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abbff12b 1.\" $OpenBSD: src/sbin/dhclient/dhclient.conf.5,v 1.19 2011/03/27 06:50:48 jmc Exp $
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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,
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26.\" LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
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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
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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.\"
abbff12b 39.Dd August 2, 2012
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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.
b998cb88 146It defaults to fifteen seconds.
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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.
b998cb88 156It defaults to three seconds.
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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.
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162The default value is ten seconds.
163The value zero requests that
b998cb88 164dhclient not wait for a link state change before timing out.
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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
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216Options in the lease can be modified before being passed to the client
217configuration script,
97aa4590 218.Xr dhclient-script 8 .
16e4ae23 219.Pp
97aa4590 220The default client configuration script
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221processes only options 1 (subnet
222mask), 3 (routers), 6 (domain name servers), 15 (domain-name), and 33
223(static routes).
224Use of option modifiers on other options will have no effect unless
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225.Xr dhclient-script 8
226the client configuration script is modified.
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227.Pp
228Several option modifiers are available.
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229.Bl -tag -width Ds
230.It Xo
231.Ic default No { Op Ar option declaration
232.Oo , Ar ... option declaration Oc }
233.Xc
234If for some set of options the client should use the value supplied by
235the server, but needs to use some default value if no value was supplied
236by the server, these values can be defined in the
237.Ic default
238statement.
239.It Xo
240.Ic supersede No { Op Ar option declaration
241.Oo , Ar ... option declaration Oc }
242.Xc
243If for some set of options the client should always use its own value
244rather than any value supplied by the server, these values can be defined
245in the
246.Ic supersede
247statement.
248.It Xo
249.Ic prepend No { Op Ar option declaration
250.Oo , Ar ... option declaration Oc }
251.Xc
252If for some set of options the client should use a value you supply,
253and then use the values supplied by the server, if any,
254these values can be defined in the
255.Ic prepend
256statement.
257The
258.Ic prepend
259statement can only be used for options which allow more than one value to
260be given.
261This restriction is not enforced \- if violated, the results are unpredictable.
262.It Xo
263.Ic append No { Op Ar option declaration
264.Oo , Ar ... option declaration Oc }
265.Xc
266If for some set of options the client should first use the values
267supplied by the server, if any, and then use values you supply, these
268values can be defined in the
269.Ic append
270statement.
271The
272.Ic append
273statement can only be used for options which allow more than one value to
274be given.
275This restriction is not enforced \- if you ignore it,
276the behaviour will be unpredictable.
277.El
278.Sh LEASE DECLARATIONS
279The lease declaration:
280.Pp
cf15b33a 281.D1 Ic lease No { Ar lease-declaration Oo Ar ... lease-declaration Oc }
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282.Pp
283The DHCP client may decide after some period of time (see
284.Sx PROTOCOL TIMING )
285that it is not going to succeed in contacting a server.
286At that time, it consults its own database of old leases and tests each one
287that has not yet timed out by pinging the listed router for that lease to
288see if that lease could work.
289It is possible to define one or more
290.Em fixed
291leases in the client configuration file for networks where there is no DHCP
292or BOOTP service, so that the client can still automatically configure its
293address.
294This is done with the
295.Ic lease
296statement.
297.Pp
298NOTE: the lease statement is also used in the
299.Pa dhclient.leases
300file in order to record leases that have been received from DHCP servers.
301Some of the syntax for leases as described below is only needed in the
302.Pa dhclient.leases
303file.
304Such syntax is documented here for completeness.
305.Pp
306A lease statement consists of the lease keyword, followed by a left
307curly brace, followed by one or more lease declaration statements,
308followed by a right curly brace.
309The following lease declarations are possible:
310.Bl -tag -width Ds
311.It Ic bootp ;
312The
313.Ic bootp
314statement is used to indicate that the lease was acquired using the
315BOOTP protocol rather than the DHCP protocol.
316It is never necessary to specify this in the client configuration file.
317The client uses this syntax in its lease database file.
318.It Ic interface Ar \&"string\&" ;
319The
320.Ic interface
321lease statement is used to indicate the interface on which the lease is valid.
322If set, this lease will only be tried on a particular interface.
323When the client receives a lease from a server, it always records the
324interface number on which it received that lease.
325If predefined leases are specified in the
326.Nm
327file, the interface should also be specified, although this is not required.
328.It Ic fixed-address Ar ip-address ;
329The
330.Ic fixed-address
331statement is used to set the IP address of a particular lease.
332This is required for all lease statements.
333The IP address must be specified as a dotted quad (e.g., 12.34.56.78).
334.It Ic filename Ar \&"string\&" ;
335The
336.Ic filename
337statement specifies the name of the boot filename to use.
338This is not used by the standard client configuration script, but is
339included for completeness.
340.It Ic server-name Ar \&"string\&" ;
341The
342.Ic server-name
343statement specifies the name of the boot server name to use.
344This is also not used by the standard client configuration script.
345.It Ic option Ar option-declaration ;
346The
347.Ic option
348statement is used to specify the value of an option supplied by the server,
349or, in the case of predefined leases declared in
68d52ec8 350.Nm ,
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351the value that the user wishes the client configuration script to use if the
352predefined lease is used.
353.It Ic medium Ar \&"media setup\&" ;
354The
355.Ic medium
356statement can be used on systems where network interfaces cannot
357automatically determine the type of network to which they are connected.
358The media setup string is a system-dependent parameter which is passed
97aa4590 359to the client configuration script when initializing the interface.
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360On
361.Ux
362and UNIX-like systems, the argument is passed on the ifconfig command line
363when configuring the interface.
364.Pp
365The DHCP client automatically declares this parameter if it used a
366media type (see the
367.Ic media
368statement) when configuring the interface in order to obtain a lease.
369This statement should be used in predefined leases only if the network
370interface requires media type configuration.
371.It Ic renew Ar date ;
372.It Ic rebind Ar date ;
373.It Ic expire Ar date ;
374The
375.Ic renew
376statement defines the time at which the DHCP client should begin trying to
377contact its server to renew a lease that it is using.
378The
379.Ic rebind
380statement defines the time at which the DHCP client should begin to try to
381contact
382.Em any
383DHCP server in order to renew its lease.
384The
385.Ic expire
386statement defines the time at which the DHCP client must stop using a lease
387if it has not been able to contact a server in order to renew it.
388.El
389.Pp
390These declarations are automatically set in leases acquired by the
391DHCP client, but must also be configured in predefined leases \- a
392predefined lease whose expiry time has passed will not be used by the
393DHCP client.
394.Pp
395Dates are specified as follows:
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396.Bd -ragged -offset indent
397.Ar <weekday>
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398.Sm off
399.Ar <year> No / Ar <month> No / Ar <day>
400.Ar <hour> : <minute> : <second>
401.Sm on
cf15b33a 402.Ed
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403.Pp
404The weekday is present to make it easy for a human to tell when a
405lease expires \- it's specified as a number from zero to six, with zero
406being Sunday.
407When declaring a predefined lease, it can always be specified as zero.
408The year is specified with the century, so it should generally be four
409digits except for really long leases.
410The month is specified as a number starting with 1 for January.
411The day of the month is likewise specified starting with 1.
412The hour is a number between 0 and 23,
413the minute a number between 0 and 59,
414and the second also a number between 0 and 59.
415.Sh ALIAS DECLARATIONS
416.Ic alias No { Ar declarations ... No }
417.Pp
418Some DHCP clients running TCP/IP roaming protocols may require that in
419addition to the lease they may acquire via DHCP, their interface also
420be configured with a predefined IP alias so that they can have a
421permanent IP address even while roaming.
422The Internet Software Consortium DHCP client doesn't support roaming with
423fixed addresses directly, but in order to facilitate such experimentation,
424the DHCP client can be set up to configure an IP alias using the
425.Ic alias
426declaration.
427.Pp
428The
429.Ic alias
430declaration resembles a lease declaration, except that options other than
431the subnet-mask option are ignored by the standard client configuration
432script, and expiry times are ignored.
433A typical alias declaration includes an interface declaration, a fixed-address
434declaration for the IP alias address, and a subnet-mask option declaration.
435A medium statement should never be included in an alias declaration.
436.Sh OTHER DECLARATIONS
437.Bl -tag -width Ds
438.It Ic reject Ar ip-address ;
439The
440.Ic reject
441statement causes the DHCP client to reject offers from servers who use
442the specified address as a server identifier.
443This can be used to avoid being configured by rogue or misconfigured DHCP
444servers, although it should be a last resort \- better to track down
445the bad DHCP server and fix it.
446.It Xo
447.Ic interface Ar \&"name\&" No { Ar declarations
448.Ar ... No }
449.Xc
450A client with more than one network interface may require different
451behaviour depending on which interface is being configured.
452All timing parameters and declarations other than lease and alias
453declarations can be enclosed in an interface declaration, and those
454parameters will then be used only for the interface that matches the
455specified name.
456Interfaces for which there is no interface declaration will use the
457parameters declared outside of any interface declaration,
458or the default settings.
459.It Xo
460.Ic media Ar \&"media setup\&"
461.Oo , Ar \&"media setup\&" , ... Oc ;
462.Xc
463The
464.Ic media
465statement defines one or more media configuration parameters which may
466be tried while attempting to acquire an IP address.
467The DHCP client will cycle through each media setup string on the list,
468configuring the interface using that setup and attempting to boot,
469and then trying the next one.
470This can be used for network interfaces which aren't capable of sensing
471the media type unaided \- whichever media type succeeds in getting a request
472to the server and hearing the reply is probably right (no guarantees).
473.Pp
474The media setup is only used for the initial phase of address
475acquisition (the DHCPDISCOVER and DHCPOFFER packets).
476Once an address has been acquired, the DHCP client will record it in its
477lease database and will record the media type used to acquire the address.
478Whenever the client tries to renew the lease, it will use that same media type.
479The lease must expire before the client will go back to cycling through media
480types.
481.It Ic script Ar \&"script-name\&" ;
482The
483.Ic script
97aa4590 484statement is used to specify the pathname of the client configuration
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485script.
486This script is used by the DHCP client to set each interface's initial
487configuration prior to requesting an address, to test the address once it
488has been offered, and to set the interface's final configuration once a
489lease has been acquired.
490If no lease is acquired, the script is used to test predefined leases, if
491any, and also called once if no valid lease can be identified.
492For more information, see
493.Xr dhclient.leases 5 .
494.El
495.Sh EXAMPLES
496The following configuration file is used on a laptop
497which has an IP alias of 192.5.5.213, and has one interface,
498ep0 (a 3Com 3C589C).
499Booting intervals have been shortened somewhat from the default, because
500the client is known to spend most of its time on networks with little DHCP
501activity.
502The laptop does roam to multiple networks.
503.Bd -literal -offset indent
504timeout 60;
505retry 60;
506reboot 10;
507select-timeout 5;
508initial-interval 2;
509reject 192.33.137.209;
510
511interface "ep0" {
512 send host-name "andare.fugue.com";
513 send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
514 send dhcp-lease-time 3600;
515 supersede domain-name "fugue.com rc.vix.com home.vix.com";
516 prepend domain-name-servers 127.0.0.1;
517 request subnet-mask, broadcast-address, time-offset, routers,
518 domain-name, domain-name-servers, host-name;
519 require subnet-mask, domain-name-servers;
520 script "/etc/dhclient-script";
521 media "media 10baseT/UTP", "media 10base2/BNC";
522}
523
524alias {
525 interface "ep0";
526 fixed-address 192.5.5.213;
527 option subnet-mask 255.255.255.255;
528}
529.Ed
530.Pp
531This is a very complicated
532.Nm
533file \- in general, yours should be much simpler.
534In many cases, it's sufficient to just create an empty
535.Nm
536file \- the defaults are usually fine.
537.Sh SEE ALSO
538.Xr dhclient.leases 5 ,
97aa4590 539.Xr dhclient-script 8 ,
846204b6 540.Xr dhcp-options 5 ,
aa4e0df8 541.Xr dhcpd.conf 5 Pq Pa pkgsrc/net/isc-dhcpd4 ,
846204b6 542.Xr dhclient 8 ,
aa4e0df8 543.Xr dhcpd 8 Pq Pa pkgsrc/net/isc-dhcpd4
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544.Pp
545RFC 2132, RFC 2131.
546.Sh AUTHORS
547.An -nosplit
548.Xr dhclient 8
549was written by
550.An Ted Lemon Aq mellon@vix.com
551under a contract with Vixie Labs.
552.Pp
553The current implementation was reworked by
554.An Henning Brauer Aq henning@openbsd.org .