rfc1541-dhcp.txt

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Network Working Group                                           R. Droms
Request for Comments: 1541                           Bucknell University
Obsoletes: 1531                                             October 1993
Category: Standards Track


                  Dynamic Host Configuration Protocol

Status of this memo

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

Abstract

   The Dynamic Host Configuration Protocol (DHCP) provides a framework
   for passing configuration information to hosts on a TCP/IP network.
   DHCP is based on the Bootstrap Protocol (BOOTP) [7], adding the
   capability of automatic allocation of reusable network addresses and
   additional configuration options [19].  DHCP captures the behavior of
   BOOTP relay agents [7, 23], and DHCP participants can interoperate
   with BOOTP participants [9].  Due to some errors introduced into RFC
   1531 in the editorial process, this memo is reissued as RFC 1541.


Table of Contents

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



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   4.1 Constructing and sending DHCP messages. . . . . . . . . . . . 21
   4.2 DHCP server administrative controls . . . . . . . . . . . . . 23
   4.3 DHCP server behavior. . . . . . . . . . . . . . . . . . . . . 24
   4.3.1 DHCPDISCOVER message. . . . . . . . . . . . . . . . . . . . 24
   4.3.2 DHCPREQUEST message . . . . . . . . . . . . . . . . . . . . 27
   4.3.3 DHCPDECLINE message . . . . . . . . . . . . . . . . . . . . 29
   4.3.4 DHCPRELEASE message . . . . . . . . . . . . . . . . . . . . 29
   4.4 DHCP client behavior. . . . . . . . . . . . . . . . . . . . . 29
   4.4.1 Initialization and allocation of network address. . . . . . 29
   4.4.2 Initialization with known network address . . . . . . . . . 33
   4.4.3 Initialization with a known DHCP server address . . . . . . 34
   4.4.4 Reacquisition and expiration. . . . . . . . . . . . . . . . 34
   4.4.5 DHCPRELEASE . . . . . . . . . . . . . . . . . . . . . . . . 35
   5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 35
   6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
   7. Security Considerations. . . . . . . . . . . . . . . . . . . . 37
   8. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 38
   A. Host Configuration Parameters  . . . . . . . . . . . . . . . . 39

List of Figures

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

List of Tables

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

1. Introduction

   The Dynamic Host Configuration Protocol (DHCP) provides configuration
   parameters to Internet hosts.  DHCP consists of two components: a
   protocol for delivering host-specific configuration parameters from a
   DHCP server to a host and a mechanism for allocation of network
   addresses to hosts.

   DHCP is built on a client-server model, where designated DHCP server
   hosts allocate network addresses and deliver configuration parameters
   to dynamically configured hosts.  Throughout the remainder of this
   document, the term "server" refers to a host providing initialization



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   parameters through DHCP, and the term "client" refers to a host
   requesting initialization parameters from a DHCP server.

   A host should not act as a DHCP server unless explicitly configured
   to do so by a system administrator.  The diversity of hardware and
   protocol implementations in the Internet would preclude reliable
   operation if random hosts were allowed to respond to DHCP requests.
   For example, IP requires the setting of many parameters within the
   protocol implementation software.  Because IP can be used on many
   dissimilar kinds of network hardware, values for those parameters
   cannot be guessed or assumed to have correct defaults.  Also,
   distributed address allocation schemes depend on a polling/defense
   mechanism for discovery of addresses that are already in use.  IP
   hosts may not always be able to defend their network addresses, so
   that such a distributed address allocation scheme cannot be
   guaranteed to avoid allocation of duplicate network addresses.

   DHCP supports three mechanisms for IP address allocation.  In
   "automatic allocation", DHCP assigns a permanent IP address to a
   host.  In "dynamic allocation", DHCP assigns an IP address to a host
   for a limited period of time (or until the host explicitly
   relinquishes the address).  In "manual allocation", a host's IP
   address is assigned by the network administrator, and DHCP is used
   simply to convey the assigned address to the host.  A particular
   network will use one or more of these mechanisms, depending on the
   policies of the network administrator.

   Dynamic allocation is the only one of the three mechanisms that
   allows automatic reuse of an address that is no longer needed by the
   host to which it was assigned.  Thus, dynamic allocation is
   particularly useful for assigning an address to a host that will be
   connected to the network only temporarily or for sharing a limited
   pool of IP addresses among a group of hosts that do not need
   permanent IP addresses.  Dynamic allocation may also be a good choice
   for assigning an IP address to a new host being permanently connected
   to a network where IP addresses are sufficiently scarce that it is
   important to reclaim them when old hosts are retired.  Manual
   allocation allows DHCP to be used to eliminate the error-prone
   process of manually configuring hosts with IP addresses in
   environments where (for whatever reasons) it is desirable to manage
   IP address assignment outside of the DHCP mechanisms.

   The format of DHCP messages is based on the format of BOOTP messages,
   to capture the BOOTP relay agent behavior described as part of the
   BOOTP specification [7, 23] and to allow interoperability of existing
   BOOTP clients with DHCP servers.  Using BOOTP relaying agents
   eliminates the necessity of having a DHCP server on each physical
   network segment.



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1.1 Related Work

   There are several Internet protocols and related mechanisms that
   address some parts of the dynamic host configuration problem.  The
   Reverse Address Resolution Protocol (RARP) [10] (through the
   extensions defined in the Dynamic RARP (DRARP) [5]) explicitly
   addresses the problem of network address discovery, and includes an
   automatic IP address assignment mechanism.  The Trivial File Transfer
   Protocol (TFTP) [20] provides for transport of a boot image from a
   boot server.  The Internet Control Message Protocol (ICMP) [16]
   provides for informing hosts of additional routers via "ICMP
   redirect" messages.  ICMP also can provide subnet mask information
   through the "ICMP mask request" message and other information through
   the (obsolete) "ICMP information request" message.  Hosts can locate
   routers through the ICMP router discovery mechanism [8].

   BOOTP is a transport mechanism for a collection of configuration
   information.  BOOTP is also extensible, and official extensions [17]
   have been defined for several configuration parameters.  Morgan has
   proposed extensions to BOOTP for dynamic IP address assignment [15].
   The Network Information Protocol (NIP), used by the Athena project at
   MIT, is a distributed mechanism for dynamic IP address assignment
   [19].  The Resource Location Protocol RLP [1] provides for location
   of higher level services.  Sun Microsystems diskless workstations use
   a boot procedure that employs RARP, TFTP and an RPC mechanism called
   "bootparams" to deliver configuration information and operating
   system code to diskless hosts.  (Sun Microsystems, Sun Workstation
   and SunOS are trademarks of Sun Microsystems, Inc.)  Some Sun
   networks also use DRARP and an auto-installation mechanism to
   automate the configuration of new hosts in an existing network.

   In other related work, the path minimum transmission unit (MTU)
   discovery algorithm can determine the MTU of an arbitrary internet
   path [14].  Comer and Droms have proposed the use of the Address
   Resolution Protocol (ARP) as a transport protocol for resource
   location and selection [6].  Finally, the Host Requirements RFCs [3,
   4] mention specific requirements for host reconfiguration and suggest
   a scenario for initial configuration of diskless hosts.

1.2 Problem definition and issues

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





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   Not all of these parameters are required for a newly initialized
   host.  A client and server may negotiate for the transmission of only
   those parameters required by the client or specific to a particular
   subnet.

   DHCP allows but does not require the configuration of host parameters
   not directly related to the IP protocol.  DHCP also does not address
   registration of newly configured hosts with the Domain Name System
   (DNS) [12, 13].

   DHCP is not intended for use in configuring routers.

1.3 Requirements

   Throughout this document, the words that are used to define the
   significance of particular requirements are capitalized.  These words
   are:

      o "MUST"

        This word or the adjective "REQUIRED" means that the
        item is an absolute requirement of this specification.

      o "MUST NOT"

        This phrase means that the item is an absolute prohibition
        of this specification.

      o "SHOULD"

        This word or the adjective "RECOMMENDED" means that there
        may exist valid reasons in particular circumstances to ignore
        this item, but the full implications should be understood and
        the case carefully weighed before choosing a different course.

      o "SHOULD NOT"

        This phrase means that there may exist valid reasons in
        particular circumstances when the listed behavior is acceptable
        or even useful, but the full implications should be understood
        and the case carefully weighed before implementing any behavior
        described with this label.









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      o "MAY"

        This word or the adjective "OPTIONAL" means that this item is