dhclient-script(8)                             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

          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/dhcp/dhclient.conf file.
          If you find that you can't make such a customization without
          customizing /etc/dhcp/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.

          After defining the make_resolv_conf function, the client
          script checks for the presence of an executable
          /etc/dhcp/dhclient-enter-hooks script, and if present, it
          invokes the script inline, using the Bourne shell '.' com-
          mand.   It also invokes all executable scripts in
          /etc/dhcp/dhclient-enter-hooks.d/* in the same way.   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 immedi-
          ately after the client script exits.

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     dhclient-script(8)                             dhclient-script(8)

          After all processing has completed, /sbin/dhclient-script
          checks for the presence of an executable
          /etc/dhcp/dhclient-exit-hooks script, which if present is
          invoked using the '.' command.  All executable scripts in
          /etc/dhcp/dhclient-exit-hooks.d/* are also invoked.   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/dhcp/dhclient-exit-hooks and
          /etc/dhcp/dhclient-exit-hooks.d/* scripts can modify the
          value of exit_status to change the exit status of dhclient-

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

          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 config-
          ured 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

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     dhclient-script(8)                             dhclient-script(8)

          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 explic-
          itly 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

          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 parame-
          ters 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 $new_static_routes.

          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 vari-
          ables 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.

     Page 3                       Plan 9             (printed 5/25/22)

     dhclient-script(8)                             dhclient-script(8)

          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

          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 inter-

          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 parame-
          ters have been passed in as with BOUND.   The client config-
          uration 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

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     dhclient-script(8)                             dhclient-script(8)

          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 sys-
          tems 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 con-
          figured, /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 con-

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

          dhclient-script(8) To learn more about Internet Systems Con-
          sortium, see https://www.isc.org.

     Page 5                       Plan 9             (printed 5/25/22)