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  inter-
       face'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, cus-
       tomizations 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  '.'  command.    It  also  invokes  all  executable  scripts  in
       /etc/dhcp/dhclient-enter-hooks.d/* in the same way.   The entire  envi-
       ronment  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/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 envi-
       ronment  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-script.

       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, $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 con-
       figure 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 vari-
       able 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  cre-
       ated, 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  vari-
       ables  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 parame-
       ters  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 Con-
       sortium  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  confus-

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

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

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