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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	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
       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_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  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 CLIENT‐
       BINDIR/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.	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 exam‐
       ple 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  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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