The client SHOULD broadcast an
 ARP reply to announce the client's new IP address and clear any
 outdated ARP cache entries in hosts on the client's subnet.

4.4.2 Initialization with known network address

 The client begins in INIT-REBOOT state and sends a DHCPREQUEST message
 with the 'ciaddr' field set to the client's network address.  The
 client may request specific configuration parameters by including the
 random transaction identifier and inserts that identifier into the
 computing the lease expiration.  The client MUST NOT incldue a 'server
 identifier' in the DHCPREQUEST message.  The client then broadcasts
 the DHCPREQUEST on the local hardware broadcast address to the 'DHCP
 server' UDP port.
 Once a DHCPACK message with an 'xid' field matching that in the
 client's DHCPREQUEST message arrives from any server, the client is

Droms [Page 33] RFC 1531 Dynamic Host Configuration Protocol October 1993

 initialized and moves to BOUND state.  The client records the lease
 expiration time as the sum of the time at which the DHCPREQUEST
 message was sent and the duration of the lease from the DHCPACK
 message.

4.4.3 Initialization with a known DHCP server address

 When the DHCP client knows the address of a DHCP server, in either
 INIT or REBOOTING state, the client may use that address in the
 DHCPDISCOVER or DHCPREQUEST rather than the IP broadcast address.  If
 the client receives no response to DHCP messages sent to the IP
 address of a known DHCP server, the DHCP client reverts to using the
 IP broadcast address.

4.4.4 Reacquisition and expiration

 The client maintains two times, T1 and T2, that specify the times at
 which the client tries to extend its lease on its network address.  T1
 is the time at which the client enters the RENEWING state and attempts
 to contact the server that originally issued the client's network
 address.  T2 is the time at which the client enters the REBINDING
 state and attempts to contact any server.
 At time T1 after the client accepts the lease on its network address,
 the client moves to RENEWING state and sends (via unicast) a
 DHCPREQUEST message to the server to extend its lease.  The client
 generates a random transaction identifier and inserts that identifier
 into the 'xid' field in the DHCPREQUEST. The client records the local
 time at which the DHCPREQUEST message is sent for computation of the
 lease expiration time.  The client MUST NOT include a 'server
 identifier' in the DHCPREQUEST message.
 Any DHCPACK messages that arrive with an 'xid' that does not match the
 When the client receives a DHCPACK from the server, the client
 computes the lease expiration time as the sum of the time at which the
 client sent the DHCPREQUEST message and the duration of the lease in
 the DHCPACK message.  The client has successfully reacquired its
 network address, returns to BOUND state and may continue network
 processing.
 If no DHCPACK arrives before time T2 (T2 > T1) before the expiration
 of the client's lease on its network address, the client moves to
 REBINDING state and sends (via broadcast) a DHCPREQUEST message to
 extend its lease.  The client sets the 'ciaddr' field in the
 DHCPREQUEST to its current network address.  The client MUST NOT
 include a 'server identifier' in the DHCPREQUEST message.
 Times T1 and T2 are configurable by the server through options.  T1

Droms [Page 34] RFC 1531 Dynamic Host Configuration Protocol October 1993

 defaults to (0.5 * duration_of_lease).  T2 defaults to (0.875 *
 duration_of_lease).  Times T1 and T2 should be chosen with some random
 "fuzz" around a fixed value, to avoid synchronization of client
 reacquisition.
 In both RENEWING and REBINDING state, if the client receives no
 response to its DHCPREQUEST message, the client should wait one-half
 the remaining time until the expiration of T1 (in RENEWING state) and
 T2 (in REBINDING state) down to a minimum of 60 seconds, before
 retransmitting the DHCPREQUEST message.
 If the lease expires before the client receives a DHCPACK, the client
 moves to INIT state, MUST immediately stop any other network
 processing and requests network initialization parameters as if the
 client were uninitialized.  If the client then receives a DHCPACK
 allocating that client its previous network address, the client SHOULD
 continue network processing.  If the client is given a new network
 address, it MUST NOT continue using the previous network address and
 SHOULD notify the local users of the problem.

4.4.5 DHCPRELEASE

 If the client no longer requires use of its assigned network address
 (e.g., the client is gracefully shut down), the client sends a
 DHCPRELEASE message to the server.  Note that the correct operation of
 DHCP does not depend on the transmission of DHCPRELEASE messages.

5. Acknowledgments

 Greg Minshall, Leo McLaughlin and John Veizades have patiently
 contributed to the the design of DHCP through innumerable discussions,
 meetings and mail conversations.  Jeff Mogul first proposed the
 client-server based model for DHCP.  Steve Deering searched the
 various IP RFCs to put together the list of network parameters
 supplied by DHCP.  Walt Wimer contributed a wealth of practical
 experience with BOOTP and wrote a document clarifying the behavior of
 BOOTP/DHCP relay agents.  Jesse Walker analyzed DHCP in detail,
 pointing out several inconsistencies in earlier specifications of the
 protocol.  Steve Alexander reviewed Walker's analysis and the fixes to
 the protocol based on Walker's work.  And, of course, all the members
 of the Dynamic Host Configuration Working Group of the IETF have
 contributed to the design of the protocol through discussion and
 review of the protocol design.

Droms [Page 35] RFC 1531 Dynamic Host Configuration Protocol October 1993

6. References

 [1] Acetta, M., "Resource Location Protocol", RFC 887, CMU, December
     1983.
 [2] Alexander, S., and R. Droms, "DHCP Options and BOOTP Vendor
     Extensions", RFC 1533, Lachman Technology, Inc., Bucknell
     University, October 1993.
 [3] Braden, R., Editor, "Requirements for Internet Hosts --
     Communication Layers", STD 3, RFC 1122, USC/Information Sciences
     Institute, October 1989.
 [4] Braden, R., Editor, "Requirements for Internet Hosts --
     Application and Support, STD 3, RFC 1123, USC/Information
     Sciences Institute, October 1989.
 [5] Brownell, D, "Dynamic Reverse Address Resolution Protocol
     (DRARP)", Work in Progress.
 [6] Comer, D., and R. Droms, "Uniform Access to Internet Directory
     Services", Proc. of ACM SIGCOMM '90 (Special issue of Computer
     Communications Review), 20(4):50--59, 1990.
 [7] Croft, B., and J. Gilmore, "Bootstrap Protocol (BOOTP)", RFC 951,
     Stanford and SUN Microsystems, September 1985.
 [8] Deering, S., "ICMP Router Discovery Messages", RFC 1256, Xerox
     PARC, September 1991.
 [9] Droms, D., "Interoperation between DHCP an BOOTP" RFC 1534,
     Bucknell University, October 1993.
[10] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A Reverse
     Address Resolution Protocol", RFC 903, Stanford, June 1984.
[11] Gray C., and D. Cheriton, "Leases: An Efficient Fault-Tolerant
     Mechanism for Distributed File Cache Consistency", In Proc. of
     the Twelfth ACM Symposium on Operating Systems Design, 1989.
[12] Mockapetris, P., "Domain Names -- Concepts and Facilities", STD
     13, RFC 1034, USC/Information Sciences Institute, November 1987.
[13] Mockapetris, P., "Domain Names -- Implementation and
     Specification", STD 13, RFC 1035, USC/Information Sciences
     Institute, November 1987.

Droms [Page 36] RFC 1531 Dynamic Host Configuration Protocol October 1993

[14] Mogul J., and S. Deering, "Path MTU Discovery", RFC 1191,
     November 1990.
[15] Morgan, R., "Dynamic IP Address Assignment for Ethernet Attached
     Hosts", Work in Progress.
[16] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
     USC/Information Sciences Institute, September 1981.
[17] Reynolds, J., "BOOTP Vendor Information Extensions", RFC 1497,
     USC/Information Sciences Institute, August 1993.
[18] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
     USC/Information Sciences Institute, July 1992.
[19] Jeffrey Schiller and Mark Rosenstein. A Protocol for the Dynamic
     Assignment of IP Addresses for use on an Ethernet. (Available
     from the Athena Project, MIT), 1989.
[20] Sollins, K., "The TFTP Protocol (Revision 2)",  RFC 783, NIC,
     June 1981.
[21] Wimer, W., "Clarifications and Extensions for the Bootstrap
     Protocol", RFC 1532, Carnegie Mellon University, October 1993.

7. Security Considerations

 DHCP is built directly on UDP and IP which are as yet inherently
 insecure.  Furthermore, DHCP is generally intended to make
 maintenance of remote and/or diskless hosts easier.  While perhaps
 not impossible, configuring such hosts with passwords or keys may be
 difficult and inconvenient.  Therefore, DHCP in its current form is
 quite insecure.
 Unauthorized DHCP servers may be easily set up.  Such servers can
 then send false and potentially disruptive information to clients
 such as incorrect or duplicate IP addresses, incorrect routing
 information (including spoof routers, etc.), incorrect domain
 nameserver addresses (such as spoof nameservers), and so on.
 Clearly, once this seed information is in place, an attacker can
 further compromise affected systems.
 Malicious DHCP clients could masquerade as legitimate clients and
 retrieve information intended for those legitimate clients.  Where
 dynamic allocation of resources is used, a malicious client could
 claim all resources for itself, thereby denying resources to
 legitimate clients.

Droms [Page 37] RFC 1531 Dynamic Host Configuration Protocol October 1993

8. Author's Address

 Ralph Droms
 Computer Science Department
 323 Dana Engineering
 Bucknell University
 Lewisburg, PA 17837
 Phone: (717) 524-1145
 EMail: droms@bucknell.edu

Droms [Page 38] RFC 1531 Dynamic Host Configuration Protocol October 1993

A. Host Configuration Parameters

 IP-layer_parameters,_per_host:_
 Be a router                     on/off                 HRC 3.1
 Non-local source routing        on/off                 HRC 3.3.5
 Policy filters for
 non-local source routing        (list)                 HRC 3.3.5
 Maximum reassembly size         integer                HRC 3.3.2
 Default TTL                     integer                HRC 3.2.1.7
 PMTU aging timeout              integer                MTU 6.6
 MTU plateau table               (list)                 MTU 7
 IP-layer_parameters,_per_interface:_
 IP address                      (address)              HRC 3.3.1.6
 Subnet mask                     (address mask)         HRC 3.3.1.6
 MTU                             integer                HRC 3.3.3
 All-subnets-MTU                 on/off                 HRC 3.3.3
 Broadcast address flavor        0x00000000/0xffffffff  HRC 3.3.6
 Perform mask discovery          on/off                 HRC 3.2.2.9
 Be a mask supplier              on/off                 HRC 3.2.2.9
 Perform router discovery        on/off                 RD 5.1
 Router solicitation address     (address)              RD 5.1
 Default routers, list of:
        router address          (address)              HRC 3.3.1.6
        preference level        integer                HRC 3.3.1.6
 Static routes, list of:
        destination             (host/subnet/net)      HRC 3.3.1.2
        destination mask        (address mask)         HRC 3.3.1.2
        type-of-service         integer                HRC 3.3.1.2
        first-hop router        (address)              HRC 3.3.1.2
        ignore redirects        on/off                 HRC 3.3.1.2
        PMTU                    integer                MTU 6.6
        perform PMTU discovery  on/off                 MTU 6.6
 Link-layer_parameters,_per_interface:_
 Trailers                       on/off                 HRC 2.3.1
 ARP cache timeout              integer                HRC 2.3.2.1
 Ethernet encapsulation         (RFC 894/RFC 1042)     HRC 2.3.3
 TCP_parameters,_per_host:_
 TTL                            integer                HRC 4.2.2.19
 Keep-alive interval            integer                HRC 4.2.3.6
 Keep-alive data size           0/1                    HRC 4.2.3.6

Key:

 MTU = Path MTU Discovery (RFC 1191, Proposed Standard)
 RD = Router Discovery (RFC 1256, Proposed Standard)

Droms [Page 39]

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