Internet Engineering Task Force R. Droms
INTERNET-DRAFT Bucknell University
June, 1992
Dynamic Host Configuration Protocol
1 Abstract
The Dynamic Host Configuration Protocol (DHCP) passes network configuration
information to participants in a TCP/IP protocol network.
2 Status of this memo
This draft document is a product of the IETF Dynamic Host Configuration
Working Group; it will be submitted to the RFC editor as a standards
document. Distribution of this memo is unlimited. It is available in both
ASCII and PostScript formats. The figures are described in PostScript and
are not included with the ASCII version of the document. Copies of the
figures are available from the author. Please respond with comments to the
host-conf@sol.cs.bucknell.edu mailing list. This document will expire on
January 1, 1993.
This document is an Internet Draft. Internet Drafts are working documents
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status of any Internet Draft.
3 Introduction
The Dynamic Host Configuration Protocol (DHCP) provides configuration
parameters to Internet hosts. DHCP consists of three components: a
INTERNET-DRAFT Dynamic Host Configuration Protocol June, 1992
protocol for delivering host--specific configuration parameters from a DHCP
server to a host; a mechanism for allocation of network addresses to hosts;
and a protocol through which a collection of DHCP servers can cooperatively
allocate network addresses from a shared pool of network addresses.
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.
DHCP uses only designated DHCP servers, rather than allowing any Internet
host to act as a DHCP server. 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 provides two forms of IP address allocation: ``automatic'' and
``dynamic.'' Automatic allocation assigns an IP address to a host newly
attached to an IP subnet. The new address is permanently assigned to the
host and cannot be recovered through DHCP. Dynamic allocation temporarily
assigns an IP address to a host. The address can be reused when no longer
needed by the host.
The format of DHCP messages is based on the format of BOOTP [6] messages,
to capture the BOOTP relay agent behavior described as part of the BOOTP
specification [6, 19] 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.
3.1 Related Work
There are several Internet protocols and related mechanisms that address
some parts of the dynamic host configuration problem. RARP [8] (through
the extensions defined in the DRARP draft RFC [4]) explicitly addresses the
problem of network address discovery, and includes an automatic IP address
assignment mechanism. TFTP [18] provides for transport of a boot image
from a boot server. ICMP [14] 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
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locate routers through the ICMP router discovery mechanism [7].
BOOTP is a transport mechanism for a collection of configuration
information. BOOTP is also extensible, and official extensions [15, 16]
have been defined for several configuration parameters. Morgan has proposed
extensions to BOOTP for dynamic IP address assignment [13]. NIP, used
by the Athena project at MIT, is a distributed mechanism for dynamic IP
address assignment [17]. RLP [1] provides for location of higher level
services. Sun Microsystems(1) 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. 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 MTU discovery algorithm can determine the
MTU of an arbitrary internet path [12]. Comer and Droms have proposed the
use of ARP as a transport protocol for resource location and selection [5].
Finally, the Host Requirements RFCs [2, 3] mention specific requirements for
host reconfiguration and suggest a scenario for initial configuration of
diskless hosts.
3.2 Problem definition and issues
DHCP is designed to supply hosts with configuration parameters as defined in
the Host Requirements RFCs. After obtaining parameters from 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.
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. A site may choose to use vendor
extensions to return information about higher level protocols to the host.
DHCP also does not address registration of newly configured hosts with
DNS[10, 11].
DHCP is not intended for use in configuring routers.
3.3 Requirements
The following list gives general requirements for DHCP.
------------------------------
1. Sun Microsystems, Sun Workstation and SunOS are trademarks of Sun
Microsystems, Inc.
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o DHCP should be a mechanism not 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.
o Hosts should require no manual configuration. Each host should be
able to discover appropriate local configuration parameters without
user intervention and incorporate those parameters into its own
configuration.
o Networks should require no hand configuration for individual hosts.
Under normal circumstances, the network manager should not have to
enter any per--host configuration parameters.
o DHCP should not require a server on each subnet. To allow for scale
and economy, DHCP must work across routers or through the intervention
of BOOTP/DHCP relay agents.
o A DHCP host must be prepared to receive multiple responses to a
request for configuration parameters. Some installations may include
multiple, overlapping DHCP servers to enhance reliability and increase
performance.
o DHCP must coexist with statically configured, non--participating hosts
and with existing network protocol implementations.
o DHCP must interoperate with the BOOTP relay agent behavior as described
by RFC 951 and by Wimer's Internet Draft.
o DHCP must interoperate with existing BOOTP clients.
The following list gives requirements specific to the transmission of the
network layer parameters. DHCP must:
o Guarantee that any specific network address will not be in use by more
than one host at a time,
o Retain host configuration across host reboot. A host should, whenever
possible, be assigned the same configuration parameters (e.g., network
address) in response to each request,
o Retain host configuration across server reboots, and, whenever
possible, a host should be assigned the same configuration parameters
despite restarts of the DHCP mechanism,
o Allow automatic assignment of configuration parameters to new hosts to
avoid hand configuration for new hosts,
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o Support fixed or permanent allocation of configuration parameters to
specific hosts.
4 Protocol Summary
From the client's point of view, DHCP is an extension of the BOOTP
mechanism. This behavior allows existing BOOTP clients to interoperate with
DHCP servers without requiring any change to the clients' initialization
software. A separate document (currently an Internet Draft) details
the interactions between BOOTP and DHCP clients and servers, as well as
interactions between specific implementations of the DHCP specification.
There are some new, optional transactions that optimize the interaction
between DHCP clients and servers that are described in sections 5 and 6.
There are two primary differences between DHCP and BOOTP. First,
DHCP defines many new vendor extensions to transmit all of the network
configuration parameters required for client operation. Second, DHCP
defines mechanisms through which DHCP servers coordinate the dynamic
allocation of network addresses to requesting clients.
DHCP extends the use of the 'htype', 'hlen' and 'chaddr' fields to allow the
use of client identifiers that are not hardware addresses. The hardware
type values defined in the ARP section of the ``Assigned Numbers'' RFC are
reserved for use when the client identifier is a hardware address. Other
client identifier types (e.g., a machine--specific serial number permanently
stored with the client) must be defined for use with DHCP.
There is also a minor difference between the format of DHCP messages and
BOOTP messages. In DHCP, the vendor extension field is extended to 312
octets to bring the minimum size of a DHCP message to 576 octets, the
minimum IP datagram size a host must be prepared to accept [2, Sec. 3.2.2,
p. 56]. DHCP clients may negotiate the use of larger DHCP messages through
the 'Maximum DHCP message size' vendor extension.
4.1 Components of the Protocol
DHCP provides three distinct services:
o A persistent, dynamic repository of configuration information for
clients.
o Dynamic allocation of configuration resources such as network layer
addresses.
o Distribution of configuration information among protocol servers.
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This document will separately describe the mechanisms through which DHCP
provides the first two of these services. A separate document will describe
the operation of the DHCP distributed database.
4.2 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 (an IP subnet number and a unique
identifier within the subnet) and the value contains the configuration
parameters for the client. For example, the key might be the pair
(IP--subnet--number, hardware--address), allowing for serial or concurrent
reuse of a hardware address on different subnets, and for hardware addresses
that may not be globally unique.
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.
4.3 Dynamic allocation of network addresses
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'' [9]. The client may extend its lease
with subsequent requests. The client may ask for a permanent assignment by
asking for an infinite lease. The client may issue a message to release the
address back to the server when the client no longer needs the address.
In some environments it will be necessary to reassign network addresses due
to exhaustion of available addresses. In such environments, the allocation
mechanism will reuse addresses whose lease has expired. The server should
use whatever information is available in the configuration information
repository to choose an address to reuse. For example, the server may
choose the least recently assigned address. As a consistency check, the
allocation mechanism will probe the reused address, e.g., with an ICMP echo
request, before allocating the address, and the client will probe the newly
received address, e.g., with ARP.
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5 The Client--Server Protocol
The DHCP protocol messages given in table 1 all have the same format, given
in Appendix B, based on the message format defined for BOOTP messages in
RFC 951. In each of these messages, the 'op' field contains BOOTREQUEST in
any message from a client to a server, and the 'op' field contains BOOTREPLY
in any message from a server to a client. The 'DHCP message type' vendor
extension indicates the type of the DHCP message.
A separate document (currently an Internet Draft) gives the vendor
extensions defined by DHCP.
5.1 Client--server interaction -- allocating a network address
The following summary of the protocol exchanges between clients and
servers refers to the DHCP protocol messages described in table 1. The
timeline diagram in figure 1 shows the timing relationships in a typical
client--server interaction, for a client requesting dynamic allocation of a
new network address.
1. The client broadcasts a DHCPDISCOVER message on its local physical
subnet. The DHCPDISCOVER message may include suggestions for specific
parameter values, e.g., network address or lease duration, from the
client. DHCP/BOOTP relay agents pass the message on to DHCP servers
not on the same physical subnet.
2. Each server may respond with a DHCPOFFER message with all configuration
parameters including network address. Servers need not reserve
the offered network address, although the protocol will work more
efficiently if the server avoids allocating the offered network address
to another client. The server unicasts the DHCPOFFER message to the
client (using the DHCP/BOOTP relay agent if necessary) if possible, or
may broadcast the message on the client's subnet.
3. The client receives DHCPOFFER message(s) from the server(s). The
client may choose to wait for multiple responses. The client chooses
one server from which to request configuration parameters, based on
offered configuration parameters in the DHCPOFFER messages. The
client broadcasts a DHCPREQUEST message, specifying the desired server,
desired network address and any other desired configuration values in
vendor extension fields. This DHCPREQUEST message is broadcast and
relayed through DHCP/BOOTP relay agents. Any DHCP/BOOTP relay agents
must ensure that any messages from this client are forwarded to the
same set of DHCP servers to ensure that the DHCPREQUEST message reaches
the selected DHCP server.
The client times out and retransmits the DHCPDISCOVER message if the
client receives no DHCPOFFER messages.
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Figure 1: Timeline diagram of messages exchanged between DHCP client and
servers when allocating a new network address
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____Message_________________________________Use__________________________
DHCPDISCOVER -- Client broadcast to locate available servers.
DHCPOFFER -- Server to client in response to DHCPDISCOVER with
offer of configuration parameters.
DHCPREQUEST -- Client broadcast to servers requesting offered
parameters from one server and implicitly declining
offers from all others.
DHCPACK -- Server to client with configuration parameters,
including committed network address.
DHCPNAK -- Server to client refusing request for configuration
parameters (e.g., requested network address already
allocated).
DHCPDECLINE -- Client to server indicating configuration parameters
(e.g., network address) invalid.
DHCPRELEASE -- Client to server relinquishing network address and
cancelling remaining lease.
Table 1: DHCP messages
4. The servers receive the DHCPREQUEST broadcast from the client. The
servers not selected in 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 server inserts a unique lease identification cookie as a vendor
extension.
If the selected server is unable to satisfy the DHCPREQUEST message
(e.g., the requested network address has been allocated), the server
responds with a DHCPNAK message.
5. The client receives the DHCPACK message with configuration parameters.
The client performs a final check on the parameters (e.g., ARP for
allocated network address), and notes the duration of the lease and the
lease identification cookie specified in the DHCPACK message. At this
point, the client is configured.
If the client detects a problem with the parameters in the DHCPACK
message, the client sends a DHCPDECLINE message to the server and
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Figure 2: Timeline diagram of messages exchanged between DHCP client and
servers when reusing a previously allocated network address
restarts the configuration process(2).
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.
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 the lease identification cookie.
5.2 Client--server interaction -- reusing a previously allocated network
address
The timeline diagram in figure 2 shows the timing relationships in a typical
client--server interaction for a client reusing a previously allocated
network address.
1. Client broadcasts a DHCPREQUEST message on its local subnet. The
DHCPREQUEST message includes the clients network address in the
'ciaddr' field and any other suggested configuration values in vendor
------------------------------
2. The client should wait a minimum of ten seconds before restarting the
configuration process to avoid excessive network traffic in case of looping.
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extensions. DHCP/BOOTP relay agents pass the message on to DHCP
servers not on the same subnet.
2. Servers with knowledge of the client's configuration parameters respond
with a DHCPACK message to the client.
If the client's request is invalid (e.g., the client has moved to a new
subnet), servers may respond with a DHCPNAK message to the client.
3. Client receives the DHCPACK message with configuration parameters. The
client performs a final check on the parameters, and notes the duration
of the lease and the lease identification cookie specified in the
DHCPACK message. At this point, the client is configured.
If the client detects a problem with the parameters in the DHCPACK
message, the client sends a DHCPDECLINE message to the server and
restarts the configuration process in INIT state, requesting a new
network address.
If the client receives a DHCPNAK message, the client restarts the
configuration process. The client restarts the configuration process
in INIT state, requesting a new network address.
The client times out and retransmits the DHCPREQUEST message if the
client receives neither a DHCPACK or a DHCPNAK message.
4. The client may choose to relinquish its lease on a network address by
sending a DHCPRELEASE message to the server. The client identifies the
lease to be released with the lease identification cookie.
Note that in this case, where the client retains its network address
locally, the client will not normally relinquish its lease during a
graceful shutdown. Only in the case where the client explicitly needs
to relinquish its lease, e.g. , the client is about to be moved to a
different subnet, will the client send a DHCPRELEASE.
5.3 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 all 1s is reserved to
represent ``infinity''. The minimum lease duration is one hour.
As clients and servers may not have synchronized clocks, times are
represented in DHCP messages as relative times, to be interpreted with
respect to the client's local clock. Representing relative times in units
of seconds in an unsigned 32 bit word gives a range of relative times from 0
to approximately 100 years, which is sufficient for the relative times to be
measured using DHCP.
The algorithm for lease duration interpretation given in 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
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return a shorter lease duration to the client than the server commits to its
local database of client information.
5.4 Host parameters in DHCP
Not all clients require initialization of all parameters listed in
Appendix A. Two techniques are used to reduce the number of parameters
transmitted from the server to the client. First, most of the
parameters have defaults defined in the Host Requirements RFCs; in lieu
of parameterization to the contrary, a client uses those default values.
Second, a client may explicitly request only certain parameters in its
initial DHCPREQUEST message.
The parameters returned to a client may still exceed the space allocated to
vendor extensions in a DHCP message. In this case, two additional vendor
extension flags (which must appear in the vendor extension area of the
message) indicate that the 'file' and 'sname' fields are to be used for
vendor extensions.
A client can fill in vendor extensions in a DHCPDISCOVER and DHCPREQUEST to
supply hints or request specific values from a server. The server fills
in vendor extensions in DHCPOFFER and DHCPACK messages to supply specific
configuration values to the client.
A client can also request specific configuration parameters without
supplying hints through the 'parameter request vector' and 'parameter
request list' vendor extensions. In the parameter request vector, a one bit
in position n in the vector represents an explicit request for the vendor
extensions parameter with tag n. The parameter request list is a list of
vendor extension tags explicitly requested by the client.
The 'ciaddr' field is to be filled in by the client only if the client has
explicit knowledge of its network address. The client can supply a hint or
a preferred network address through the IP address vendor extension.
If a server receives a DHCPREQUEST message with an invalid 'ciaddr', the
server responds 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' vendor extension.
The client should specify the largest acceptable DHCP message with the 'DHCP
message size' vendor extension to ensure that the server can transmit all
the appropriate parameters in a single DHCP message.
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5.5 Use of DHCP in hosts with multiple interfaces
A host with multiple network interfaces must use DHCP through each interface
independently to obtain configuration information parameters for those
separate interfaces.
5.6 Use of DHCP by hosts
A host should use DHCP to reacquire or verify its IP address and network
parameters whenever the local network parameters may have changed; e.g.,
at system boot time or after a disconnection from the local network, as
the local network configuration may change without the host's or user's
knowledge.
If a host has knowledge of a previous network address and is unable to
contact a local DHCP server, the host may continue to use the previous
network address until the lease for that address expires.
6 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, and that the
server will independently employ the server--server protocol to obtain more
network addresses when necessary. Each server also maintains a database of
allocated addresses and leases in local permanent storage.
6.1 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 vendor
extension information in the variable format vendor extension area. The
vendor extension area includes first a four--octet 'magic cookie', the
vendor extensions and an 'end' tag, which indicates the end of the vendor
extensions.
DHCP uses UDP as its transport protocol. DHCP messages from a client to a
server are sent to the 'DHCP server' port (67), and DHCP messages from a
server to a client are sent to the 'DHCP client' port (68).
DHCP messages broadcast by a client prior to that client obtaining its IP
address must have the source address field in the IP header set to 0.
If the 'giaddr' field in a DHCP message from a client is non--zero, the
server sends any return messages to the 'DHCP client' port on the DHCP
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relaying agent whose address appears in 'giaddr'. If the 'giaddr' field
is zero, the client is on the same subnet, and the server sends any
return messages to either the client's network address (if known) or to the
client's hardware address or to the local subnet broadcast address.
6.2 DHCP server behavior
A DHCP server processes incoming DHCP messages from a client based on the
current state of the binding for that client. A DHCP server can receive the
following messages from a client:
o DHCPDISCOVER
o DHCPREQUEST
o DHCPDECLINE
o DHCPRELEASE
Table 2 gives the use of the fields and vendor extensions in a DHCP message
by a server. The remainder of this section describes the action of the DHCP
server for each possible incoming message.
6.2.1 DHCPDISCOVER message
When a server receives a DHCPDISCOVER message from a client, the server
chooses a network address for the requesting client. If no address is
available, the server may choose to report the problem to the system
administrator and may choose to reply to the client with a DHCPNAK message.
If the server chooses to respond to the client, it may include an error
message in the 'message' vendor extension. If an address is available, the
new address should be chosen as follows:
o The client's previous address as recorded in the client's binding, if
that address is in the server's pool of available addresses and not
already allocated, else
o The address requested in the 'Requested IP Address' vendor extension,
if that address is valid and not already allocated, else
o A new address allocated from the server's pool of available addresses.
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Field_________DHCPOFFER____________DHCPACK_____________DHCPNAK______________
'op' BOOTREPLY BOOTREPLY BOOTREPLY
'htype' (From ``Assigned Numbers'' RFC; 0 implies ``other type
identifier'')
'hlen' (Hardware adress length in octets)
'hops' 0 0 0
'xid' 'xid' from client 'xid' from 'xid' from
DHCPDISCOVER client DHCPREQUEST client DHCPREQUEST
message message message
secs 0 0 0
'ciaddr' 0 0 0
'yiaddr' IP address offered IP address as- 0
to client signed to client
'siaddr' IP address of IP address of IP address of
server server server
'giaddr' 0 0 0
'chaddr' 'chaddr' from 'chaddr' from 'chaddr' from
client DHCP- client DHCPREQUEST client DHCPREQUEST
DISCOVER message message message
'sname' Server host Server host (unused)
name or vendor name or vendor
extensions extensions
'file' Client boot file Client boot file (unused)
name or vendor name or vendor
extensions extensions
'vend' Vendor extensions Vendor extensions
Vendor_extension_________DHCPOFFER________DHCPACK__________DHCPNAK__________
Requested IP address MAY NOT MAY NOT MAY NOT
IP address lease time MUST MUST MAY NOT
Use 'file'/'sname' MAY MAY MAY NOT
fields
DHCP message type DHCPOFFER DHCPACK DHCPNAK
Lease identifier MUST MUST MAY NOT
cookie
Parameter request MAY NOT MAY NOT MAY NOT
vector
Parameter request list MAY NOT MAY NOT MAY NOT
NIS (YP) domain MAY MAY MAY NOT
NIS (YP) servers MAY MAY MAY NOT
NTP (RFC 1129) servers MAY MAY MAY NOT
Message MAY MAY MAY
All others MAY MAY MAY NOT
Table 2: Fields and vendor extensions used by DHCP servers
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While not required for correct operation of DHCP, the server should arrange
to avoid reusing the selected network address soon after offering the
address to the client. The server may choose to record the address as
offered to the client.
The server must also choose an expiration time for the lease, as follows:
o If the client has not requested a specific lease in the DHCPDISCOVER
message and the client already has an assigned network address, the
server returns the existing lease expiration time.
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.
o If the client has requested a specific lease in the DHCPDISCOVER
message (regardless of whether the client has an assigned network
address), the server may choose either to return the requested lease
(if the lease is acceptable to local policy) or select another lease.
Once the network address and lease have been determined, the server
constructs a DHCPOFFER message with the offered initialization parameters:
o The client's network address and subnet mask.
o The expiration time for the client's lease.
o Parameters requested by the client.
o Parameters with non--default values on the client's subnet.
The server inserts the 'xid' field from the DHCPDISCOVER message into the
'xid' field of the DHCPOFFER message and sends the DHCPOFFER message to the
requesting client.
6.2.2 DHCPREQUEST message
If the server is identified in the 'server identifier' vendor extension
in the DHCPREQUEST message or if there is no 'server identifier' vendor
extension, the server checks to confirm that the requested parameters are
acceptable and returns a DHCPACK message to the requesting client with
the client's initialization parameters. The server records the allocated
address and lease duration in the server database. The server should
also record the client's initialization parameters for reuse in response to
subsequent requests from the client.
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If the requested parameters are unacceptable, e.g., the requested lease time
is unacceptable to local policy, the server sends a DHCPNAK message to the
client. The server may choose to return an error message in the 'message'
vendor extension.
If a different server is identified in the 'server identifier' field, the
client has selected a different server form which to obtain configuration
parameters. The server may discard any hints about the client.
Note that the client may choose to collect several DHCPOFFER messages
and select the ``best'' offer. The client indicates its selection by
identifying the offering server in the DHCPREQUEST message. If the
client receives no acceptable offers, the client may choose to try another
DHCPDISCOVER message. Therefore, the servers may not receive a specific
DHCPREQUEST from which they can decide whether or not the client has
accepted the offer. Because the servers have not committed any network
address assignments on the basis of a DHCPOFFER, servers are free to
reuse offered network addresses in response to subsequent requests. As
an implementation detail, servers should try to arrange to avoid reusing
offered addresses and may use an implementation--specific timeout mechanism
to decide when to reuse an offered address.
6.2.3 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 marks the network address as not allocated and may notify
the local system administrator of a possible configuration problem.
6.2.4 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.
6.3 Client behavior
Figure 3 gives a state--transition diagram for a DHCP client. A client can
receive the following messages from a server:
o DHCPOFFER
o DHCPACK
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Figure 3: State--transition diagram for DHCP clients
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o DHCPNAK
Table 3 gives the use of the fields and vendor extensions in a DHCP message
by a client. The remainder of this section describes the action of the DHCP
client for each possible incoming message.
6.3.1 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 all
0s. The client may include a suggested network address and time for the
duration of the lease and requests for any other specific parameters the
client might need in the vendor extensions. Any vendor extension fields
are interpreted by the server as requests for specific parameters and will
be filled in and returned in the server's DHCPOFFER. The client generates
and records a random transaction identifier and inserts that identifier into
the 'xid' field. The client records its own local time for later use in
computing the lease expiration. The client then broadcasts the DHCPDISCOVER
on the local hardware broadcast address to the all-ones IP broadcast address
and 'DHCP server' UDP port.
AUTHOR'S NOTE: The client must implement a timeout and
retransmission with exponential backoff algorithm for the receipt
of the DHCPOFFER message.
If the 'xid' of an arriving DHCPOFFER message does not match the 'xid' of
the most recent DHCPDISCOVER message, the DHCPOFFER message is silently
discarded. Any arriving DHCPACK messages are 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 DHCPOFFER
message. The time over which the client collects messages and the mechanism
used to select one DHCPOFFER are implementation dependent. The client may
perform a check on the suggested address to ensure that the address is not
already in use. For example, if the client is on a network that supports
ARP, the client may issue an ARP request for the suggested request(3). If
the network address appears to be in use, the client sends a DHCPDECLINE
------------------------------
3. When ARPing 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.
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Field DHCPDISCOVER DHCPREQUEST DHCPDECLINE,
______________________________________________________DHCPRELEASE___________
'op' BOOTREQUEST BOOTREQUEST BOOTREQUEST
'htype' (From ``Assigned Numbers'' RFC; 0 implies ``other type
identifier'')
'hlen' (Hardware adress length in octets)
'hops' 0 0 0
'xid' selected by client selected by client selected by client
'secs' (opt.) (opt.) 0
'ciaddr' 0 (if known) 0
'yiaddr' 0 0 0
'siaddr' 0 0 0
'giaddr' 0 0 0
'chaddr' client's hard- client's hard- client's hard-
ware address or ware address or ware address or
identifier identifier identifier
'sname' Vendor extensions Vendor extensions (unused)
(opt.) (opt.)
'file' Vendor extensions Vendor extensions (unused)
(opt.) (opt.)
'vend' Vendor extensions Vendor extensions (unused)
Vendor extension DHCPDISCOVER DHCPREQUEST DHCPDECLINE,
______________________________________________________DHCPRELEASE________
Requested IP address MAY MAY MAY NOT
IP address lease time MAY MAY MAY NOT
Use 'file'/'sname' fields MAY MAY MAY
DHCP message type DHCPDISCOVER DHCPREQUEST DHCPDECLINE/
DHCPRELEASE
Lease identifier cookie MAY NOT MAY NOT MAY NOT
Parameter request vector MAY MAY MAY NOT
Parameter request list MAY MAY MAY NOT
NIS (YP) domain MAY NOT MAY NOT MAY NOT
NIS (YP) servers MAY NOT MAY NOT MAY NOT
NTP (RFC 1129) servers MAY NOT MAY NOT MAY NOT
Message MAY NOT MAY NOT MAY NOT
All others MAY NOT MAY NOT MAY NOT
Table 3: Fields and vendor extensions used by DHCP clients
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message to the server and waits for another DHCPOFFER. As the client does
not have a valid network address, the client must broadcast the DHCPDECLINE
message.
If the parameters are acceptable, the client records the address of the
server that supplied the parameters from the 'siaddr' field and sends that
address in the 'server identifier' field of a DHCPREQUEST broadcast message.
Once the DHCPACK message from the server arrives, the client is initialized
and moves to BOUND state. The DHCPREQUEST message contains the same 'xid'
as the DHCPOFFER message. The client records the lease expiration time as
the sum of the time at which the original request was sent and the duration
of the lease from the DHCPOFFER message.
6.3.2 Initialization with known network address
The client begins in REBOOTING state and sends a DHCPREQUEST message with
the 'ciaddr' field set to the client's network address. The client may
include requests for for any other specific parameters the client might need
in the vendor extensions. Any vendor extension fields are interpreted
by the server as requests for specific parameters and will be filled in
and returned in the server's DHCPOFFER. 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 DHCPREQUEST on the local
hardware broadcast address to the 'DHCP server' UDP port.
AUTHOR'S NOTE: The client must implement a timeout and
retransmission with exponential backoff algorithm for the receipt
of the DHCPOFFER message.
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.
6.3.3 Reacquisition and Expiration
At time T1 before the expiration of the client's lease on its network
address, the client moves to RENEWING state and sends (via unicast) a
DHCPREQUEST message to the server to extend its lease. The client generates
a random transaction identifier and inserts that identifier into the 'xid'
field in the DHCPREQUEST. The client records the local time at which the
DHCPREQUEST message is sent for computation of the lease expiration time.
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Any DHCPACK messages that arrive with an 'xid' that does not match the 'xid'
of the client's DHCPREQUEST message are silently discarded. When the client
receives a DHCPACK from the server, the client computes the lease expiration
time as the sum of the time at which the client sent the DHCPREQUEST message
and the duration of the lease in the DHCPACK message. The client has
successfully reacquired its network address, returns to BOUND state and may
continue network processing.
If no DHCPACK arrives before time T2 (T2 < T1) before the expiration of the
client's lease on its network address, the client moves to REBINDING state
and sends (via broadcast) a DHCPREQUEST message to extend its lease. The
client sets the 'ciaddr' field in the DHCPREQUEST to its current network
address.
AUTHOR'S NOTE: The client must implement a timeout and
retransmission with exponential backoff algorithm to retry the
unicast and broadcast DHCPDISCOVER messages.
Times T1 and T2 are configurable by the server through vendor
extensions. 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.
If the lease expires before the client receives a DHCPACK, the client
moves to INIT state, must immediately stop any other network processing
and request 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 may continue network processing.
If the client is given a new network address, it may not continue using the
previous network address and must notify the local users of the problem.
6.3.4 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.
7 Security Considerations
This document does not address security issues.
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8 Acknowledgments
Greg Minshall, Leo McLaughlin and John Veizades have patiently contributed
to the the design of DHCP through innumerable discussions, meetings and mail
conversations. Jeff Mogul first proposed the client--server based model
for DHCP. Steve Deering searched the various IP RFCs to put together the
list of network parameters supplied by DHCP. Walt Wimer contributed a
wealth of practical experience with BOOTP and wrote a document clarifying
the behavior of BOOTP/DHCP relay agents. Jesse Walker analyzed DHCP in
detail, pointing out several inconsistencies in earlier specifications of
the protocol. Steve Alexander reviewed Walker's analysis and the fixes to
the protocol based on Walker's work. And, of course, all the members of the
Dynamic Host Configuration Working Group of the IETF have contributed to the
design of the protocol through discussion and review of the protocol design.
9 Author's Address
Ralph Droms
Computer Science Department
323 Dana Engineering
Bucknell University
Lewisburg, PA 17837
(717) 524--1145
droms@bucknell.edu
References
[1] M. Acetta. Resource Location Protocol. RFC 887, NIC, December 1983.
[2] R. Braden (Ed.). Requirements for Internet Hosts -- Communication
Layers. RFC 1122, NIC, October 1989.
[3] R. Braden (Ed.). Requirements for Internet Hosts -- Application and
Support. RFC 1123, NIC, October 1989.
[4] David Brownell. Dynamic Reverse Address Resolution Protocol (DRARP).
RFC DRAFT, NIC, 1989.
[5] D. Comer 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.
[6] B Croft and J. Gilmore. Bootstrap Protocol (BOOTP). RFC 951, NIC,
September 1985.
[7] S. Deering. ICMP Router Discovery Messages. RFC 1256, NIC, September
1991.
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INTERNET-DRAFT Dynamic Host Configuration Protocol June, 1992
[8] R. Finlayson, T. Mann, J. Mogul, and M. Theimer. A Reverse Address
Resolution Protocol. RFC 903, NIC, June 1984.
[9] C. G. Gray and D. R. Cheriton. Leases: An Efficient Fault-Tolerant
Mechanism for Distributed File Cache Consistency. In Proc. of the
Twelfth ACM Symposium on Operating Systems Design, 1989.
[10] P. Mockapetris. Domain Names -- Concepts and Facilities. RFC 1034,
NIC, November 1987.
[11] P. Mockapetris. Domain Names -- Implementation and Specification. RFC
1035, NIC, November 1987.
[12] J. Mogul and S. Deering. Path MTU Discovery. RFC 1191, NIC, November
1990.
[13] R. L. Morgan. Dynamic IP Address Assignment for Ethernet Attached
Hosts. RFC DRAFT, NIC, 1989.
[14] J. Postel. Internet Control Message Protocol. RFC 792, NIC, September
1981.
[15] P. Prindeville. BOOTP Vendor Information Extensions. RFC 1048, NIC,
February 1988.
[16] J. Reynolds. BOOTP Vendor Information Extensions. RFC 1084, NIC,
December 1988.
[17] 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.
[18] K. Sollins. The TFTP Protocol (Revision 2). RFC 783, NIC, June 1981.
[19] W. Wimer. Clarifications and Extensions for the Bootstrap Protocol.
Internet Draft, NIC, 1991.
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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 all 0s/all 1s 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:
HRC = Requirements for Internet Hosts -- Communication Layers
(RFC 1122)
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MTU = Path MTU Discovery (RFC 1191, Proposed Standard)
RD = Router Discovery (RFC 1256, Proposed Standard)
B Format of a DHCP message
_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. '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 in relay--agent booting.
xid 4 Transaction ID, a random number chosen by the
client, used by the client and server to associate
messages and responses between a client and a
server.
secs 2 Filled in by client, seconds elapsed since client
started trying to boot.
-- 2 Unused.
ciaddr 4 Client IP address; filled in by client in
DHCPDISCOVER if known.
yiaddr 4 'your' (client) IP address; filled by server if
client doesn't know its own address ('ciaddr' was
0).
siaddr 4 Server IP address; returned in DHCPOFFER, DHCPACK
and DHCPNAK by server.
giaddr 4 Relay agent IP address, used in optional relay--
agent booting.
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.
vend 312 Optional vendor extensions field. See the vendor
extension documents for a list of defined vendor
extensions.
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