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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 in-
       terface'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 de-
       tails).   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 with-
       out 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.

       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_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 and /etc/dhcp/dhclient-exit-
       hooks.d/* scripts can modify the value  of  exit_status  to  change  the  exit  status  of
       dhclient-script.

OPERATION
       When  dhclient  needs to invoke the client configuration script, it defines a set of vari-
       ables 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 li-
       brary, 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 op-
       tion 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.  Options from a non-
       default  universe  will  have  the universe name prepended to the option name, for example
       $new_dhcp6_server_id.  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, for 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 desir-
       able.

       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_, and $requested_ there is another  set  of  vari-
       ables 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.   Other-
       wise, 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, ex-
       cept 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 ex-
       pired.   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 uncon-
       figure 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 be-
       lieved it should commence DHCP.  What dhclient-script should do in this situation  is  en-
       tirely 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  con-
       figuration  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  in-
       cluded  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  in-
       tended 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.    As-
       suming  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)   To   learn   more   about   Internet    Systems    Consortium,    see
       https://www.isc.org.

                                                                               dhclient-script(8)

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