Provided by: isc-dhcp-client_4.3.3-5ubuntu12_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
       /etc/dhcp/dhclient.conf file.   If you find that  you  can't  make  such  a  customization
       without  customizing  /etc/dhcp/dhclient.conf  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.

       After defining the make_resolv_conf function, the client script checks for the presence of
       an executable /etc/dhcp/dhclient-enter-hooks script, and if present, it invokes the script
       inline, using the Bourne shell ´.´ command.   It also invokes all  executable  scripts  in
       /etc/dhcp/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  /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/dhcp/dhclient-exit-hooks script, which if present is invoked using the '.'
       command.   All  executable  scripts in /etc/dhcp/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   /etc/dhcp/dhclient-exit-hooks   and
       /etc/dhcp/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  /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,


       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.  The
       options  that  the client explicitly requested via a PRL or ORO option are passed with the
       same option name as above but prepended with requested_ and with a value of 1, or  example
       requested_subnet_mask=1.   No  such  variable  is defined for options not requested by the
       client  or  options  that  don't  require  a  request  option,  such  as  the  ip  address
       (*_ip_address) or expiration time (*_expiry).

       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_,  and  $requested_  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)   To   learn   more   about   Internet    Systems    Consortium,    see