Provided by: dhcp-client_2.0pl5-19.4_i386 bug


       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
       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 dhclient-script 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
       nonzero  value, and /etc/dhclient-script will exit with that error code
       immediately after the client script exits.

       After all processing has completed, /etc/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  is  passed
       in  the  exit_status  shell  variable,  and  will always be zero if the
       script succeeded at the task for which it was invoked.


       When dhclient needs to  invoke  the  client  configuration  script,  it
       writes  a  shell script into /tmp which defines a variety of variables.
       In all cases, $reason is set to the name of the reason why  the  script
       has  been  invoked.    The  following  reasons  are  currently defined:


       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
       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.


       The DHCP client is requesting that an address that has been offered  to
       it  be  checked  to see if somebody else is using it, by sending an ARP
       request for that address.   It’s not clear how to implement this, so no
       examples   exist   yet.     The  IP  address  to  check  is  passed  in
       $new_ip_address, and the interface name is passed in $interface.


       The DHCP client wants to know if a response to  the  ARP  request  send
       using  ARPSEND  has been received.   If one has, the script should exit
       with a nonzero status, indicating that the offered address has  already
       been requested and should be declined.   $new_ip_address and $interface
       are set as with ARPSEND.


       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
       variable names start with new_.   So for example, the new  subnet  mask
       would be passed in $new_subnet_mask.

       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,
       separated   by   spaces).     A  default  route  should  be  set  using
       $new_routers,  and  static  routes  may  need  to  be  set   up   using

       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.


       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


       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
       parameters 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,
       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.


       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


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


       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   To  learn  more  about Vixie Enterprises, see