.\"	$Id: dhclient.conf.5,v 1.17.84.2 2007/05/23 23:30:32 each Exp $

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.\" Copyright (c) 2004,2007 by Internet Systems Consortium, Inc. ("ISC")

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.\" This software has been written for Internet Software Consortium

.\" by Ted Lemon in cooperation with Vixie Enterprises and Nominum, Inc.

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.\" ``http://www.isc.org/''.  To learn more about Vixie Enterprises,

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.\" $Id: dhclient.conf.5,v 1.17.84.2 2007/05/23 23:30:32 each Exp $

.\"

.TH dhclient.conf 5

.SH NAME

dhclient.conf - DHCP client configuration file

.SH DESCRIPTION

The dhclient.conf file contains configuration information for

.IR dhclient,

the Internet Systems Consortium DHCP Client.

.PP

The dhclient.conf file is a free-form ASCII text file.   It is parsed by

the recursive-descent parser built into dhclient.   The file may contain

extra tabs and newlines for formatting purposes.  Keywords in the file

are case-insensitive.   Comments may be placed anywhere within the

file (except within quotes).   Comments begin with the # character and

end at the end of the line.

.PP

The dhclient.conf file can be used to configure the behaviour of the

client in a wide variety of ways: protocol timing, information

requested from the server, information required of the server,

defaults to use if the server does not provide certain information,

values with which to override information provided by the server, or

values to prepend or append to information provided by the server.

The configuration file can also be preinitialized with addresses to

use on networks that don't have DHCP servers.

.SH PROTOCOL TIMING

The timing behaviour of the client need not be configured by the user.

If no timing configuration is provided by the user, a fairly

reasonable timing behaviour will be used by default - one which

results in fairly timely updates without placing an inordinate load on

the server.

.PP

The following statements can be used to adjust the timing behaviour of

the DHCP client if required, however:

.PP

.I The

.B timeout

.I statement

.PP

.B timeout

.I time

.B ;

.PP

The

.I timeout

statement determines the amount of time that must pass between the

time that the client begins to try to determine its address and the

time that it decides that it's not going to be able to contact a

server.   By default, this timeout is sixty seconds.   After the

timeout has passed, if there are any static leases defined in the

configuration file, or any leases remaining in the lease database that

have not yet expired, the client will loop through these leases

attempting to validate them, and if it finds one that appears to be

valid, it will use that lease's address.   If there are no valid

static leases or unexpired leases in the lease database, the client

will restart the protocol after the defined retry interval.

.PP

.I The

.B retry

.I statement

.PP

 \fBretry \fItime\fR\fB;\fR

.PP

The

.I retry

statement determines the time that must pass after the client has

determined that there is no DHCP server present before it tries again

to contact a DHCP server.   By default, this is five minutes.

.PP

.I The

.B select-timeout

.I statement

.PP

 \fBselect-timeout \fItime\fR\fB;\fR

.PP

It is possible (some might say desirable) for there to be more than

one DHCP server serving any given network.   In this case, it is

possible that a client may be sent more than one offer in response to

its initial lease discovery message.   It may be that one of these

offers is preferable to the other (e.g., one offer may have the

address the client previously used, and the other may not).

.PP

The

.I select-timeout

is the time after the client sends its first lease discovery request

at which it stops waiting for offers from servers, assuming that it

has received at least one such offer.   If no offers have been

received by the time the

.I select-timeout

has expired, the client will accept the first offer that arrives.

.PP

By default, the select-timeout is zero seconds - that is, the client

will take the first offer it sees.

.PP

.I The

.B reboot

.I statement

.PP

 \fBreboot \fItime\fR\fB;\fR

.PP

When the client is restarted, it first tries to reacquire the last

address it had.   This is called the INIT-REBOOT state.   If it is

still attached to the same network it was attached to when it last

ran, this is the quickest way to get started.   The

.I reboot

statement sets the time that must elapse after the client first tries

to reacquire its old address before it gives up and tries to discover

a new address.   By default, the reboot timeout is ten seconds.

.PP

.I The

.B backoff-cutoff

.I statement

.PP

 \fBbackoff-cutoff \fItime\fR\fB;\fR

.PP

The client uses an exponential backoff algorithm with some randomness,

so that if many clients try to configure themselves at the same time,

they will not make their requests in lockstep.   The

.I backoff-cutoff

statement determines the maximum amount of time that the client is

allowed to back off, the actual value will be evaluated randomly between

1/2 to 1 1/2 times the \fItime\fR specified.   It defaults to two minutes.

.PP

.I The

.B initial-interval

.I statement

.PP

 \fBinitial-interval \fItime\fR\fB;\fR

.PP

The

.I initial-interval

statement sets the amount of time between the first attempt to reach a

server and the second attempt to reach a server.  Each time a message

is sent, the interval between messages is incremented by twice the

current interval multiplied by a random number between zero and one.

If it is greater than the backoff-cutoff amount, it is set to that

amount.  It defaults to ten seconds.

.SH LEASE REQUIREMENTS AND REQUESTS

The DHCP protocol allows the client to request that the server send it

specific information, and not send it other information that it is not

prepared to accept.   The protocol also allows the client to reject

offers from servers if they don't contain information the client

needs, or if the information provided is not satisfactory.

.PP

There is a variety of data contained in offers that DHCP servers send

to DHCP clients.  The data that can be specifically requested is what

are called \fIDHCP Options\fR.  DHCP Options are defined in

 \fBdhcp-options(5)\fR.

.PP

.I The

.B request

.I statement

.PP

 \fBrequest [ \fIoption\fR ] [\fB,\fI ... \fIoption\fR ]\fB;\fR

.PP

The request statement causes the client to request that any server

responding to the client send the client its values for the specified

options.   Only the option names should be specified in the request

statement - not option parameters.   By default, the DHCP server

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

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

and ntp-servers options.

.PP

In some cases, it may be desirable to send no parameter request list

at all.   To do this, simply write the request statement but specify

no parameters:

.PP

.nf

	request;

.fi

.PP

.I The

.B require

.I statement

.PP

 \fBrequire [ \fIoption\fR ] [\fB,\fI ... \fIoption ]\fB;\fR

.PP

The require statement lists options that must be sent in order for an

offer to be accepted.   Offers that do not contain all the listed

options will be ignored.

.PP

.I The

.B send

.I statement

.PP

 \fBsend { [ \fIoption declaration\fR ]

[\fB,\fI ... \fIoption declaration\fR ]\fB}\fR

.PP

The send statement causes the client to send the specified options to

the server with the specified values.  These are full option

declarations as described in \fBdhcp-options(5)\fR.  Options that are

always sent in the DHCP protocol should not be specified here, except

that the client can specify a \fBrequested-lease-time\fR option other

than the default requested lease time, which is two hours.  The other

obvious use for this statement is to send information to the server

that will allow it to differentiate between this client and other

clients or kinds of clients.

.SH DYNAMIC DNS

The client now has some very limited support for doing DNS updates

when a lease is acquired.   This is prototypical, and probably doesn't

do what you want.   It also only works if you happen to have control

over your DNS server, which isn't very likely.

.PP

To make it work, you have to declare a key and zone as in the DHCP

server (see \fBdhcpd.conf\fR(5) for details).   You also need to

configure the fqdn option on the client, as follows:

.PP

.nf

  send fqdn.fqdn "grosse.fugue.com.";

  send fqdn.encoded on;

  send fqdn.server-update off;

.fi

.PP

The \fIfqdn.fqdn\fR option \fBMUST\fR be a fully-qualified domain

name.   You \fBMUST\fR define a zone statement for the zone to be

updated.   The \fIfqdn.encoded\fR option may need to be set to

\fIon\fR or \fIoff\fR, depending on the DHCP server you are using.

.PP

.I The

.B do-forward-updates

.I statement

.PP

 \fBdo-forward-updates [ \fIflag\fR ] \fB;\fR

.PP

If you want to do DNS updates in the DHCP client

script (see \fBdhclient-script(8)\fR) rather than having the

DHCP client do the update directly (for example, if you want to

use SIG(0) authentication, which is not supported directly by the

DHCP client, you can instruct the client not to do the update using

the \fBdo-forward-updates\fR statement.   \fIFlag\fR should be \fBtrue\fR

if you want the DHCP client to do the update, and \fBfalse\fR if

you don't want the DHCP client to do the update.   By default, the DHCP

client will do the DNS update.

.SH OPTION MODIFIERS

In some cases, a client may receive option data from the server which

is not really appropriate for that client, or may not receive

information that it needs, and for which a useful default value

exists.   It may also receive information which is useful, but which

needs to be supplemented with local information.   To handle these

needs, several option modifiers are available.

.PP

.I The

.B default

.I statement

.PP

 \fBdefault [ \fIoption declaration\fR ] \fB;\fR

.PP

If for some option the client should use the value supplied by

the server, but needs to use some default value if no value was supplied

by the server, these values can be defined in the

.B default

statement.

.PP

.I The

.B supersede

.I statement

.PP

 \fBsupersede [ \fIoption declaration\fR ] \fB;\fR

.PP

If for some option the client should always use a locally-configured

value or values rather than whatever is supplied by the server, these

values can be defined in the 

.B supersede

statement.

.PP

.I The

.B prepend

.I statement

.PP

 \fBprepend [ \fIoption declaration\fR ] \fB;\fR

.PP

If for some set of options the client should use a value you

supply, and then use the values supplied by

the server, if any, these values can be defined in the

.B prepend

statement.   The

.B prepend

statement can only be used for options which

allow more than one value to be given.   This restriction is not

enforced - if you ignore it, the behaviour will be unpredictable.

.PP

.I The

.B append

.I statement

.PP

 \fBappend [ \fIoption declaration\fR ] \fB;\fR

.PP

If for some set of options the client should first use the values

supplied by the server, if any, and then use values you supply, these

values can be defined in the

.B append

statement.   The

.B append

statement can only be used for options which

allow more than one value to be given.   This restriction is not

enforced - if you ignore it, the behaviour will be unpredictable.

.SH LEASE DECLARATIONS

.PP

.I The

.B lease

.I declaration

.PP

 \fBlease {\fR \fIlease-declaration\fR [ ... \fIlease-declaration ] \fB}\fR

.PP

The DHCP client may decide after some period of time (see \fBPROTOCOL

TIMING\fR) that it is not going to succeed in contacting a

server.   At that time, it consults its own database of old leases and

tests each one that has not yet timed out by pinging the listed router

for that lease to see if that lease could work.   It is possible to

define one or more \fIfixed\fR leases in the client configuration file

for networks where there is no DHCP or BOOTP service, so that the

client can still automatically configure its address.   This is done

with the

.B lease

statement.

.PP

NOTE: the lease statement is also used in the dhclient.leases file in

order to record leases that have been received from DHCP servers.

Some of the syntax for leases as described below is only needed in the

dhclient.leases file.   Such syntax is documented here for

completeness.

.PP

A lease statement consists of the lease keyword, followed by a left

curly brace, followed by one or more lease declaration statements,

followed by a right curly brace.   The following lease declarations

are possible:

.PP

 \fBbootp;\fR

.PP

The

.B bootp

statement is used to indicate that the lease was acquired using the

BOOTP protocol rather than the DHCP protocol.   It is never necessary

to specify this in the client configuration file.   The client uses

this syntax in its lease database file.

.PP

 \fBinterface\fR \fB"\fR\fIstring\fR\fB";\fR

.PP

The

.B interface

lease statement is used to indicate the interface on which the lease

is valid.   If set, this lease will only be tried on a particular

interface.   When the client receives a lease from a server, it always

records the interface number on which it received that lease.

If predefined leases are specified in the dhclient.conf file, the

interface should also be specified, although this is not required.

.PP

 \fBfixed-address\fR \fIip-address\fR\fB;\fR

.PP

The

.B fixed-address

statement is used to set the ip address of a particular lease.   This

is required for all lease statements.   The IP address must be

specified as a dotted quad (e.g., 12.34.56.78).

.PP

 \fBfilename "\fR\fIstring\fR\fB";\fR

.PP

The

.B filename

statement specifies the name of the boot filename to use.   This is

not used by the standard client configuration script, but is included

for completeness.

.PP

 \fBserver-name "\fR\fIstring\fR\fB";\fR

.PP

The

.B server-name

statement specifies the name of the boot server name to use.   This is

also not used by the standard client configuration script.

.PP

 \fBoption\fR \fIoption-declaration\fR\fB;\fR

.PP

The

.B option

statement is used to specify the value of an option supplied by the

server, or, in the case of predefined leases declared in

dhclient.conf, the value that the user wishes the client configuration

script to use if the predefined lease is used.

.PP

 \fBscript "\fIscript-name\fB";\fR

.PP

The

.B script

statement is used to specify the pathname of the dhcp client

configuration script.  This script is used by the dhcp client to set

each interface's initial configuration prior to requesting an address,

to test the address once it has been offered, and to set the

interface's final configuration once a lease has been acquired.   If

no lease is acquired, the script is used to test predefined leases, if

any, and also called once if no valid lease can be identified.   For

more information, see

.B dhclient-script(8).

.PP

 \fBvendor option space "\fIname\fB";\fR

.PP

The

.B vendor option space

statement is used to specify which option space should be used for

decoding the vendor-encapsulate-options option if one is received.

The \fIdhcp-vendor-identifier\fR can be used to request a specific

class of vendor options from the server.   See

.B dhcp-options(5)

for details.

.PP

 \fBmedium "\fImedia setup\fB";\fR

.PP

The

.B medium

statement can be used on systems where network interfaces cannot

automatically determine the type of network to which they are

connected.  The media setup string is a system-dependent parameter

which is passed to the dhcp client configuration script when

initializing the interface.  On Unix and Unix-like systems, the

argument is passed on the ifconfig command line when configuring the

interface.

.PP

The dhcp client automatically declares this parameter if it uses a

media type (see the

.B media

statement) when configuring the interface in order to obtain a lease.

This statement should be used in predefined leases only if the network

interface requires media type configuration.

.PP

 \fBrenew\fR \fIdate\fB;\fR

.PP

 \fBrebind\fR \fIdate\fB;\fR

.PP

 \fBexpire\fR \fIdate\fB;\fR

.PP

The \fBrenew\fR statement defines the time at which the dhcp client

should begin trying to contact its server to renew a lease that it is

using.   The \fBrebind\fR statement defines the time at which the dhcp

client should begin to try to contact \fIany\fR dhcp server in order

to renew its lease.   The \fBexpire\fR statement defines the time at

which the dhcp client must stop using a lease if it has not been able

to contact a server in order to renew it.

.PP

These declarations are automatically set in leases acquired by the

DHCP client, but must also be configured in predefined leases - a

predefined lease whose expiry time has passed will not be used by the

DHCP client.

.PP

Dates are specified as follows:

.PP

 \fI<weekday> <year>\fB/\fI<month>\fB/\fI<day>

<hour>\fB:\fI<minute>\fB:\fI<second>\fR

.PP

The weekday is present to make it easy for a human to tell when a

lease expires - it's specified as a number from zero to six, with zero

being Sunday.  When declaring a predefined lease, it can always be

specified as zero.  The year is specified with the century, so it

should generally be four digits except for really long leases.  The

month is specified as a number starting with 1 for January.  The day

of the month is likewise specified starting with 1.  The hour is a

number between 0 and 23, the minute a number between 0 and 59, and the

second also a number between 0 and 59.

.SH ALIAS DECLARATIONS

 \fBalias { \fI declarations ... \fB}\fR

.PP

Some DHCP clients running TCP/IP roaming protocols may require that in

addition to the lease they may acquire via DHCP, their interface also

be configured with a predefined IP alias so that they can have a

permanent IP address even while roaming.   The Internet Systems

Consortium DHCP client doesn't support roaming with fixed addresses

directly, but in order to facilitate such experimentation, the dhcp

client can be set up to configure an IP alias using the

.B alias

declaration.

.PP

The alias declaration resembles a lease declaration, except that

options other than the subnet-mask option are ignored by the standard

client configuration script, and expiry times are ignored.  A typical

alias declaration includes an interface declaration, a fixed-address

declaration for the IP alias address, and a subnet-mask option

declaration.   A medium statement should never be included in an alias

declaration.

.SH OTHER DECLARATIONS

 \fBreject \fIcidr-ip-address\fR [\fB,\fR \fI...\fB \fIcidr-ip-address\fR ] \fB;\fR

.PP

The

.B reject

statement causes the DHCP client to reject offers from

servers whose server identifier matches any of the specified hosts or

subnets.  This can be used to avoid being configured by rogue or

misconfigured dhcp servers, although it should be a last resort -

better to track down the bad DHCP server and fix it.

.PP

The \fIcidr-ip-address\fR configuration type is of the

form \fIip-address\fR[\fB/\fIprefixlen\fR], where \fIip-address\fR is a

dotted quad IP address, and \fRprefixlen\fR is the CIDR prefix length of

the subnet, counting the number of significant bits in the netmask starting

from the leftmost end.  Example configuration syntax:

.PP

 \fIreject\fR 192.168.0.0\fB/\fR16\fB,\fR 10.0.0.5\fB;\fR

.PP

The above example would cause offers from any server identifier in the

entire RFC 1918 "Class C" network 192.168.0.0/16, or the specific

single address 10.0.0.5, to be rejected.

.PP

 \fBinterface "\fIname\fB" { \fIdeclarations ... \fB }

.PP

A client with more than one network interface may require different

behaviour depending on which interface is being configured.   All

timing parameters and declarations other than lease and alias

declarations can be enclosed in an interface declaration, and those

parameters will then be used only for the interface that matches the

specified name.   Interfaces for which there is no interface

declaration will use the parameters declared outside of any interface

declaration, or the default settings.

.PP

.B Note well:

ISC dhclient only maintains one list of interfaces, which is either

determined at startup from command line arguments, or otherwise is

autodetected.  If you supplied the list of interfaces on the command

line, this configuration clause will add the named interface to the

list in such a way that will cause it to be configured by DHCP.  Which

may not be the result you had intended.  This is an undesirable side

effect that will be addressed in a future release.

.PP

 \fBpseudo "\fIname\fR" "\fIreal-name\fB" { \fIdeclarations ... \fB }

.PP

Under some circumstances it can be useful to declare a pseudo-interface 

and have the DHCP client acquire a configuration for that interface.

Each interface that the DHCP client is supporting normally has a DHCP

client state machine running on it to acquire and maintain its lease.

A pseudo-interface is just another state machine running on the

interface named \fIreal-name\fR, with its own lease and its own

state.   If you use this feature, you must provide a client identifier

for both the pseudo-interface and the actual interface, and the two

identifiers must be different.   You must also provide a separate

client script for the pseudo-interface to do what you want with the IP

address.   For example:

.PP

.nf

	interface "ep0" {

		send dhcp-client-identifier "my-client-ep0";

	}

	pseudo "secondary" "ep0" {

		send dhcp-client-identifier "my-client-ep0-secondary";

		script "/etc/dhclient-secondary";

	}

.fi

.PP

The client script for the pseudo-interface should not configure the

interface up or down - essentially, all it needs to handle are the

states where a lease has been acquired or renewed, and the states

where a lease has expired.   See \fBdhclient-script(8)\fR for more

information.

.PP

 \fBmedia "\fImedia setup\fB"\fI [ \fB, "\fImedia setup\fB", \fI... ]\fB;\fR

.PP

The

.B media

statement defines one or more media configuration parameters which may

be tried while attempting to acquire an IP address.   The dhcp client

will cycle through each media setup string on the list, configuring

the interface using that setup and attempting to boot, and then trying

the next one.   This can be used for network interfaces which aren't

capable of sensing the media type unaided - whichever media type

succeeds in getting a request to the server and hearing the reply is

probably right (no guarantees).

.PP

The media setup is only used for the initial phase of address

acquisition (the DHCPDISCOVER and DHCPOFFER packets).   Once an

address has been acquired, the dhcp client will record it in its lease

database and will record the media type used to acquire the address.

Whenever the client tries to renew the lease, it will use that same

media type.   The lease must expire before the client will go back to

cycling through media types.

.PP

 \fBbootp-broadcast-always;\fR

.PP

The

.B bootp-broadcast-always

statement instructs dhclient to always set the bootp broadcast flag in

request packets, so that servers will always broadcast replies.

This is equivalent to supplying the dhclient -B argument, and has

the same effect as specifying 'always-broadcast' in the server's dhcpd.conf.

This option is provided as an extension to enable dhclient to work

on IBM s390 Linux guests.

.PP

.SH SAMPLE

The following configuration file is used on a laptop running NetBSD

1.3.   The laptop has an IP alias of 192.5.5.213, and has one

interface, ep0 (a 3com 3C589C).   Booting intervals have been

shortened somewhat from the default, because the client is known to

spend most of its time on networks with little DHCP activity.   The

laptop does roam to multiple networks.

.nf

timeout 60;

retry 60;

reboot 10;

select-timeout 5;

initial-interval 2;

reject 192.33.137.209;

interface "ep0" {

    send host-name "andare.fugue.com";

    send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;

    send dhcp-lease-time 3600;

    supersede domain-name "fugue.com rc.vix.com home.vix.com";

    prepend domain-name-servers 127.0.0.1;

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

	    domain-name, domain-name-servers, host-name;

    require subnet-mask, domain-name-servers;

    script "CLIENTBINDIR/dhclient-script";

    media "media 10baseT/UTP", "media 10base2/BNC";

}

alias {

  interface "ep0";

  fixed-address 192.5.5.213;

  option subnet-mask 255.255.255.255;

}

.fi

This is a very complicated dhclient.conf file - in general, yours

should be much simpler.   In many cases, it's sufficient to just

create an empty dhclient.conf file - the defaults are usually fine.

.SH SEE ALSO

dhcp-options(5), dhcp-eval(5), dhclient.leases(5), dhcpd(8), dhcpd.conf(5),

RFC2132, RFC2131.

.SH AUTHOR

.B dhclient(8)

was written by Ted Lemon

under a contract with Vixie Labs.   Funding

for this project was provided by Internet Systems Consortium.

Information about Internet Systems Consortium can be found at

.B http://www.isc.org.