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dhclient(8)									      dhclient(8)

       dhclient - Dynamic Host Configuration Protocol Client

       dhclient [ -p port ] [ -d ] [ -q ] [ -1 ] [ -r ] [ -lf lease-file ] [ -pf pid-file ] [ -cf
       config-file ] [ -sf script-file ] [ -s server ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [ if0 [
       ...ifN ] ]

       The  Internet  Software Consortium DHCP Client, dhclient, provides a means for configuring
       one or more network interfaces using the Dynamic Host Configuration Protocol, BOOTP proto-
       col, or if these protocols fail, by statically assigning an address.

       The  DHCP  protocol allows a host to contact a central server which maintains a list of IP
       addresses which may be assigned on one or more subnets.	 A DHCP  client  may  request  an
       address from this pool, and then use it on a temporary basis for communication on network.
       The DHCP protocol also provides a mechanism whereby a client can learn  important  details
       about  the  network to which it is attached, such as the location of a default router, the
       location of a name server, and so on.

       On startup, dhclient reads the dhclient.conf for  configuration	instructions.	 It  then
       gets a list of all the network interfaces that are configured in the current system.   For
       each interface, it attempts to configure the interface using the DHCP protocol.

       In order to keep track of leases across system reboots and server restarts, dhclient keeps
       a  list of leases it has been assigned in the dhclient.leases(5) file.	On startup, after
       reading the dhclient.conf file, dhclient reads the dhclient.leases  file  to  refresh  its
       memory about what leases it has been assigned.

       When  a new lease is acquired, it is appended to the end of the dhclient.leases file.   In
       order to prevent the file from becoming arbitrarily large, from time to time dhclient cre-
       ates  a	new dhclient.leases file from its in-core lease database.  The old version of the
       dhclient.leases file is retained under the  name  dhclient.leases~  until  the  next  time
       dhclient rewrites the database.

       Old  leases  are kept around in case the DHCP server is unavailable when dhclient is first
       invoked (generally during the initial system boot process).   In that  event,  old  leases
       from  the  dhclient.leases  file  which	have  not yet expired are tested, and if they are
       determined to be valid, they are used until either they expire or the DHCP server  becomes

       A  mobile host which may sometimes need to access a network on which no DHCP server exists
       may be preloaded with a lease for a fixed address on that network.   When all attempts  to
       contact	a DHCP server have failed, dhclient will try to validate the static lease, and if
       it succeeds, will use that lease until it is restarted.

       A mobile host may also travel to some networks on which DHCP is not  available  but  BOOTP
       is.    In  that case, it may be advantageous to arrange with the network administrator for
       an entry on the BOOTP database, so that the host can boot quickly on that  network  rather
       than cycling through the list of old leases.

       The names of the network interfaces that dhclient should attempt to configure may be spec-
       ified on the command line.  If no interface  names  are	specified  on  the  command  line
       dhclient will normally identify all network interfaces, elimininating non-broadcast inter-
       faces if possible, and attempt to configure each interface.

       It is also possible to specify interfaces by  name  in  the  dhclient.conf(5)  file.    If
       interfaces  are specified in this way, then the client will only configure interfaces that
       are either specified in the configuration file or on the command line, and will ignore all
       other interfaces.

       If the DHCP client should listen and transmit on a port other than the standard (port 68),
       the -p flag may used.  It should be followed by the udp port number that  dhclient  should
       use.   This is mostly useful for debugging purposes.  If a different port is specified for
       the client to listen on and transmit on, the client will also use a different  destination
       port - one greater than the specified destination port.

       The  DHCP  client normally transmits any protocol messages it sends before acquiring an IP
       address to,, the IP limited broadcast address.	For  debugging	purposes,
       it  may be useful to have the server transmit these messages to some other address.   This
       can be specified with the -s flag, followed by the IP address or domain name of the desti-

       For  testing  purposes,	the  giaddr field of all packets that the client sends can be set
       using the -g flag, followed by the IP address to send.	This is only useful for  testing,
       and should not be expected to work in any consistent or useful way.

       The  DHCP client will normally run in the foreground until it has configured an interface,
       and then will revert to running in the background.   To run force dhclient to  always  run
       as a foreground process, the -d flag should be specified.  This is useful when running the
       client under a debugger, or when running it out of inittab on System V systems.

       The client normally prints a startup message and displays the  protocol	sequence  to  the
       standard  error	descriptor  until it has acquired an address, and then only logs messages
       using the syslog (3) facility.	The -q flag prevents any messages other than errors  from
       being printed to the standard error descriptor.

       The  client  normally  doesn't release the current lease as it is not required by the DHCP
       protocol.  Some cable ISPs require their clients to notify the  server  if  they  wish  to
       release	an  assigned  IP address.  The -r flag explicitly releases the current lease, and
       once the lease has been released, the client exits.

       The -1 flag cause dhclient to try once to get a lease.  If it fails, dhclient  exits  with
       exit code two.

       The  DHCP  client normally gets its configuration information from /etc/dhclient.conf, its
       lease database from /var/lib/dhcp/dhclient.leases, stores its process ID in a file  called
       /var/run/dhclient.pid, and configures the network interface using /sbin/dhclient-script To
       specify different names and/or locations for these files, use the -cf, -lf,  -pf  and  -sf
       flags,  respectively,  followed by the name of the file.   This can be particularly useful
       if, for example, /var/lib/dhcp or /var/run has not yet been mounted when the  DHCP  client
       is started.

       The DHCP client normally exits if it isn't able to identify any network interfaces to con-
       figure.	 On laptop computers and other computers with hot-swappable I/O buses, it is pos-
       sible  that  a broadcast interface may be added after system startup.   The -w flag can be
       used to cause the client not to exit when it  doesn't  find  any  such  interfaces.    The
       omshell	(8)  program  can  then be used to notify the client when a network interface has
       been added or removed, so that the client can attempt to configure an IP address  on  that

       The  DHCP  client  can be directed not to attempt to configure any interfaces using the -n
       flag.   This is most likely to be useful in combination with the -w flag.

       The client can also be instructed to become a  daemon  immediately,  rather  than  waiting
       until it has acquired an IP address.   This can be done by supplying the -nw flag.

       The syntax of the dhclient.conf(8) file is discussed seperately.

       The  DHCP client provides some ability to control it while it is running, without stopping
       it.  This capability is provided using OMAPI, an  API  for  manipulating  remote  objects.
       OMAPI  clients  connect to the client using TCP/IP, authenticate, and can then examine the
       client's current status and make changes to it.

       Rather than implementing the underlying OMAPI protocol directly, user programs should  use
       the  dhcpctl  API  or OMAPI itself.   Dhcpctl is a wrapper that handles some of the house-
       keeping chores that OMAPI does not do automatically.   Dhcpctl and OMAPI are documented in
       dhcpctl(3)  and	omapi(3).    Most  things  you'd  want	to do with the client can be done
       directly using the omshell(1) command, rather than having to write a special program.

       The control object allows you to shut the client down, releasing all leases that it  holds
       and  deleting any DNS records it may have added.  It also allows you to pause the client -
       this unconfigures any interfaces the client is using.   You can	then  restart  it,  which
       causes  it to reconfigure those interfaces.   You would normally pause the client prior to
       going into hibernation or sleep on a laptop computer.   You would then resume it after the
       power  comes back.  This allows PC cards to be shut down while the computer is hibernating
       or sleeping, and then reinitialized to their previous state once the computer comes out of
       hibernation or sleep.

       The control object has one attribute - the state attribute.   To shut the client down, set
       its state attribute to 2.   It will automatically do a DHCPRELEASE.   To pause it, set its
       state attribute to 3.   To resume it, set its state attribute to 4.

       /sbin/dhclient-script,	       /etc/dhclient.conf,	   /var/lib/dhcp/dhclient.leases,
       /var/run/dhclient.pid, /var/lib/dhcp/dhclient.leases~.

       dhcpd(8), dhcrelay(8), dhclient-script (8), dhclient.conf(5), dhclient.leases(5).

       dhclient(8) has been written for the Internet Software Consortium by Ted Lemon in coopera-
       tion  with  Vixie  Enterprises.	To learn more about the Internet Software Consortium, see
       http://www.isc.org To learn more about Vixie Enterprises, see http://www.vix.com.

       This client was substantially modified and enhanced by Elliot Poger for use on Linux while
       he was working on the MosquitoNet project at Stanford.

       The  current version owes much to Elliot's Linux enhancements, but was substantially reor-
       ganized and partially rewritten by Ted Lemon so as to use the  same  networking	framework
       that  the Internet Software Consortium DHCP server uses.   Much system-specific configura-
       tion code was moved into a shell script so that as support for more operating  systems  is
       added, it will not be necessary to port and maintain system-specific configuration code to
       these operating systems - instead, the shell script can invoke the native tools to  accom-
       plish the same purpose.

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