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
       ETCDIR/dhclient.conf file.   If you find that you can't make such a
       customization without customizing ETCDIR/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 ETCDIR/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
       CLIENTBINDIR/dhclient-script will exit with that error code immediately
       after the client script exits.

       After all processing has completed, CLIENTBINDIR/dhclient-script checks
       for the presence of an executable ETCDIR/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
       ETCDIR/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

       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

       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.  Options from a non-default universe will have the
       universe name prepended to the option name, for example
       $new_dhcp6_server_id.  The options that the client explicitly requested
       via a PRL or ORO option are passed with the same option name as above but
       prepended with requested_ and with a value of 1, for example
       requested_subnet_mask=1.  No such variable is defined for options not
       requested by the client or options that don't require a request option,
       such as the ip address (*_ip_address) or expiration time (*_expiry).

       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_, and
       $requested_ 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 BOUND.

       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 dhclient has been informed to shut down gracefully, the dhclient-
       script should unconfigure or shutdown the interface as appropriate.

       The dhclient has been executed using the -r flag, indicating that the
       administrator wishes it to release its lease(s).  dhclient-script should
       unconfigure or shutdown the interface.

       No-Broadcast-Interfaces...dhclient was unable to find any interfaces upon
       which it believed it should commence DHCP.  What dhclient-script should
       do in this situation is entirely up to the implementor.

       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

       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 confusing.

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

       dhclient-script(8) To learn more about Internet Systems Consortium, see