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DHCLIENT.CONF(5)            BSD File Formats Manual           DHCLIENT.CONF(5)

     dhclient.conf -- DHCP client configuration file

     The dhclient.conf file contains configuration information for

     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 con-
     tain 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
     append 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
     timing behaviour will be used by default - one which results in fairly
     timely 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
             address.  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 minutes.

     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.

             The value of reboot is also used to control how long a client
             will attempt unicast renewal requests before falling back to
             using broadcast requests.

             By default, the reboot timeout is ten seconds.

     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 fif-
             teen seconds.

     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 three 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 ten sec-
             onds.  The value zero 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).

     ignore [option, ...];
             The ignore statement causes the client to discard values provided
             by the server for the specified options.  Only option names
             should be specified in the ignore statement - not option values.
             Only the last ignore statement has any effect, overriding any
             previous ignore statement.

     request [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 option names should be specified in
             the request statement - not option values.  Only the last request
             statement has any effect, overriding any previous request state-

     require [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.  Only option names should be
             specified in the require statement - not option values.  Only the
             last require statement has any effect, overriding any previous
             require statement.

     send option option-value;
             The send statement causes the client to send the specified option
             and value to the server.  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.

     Options in the lease can be modified before being accepted.

     Several option modifiers are available.

     default option option-value;
             Use option-value for the given option, if no value is supplied by
             the server.

     supersede option option-value;
             Use option-value for the given option, regardless of the value
             supplied by the server.

     prepend option option-value;
             Use option-value for the given option, and then use the value
             supplied by the server.  prepend can only be used for options
             which allow more than one value to be given.  The restriction is
             not enforced - if violated, the results are unpredictable.

     append option option-value;
             Use option-value for the given option, after first using the
             value supplied by the server.  append can only be used for
             options which allow more than one value to be given.  The
             restriction is not enforced - if violated, the results are unpre-

     The lease declaration:

           lease { 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
     /var/db/dhclient.leases.<IFNAME> 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 /var/db/dhclient.leases.<IFNAME>
     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
             acquired 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 speci-
             fied, 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, but is included
             for completeness.

     server-name "string";
             The server-name statement specifies the name of the boot server
             name to use.  This is not used by the standard client, but is
             included for completeness.

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

     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.

     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" { declaration; ...; }
             A client with more than one network interface may require differ-
             ent behaviour depending on which interface is being configured.
             All timing parameters and declarations other than lease declara-
             tions 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.

     The following configuration file is used on a laptop which has one inter-
     face, ep0 (a 3Com 3C589C).  Booting intervals have been shortened some-
     what 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 mul-
     tiple 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;

     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)

     R. Droms, Dynamic Host Configuration Protocol, RFC 2131, March 1997.

     S. Alexander and R. Droms, DHCP Options and BOOTP Vendor Extensions, RFC
     2132, March 1997.

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

     The current implementation was reworked by Henning Brauer

BSD                             March 30, 2017                             BSD