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RedHat 9 (Linux i386) - man page for dhclient-script (redhat section 8)

dhclient-script(8)							       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
       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
       /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
       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/dhclient-exit-hooks script, which if present is invoked using the '.' com-
       mand.  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  vari-
       ables  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, 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 broad-
       cast address of 255.255.255.255.   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.

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 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.

       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-
       able.

       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, seperated 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 parameters 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 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.

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 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.

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 the Internet Software Consortium by Ted Lemon in
       cooperation with Vixie Enterprises.  To learn more about the Internet Software Consortium,
       see http://www.isc.org.	To learn more about Vixie Enterprises, see http://www.vix.com.

									       dhclient-script(8)


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