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dhclient(8)							   dhclient(8)

NAME
       dhclient	- Dynamic Host Configuration Protocol Client

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

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

OPERATION
       The DHCP	protocol allows	a host to contact a central server which main-
       tains a list of IP addresses which may be assigned on one or more  sub-
       nets.	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 instruc-
       tions.	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   creates   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 available.

       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	 data-
       base,  so  that	the  host can boot quickly on that network rather than
       cycling through the list	of old leases.

COMMAND	LINE
       The names of the	network	interfaces that	 dhclient  should  attempt  to
       configure  may be specified on the command line.	 If no interface names
       are specified on	the command line dhclient will normally	 identify  all
       network	interfaces,  eliminating non-broadcast interfaces 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.

       The -D flag causes dhclient to save the script it creates  for  use  in
       conjunction with	dhclient-script	in /tmp.

       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	desti-
       nation port - one greater than the specified destination	port.

       The DHCP	client normally	 transmits  any	 protocol  messages  it	 sends
       before  acquiring  an  IP  address  to, 255.255.255.255,	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 destination.

       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.

       On FreeBSD, dhclient can	be enabled to automatically  handle  the  link
       status  of  the	network	card. Normally polling is done every five sec-
       onds. The polling interval can be set using the -i  flag,  followed  by
       the numbers of seconds. Minimum is one second.

       The  DHCP  client will normally run in the foreground until it has con-
       figured an interface, and then will revert  to  running	in  the	 back-
       ground.	  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.   -v  flag  turns  on	all  messages.
       Opposite	of -q .

       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/db/dhclient.leases,
       stores its process ID in	a file called /var/run/dhclient.pid, and  con-
       figures	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/db 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 configure.	  On laptop computers and other	computers with
       hot-swappable  I/O buses, it is possible	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  net-
       work  interface	has  been  added  or  removed,	so that	the client can
       attempt to configure an IP address on that interface.

       The DHCP	client can be directed not to attempt to configure any	inter-
       faces using the -n flag.	  This is most likely to be useful in combina-
       tion 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.

CONFIGURATION
       The syntax of the dhclient.conf(5) file is discussed separately.

OMAPI
       The DHCP	client provides	some ability to	control	it while  it  is  run-
       ning, 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	housekeeping 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
       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	inter-
       faces  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.

FILES
       /sbin/dhclient-script,	/etc/dhclient.conf,   /var/db/dhclient.leases,
       /var/run/dhclient.pid, /var/db/dhclient.leases~.

SEE ALSO
       dhclient.conf(5), dhclient.leases(5), dhclient-script(8).

AUTHOR
       dhclient(8)  has	 been  written	for Internet Systems Consortium	by Ted
       Lemon in	cooperation with  Vixie	 Enterprises.	To  learn  more	 about
       Internet	Systems	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 Stan-
       ford.

       The current version owes	much to	Elliot's Linux enhancements,  but  was
       substantially reorganized and partially rewritten by Ted	Lemon so as to
       use the same networking framework that the Internet Systems  Consortium
       DHCP  server  uses.   Much system-specific configuration	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 accomplish	the same purpose.

								   dhclient(8)

NAME | SYNOPSIS | DESCRIPTION | OPERATION | COMMAND LINE | CONFIGURATION | OMAPI | THE CONTROL OBJECT | FILES | SEE ALSO | AUTHOR

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