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dhclient(8)		    System Manager's Manual		   dhclient(8)

       dhclient	- Dynamic Host Configuration Protocol Client

       dhclient	 [  -p	port ] [ -D ] [	-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  protocol,	 or if these protocols
       fail, by	statically assigning an	address.

       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  ar-
       bitrarily   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  be-
       comes 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  ar-
       range  with  the	 network administrator for an entry on the BOOTP data-
       base, so	that the host can boot quickly on that network rather than cy-
       cling through the list of old leases.

       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 be-
       fore acquiring an IP address to,, the IP	limited	broad-
       cast  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.

       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

       The client normally prints a startup message and	displays the  protocol
       sequence	 to the	standard error descriptor until	it has acquired	an ad-
       dress, 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  re-
       quired  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/, 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  at-
       tempt 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.

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

       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  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 re-
       sume it,	set its	state attribute	to 4.

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

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

       dhclient(8) has been written for	the Internet  Software	Consortium  by
       Ted  Lemon  in cooperation with Vixie Enterprises.  To learn more about
       the Internet Software Consortium, see	To learn  more
       about Vixie Enterprises,	see

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

       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 Software 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.



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