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, 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.

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 OPERA-
       TION 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_status shell variable, and will always be zero if the script succeeded at the task for which it was invoked.   The rest of	the  envi-
       ronment	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
       /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 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 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 explic-
       itly  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 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 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 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, process-
       ing 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 param-
       eters 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  cur-
       rently  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 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.

       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)