1.\" $OpenBSD: src/sbin/dhclient/dhclient.conf.5,v 1.24 2012/10/27 23:08:53 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 9, 2014 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 . 50.Pp 51The 52.Nm 53file is a free-form ASCII text file. 54It is parsed by the recursive-descent parser built into 55.Xr dhclient 8 . 56The file may contain extra tabs and newlines for formatting purposes. 57Keywords in the file are case-insensitive. 58Comments may be placed anywhere within the file (except within quotes). 59Comments begin with the 60.Sq # 61character and end at the end of the line. 62.Pp 63The 64.Nm 65file can be used to configure the behaviour of the client in a wide variety 66of ways: protocol timing, information requested from the server, information 67required of the server, defaults to use if the server does not provide 68certain information, values with which to override information provided by 69the server, or values to prepend or append to information provided by the 70server. 71The configuration file can also be preinitialized with addresses to 72use on networks that don't have DHCP servers. 73.Sh PROTOCOL TIMING 74The timing behaviour of the client need not be configured by the user. 75If no timing configuration is provided by the user, a fairly 76reasonable timing behaviour will be used by default \- one which 77results in fairly timely updates without placing an inordinate load on 78the server. 79.Pp 80The following statements can be used to adjust the timing behaviour of 81the DHCP client if required, however: 82.Bl -tag -width Ds 83.It Ic timeout Ar time ; 84The 85.Ic timeout 86statement determines the amount of time that must pass between the 87time that the client begins to try to determine its address and the 88time that it decides that it's not going to be able to contact a server. 89By default, this timeout is sixty seconds. 90After the timeout has passed, if there are any static leases defined in the 91configuration file, or any leases remaining in the lease database that 92have not yet expired, the client will loop through these leases 93attempting to validate them, and if it finds one that appears to be 94valid, it will use that lease's address. 95If there are no valid static leases or unexpired leases in the lease database, 96the client will restart the protocol after the defined retry interval. 97.It Ic retry Ar time ; 98The 99.Ic retry 100statement determines the time that must pass after the client has 101determined that there is no DHCP server present before it tries again 102to contact a DHCP server. 103By default, this is five minutes. 104.It Ic select-timeout Ar time ; 105It is possible (some might say desirable) for there to be more than 106one DHCP server serving any given network. 107In this case, it is possible that a client may be sent more than one offer 108in response to its initial lease discovery message. 109It may be that one of these offers is preferable to the other 110(e.g., one offer may have the address the client previously used, 111and the other may not). 112.Pp 113The 114.Ic select-timeout 115is the time after the client sends its first lease discovery request 116at which it stops waiting for offers from servers, assuming that it 117has received at least one such offer. 118If no offers have been received by the time the 119.Ic select-timeout 120has expired, the client will accept the first offer that arrives. 121.Pp 122By default, the 123.Ic select-timeout 124is zero seconds \- that is, the client will take the first offer it sees. 125.It Ic reboot Ar time ; 126When the client is restarted, it first tries to reacquire the last 127address it had. 128This is called the INIT-REBOOT state. 129If it is still attached to the same network it was attached to when it last 130ran, this is the quickest way to get started. 131The 132.Ic reboot 133statement sets the time that must elapse after the client first tries 134to reacquire its old address before it gives up and tries to discover 135a new address. 136By default, the reboot timeout is ten seconds. 137.It Ic backoff-cutoff Ar time ; 138The client uses an exponential backoff algorithm with some randomness, 139so that if many clients try to configure themselves at the same time, 140they will not make their requests in lockstep. 141The 142.Ic backoff-cutoff 143statement determines the maximum amount of time that the client is 144allowed to back off. 145It defaults to fifteen seconds. 146.It Ic initial-interval Ar time ; 147The 148.Ic initial-interval 149statement sets the amount of time between the first attempt to reach a 150server and the second attempt to reach a server. 151Each time a message is sent, the interval between messages is incremented by 152twice the current interval multiplied by a random number between zero and one. 153If it is greater than the backoff-cutoff amount, it is set to that 154amount. 155It defaults to three seconds. 156.It Ic link-timeout Ar time ; 157The 158.Ic link-timeout 159statement sets the amount of time to wait for an interface link before timing 160out. 161The default value is ten seconds. 162The value zero requests that 163dhclient not wait for a link state change before timing out. 164.El 165.Sh LEASE REQUIREMENTS AND REQUESTS 166The DHCP protocol allows the client to request that the server send it 167specific information, and not send it other information that it is not 168prepared to accept. 169The protocol also allows the client to reject offers from servers if they 170don't contain information the client needs, or if the information provided 171is not satisfactory. 172.Pp 173There is a variety of data contained in offers that DHCP servers send 174to DHCP clients. 175The data that can be specifically requested is what are called 176.Em DHCP Options . 177DHCP Options are defined in 178.Xr dhcp-options 5 . 179.Bl -tag -width Ds 180.It Ic ignore Op Ar option 181The 182.Ic ignore 183statement causes the client to discard values provided by the server for 184the specified options. 185Only the option names should be specified in the ignore statement \- not 186option parameters. 187.It Ic request Op Ar option , ... ; 188The 189.Ic request 190statement causes the client to request that any server responding to the 191client send the client its values for the specified options. 192Only option names should be specified in the request statement \- not 193option values. 194Only the last 195.Ic request 196statement has any effect, overriding any previous 197.Ic request 198statement. 199.It Ic require Op Ar option , ... ; 200The 201.Ic require 202statement lists options that must be sent in order for an offer to be accepted. 203Offers that do not contain all the listed options will be ignored. 204Only option names should be specified in the require statement \- not 205option values. 206Only the last 207.Ic require 208statement has any effect, overriding any previous 209.Ic require 210statement. 211.It Ic send Ar option option-value ; 212The 213.Ic send 214statement causes the client to send the specified option and value 215to the server. 216Options that are always sent in the DHCP protocol should not be specified 217here. 218One use for this statement is to send information to the server 219that will allow it to differentiate between this client and other 220clients or kinds of clients. 221.El 222.Sh OPTION MODIFIERS 223Options in the lease can be modified before being passed to the client 224configuration script, 225.Xr dhclient-script 8 . 226.Pp 227The default client configuration script 228processes only options 1 (subnet 229mask), 3 (routers), 6 (domain name servers), 15 (domain-name). 230Use of option modifiers on other options will have no effect unless 231.Xr dhclient-script 8 232the client configuration script is modified. 233.Pp 234Several option modifiers are available. 235.Bl -tag -width Ds 236.It Ic default Ar option option-value ; 237Use 238.Ar option-value 239for the given 240.Ar option , 241if no value is supplied by the server. 242.It Ic supersede Ar option option-value ; 243Use 244.Ar option-value 245for the given 246.Ar option , 247regardless of the value supplied by the server. 248.It Ic prepend Ar option option-value ; 249Use 250.Ar option-value 251for the given 252.Ar option , 253and then use the value supplied by the server. 254.Ic prepend 255can only be used for options which allow more than one value to be given. 256The restriction is not enforced \- if violated, the results are unpredictable. 257.It Ic append Ar option option-value ; 258Use 259.Ar option-value 260for the given 261.Ar option , 262after first using the value supplied by the server. 263.Ic append 264can only be used for options which allow more than one value to be given. 265The restriction is not enforced \- if violated, the results are unpredictable. 266.El 267.Sh LEASE DECLARATIONS 268The lease declaration: 269.Pp 270.D1 Ic lease No { Ar lease-declaration ; ... ; No } 271.Pp 272The DHCP client may decide after some period of time (see 273.Sx PROTOCOL TIMING ) 274that it is not going to succeed in contacting a server. 275At that time, it consults its own database of old leases and tests each one 276that has not yet timed out by pinging the listed router for that lease to 277see if that lease could work. 278It is possible to define one or more 279.Em fixed 280leases in the client configuration file for networks where there is no DHCP 281or BOOTP service, so that the client can still automatically configure its 282address. 283This is done with the 284.Ic lease 285statement. 286.Pp 287NOTE: the lease statement is also used in the 288.Pa /var/db/dhclient.leases. Ns Aq Ar IFNAME 289file in order to record leases that have been received from DHCP servers. 290Some of the syntax for leases as described below is only needed in the 291.Pa /var/db/dhclient.leases. Ns Aq Ar IFNAME 292file. 293Such syntax is documented here for completeness. 294.Pp 295A lease statement consists of the lease keyword, followed by a left 296curly brace, followed by one or more lease declaration statements, 297followed by a right curly brace. 298The following lease declarations are possible: 299.Pp 300.Bl -tag -width Ds -compact 301.It Ic bootp ; 302The 303.Ic bootp 304statement is used to indicate that the lease was acquired using the 305BOOTP protocol rather than the DHCP protocol. 306It is never necessary to specify this in the client configuration file. 307The client uses this syntax in its lease database file. 308.Pp 309.It Ic interface Qq Ar string ; 310The 311.Ic interface 312lease statement is used to indicate the interface on which the lease is valid. 313If set, this lease will only be tried on a particular interface. 314When the client receives a lease from a server, it always records the 315interface number on which it received that lease. 316If predefined leases are specified in the 317.Nm 318file, the interface should also be specified, although this is not required. 319.Pp 320.It Ic fixed-address Ar ip-address ; 321The 322.Ic fixed-address 323statement is used to set the IP address of a particular lease. 324This is required for all lease statements. 325The IP address must be specified as a dotted quad (e.g., 12.34.56.78). 326.Pp 327.It Ic filename Qq Ar string ; 328The 329.Ic filename 330statement specifies the name of the boot filename to use. 331This is not used by the standard client, but is included for completeness. 332.Pp 333.It Ic server-name Qq Ar string ; 334The 335.Ic server-name 336statement specifies the name of the boot server name to use. 337This is not used by the standard client, but is included for completeness. 338.Pp 339.It Ic option Ar option option-value ; 340The 341.Ic option 342statement is used to specify the value of an option supplied by the server, 343or, in the case of predefined leases declared in 344.Nm , 345the value that the user wishes the client to use if the 346predefined lease is used. 347.Pp 348.It Ic renew Ar date ; 349.It Ic rebind Ar date ; 350.It Ic expire Ar date ; 351The 352.Ic renew 353statement defines the time at which the DHCP client should begin trying to 354contact its server to renew a lease that it is using. 355The 356.Ic rebind 357statement defines the time at which the DHCP client should begin to try to 358contact 359.Em any 360DHCP server in order to renew its lease. 361The 362.Ic expire 363statement defines the time at which the DHCP client must stop using a lease 364if it has not been able to contact a server in order to renew it. 365.El 366.Pp 367These declarations are automatically set in leases acquired by the 368DHCP client, but must also be configured in predefined leases \- a 369predefined lease whose expiry time has passed will not be used by the 370DHCP client. 371.Pp 372Dates are specified as follows: 373.Pp 374.D1 <weekday> <year>/<month>/<day> <hour>:<minute>:<second> 375.Pp 376The weekday is present to make it easy for a human to tell when a 377lease expires \- it's specified as a number from zero to six, with zero 378being Sunday. 379When declaring a predefined lease, it can always be specified as zero. 380The year is specified with the century, so it should generally be four 381digits except for really long leases. 382The month is specified as a number starting with 1 for January. 383The day of the month is likewise specified starting with 1. 384The hour is a number between 0 and 23, 385the minute a number between 0 and 59, 386and the second also a number between 0 and 59. 387.Sh OTHER DECLARATIONS 388.Bl -tag -width Ds 389.It Ic reject Ar ip-address ; 390The 391.Ic reject 392statement causes the DHCP client to reject offers from servers who use 393the specified address as a server identifier. 394This can be used to avoid being configured by rogue or misconfigured DHCP 395servers, although it should be a last resort \- better to track down 396the bad DHCP server and fix it. 397.It Ic interface Qo Ar name Qc No { Ar declaration ; ... ; No } 398A client with more than one network interface may require different 399behaviour depending on which interface is being configured. 400All timing parameters and declarations other than lease 401declarations can be enclosed in an interface declaration, and those 402parameters will then be used only for the interface that matches the 403specified name. 404Interfaces for which there is no interface declaration will use the 405parameters declared outside of any interface declaration, 406or the default settings. 407.It Ic script Ar \&"script-name\&" ; 408The 409.Ic script 410statement is used to specify the pathname of the client configuration 411script. 412This script is used by the DHCP client to set each interface's initial 413configuration prior to requesting an address, to test the address once it 414has been offered, and to set the interface's final configuration once a 415lease has been acquired. 416If no lease is acquired, the script is used to test predefined leases, if 417any, and also called once if no valid lease can be identified. 418For more information, see 419.Xr dhclient.leases 5 . 420.El 421.Sh EXAMPLES 422The following configuration file is used on a laptop 423which has one interface, ep0 (a 3Com 3C589C). 424Booting intervals have been shortened somewhat from the default, because 425the client is known to spend most of its time on networks with little DHCP 426activity. 427The laptop does roam to multiple networks. 428.Bd -literal -offset indent 429timeout 60; 430retry 60; 431reboot 10; 432select-timeout 5; 433initial-interval 2; 434reject 192.33.137.209; 435 436interface "ep0" { 437 send host-name "andare.fugue.com"; 438 send dhcp-client-identifier 1:0:a0:24:ab:fb:9c; 439 send dhcp-lease-time 3600; 440 supersede domain-name "fugue.com rc.vix.com home.vix.com"; 441 prepend domain-name-servers 127.0.0.1; 442 request subnet-mask, broadcast-address, time-offset, routers, 443 domain-name, domain-name-servers, host-name; 444 require subnet-mask, domain-name-servers; 445 script "/etc/dhclient-script"; 446} 447.Ed 448.Pp 449This is a very complicated 450.Nm 451file \- in general, yours should be much simpler. 452In many cases, it's sufficient to just create an empty 453.Nm 454file \- the defaults are usually fine. 455.Sh SEE ALSO 456.Xr dhclient.leases 5 , 457.Xr dhcp-options 5 , 458.Xr dhcpd.conf 5 Pq Pa net/isc-dhcp42-server , 459.Xr dhclient 8 , 460.Xr dhclient-script 8 , 461.Xr dhcpd 8 Pq Pa net/isc-dhcp42-server 462.Sh STANDARDS 463.Rs 464.%A R. Droms 465.%D March 1997 466.%R RFC 2131 467.%T Dynamic Host Configuration Protocol 468.Re 469.Pp 470.Rs 471.%A S. Alexander 472.%A R. Droms 473.%D March 1997 474.%R RFC 2132 475.%T DHCP Options and BOOTP Vendor Extensions 476.Re 477.Sh AUTHORS 478.An -nosplit 479.Xr dhclient 8 480was written by 481.An Ted Lemon Aq Mt mellon@vix.com 482under a contract with Vixie Labs. 483.Pp 484The current implementation was reworked by 485.An Henning Brauer Aq Mt henning@openbsd.org . 486