trusty (8) dhclient-script.8.gz

Provided by: isc-dhcp-client_4.2.4-7ubuntu12.13_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 ETCDIR/dhclient.conf file.   If
       you find that you can't make such a customization without customizing ETCDIR/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
       ETCDIR/dhclient-enter-hooks script, and if present, it invokes the script inline, using the Bourne  shell
       ´.´ command.   It also invokes all executable scripts in ETCDIR/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 CLIENTBINDIR/dhclient-script will
       exit with that error code immediately after the client script exits.

       After all processing has completed, CLIENTBINDIR/dhclient-script checks for the presence of an executable
       ETCDIR/dhclient-exit-hooks script, which if present is invoked using the  '.'  command.   All  executable
       scripts  in ETCDIR/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 ETCDIR/dhclient-exit-hooks and ETCDIR/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 CLIENTBINDIR/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.

       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_, 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) 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)