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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	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/dhclient.conf  file.   If you find that you	can't make such	a cus-
       tomization without customizing /etc/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 /etc/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  non-
       zero  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/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
       /etc/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  de-
       fines  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,  EX-
       PIRE, FAIL and TIMEOUT.

       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 re-
       quired 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  op-
       tions  acquired	from  the  server are passed using the option name de-
       scribed in dhcp-options,	except that dashes ('-') are replaced  by  un-
       derscores  ('_')	 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.

       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,  RE-
       BIND,  or REBOOT	states,	but is not required, and indeed	may not	be de-

       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_  in-
       stead 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, ex-
       cept that in addition to	all the	variables starting with	 $new_,	 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

       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 re-
       boot.   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  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, al-
       though the script files for similar operating systems may be similar or
       even  identical.	   The	script files included in the Internet Software
       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 re-
       peatedly	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.conf(5), dhclient.leases(5), dhclient(8).

       dhclient-script(8) has been written for the Internet  Software  Consor-
       tium by Ted Lemon in cooperation	with Vixie Enterprises.	 To learn more
       about the Internet Software  Consortium,	 see   To
       learn more about	Vixie Enterprises, see



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