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dhclient-script(8)          System Manager's Manual         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  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,  and  to  handle  DHCP  options  not handled by
       default.
       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/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.
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/dhcp/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 /usr/sbin/dhclient-script will exit with that  error
       code immediately after the client script exits.
       After  all  processing  has completed, /usr/sbin/dhclient-script checks
       for the presence of an executable /etc/dhcp/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_sta-
       tus  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 script can modify the valid of  exit_sta-
       tus to change the exit status of dhclient-script.
       Immediately  after  dhclient  brings  an  interface  UP  with  a new IP
       address, subnet mask, and routes, in the REBOOT/BOUND states,  it  will
       check  for  the  existence of an executable /etc/dhcp/dhclient-up-hooks
       script, and source it if found. This script can handle DHCP options  in
       the  environment  that  are  not  handled  by default. A per-interface.
       /etc/dhcp/dhclient-${IF}-up-hooks  script  will  override  the  generic
       script and be sourced when interface $IF has been brought up.
       Immediately  before  dhclient brings an interface DOWN, removing its IP
       address, subnet mask, and routes, in the STOP/RELEASE  states, it  will
       check  for the existence of an executable /etc/dhcp/dhclient-down-hooks
       script, and source it if found. This script can handle DHCP options  in
       the  environment  that  are  not  handled  by  default. A per-interface
       /etc/dhcp/dhclient-${IF}-down-hooks script will  override  the  generic
       script and be sourced when interface $IF is about to be brought down.
OPERATION
       When  dhclient  needs  to  invoke  the  client configuration script, it
       defines a set  of  variables  in  the  environment,  and  then  invokes
       /usr/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, STOP, RELEASE, NBI 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 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.  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  exam-
       ple  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
       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_, 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.
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 parame-
       ters  from  the last lease tested should be deconfigured.   This can be
       handled in the same way as EXPIRE.
STOP
       The dhclient has been informed to shut down gracefully,  the  dhclient-
       script should unconfigure or shutdown the interface as appropriate.
RELEASE
       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.
NBI
       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.
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.
       Normally,  if  dhclient  was  compiled with libcap-ng support, dhclient
       drops most capabilities immediately upon startup.  While  more  secure,
       this  greatly  restricts the additional actions that hooks in dhclient-
       script can take. For example, any daemons that  dhclient-script  starts
       or restarts will inherit the restricted capabilities as well, which may
       interfere with their correct operation.  Thus, the -nc  option  can  be
       used to prevent dhclient from dropping capabilities.
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 https://www.isc.org.   To  learn  more
       about Vixie Enterprises, see http://www.vix.com.

                                                            dhclient-script(8)