1.\" $OpenBSD: src/sbin/dhclient/dhclient.conf.5,v 1.21 2011/04/09 19:53:00 krw Exp $ 2.\" 3.\" Copyright (c) 1997 The Internet Software Consortium. 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of The Internet Software Consortium nor the names 16.\" of its contributors may be used to endorse or promote products derived 17.\" from this software without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM AND 20.\" CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, 21.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF 22.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 23.\" DISCLAIMED. IN NO EVENT SHALL THE INTERNET SOFTWARE CONSORTIUM OR 24.\" CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 25.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 26.\" LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF 27.\" USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND 28.\" ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, 29.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT 30.\" OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 31.\" SUCH DAMAGE. 32.\" 33.\" This software has been written for the Internet Software Consortium 34.\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie 35.\" Enterprises. To learn more about the Internet Software Consortium, 36.\" see ``http://www.isc.org/isc''. To learn more about Vixie 37.\" Enterprises, see ``http://www.vix.com''. 38.\" 39.Dd August 4, 2012 40.Dt DHCLIENT.CONF 5 41.Os 42.Sh NAME 43.Nm dhclient.conf 44.Nd DHCP client configuration file 45.Sh DESCRIPTION 46The 47.Nm 48file contains configuration information for 49.Xr dhclient 8 , 50the Internet Software Consortium DHCP Client. 51.Pp 52The 53.Nm 54file is a free-form ASCII text file. 55It is parsed by the recursive-descent parser built into 56.Xr dhclient 8 . 57The file may contain extra tabs and newlines for formatting purposes. 58Keywords in the file are case-insensitive. 59Comments may be placed anywhere within the file (except within quotes). 60Comments begin with the 61.Sq # 62character and end at the end of the line. 63.Pp 64The 65.Nm 66file can be used to configure the behaviour of the client in a wide variety 67of ways: protocol timing, information requested from the server, information 68required of the server, defaults to use if the server does not provide 69certain information, values with which to override information provided by 70the server, or values to prepend or append to information provided by the 71server. 72The configuration file can also be preinitialized with addresses to 73use on networks that don't have DHCP servers. 74.Sh PROTOCOL TIMING 75The timing behaviour of the client need not be configured by the user. 76If no timing configuration is provided by the user, a fairly 77reasonable timing behaviour will be used by default \- one which 78results in fairly timely updates without placing an inordinate load on 79the server. 80.Pp 81The following statements can be used to adjust the timing behaviour of 82the DHCP client if required, however: 83.Bl -tag -width Ds 84.It Ic timeout Ar time ; 85The 86.Ic timeout 87statement determines the amount of time that must pass between the 88time that the client begins to try to determine its address and the 89time that it decides that it's not going to be able to contact a server. 90By default, this timeout is sixty seconds. 91After the timeout has passed, if there are any static leases defined in the 92configuration file, or any leases remaining in the lease database that 93have not yet expired, the client will loop through these leases 94attempting to validate them, and if it finds one that appears to be 95valid, it will use that lease's address. 96If there are no valid static leases or unexpired leases in the lease database, 97the client will restart the protocol after the defined retry interval. 98.It Ic retry Ar time ; 99The 100.Ic retry 101statement determines the time that must pass after the client has 102determined that there is no DHCP server present before it tries again 103to contact a DHCP server. 104By default, this is five minutes. 105.It Ic select-timeout Ar time ; 106It is possible (some might say desirable) for there to be more than 107one DHCP server serving any given network. 108In this case, it is possible that a client may be sent more than one offer 109in response to its initial lease discovery message. 110It may be that one of these offers is preferable to the other 111(e.g., one offer may have the address the client previously used, 112and the other may not). 113.Pp 114The 115.Ic select-timeout 116is the time after the client sends its first lease discovery request 117at which it stops waiting for offers from servers, assuming that it 118has received at least one such offer. 119If no offers have been received by the time the 120.Ic select-timeout 121has expired, the client will accept the first offer that arrives. 122.Pp 123By default, the 124.Ic select-timeout 125is zero seconds \- that is, the client will take the first offer it sees. 126.It Ic reboot Ar time ; 127When the client is restarted, it first tries to reacquire the last 128address it had. 129This is called the INIT-REBOOT state. 130If it is still attached to the same network it was attached to when it last 131ran, this is the quickest way to get started. 132The 133.Ic reboot 134statement sets the time that must elapse after the client first tries 135to reacquire its old address before it gives up and tries to discover 136a new address. 137By default, the reboot timeout is ten seconds. 138.It Ic backoff-cutoff Ar time ; 139The client uses an exponential backoff algorithm with some randomness, 140so that if many clients try to configure themselves at the same time, 141they will not make their requests in lockstep. 142The 143.Ic backoff-cutoff 144statement determines the maximum amount of time that the client is 145allowed to back off. 146It defaults to fifteen seconds. 147.It Ic initial-interval Ar time ; 148The 149.Ic initial-interval 150statement sets the amount of time between the first attempt to reach a 151server and the second attempt to reach a server. 152Each time a message is sent, the interval between messages is incremented by 153twice the current interval multiplied by a random number between zero and one. 154If it is greater than the backoff-cutoff amount, it is set to that 155amount. 156It defaults to three seconds. 157.It Ic link-timeout Ar time ; 158The 159.Ic link-timeout 160statement sets the amount of time to wait for an interface link before timing 161out. 162The default value is ten seconds. 163The value zero requests that 164dhclient not wait for a link state change before timing out. 165.El 166.Sh LEASE REQUIREMENTS AND REQUESTS 167The DHCP protocol allows the client to request that the server send it 168specific information, and not send it other information that it is not 169prepared to accept. 170The protocol also allows the client to reject offers from servers if they 171don't contain information the client needs, or if the information provided 172is not satisfactory. 173.Pp 174There is a variety of data contained in offers that DHCP servers send 175to DHCP clients. 176The data that can be specifically requested is what are called 177.Em DHCP Options . 178DHCP Options are defined in 179.Xr dhcp-options 5 . 180.Bl -tag -width Ds 181.It Xo 182.Ic request Op Ar option 183.Oo , Ar ... option Oc ; 184.Xc 185The 186.Ic request 187statement causes the client to request that any server responding to the 188client send the client its values for the specified options. 189Only the option names should be specified in the request statement \- not 190option parameters. 191.It Xo 192.Ic require Op Ar option 193.Oo , Ar ... option Oc ; 194.Xc 195The 196.Ic require 197statement lists options that must be sent in order for an offer to be accepted. 198Offers that do not contain all the listed options will be ignored. 199.It Xo 200.Ic send No { Op Ar option declaration 201.Oo , Ar ... option declaration Oc } 202.Xc 203The 204.Ic send 205statement causes the client to send the specified options to the server with 206the specified values. 207These are full option declarations as described in 208.Xr dhcp-options 5 . 209Options that are always sent in the DHCP protocol should not be specified 210here. 211One use for this statement is to send information to the server 212that will allow it to differentiate between this client and other 213clients or kinds of clients. 214.El 215.Sh OPTION MODIFIERS 216Options in the lease can be modified before being passed to the client 217configuration script, 218.Xr dhclient-script 8 . 219.Pp 220The default client configuration script 221processes only options 1 (subnet 222mask), 3 (routers), 6 (domain name servers), 15 (domain-name). 223Use of option modifiers on other options will have no effect unless 224.Xr dhclient-script 8 225the client configuration script is modified. 226.Pp 227Several option modifiers are available. 228.Bl -tag -width Ds 229.It Xo 230.Ic default No { Op Ar option declaration 231.Oo , Ar ... option declaration Oc } 232.Xc 233If for some set of options the client should use the value supplied by 234the server, but needs to use some default value if no value was supplied 235by the server, these values can be defined in the 236.Ic default 237statement. 238.It Xo 239.Ic supersede No { Op Ar option declaration 240.Oo , Ar ... option declaration Oc } 241.Xc 242If for some set of options the client should always use its own value 243rather than any value supplied by the server, these values can be defined 244in the 245.Ic supersede 246statement. 247.It Xo 248.Ic prepend No { Op Ar option declaration 249.Oo , Ar ... option declaration Oc } 250.Xc 251If for some set of options the client should use a value you supply, 252and then use the values supplied by the server, if any, 253these values can be defined in the 254.Ic prepend 255statement. 256The 257.Ic prepend 258statement can only be used for options which allow more than one value to 259be given. 260This restriction is not enforced \- if violated, the results are unpredictable. 261.It Xo 262.Ic append No { Op Ar option declaration 263.Oo , Ar ... option declaration Oc } 264.Xc 265If for some set of options the client should first use the values 266supplied by the server, if any, and then use values you supply, these 267values can be defined in the 268.Ic append 269statement. 270The 271.Ic append 272statement can only be used for options which allow more than one value to 273be given. 274This restriction is not enforced \- if you ignore it, 275the behaviour will be unpredictable. 276.El 277.Sh LEASE DECLARATIONS 278The lease declaration: 279.Pp 280.D1 Ic lease No { Ar lease-declaration Oo Ar ... lease-declaration Oc } 281.Pp 282The DHCP client may decide after some period of time (see 283.Sx PROTOCOL TIMING ) 284that it is not going to succeed in contacting a server. 285At that time, it consults its own database of old leases and tests each one 286that has not yet timed out by pinging the listed router for that lease to 287see if that lease could work. 288It is possible to define one or more 289.Em fixed 290leases in the client configuration file for networks where there is no DHCP 291or BOOTP service, so that the client can still automatically configure its 292address. 293This is done with the 294.Ic lease 295statement. 296.Pp 297NOTE: the lease statement is also used in the 298.Pa dhclient.leases 299file in order to record leases that have been received from DHCP servers. 300Some of the syntax for leases as described below is only needed in the 301.Pa dhclient.leases 302file. 303Such syntax is documented here for completeness. 304.Pp 305A lease statement consists of the lease keyword, followed by a left 306curly brace, followed by one or more lease declaration statements, 307followed by a right curly brace. 308The following lease declarations are possible: 309.Bl -tag -width Ds 310.It Ic bootp ; 311The 312.Ic bootp 313statement is used to indicate that the lease was acquired using the 314BOOTP protocol rather than the DHCP protocol. 315It is never necessary to specify this in the client configuration file. 316The client uses this syntax in its lease database file. 317.It Ic interface Ar \&"string\&" ; 318The 319.Ic interface 320lease statement is used to indicate the interface on which the lease is valid. 321If set, this lease will only be tried on a particular interface. 322When the client receives a lease from a server, it always records the 323interface number on which it received that lease. 324If predefined leases are specified in the 325.Nm 326file, the interface should also be specified, although this is not required. 327.It Ic fixed-address Ar ip-address ; 328The 329.Ic fixed-address 330statement is used to set the IP address of a particular lease. 331This is required for all lease statements. 332The IP address must be specified as a dotted quad (e.g., 12.34.56.78). 333.It Ic filename Ar \&"string\&" ; 334The 335.Ic filename 336statement specifies the name of the boot filename to use. 337This is not used by the standard client configuration script, but is 338included for completeness. 339.It Ic server-name Ar \&"string\&" ; 340The 341.Ic server-name 342statement specifies the name of the boot server name to use. 343This is also not used by the standard client configuration script. 344.It Ic option Ar option-declaration ; 345The 346.Ic option 347statement is used to specify the value of an option supplied by the server, 348or, in the case of predefined leases declared in 349.Nm , 350the value that the user wishes the client configuration script to use if the 351predefined lease is used. 352.It Ic renew Ar date ; 353.It Ic rebind Ar date ; 354.It Ic expire Ar date ; 355The 356.Ic renew 357statement defines the time at which the DHCP client should begin trying to 358contact its server to renew a lease that it is using. 359The 360.Ic rebind 361statement defines the time at which the DHCP client should begin to try to 362contact 363.Em any 364DHCP server in order to renew its lease. 365The 366.Ic expire 367statement defines the time at which the DHCP client must stop using a lease 368if it has not been able to contact a server in order to renew it. 369.El 370.Pp 371These declarations are automatically set in leases acquired by the 372DHCP client, but must also be configured in predefined leases \- a 373predefined lease whose expiry time has passed will not be used by the 374DHCP client. 375.Pp 376Dates are specified as follows: 377.Bd -ragged -offset indent 378.Ar <weekday> 379.Sm off 380.Ar <year> No / Ar <month> No / Ar <day> 381.Ar <hour> : <minute> : <second> 382.Sm on 383.Ed 384.Pp 385The weekday is present to make it easy for a human to tell when a 386lease expires \- it's specified as a number from zero to six, with zero 387being Sunday. 388When declaring a predefined lease, it can always be specified as zero. 389The year is specified with the century, so it should generally be four 390digits except for really long leases. 391The month is specified as a number starting with 1 for January. 392The day of the month is likewise specified starting with 1. 393The hour is a number between 0 and 23, 394the minute a number between 0 and 59, 395and the second also a number between 0 and 59. 396.Sh OTHER DECLARATIONS 397.Bl -tag -width Ds 398.It Ic reject Ar ip-address ; 399The 400.Ic reject 401statement causes the DHCP client to reject offers from servers who use 402the specified address as a server identifier. 403This can be used to avoid being configured by rogue or misconfigured DHCP 404servers, although it should be a last resort \- better to track down 405the bad DHCP server and fix it. 406.It Xo 407.Ic interface Ar \&"name\&" No { Ar declarations 408.Ar ... No } 409.Xc 410A client with more than one network interface may require different 411behaviour depending on which interface is being configured. 412All timing parameters and declarations other than lease 413declarations can be enclosed in an interface declaration, and those 414parameters will then be used only for the interface that matches the 415specified name. 416Interfaces for which there is no interface declaration will use the 417parameters declared outside of any interface declaration, 418or the default settings. 419.It Ic script Ar \&"script-name\&" ; 420The 421.Ic script 422statement is used to specify the pathname of the client configuration 423script. 424This script is used by the DHCP client to set each interface's initial 425configuration prior to requesting an address, to test the address once it 426has been offered, and to set the interface's final configuration once a 427lease has been acquired. 428If no lease is acquired, the script is used to test predefined leases, if 429any, and also called once if no valid lease can be identified. 430For more information, see 431.Xr dhclient.leases 5 . 432.El 433.Sh EXAMPLES 434The following configuration file is used on a laptop 435which has one interface, ep0 (a 3Com 3C589C). 436Booting intervals have been shortened somewhat from the default, because 437the client is known to spend most of its time on networks with little DHCP 438activity. 439The laptop does roam to multiple networks. 440.Bd -literal -offset indent 441timeout 60; 442retry 60; 443reboot 10; 444select-timeout 5; 445initial-interval 2; 446reject 192.33.137.209; 447 448interface "ep0" { 449 send host-name "andare.fugue.com"; 450 send dhcp-client-identifier 1:0:a0:24:ab:fb:9c; 451 send dhcp-lease-time 3600; 452 supersede domain-name "fugue.com rc.vix.com home.vix.com"; 453 prepend domain-name-servers 127.0.0.1; 454 request subnet-mask, broadcast-address, time-offset, routers, 455 domain-name, domain-name-servers, host-name; 456 require subnet-mask, domain-name-servers; 457 script "/etc/dhclient-script"; 458} 459.Ed 460.Pp 461This is a very complicated 462.Nm 463file \- in general, yours should be much simpler. 464In many cases, it's sufficient to just create an empty 465.Nm 466file \- the defaults are usually fine. 467.Sh SEE ALSO 468.Xr dhclient.leases 5 , 469.Xr dhclient-script 8 , 470.Xr dhcp-options 5 , 471.Xr dhcpd.conf 5 Pq Pa pkgsrc/net/isc-dhcpd4 , 472.Xr dhclient 8 , 473.Xr dhcpd 8 Pq Pa pkgsrc/net/isc-dhcpd4 474.Pp 475RFC 2132, RFC 2131. 476.Sh AUTHORS 477.An -nosplit 478.Xr dhclient 8 479was written by 480.An Ted Lemon Aq mellon@vix.com 481under a contract with Vixie Labs. 482.Pp 483The current implementation was reworked by 484.An Henning Brauer Aq henning@openbsd.org . 485