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NAME    [Toc]    [Back]

     dhclient-script - DHCP client network configuration script

DESCRIPTION    [Toc]    [Back]

     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.

     In general, customizations specific to a particular computer
should be
     done in the /etc/dhclient.conf file.

OPERATION    [Toc]    [Back]

     When  dhclient(8)  needs  to invoke the client configuration
script, it sets
     up   a   number   of   environment   variables   and    runs
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,

     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
               as  required in order to send packets prior to receiving an actual
 address.  This means configuring  the  interface with an IP
               address  of  and  a  broadcast address of
               The interface name is passed  in  $interface,  and
the media type
               in $medium.

               If    an   IP   alias   has   been   declared   in
dhclient.conf(5), its address
 will be  passed  in  $alias_ip_address,  and
that IP alias
               should  be  deleted from the interface, along with
any routes to

     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
               sent 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
               $new_ip_address  and  $interface  are  set as with

     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(5),  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

               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

               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

     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

     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

     TIMEOUT    The  DHCP  client  has been unable to contact any
DHCP servers.
               However, an old lease has been identified, and its
               have  been  passed  in  as with BOUND.  The client
               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
     (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
     listed   in   $new_routers,  as  well  as  those  listed  in
     but current scripts do not do this.

SEE ALSO    [Toc]    [Back]

     dhclient.conf(5), dhclient.leases(5), dhclient(8), dhcpd(8),

AUTHORS    [Toc]    [Back]

     The  original version of dhclient-script was written for the
     Software Consortium by Ted Lemon <mellon@fugue.com> in cooperation with
     Vixie Enterprises.

     The OpenBSD implementation of dhclient-script was written by
     Kenneth R. Westerback <krw@openbsd.org>.

BUGS    [Toc]    [Back]

     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.

OpenBSD     3.6                         January      1,      1997
[ Back ]
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