Provided by: isc-dhcp-client_4.2.4-7ubuntu12.13_amd64 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
       ETCDIR/dhclient.conf file.   If you find that you can't make such a customization  without
       customizing  ETCDIR/dhclient.conf  or  using the enter and exit hooks, please submit a bug


       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.

       After defining the make_resolv_conf function, the client script checks for the presence of
       an  executable  ETCDIR/dhclient-enter-hooks  script, and if present, it invokes the script
       inline, using the Bourne shell ´.´ command.   It also invokes all  executable  scripts  in
       ETCDIR/dhclient-enter-hooks.d/* in the same way.   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 CLIENTBINDIR/dhclient-
       script will exit with that error code immediately after the client script exits.

       After  all  processing has completed, CLIENTBINDIR/dhclient-script checks for the presence
       of an executable ETCDIR/dhclient-exit-hooks script, which if present is invoked using  the
       '.'  command.   All executable scripts in ETCDIR/dhclient-exit-hooks.d/* are also invoked.
       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   ETCDIR/dhclient-exit-hooks   and
       ETCDIR/dhclient-exit-hooks.d/* scripts can modify the value of exit_status to  change  the
       exit status of dhclient-script.


       When  dhclient  needs  to  invoke  the  client  configuration  script, it defines a set of
       variables in the environment,  and  then  invokes  CLIENTBINDIR/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,


       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  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.

       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

       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 $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.


       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.


       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  dhclient  has  been  informed  to  shut  down  gracefully, the dhclient-script should
       unconfigure or shutdown the interface as appropriate.


       The dhclient has been executed using the -r flag, indicating that the administrator wishes
       it to release its lease(s).  dhclient-script should unconfigure or shutdown the interface.


       No-Broadcast-Interfaces...dhclient  was  unable  to  find  any  interfaces  upon  which it
       believed it should commence DHCP.  What dhclient-script should do  in  this  situation  is
       entirely up to the implementor.


       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

       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 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.


       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.


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


       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  To learn more about Vixie Enterprises, see