bionic (8) dhclient-script.8.gz

Provided by: isc-dhcp-client_4.3.5-3ubuntu7.4_amd64 bug

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.

       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  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_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  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 $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.
       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 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_,  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, 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
       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 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)