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dhclient-script(8)          System Manager's Manual         dhclient-script(8)

       dhclient-script - DHCP client network configuration script

       The DHCP client network configuration script is invoked from time to
       time by dhclient(8).  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.

       This script is not meant to be customized by the end user.  If local
       customizations are needed, they should be possible using the enter and
       exit hooks provided (see HOOKS for details).   These hooks will allow
       the user to override the default behaviour of the client in creating a
       /etc/resolv.conf file.

       No standard client script exists for some operating systems, even
       though the actual client may work, so a pioneering user may well need
       to create a new script or modify an existing one.  In general,
       customizations specific to a particular computer should be done in the
       /etc/dhclient.conf file.   If you find that you can't make such a
       customization without customizing /etc/dhclient.conf or using the enter
       and exit hooks, please submit a bug report.

       When it starts, the client script first defines a shell function,
       make_resolv_conf , which is later used to create the /etc/resolv.conf
       file.   To override the default behaviour, redefine this function in
       the enter hook script.

       On after defining the make_resolv_conf function, the client script
       checks for the presence of an executable /etc/dhclient-enter-hooks
       script, and if present, it invokes the script inline, using the Bourne
       shell '.' command.   The entire environment documented under OPERATION
       is available to this script, which may modify the environment if needed
       to change the behaviour of the script.   If an error occurs during the
       execution of the script, it can set the exit_status variable to a
       nonzero value, and /sbin/dhclient-script will exit with that error code
       immediately after the client script exits.

       After all processing has completed, /sbin/dhclient-script checks for
       the presence of an executable /etc/dhclient-exit-hooks script, which if
       present is invoked using the '.' command.  The exit status of dhclient-
       script will be passed to dhclient-exit-hooks in the exit_status shell
       variable, and will always be zero if the script succeeded at the task
       for which it was invoked.   The rest of the environment as described
       previously for dhclient-enter-hooks is also present.   The
       /etc/dhclient-exit-hooks script can modify the valid of exit_status to
       change the exit status of dhclient-script.

       When dhclient needs to invoke the client configuration script, it
       defines a set of variables in the environment, and then invokes
       CLIENTBINDIR/dhclient-script. In all cases, $reason is set to the name
       of the reason why the script has been invoked.   The following reasons
       are currently defined: MEDIUM, PREINIT, BOUND, RENEW, REBIND, REBOOT,

       The DHCP client is requesting that an interface's media type be set.
       The interface name is passed in $interface, and the media type is
       passed in $medium.

       The DHCP client is requesting that an interface be configured as
       required in order to send packets prior to receiving an actual address.
       For clients which use the BSD socket library, this means configuring
       the interface with an IP address of and a broadcast address of   For other clients, it may be possible to simply
       configure the interface up without actually giving it an IP address at
       all.   The interface name is passed in $interface, and the media type
       in $medium.

       If an IP alias has been declared in dhclient.conf, its address will be
       passed in $alias_ip_address, and that ip alias should be deleted from
       the interface, along with any routes to it.

       The DHCP client has done an initial binding to a new address.   The new
       ip address is passed in $new_ip_address, and the interface name is
       passed in $interface.   The media type is passed in $medium.   Any
       options acquired from the server are passed using the option name
       described in dhcp-options, except that dashes ('-') are replaced by
       underscores ('_') in order to make valid shell variables, and the
       variable names start with new_.   So for example, the new subnet mask
       would be passed in $new_subnet_mask.

       Before actually configuring the address, dhclient-script should somehow
       ARP for it and exit with a nonzero status if it receives a reply.   In
       this case, the client will send a DHCPDECLINE message to the server and
       acquire a different address.   This may also be done in the RENEW,
       REBIND, or REBOOT states, but is not required, and indeed may not be

       When a binding has been completed, a lot of network parameters are
       likely to need to be set up.   A new /etc/resolv.conf needs to be
       created, using the values of $new_domain_name and
       $new_domain_name_servers (which may list more than one server,
       separated by spaces).   A default route should be set using
       $new_routers, and static routes may need to be set up using

       If an IP alias has been declared, it must be set up here.   The alias
       IP address will be written as $alias_ip_address, and other DHCP options
       that are set for the alias (e.g., subnet mask) will be passed in
       variables named as described previously except starting with $alias_
       instead of $new_.   Care should be taken that the alias IP address not
       be used if it is identical to the bound IP address ($new_ip_address),
       since the other alias parameters may be incorrect in this case.

       When a binding has been renewed, the script is called as in BOUND,
       except that in addition to all the variables starting with $new_, there
       is another set of variables starting with $old_.  Persistent settings
       that may have changed need to be deleted - for example, if a local
       route to the bound address is being configured, the old local route
       should be deleted.  If the default route has changed, the old default
       route should be deleted.  If the static routes have changed, the old
       ones should be deleted.  Otherwise, processing can be done as with

       The DHCP client has rebound to a new DHCP server.  This can be handled
       as with RENEW, except that if the IP address has changed, the ARP table
       should be cleared.

       The DHCP client has successfully reacquired its old address after a
       reboot.   This can be processed as with BOUND.

       The DHCP client has failed to renew its lease or acquire a new one, and
       the lease has expired.   The IP address must be relinquished, and all
       related parameters should be deleted, as in RENEW and REBIND.

       The DHCP client has been unable to contact any DHCP servers, and any
       leases that have been tested have not proved to be valid.   The
       parameters from the last lease tested should be deconfigured.   This
       can be handled in the same way as EXPIRE.

       The DHCP client has been unable to contact any DHCP servers.  However,
       an old lease has been identified, and its parameters have been passed
       in as with BOUND.   The client configuration script should test these
       parameters and, if it has reason to believe they are valid, should exit
       with a value of zero.   If not, it should exit with a nonzero value.

       The usual way to test a lease is to set up the network as with REBIND
       (since this may be called to test more than one lease) and then ping
       the first router defined in $routers.  If a response is received, the
       lease must be valid for the network to which the interface is currently
       connected.   It would be more complete to try to ping all of the
       routers listed in $new_routers, as well as those listed in
       $new_static_routes, but current scripts do not do this.

       Each operating system should generally have its own script file,
       although the script files for similar operating systems may be similar
       or even identical.   The script files included in Internet Systems
       Consortium DHCP distribution appear in the distribution tree under
       client/scripts, and bear the names of the operating systems on which
       they are intended to work.

       If more than one interface is being used, there's no obvious way to
       avoid clashes between server-supplied configuration parameters - for
       example, the stock dhclient-script rewrites /etc/resolv.conf.   If more
       than one interface is being configured, /etc/resolv.conf will be
       repeatedly initialized to the values provided by one server, and then
       the other.   Assuming the information provided by both servers is
       valid, this shouldn't cause any real problems, but it could be

       dhclient(8), dhcpd(8), dhcrelay(8), dhclient.conf(5) and

       dhclient-script(8) has been written for Internet Systems Consortium by
       Ted Lemon in cooperation with Vixie Enterprises.  To learn more about
       Internet Systems Consortium, see http://www.isc.org. To learn more
       about Vixie Enterprises, see http://www.vix.com.