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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
       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

       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  ETCDIR/dhclient-enter-hooks  script, and if present, it invokes the script
       inline, using the Bourne shell ´.´ command.   It also invokes all  executable  scripts  in
       ETCDIR/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 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.   All executable scripts in ETCDIR/dhclient-exit-hooks.d/* are also invoked.
       The exit status of dhclient-script will be passed to dhclient-exit-hooks in the  exit_sta‐
       tus  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 and ETCDIR/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  vari‐
       ables  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, EXPIRE,

       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 broad‐
       cast address of   For other clients, it may be possible to simply config‐
       ure 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 under‐
       scores (´_´) 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.  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, or 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 desir‐

       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 $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.   Other‐
       wise, 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 uncon‐
       figure 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  con‐
       figuration  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 confusing.

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

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


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