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+
+
+
+
+
+
+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
+
+
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+ 4.1 Constructing and sending DHCP messages. . . . . . . . . . . . 22
+ 4.2 DHCP server administrative controls . . . . . . . . . . . . . 25
+ 4.3 DHCP server behavior. . . . . . . . . . . . . . . . . . . . . 26
+ 4.4 DHCP client behavior. . . . . . . . . . . . . . . . . . . . . 34
+ 5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .42
+ 6. References . . . . . . . . . . . . . . . . . . . . . . . . . .42
+ 7. Security Considerations. . . . . . . . . . . . . . . . . . . .43
+ 8. Author's Address . . . . . . . . . . . . . . . . . . . . . . .44
+ A. Host Configuration Parameters . . . . . . . . . . . . . . . .45
+List of Figures
+ 1. Format of a DHCP message . . . . . . . . . . . . . . . . . . . 9
+ 2. Format of the 'flags' field. . . . . . . . . . . . . . . . . . 11
+ 3. Timeline diagram of messages exchanged between DHCP client and
+ servers when allocating a new network address. . . . . . . . . 15
+ 4. Timeline diagram of messages exchanged between DHCP client and
+ servers when reusing a previously allocated network address. . 18
+ 5. State-transition diagram for DHCP clients. . . . . . . . . . . 34
+List of Tables
+ 1. Description of fields in a DHCP message. . . . . . . . . . . . 10
+ 2. DHCP messages. . . . . . . . . . . . . . . . . . . . . . . . . 14
+ 3. Fields and options used by DHCP servers. . . . . . . . . . . . 28
+ 4. Client messages from various states. . . . . . . . . . . . . . 33
+ 5. Fields and options used by DHCP clients. . . . . . . . . . . . 37
+
+1. Introduction
+
+ The Dynamic Host Configuration Protocol (DHCP) provides configuration
+ parameters to Internet hosts. DHCP consists of two components: a
+ protocol for delivering host-specific configuration parameters from a
+ DHCP server to a host and a mechanism for allocation of network
+ addresses to hosts.
+
+ DHCP is built on a client-server model, where designated DHCP server
+ hosts allocate network addresses and deliver configuration parameters
+ to dynamically configured hosts. Throughout the remainder of this
+ document, the term "server" refers to a host providing initialization
+ parameters through DHCP, and the term "client" refers to a host
+ requesting initialization parameters from a DHCP server.
+
+ A host should not act as a DHCP server unless explicitly configured
+ to do so by a system administrator. The diversity of hardware and
+ protocol implementations in the Internet would preclude reliable
+ operation if random hosts were allowed to respond to DHCP requests.
+ For example, IP requires the setting of many parameters within the
+ protocol implementation software. Because IP can be used on many
+ dissimilar kinds of network hardware, values for those parameters
+ cannot be guessed or assumed to have correct defaults. Also,
+ distributed address allocation schemes depend on a polling/defense
+
+
+
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+
+ 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.
+
+
+
+
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+
+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.
+
+
+
+
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+ 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.
+
+
+
+
+
+
+
+
+
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+ 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.
+
+
+
+
+
+
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+ 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.
+
+
+
+
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+
+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."
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
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+ 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.
+
+
+
+
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+ 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
+
+
+
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+ 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
+
+
+
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+ 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.
+
+
+
+
+
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+
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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]
+\f
+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
+
+
+
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+
+
+ 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.
+
+
+
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+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
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+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
+ 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 and BOOTP", RFC 1534,
+ Bucknell University, October 1993.
+
+
+
+
+
+Droms Standards Track [Page 42]
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+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.
+
+
+
+
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+
+
+ 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
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+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)
+
+
+
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+\f
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