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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  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.

       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, cus-
       tomizations specific to a particular computer should  be  done  in  the
       /etc/dhclient.conf  file.   If you find that you can't make such a cus-
       tomization without customizing dhclient-script 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/dhclient-enter-hooks
       script, and if present, it invokes the script inline, using the  Bourne
       shell  '.' command.   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 /etc/dhclient-script will exit with that error code
       immediately after the client script exits.

       After all processing has completed, /etc/dhclient-script checks for the
       presence  of  an  executable  /etc/dhclient-exit-hooks script, which if
       present is invoked using the '.' command.   The exit status  is  passed
       in  the  exit_status  shell  variable,  and  will always be zero if the
       script succeeded at the task for which it was invoked.

OPERATION
       When dhclient needs to  invoke  the  client  configuration  script,  it
       writes  a  shell script into /tmp which defines a variety of variables.
       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,  ARPCHECK,  ARPSEND,  BOUND,  RENEW,  REBIND,  REBOOT,
       EXPIRE, FAIL 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 con-
       figure 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.

ARPSEND
       The DHCP client is requesting that an address that has been offered  to
       it  be  checked  to see if somebody else is using it, by sending an ARP
       request for that address.   It's not clear how to implement this, so no
       examples   exist   yet.     The  IP  address  to  check  is  passed  in
       $new_ip_address, and the interface name is passed in $interface.

ARPCHECK
       The DHCP client wants to know if a response to  the  ARP  request  send
       using  ARPSEND  has been received.   If one has, the script should exit
       with a nonzero status, indicating that the offered address has  already
       been requested and should be declined.   $new_ip_address and $interface
       are set as with ARPSEND.

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.

       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, seperated 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  vari-
       ables  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 parame-
       ters  from  the last lease tested should be deconfigured.   This can be
       handled in the same way as EXPIRE.

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 the Internet Software
       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 confus-
       ing.

SEE ALSO
       dhclient(8),    dhcpd(8),     dhcrelay(8),     dhclient.conf(5)     and
       dhclient.leases(5).

AUTHOR
       dhclient-script(8)  has  been written for the Internet Software Consor-
       tium by Ted Lemon <mellonATfugue.com> in cooperation with  Vixie  Enter-
       prises.   To  learn  more  about  the Internet Software Consortium, see
       http://www.vix.com/isc.  To learn more  about  Vixie  Enterprises,  see
       http://www.vix.com.



                                                            dhclient-script(8)