dhclient-script(8)					      System Manager's Manual						dhclient-script(8)

NAME

       dhclient-script - DHCP client network configuration script

DESCRIPTION

       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, customizations specific to a particular computer should be done in the
       /etc/dhclient.conf file.   If you find that you can't make such a customization without customizing /etc/dhclient.conf or using	the  enter
       and exit hooks, please submit a bug report.

HOOKS

       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 OPERA-
       TION 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/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.

OPERATION

       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, EXPIRE, FAIL and TIMEOUT.

MEDIUM

       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.

PREINIT

       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 0.0.0.0 and a broadcast address of
       255.255.255.255.    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.

BOUND

       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.

       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 desirable.

       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 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.

RENEW

       When  a binding has been renewed, the script is called as in BOUND, except 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
       BOUND.

REBIND

       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.

REBOOT

       The DHCP client has successfully reacquired its old address after a reboot.   This can be processed as with BOUND.

EXPIRE

       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.

FAIL

       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 param-
       eters from the last lease tested should be deconfigured.   This can be handled in the same way as EXPIRE.

TIMEOUT

       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 cur-
       rently 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.

FILES

       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.

BUGS

       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.

SEE ALSO

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

AUTHOR

       dhclient-script(8) has been written for Internet Systems Consortium by Ted Lemon in cooperation with  Vixie  Enterprises.   To  learn  more
       about Internet Systems Consortium, see http://www.isc.org.  To learn more about Vixie Enterprises, see http://www.vix.com.

																dhclient-script(8)