man > dhclient-script(8)

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  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 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.

       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 in‐
       line, 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  exe‐
       cutable  /etc/dhcp/dhclient-exit-hooks script, which if present is invoked using the '.' com‐
       mand.  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_status shell
       variable,  and  will  always be zero if the script succeeded at the task for which it was in‐
       voked.   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 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  $inter‐
       face, 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  or‐
       der  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 ex‐
       piration 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 DHCPDE‐
       CLINE 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 de‐
       fault 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 $new_.   Care should be taken that the alias IP address not be used if it is iden‐
       tical  to the bound IP address ($new_ip_address), since the other alias parameters may be in‐
       correct 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 ex‐
       ample, 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  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 de‐
       configured.   This can be handled in the same way as EXPIRE.

STOP
       The  dhclient has been informed to shut down gracefully, the dhclient-script should unconfig‐
       ure 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.

V6ONLY
       The  DHCPv4 client has received a requested valid v6-only-preferred option. The system should
       disable IPv4 on the interface. On its side the dhclient waits for  V6ONLY_WAIT  seconds  (the
       timer  is  carried  by  the option with a minimum of MIN_V6ONLY_WAIT) before returning in the
       INIT state.

       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.

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)