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dhclient-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, 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.


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 ’.’ 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
       /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.
       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 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.


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 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.


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
       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 parame-
       ters 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 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 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.


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  confus-
       ing.


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)

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