.\"	dhclient.conf.5.\".\" Copyright (c) 1996-2001 Internet Software Consortium..\" Redistribution and use in source and binary forms, with or without.\" modification, are permitted provided that the following conditions.\" are met:.\".\" 1. Redistributions of source code must retain the above copyright.\"    notice, this list of conditions and the following disclaimer..\" 2. Redistributions in binary form must reproduce the above copyright.\"    notice, this list of conditions and the following disclaimer in the.\"    documentation and/or other materials provided with the distribution..\" 3. Neither the name of The Internet Software Consortium nor the names.\"    of its contributors may be used to endorse or promote products derived.\"    from this software without specific prior written permission..\".\" THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM AND.\" CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE.\" DISCLAIMED.  IN NO EVENT SHALL THE INTERNET SOFTWARE CONSORTIUM OR.\" CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT.\" LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF.\" USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND.\" ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT.\" OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF.\" SUCH DAMAGE..\".\" This software has been written for the Internet Software Consortium.\" by Ted Lemon in cooperation with Vixie Enterprises and Nominum, Inc..\" To learn more about the Internet Software Consortium, see.\" ``http://www.isc.org/''.  To learn more about Vixie Enterprises,.\" see ``http://www.vix.com''.   To learn more about Nominum, Inc., see.\" ``http://www.nominum.com''..TH dhclient.conf 5.SH NAMEdhclient.conf - DHCP client configuration file.SH DESCRIPTIONThe dhclient.conf file contains configuration information for.IR dhclient,the Internet Software Consortium DHCP Client..PPThe dhclient.conf file is a free-form ASCII text file.   It is parsed bythe recursive-descent parser built into dhclient.   The file may containextra tabs and newlines for formatting purposes.  Keywords in the fileare case-insensitive.   Comments may be placed anywhere within thefile (except within quotes).   Comments begin with the # character andend at the end of the line..PPThe dhclient.conf file can be used to configure the behaviour of theclient in a wide variety of ways: protocol timing, informationrequested 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, orvalues to prepend or append to information provided by the server.The configuration file can also be preinitialized with addresses touse on networks that don't have DHCP servers..SH PROTOCOL TIMINGThe timing behaviour of the client need not be configured by the user.If no timing configuration is provided by the user, a fairlyreasonable timing behaviour will be used by default - one whichresults in fairly timely updates without placing an inordinate load onthe server..PPThe following statements can be used to adjust the timing behaviour ofthe DHCP client if required, however:.PP.I The.B timeout.I statement.PP.B timeout.I time.B ;.PPThe.I timeoutstatement determines the amount of time that must pass between thetime that the client begins to try to determine its address and thetime that it decides that it's not going to be able to contact aserver.   By default, this timeout is sixty seconds.   After thetimeout has passed, if there are any static leases defined in theconfiguration file, or any leases remaining in the lease database thathave not yet expired, the client will loop through these leasesattempting to validate them, and if it finds one that appears to bevalid, it will use that lease's address.   If there are no validstatic leases or unexpired leases in the lease database, the clientwill restart the protocol after the defined retry interval..PP.I The.B retry.I statement.PP \fBretry \fItime\fR\fB;\fR.PPThe.I retrystatement determines the time that must pass after the client hasdetermined that there is no DHCP server present before it tries againto 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.PPIt is possible (some might say desirable) for there to be more thanone DHCP server serving any given network.   In this case, it ispossible that a client may be sent more than one offer in response toits initial lease discovery message.   It may be that one of theseoffers is preferable to the other (e.g., one offer may have theaddress the client previously used, and the other may not)..PPThe.I select-timeoutis the time after the client sends its first lease discovery requestat which it stops waiting for offers from servers, assuming that ithas received at least one such offer.   If no offers have beenreceived by the time the.I select-timeouthas expired, the client will accept the first offer that arrives..PPBy default, the select-timeout is zero seconds - that is, the clientwill take the first offer it sees..PP.I The.B reboot.I statement.PP \fBreboot \fItime\fR\fB;\fR.PPWhen the client is restarted, it first tries to reacquire the lastaddress it had.   This is called the INIT-REBOOT state.   If it isstill attached to the same network it was attached to when it lastran, this is the quickest way to get started.   The.I rebootstatement sets the time that must elapse after the client first triesto reacquire its old address before it gives up and tries to discovera 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.PPThe 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-cutoffstatement determines the maximum amount of time that the client isallowed to back off.   It defaults to two minutes..PP.I The.B initial-interval.I statement.PP \fBinitial-interval \fItime\fR\fB;\fR.PPThe.I initial-intervalstatement sets the amount of time between the first attempt to reach aserver and the second attempt to reach a server.  Each time a messageis sent, the interval between messages is incremented by twice thecurrent interval multiplied by a random number between zero and one.If it is greater than the backoff-cutoff amount, it is set to thatamount.  It defaults to ten seconds..SH LEASE REQUIREMENTS AND REQUESTSThe DHCP protocol allows the client to request that the server send itspecific information, and not send it other information that it is notprepared to accept.   The protocol also allows the client to rejectoffers from servers if they don't contain information the clientneeds, or if the information provided is not satisfactory..PPThere is a variety of data contained in offers that DHCP servers sendto DHCP clients.  The data that can be specifically requested is whatare 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.PPThe request statement causes the client to request that any serverresponding to the client send the client its values for the specifiedoptions.   Only the option names should be specified in the requeststatement - not option parameters.   By default, the DHCP serverrequests the subnet-mask, broadcast-address, time-offset, routers,domain-name, domain-name-servers and host-name options. .PPIn some cases, it may be desirable to send no parameter request listat all.   To do this, simply write the request statement but specifyno parameters:.PP.nf	request;.fi.PP.I The.B require.I statement.PP \fBrequire [ \fIoption\fR ] [\fB,\fI ... \fIoption ]\fB;\fR.PPThe require statement lists options that must be sent in order for anoffer to be accepted.   Offers that do not contain all the listedoptions will be ignored..PP.I The.B send.I statement.PP \fBsend { [ \fIoption declaration\fR ][\fB,\fI ... \fIoption declaration\fR ]\fB}\fR.PPThe send statement causes the client to send the specified options tothe server with the specified values.  These are full optiondeclarations as described in \fBdhcp-options(5)\fR.  Options that arealways sent in the DHCP protocol should not be specified here, exceptthat the client can specify a \fBrequested-lease-time\fR option otherthan the default requested lease time, which is two hours.  The otherobvious use for this statement is to send information to the serverthat will allow it to differentiate between this client and otherclients or kinds of clients..SH DYNAMIC DNSThe client now has some very limited support for doing DNS updateswhen a lease is acquired.   This is prototypical, and probably doesn'tdo what you want.   It also only works if you happen to have controlover your DNS server, which isn't very likely..PPTo make it work, you have to declare a key and zone as in the DHCPserver (see \fBdhcpd.conf\fR(5) for details).   You also need toconfigure 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.PPThe \fIfqdn.fqdn\fR option \fBMUST\fR be a fully-qualified domainname.   You \fBMUST\fR define a zone statement for the zone to beupdated.   The \fIfqdn.encoded\fR option may need to be set to\fIon\fR or \fIoff\fR, depending on the DHCP server you are using..SH OPTION MODIFIERSIn some cases, a client may receive option data from the server whichis not really appropriate for that client, or may not receiveinformation that it needs, and for which a useful default valueexists.   It may also receive information which is useful, but whichneeds to be supplemented with local information.   To handle theseneeds, several option modifiers are available..PP.I The.B default.I statement.PP \fBdefault [ \fIoption declaration\fR ] \fB;\fR.PPIf for some option the client should use the value supplied bythe server, but needs to use some default value if no value was suppliedby the server, these values can be defined in the.B defaultstatement..PP.I The.B supersede.I statement.PP \fBsupersede [ \fIoption declaration\fR ] \fB;\fR.PPIf for some option the client should always use a locally-configuredvalue or values rather than whatever is supplied by the server, thesevalues can be defined in the .B supersedestatement..PP.I The.B prepend.I statement.PP \fBprepend [ \fIoption declaration\fR ] \fB;\fR.PPIf for some set of options the client should use a value yousupply, and then use the values supplied bythe server, if any, these values can be defined in the.B prependstatement.   The.B prependstatement can only be used for options whichallow more than one value to be given.   This restriction is notenforced - if you ignore it, the behaviour will be unpredictable..PP.I The.B append.I statement.PP \fBappend [ \fIoption declaration\fR ] \fB;\fR.PPIf for some set of options the client should first use the valuessupplied by the server, if any, and then use values you supply, thesevalues can be defined in the