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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  pio-
       neering  user  may well need to create a new script or modify an existing one.  In general, customizations spe-
       cific 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 /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  environ-
       ment,  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 $inter-
       face, 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 inter-
       face 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 sta-
       tus 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 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_, 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  parameters  have been passed in as with BOUND.   The client configuration script should test these parame-
       ters 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 operat-
       ing 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  con-
       figuration  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.

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)