MirBSD manpage: dhclient.conf(5)

DHCLIENT.CONF(5)             BSD Reference Manual             DHCLIENT.CONF(5)


     dhclient.conf - DHCP client configuration file


     The dhclient.conf file contains configuration information for
     dhclient(8), the Internet Software Consortium DHCP Client.

     The dhclient.conf file is a free-form ASCII text file. It is parsed by
     the recursive-descent parser built into dhclient(8). 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.

     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 ap-
     pend 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.


     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 tim-
     ing behaviour will be used by default - one which results in fairly time-
     ly updates without placing an inordinate load on the server.

     The following statements can be used to adjust the timing behaviour of
     the DHCP client if required, however:

     timeout time;
             The 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 ad-
             dress. 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.

     retry time;
             The 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

     select-timeout time;
             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).

             The 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 select-timeout
             has expired, the client will accept the first offer that arrives.

             By default, the select-timeout is zero seconds - that is, the
             client will take the first offer it sees.

     reboot time;
             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 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

     backoff-cutoff time;
             The client uses an exponential backoff algorithm with some ran-
             domness, so that if many clients try to configure themselves at
             the same time, they will not make their requests in lockstep. The
             backoff-cutoff statement determines the maximum amount of time
             that the client is allowed to back off. It defaults to 15

     initial-interval time;
             The 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.

     link-timeout time;
             The link-timeout statement sets the amount of time to wait for an
             interface link before timing out. The default value is 10
             seconds, but the special value 0 requests that dhclient not wait
             for a link state change before timing out.


     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.

     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 DHCP Options. DHCP Options are defined in dhcp-options(5).

     request [option] [, ... option];
             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.

     require [option] [, ... option];
             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.

     send { [option declaration] [, ... option declaration] }
             The send statement causes the client to send the specified op-
             tions to the server with the specified values. These are full op-
             tion declarations as described in dhcp-options(5). Options that
             are always sent in the DHCP protocol should not be specified
             here. One 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.


     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.

     default { [option declaration] [, ... option declaration] }
             If for some set of options the client should use the value sup-
             plied 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 default statement.

     supersede { [option declaration] [, ... option declaration] }
             If for some set of options the client should always use its own
             value rather than any value supplied by the server, these values
             can be defined in the supersede statement.

     prepend { [option declaration] [, ... option declaration] }
             If for some set of options the client should use a value you sup-
             ply, and then use the values supplied by the server, if any,
             these values can be defined in the prepend statement. The prepend
             statement can only be used for options which allow more than one
             value to be given. This restriction is not enforced - if violat-
             ed, the results are unpredictable.

     append { [option declaration] [, ... option declaration] }
             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 append statement. The append
             statement can only be used for options which allow more than one
             value to be given. This restriction is not enforced - if you ig-
             nore it, the behaviour will be unpredictable.


     The lease declaration:

           lease { lease-declaration [... lease-declaration] }

     The DHCP client may decide after some period of time (see PROTOCOL
     TIMING) 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 fixed
     leases in the client configuration file for networks where there is no
     DHCP or BOOTP service, so that the client can still automatically config-
     ure its address. This is done with the lease statement.

     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.

     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:

     bootp;  The bootp statement is used to indicate that the lease was ac-
             quired 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.

     interface "string";
             The 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.

     fixed-address ip-address;
             The 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.,

     filename "string";
             The 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.

     server-name "string";
             The server-name statement specifies the name of the boot server
             name to use. This is also not used by the standard client confi-
             guration script.

     option option-declaration;
             The option statement is used to specify the value of an option
             supplied by the server, or, in the case of predefined leases de-
             clared in dhclient.conf, the value that the user wishes the
             client configuration script to use if the predefined lease is

     script "script-name";
             The 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 re-
             questing an address, to test the address once it has been of-
             fered, 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

     medium "media setup";
             The medium statement can be used on systems where network inter-
             faces 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 confi-
             guring the interface.

             The DHCP client automatically declares this parameter if it used
             a media type (see the media statement) when configuring the in-
             terface in order to obtain a lease. This statement should be used
             in predefined leases only if the network interface requires media
             type configuration.

     renew date;

     rebind date;

     expire date;
             The renew 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 rebind statement defines the time at which the
             DHCP client should begin to try to contact any DHCP server in
             order to renew its lease. The expire 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.

     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.

     Dates are specified as follows:

     <weekday> <year>/<month>/<day> <hour>:<minute>:<second>

     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.


     alias { declarations ... }

     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 Software Consortium DHCP
     client doesn't support roaming with fixed addresses directly, but in ord-
     er to facilitate such experimentation, the DHCP client can be set up to
     configure an IP alias using the alias declaration.

     The alias declaration resembles a lease declaration, except that options
     other than the subnet-mask option are ignored by the standard client con-
     figuration script, and expiry times are ignored. A typical alias declara-
     tion 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.


     reject ip-address;
             The reject statement causes the DHCP client to reject offers from
             servers who use the specified address as a server identifier.
             This can be used to avoid being configured by rogue or misconfig-
             ured DHCP servers, although it should be a last resort - better
             to track down the bad DHCP server and fix it.

     interface "name" { declarations ... }
             A client with more than one network interface may require dif-
             ferent behaviour depending on which interface is being config-
             ured. 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.

     media "media setup" [, "media setup", ...];
             The 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

             The media setup is only used for the initial phase of address ac-
             quisition (the DHCPDISCOVER and DHCPOFFER packets). Once an ad-
             dress 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.


     The following configuration file is used on a laptop which has an IP
     alias of, 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.

           timeout 60;
           retry 60;
           reboot 10;
           select-timeout 5;
           initial-interval 2;

           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;
               request subnet-mask, broadcast-address, time-offset, routers,
                       domain-name, domain-name-servers, host-name;
               require subnet-mask, domain-name-servers;
               script "/etc/dhclient-script";
               media "media 10baseT/UTP", "media 10base2/BNC";

           alias {
             interface "ep0";
             option subnet-mask;

     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.


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

     RFC 2132, RFC 2131.


     dhclient(8) was written by Ted Lemon <mellon@vix.com> under a contract
     with Vixie Labs.

     The current implementation was reworked by Henning Brauer

MirBSD #10-current             January 1, 1997                               5

Generated on 2022-12-24 01:00:14 by $MirOS: src/scripts/roff2htm,v 1.113 2022/12/21 23:14:31 tg Exp $ — This product includes material provided by mirabilos.

These manual pages and other documentation are copyrighted by their respective writers; their sources are available at the project’s CVSweb, AnonCVS and other mirrors. The rest is Copyright © 2002–2022 MirBSD.

This manual page’s HTML representation is supposed to be valid XHTML/1.1; if not, please send a bug report — diffs preferred.

Kontakt / Impressum & Datenschutzerklärung