dhclient-script(8)                                                                                            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 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, 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, customizations specific to a particular com-
       puter 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_sta-
       tus 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.  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  script  can  modify  the  valid of exit_status 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  /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  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.

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 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
       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  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 pre-
       viously 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 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  parameters  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 parame-
       ters 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 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.

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 con-
       figured,  /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  con-
       fusing.

       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)