.\"	dhclient.8

.\"

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.\" Copyright (c) 1996-2003 by Internet Software Consortium

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.\"   Internet Systems Consortium, Inc.

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.\" $Id: dhclient.8,v 1.18.116.4 2007/05/23 23:30:32 each Exp $

.\"

.TH dhclient 8

.SH NAME

dhclient - Dynamic Host Configuration Protocol Client

.SH SYNOPSIS

.B dhclient

[

.B -p

.I port

]

[

.B -d

]

[

.B -e

.I VAR=value

]

[

.B -q

]

[

.B -1

]

[

.B -r

]

[

.B -x

]

[

.B -lf

.I lease-file

]

[

.B -pf

.I pid-file

]

[

.B -cf

.I config-file

]

[

.B -sf

.I script-file

]

[

.B -s

server

]

[

.B -g

relay

]

[

.B -n

]

[

.B -nw

]

[

.B -w

]

[

.B -B

]

[

.B -I

.I dhcp-client-identifier

]

[

.B -H

.I host-name

.R |

.B -F fqdn.fqdn

]

[

.B -V

.I vendor-class-identifier

]

[

.B -R

.I request option list

]

[

.B -T

.I timeout

]

[

.I if0

[

.I ...ifN

]

]

.SH DESCRIPTION

The Internet Systems Consortium DHCP Client, dhclient, provides a

means for configuring one or more network interfaces using the Dynamic

Host Configuration Protocol, BOOTP protocol, or if these protocols

fail, by statically assigning an address.

.SH OPERATION

.PP

The DHCP protocol allows a host to contact a central server which

maintains a list of IP addresses which may be assigned on one or more

subnets.   A DHCP client may request an address from this pool, and

then use it on a temporary basis for communication on network.   The

DHCP protocol also provides a mechanism whereby a client can learn

important details about the network to which it is attached, such as

the location of a default router, the location of a name server, and

so on.

.PP

On startup, dhclient reads the

.IR dhclient.conf

for configuration instructions.   It then gets a list of all the

network interfaces that are configured in the current system.   For

each interface, it attempts to configure the interface using the DHCP

protocol.

.PP

In order to keep track of leases across system reboots and server

restarts, dhclient keeps a list of leases it has been assigned in the

dhclient.leases(5) file.   On startup, after reading the dhclient.conf

file, dhclient reads the dhclient.leases file to refresh its memory

about what leases it has been assigned.

.PP

When a new lease is acquired, it is appended to the end of the

dhclient.leases file.   In order to prevent the file from becoming

arbitrarily large, from time to time dhclient creates a new

dhclient.leases file from its in-core lease database.  The old version

of the dhclient.leases file is retained under the name

.IR dhclient.leases~

until the next time dhclient rewrites the database.

.PP

Old leases are kept around in case the DHCP server is unavailable when

dhclient is first invoked (generally during the initial system boot

process).   In that event, old leases from the dhclient.leases file

which have not yet expired are tested, and if they are determined to

be valid, they are used until either they expire or the DHCP server

becomes available.

.PP

A mobile host which may sometimes need to access a network on which no

DHCP server exists may be preloaded with a lease for a fixed

address on that network.   When all attempts to contact a DHCP server

have failed, dhclient will try to validate the static lease, and if it

succeeds, will use that lease until it is restarted.

.PP

A mobile host may also travel to some networks on which DHCP is not

available but BOOTP is.   In that case, it may be advantageous to

arrange with the network administrator for an entry on the BOOTP

database, so that the host can boot quickly on that network rather

than cycling through the list of old leases.

.PP

The names of the network interfaces that dhclient should attempt to

configure may be specified on the command line.  If no interface names

are specified on the command line dhclient will normally identify all

network interfaces, eliminating non-broadcast interfaces if

possible, and attempt to configure each interface.

.PP

It is also possible to specify interfaces by name in the

.B dhclient.conf(5)

file.   If interfaces are specified in this way, then the client will

only configure interfaces that are either specified in the

configuration file or on the command line, and will ignore all other

interfaces.

.SH OPTIONS

.TP

.BI \-p\ <port\ number>

The UDP port number the DHCP client should listen and transmit on.  If

unspecified,

.B dhclient

uses the default port 68.  This option is mostly useful for debugging

purposes.  If a different port is specified for the client to listen and

transmit on, the client will also use a different destination port - one

greater than the specified destination port.

.TP

.BI \-d

Force

.B dhclient

to run as a foreground process.  This is useful when running the client

under a debugger, or when running it out of inittab on System V systems.

.TP

.BI \-e\ VAR=value

Define additional environment variables for the environment where

dhclient-script executes.  You may specify multiple

.B \-e

options on the command line.

.TP

.BI \-q

Suppress all terminal and log output except error messages.

.TP

.BI \-1

Try one to get a lease.  On failure, exit with code 2.

.TP

.BI \-r

Tell

.B dhclient

to release the current lease it has from the server.  This is not required

by the DHCP protocol, but some ISPs require their clients to notify the

server if they wish to release an assigned IP address.

.TP

.BI \-lf\ <lease-file>

Path to the lease database file.  If unspecified, the default

.B DBDIR/dhclient.leases

is used.

.TP

.BI \-pf\ <pid-file>

Path to the process ID file.  If unspecified, the default

.B RUNDIR/dhclient.pid

is used.

.TP

.BI \-cf\ <config-file>

Path to the client configuration file.  If unspecified, the default

.B ETCDIR/dhclient.conf

is used.

.TP

.BI \-sf\ <script-file>

Path to the network configuration script invoked by

.B dhclient

when it gets a lease.  If unspecified, the default

.B CLIENTBINDIR/dhclient-script

is used.

.TP

.BI \-s\ <server>

Specifiy the server IP address or fully qualified domain name to transmit

DHCP protocol messages to.  Normally,

.B dhclient

transmits these messages to 255.255.255.255 (the IP limited broadcast

address).  Overriding this is mostly useful for debugging purposes.

.TP

.BI \-g\ <relay>

Only for debugging.  Set the giaddr field of all packets the client

sends to the IP address specified.  This should not be expected to work

in any consistent or useful way.

.TP

.BI \-n

Do not configure any interfaces.  Most useful combined with the

.B -w

option.

.TP

.BI \-nw

Become a daemon process immediately (nowait) rather than waiting until an IP

address has been acquired.

.TP

.BI \-w

Keep running even if no network interfaces are found.  The

.B omshell

program can be used to notify the client when a network interface has been

added or removed so it can attempt to configure an IP address on that

interface.

.TP

.BI \-B

Set the BOOTP broadcast flag in request packets so servers will always

broadcast replies.

.TP

.BI \-I\ <dhcp-client-identifier>

Specify the dhcp-client-identifier option to send to the DHCP server.

.TP

.BI \-H\ <host-name>

Specify the host-name option to send to the DHCP server.  The host-name

string only contains the client's hostname prefix, to which the server will

append the ddns-domainname or domain-name options, if any, to derive the

fully qualified domain name of the client.  The

.B -H

option cannot be used with the

.B -F

option.

.TP

.BI \-F\ <fqdn.fqdn>

Specify the fqdn.fqdn option to send to the DHCP server.  This option cannot

be used with the

.B -H

option.  The fqdn.fqdn option must specify the complete domain name of the

client host, which the server may use for dynamic DNS updates.

.TP

.BI \-V\ <vendor-class-identifier>

Specify the vendor-class-identifier option to send to the DHCP server.

.TP

.BI \-R\ <option>[,<option>...]

Specify the list of options the client is to request from the server.  The

option list must be a single string consisting of option names separated

by at least one command and optional space characters.  The default option

list is:

.BR

    subnet-mask, broadcast-address, time-offset, routers,

.BR

    domain-name, domain-name-servers, host-name, nis-domain,

.BR

    nis-servers, ntp-servers

The

.B -R

option does not append options to the default request, it overrides the

default request list.  Keep this in mind if you want to request an

additional option besides the default request list.  You will have to

specify all option names for the

.B -R

parameter.

.TP

.BI \-T\ <timeout>

Specify the time after which

.B dhclient

will decide that no DHCP servers can be contacted when no responses have been

received.

.PP

If the client is killed by a signal (for example at shutdown or reboot)

it won't execute the

.B dhclient-script (8)

at exit. However if you shut the client down gracefully with

.B -r

or

.B -x

it will execute

.B dhclient-script (8)

at shutdown with the specific reason for calling the script set.

.PP

.SH CONFIGURATION

The syntax of the dhclient.conf(5) file is discussed separately.

.SH OMAPI

The DHCP client provides some ability to control it while it is

running, without stopping it.  This capability is provided using OMAPI,

an API for manipulating remote objects.  OMAPI clients connect to the

client using TCP/IP, authenticate, and can then examine the client's

current status and make changes to it. 

.PP

Rather than implementing the underlying OMAPI protocol directly, user

programs should use the dhcpctl API or OMAPI itself.   Dhcpctl is a

wrapper that handles some of the housekeeping chores that OMAPI does

not do automatically.   Dhcpctl and OMAPI are documented in \fBdhcpctl(3)\fR

and \fBomapi(3)\fR.   Most things you'd want to do with the client can

be done directly using the \fBomshell(1)\fR command, rather than

having to write a special program.

.SH THE CONTROL OBJECT

The control object allows you to shut the client down, releasing all

leases that it holds and deleting any DNS records it may have added.

It also allows you to pause the client - this unconfigures any

interfaces the client is using.   You can then restart it, which

causes it to reconfigure those interfaces.   You would normally pause

the client prior to going into hibernation or sleep on a laptop

computer.   You would then resume it after the power comes back.

This allows PC cards to be shut down while the computer is hibernating

or sleeping, and then reinitialized to their previous state once the

computer comes out of hibernation or sleep.

.PP

The control object has one attribute - the state attribute.   To shut

the client down, set its state attribute to 2.   It will automatically

do a DHCPRELEASE.   To pause it, set its state attribute to 3.   To

resume it, set its state attribute to 4.

.PP

.SH FILES

.B CLIENTBINDIR/dhclient-script,

.B ETCDIR/dhclient.conf, DBDIR/dhclient.leases, RUNDIR/dhclient.pid,

.B DBDIR/dhclient.leases~.

.SH SEE ALSO

dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5),

dhclient.leases(5), dhcp-eval(5).

.SH AUTHOR

.B dhclient(8)

has been written for Internet Systems Consortium

by Ted Lemon in cooperation with Vixie

Enterprises.  To learn more about Internet Systems Consortium,

see

.B http://www.isc.org

To learn more about Vixie

Enterprises, see

.B http://www.vix.com.

.PP

This client was substantially modified and enhanced by Elliot Poger

for use on Linux while he was working on the MosquitoNet project at

Stanford.

.PP

The current version owes much to Elliot's Linux enhancements, but

was substantially reorganized and partially rewritten by Ted Lemon

so as to use the same networking framework that the Internet Systems

Consortium DHCP server uses.   Much system-specific configuration code

was moved into a shell script so that as support for more operating

systems is added, it will not be necessary to port and maintain

system-specific configuration code to these operating systems - instead,

the shell script can invoke the native tools to accomplish the same

purpose.

.PP