Switch to SpeakEasy.net DSL

The Modular Manual Browser

Home Page
Manual: (NetBSD-6.1.5)
Apropos / Subsearch:
optional field

dhclient.conf(5)              File Formats Manual             dhclient.conf(5)

       dhclient.conf - DHCP client configuration file

       The dhclient.conf file contains configuration information for dhclient,
       the Internet Systems 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.   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 behavior 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 behavior of the client need not be configured by the user.
       If no timing configuration is provided by the user, a fairly reasonable
       timing behavior 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 behavior of
       the DHCP client if required, however:

       The timeout statement

       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.

       The retry statement

        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.

       The select-timeout statement

        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.

       The reboot statement

        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 seconds.

       The backoff-cutoff statement

        backoff-cutoff time;

       The client uses an exponential backoff algorithm with some randomness,
       so that if many clients try to configure themselves at the same time,
       they will not make their requests in lock-step.   The backoff-cutoff
       statement determines the maximum amount of time that the client is
       allowed to back off, the actual value will be evaluated randomly
       between 1/2 to 1 1/2 times the time specified.   It defaults to two

       The initial-interval statement

        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.

       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

       The request statement

        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.   By default, the DHCP client
       requests the subnet-mask, broadcast-address, time-offset, routers,
       domain-name, domain-name-servers and host-name options.

       In some cases, it may be desirable to send no parameter request list at
       all.   To do this, simply write the request statement but specify no


       The require statement

        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.

       The send statement

        send { [ option declaration ] [, ... option declaration ]}

       The send statement causes the client to send the specified options to
       the server with the specified values.  These are full option
       declarations as described in dhcp-options(5).  Options that are always
       sent in the DHCP protocol should not be specified here, except that the
       client can specify a requested-lease-time option other than the default
       requested lease time, which is two hours.  The other obvious 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

       The client now has some very limited support for doing DNS updates when
       a lease is acquired.   This is prototypical, and probably doesn't do
       what you want.   It also only works if you happen to have control over
       your DNS server, which isn't very likely.

       To make it work, you have to declare a key and zone as in the DHCP
       server (see dhcpd.conf(5) for details).   You also need to configure
       the fqdn option on the client, as follows:

         send fqdn.fqdn "grosse.fugue.com.";
         send fqdn.encoded on;
         send fqdn.server-update off;

       The fqdn.fqdn option MUST be a fully-qualified domain name.   You MUST
       define a zone statement for the zone to be updated.   The fqdn.encoded
       option may need to be set to on or off, depending on the DHCP server
       you are using.

       The do-forward-updates statement

        do-forward-updates [ flag ] ;

       If you want to do DNS updates in the DHCP client script (see dhclient-
       script(8)) rather than having the DHCP client do the update directly
       (for example, if you want to use SIG(0) authentication, which is not
       supported directly by the DHCP client, you can instruct the client not
       to do the update using the do-forward-updates statement.   Flag should
       be true if you want the DHCP client to do the update, and false if you
       don't want the DHCP client to do the update.   By default, the DHCP
       client will do the DNS update.

       The omapi port statement

        omapi port [ port ] ;

       The omapi port statement causes the DHCP client to set up an OMAPI
       listener on the specified port.   Only one such port should be
       specified.  The OMAPI listener will be sensitive to connections from
       any IP address, so it is important to also set up an OMAPI key to
       protect the client from unauthorized changes.

       The omapi key statement

        omapi key [  key-id  ] ;

       The omapi key statement causes the DHCP client to check any incoming
       OMAPI messages to make sure that they are signed by the specified key.
       If a message is not signed by this key, it is rejected with an "invalid
       key" error.

       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.

       The default statement

        default [ option declaration ] ;

       If for some option the client should use the value supplied 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.

       The supersede statement

        supersede [ option declaration ] ;

       If for some option the client should always use a locally-configured
       value or values rather than whatever is supplied by the server, these
       values can be defined in the supersede statement.

       The prepend statement

        prepend [ option declaration ] ;

       If for some set of options the client should use a value you supply,
       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 you ignore it, the behavior will
       be unpredictable.

       The append statement

        append [ 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 ignore it, the
       behavior 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 configure its address.   This is done with the lease

       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

       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


       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 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 configuration

        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 declared in
       dhclient.conf, the value that the user wishes the client configuration
       script to use if the predefined lease is used.

        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 requesting an address,
       to test the address once it has been offered, 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 dhclient-script(8).

        vendor option space "name";

       The vendor option space statement is used to specify which option space
       should be used for decoding the vendor-encapsulate-options option if
       one is received.  The dhcp-vendor-identifier can be used to request a
       specific class of vendor options from the server.   See dhcp-options(5)
       for details.

        medium "media setup";

       The medium statement can be used on systems where network interfaces
       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 configuring the

       The dhcp client automatically declares this parameter if it uses a
       media type (see the media statement) when configuring the interface 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 Systems
       Consortium DHCP client doesn't support roaming with fixed addresses
       directly, but in order to facilitate such experimentation, the dhcp
       client can be set up to configure an IP alias using the alias

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

        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 misconfigured 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 different
       behavior depending on which interface is being configured.   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.

       Note well: ISC dhclient only maintains one list of interfaces, which is
       either determined at startup from command line arguments, or otherwise
       is autodetected.  If you supplied the list of interfaces on the command
       line, this configuration clause will add the named interface to the
       list in such a way that will cause it to be configured by DHCP.  Which
       may not be the result you had intended.  This is an undesirable side
       effect that will be addressed in a future release.

        pseudo "name" "real-name" { declarations ...  }

       Under some circumstances it can be useful to declare a pseudo-interface
       and have the DHCP client acquire a configuration for that interface.
       Each interface that the DHCP client is supporting normally has a DHCP
       client state machine running on it to acquire and maintain its lease.
       A pseudo-interface is just another state machine running on the
       interface named real-name, with its own lease and its own state.   If
       you use this feature, you must provide a client identifier for both the
       pseudo-interface and the actual interface, and the two identifiers must
       be different.   You must also provide a separate client script for the
       pseudo-interface to do what you want with the IP address.   For

            interface "ep0" {
                 send dhcp-client-identifier "my-client-ep0";
            pseudo "secondary" "ep0" {
                 send dhcp-client-identifier "my-client-ep0-secondary";
                 script "/etc/dhclient-secondary";

       The client script for the pseudo-interface should not configure the
       interface up or down - essentially, all it needs to handle are the
       states where a lease has been acquired or renewed, and the states where
       a lease has expired.   See dhclient-script(8) for more information.

        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 guarantees).

       The media setup is only used for the initial phase of address
       acquisition (the DHCPDISCOVER and DHCPOFFER packets).   Once an address
       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 running NetBSD
       1.3.   The laptop 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 "CLIENTBINDIR/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.

       dhcp-options(5), dhclient.leases(5), dhcpd(8), dhcpd.conf(5), RFC2132,

       dhclient(8) was written by Ted Lemon under a contract with Vixie Labs.
       Funding for this project was provided by Internet Systems Consortium.
       Information about Internet Systems Consortium can be found at