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DHCLIENT(8)		FreeBSD	System Manager's Manual		   DHCLIENT(8)

     dhclient -- Dynamic Host Configuration Protocol (DHCP) client

     dhclient [-dnrv] [-c file]	[-i options] interface

     dhclient uses the Dynamic Host Configuration Protocol (DHCP), or its pre-
     decessor BOOTP, to	configure a network interface.	Information typically
     provided via DHCP includes	IPv4 address and subnet	mask, default route,
     and domain	name server.

     dhclient can be run on the	command	line, or at boot time by specifying
     `dhcp' in the appropriate hostname.if(5) file.

     The options are as	follows:

     -c	file
	     Specify an	alternate location to /etc/dhclient.conf for the con-
	     figuration	file.  If file is the empty string then	no configura-
	     tion file is read.

     -d	     Do	not daemonize.	If this	option is specified, dhclient will run
	     in	the foreground and log to stderr.

     -i	options
	     dhclient will ignore values provided by leases for	the options
	     specified.	 This list will	supplement ignore statements in
	     dhclient.conf(5).	options	must be	a comma	separated list of
	     valid option names.

     -n	     Configtest	mode.  Only check the configuration file for validity.

     -r	     Release the current lease back to the server it came from.
	     dhclient exits after removing the active lease from
	     /var/db/dhclient.leases.<IFNAME>, deleting	the address the	lease
	     caused to be added	to the interface, and sending a	DHCPRELEASE
	     packet to the server that supplied	the lease.

	     If	there is no dhclient controlling the specified interface, or
	     dhclient has no active lease configured, no action	is performed.

     -v	     Causes dhclient to	show more information about interactions with
	     the DHCP server and what network configuration changes are	at-
	     tempted after accepting a lease.  -v is implied if	either -d or
	     -n	is present.

     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 the network.	The DHCP pro-
     tocol also	provides a mechanism whereby a client can learn	important de-
     tails 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	/etc/dhclient.conf for configuration instruc-
     tions.  It	then attempts to configure the network interface interface
     with DHCP.	 The special value "egress" may	be used	instead	of a network
     interface name.  In this case dhclient will look for the network inter-
     face currently in the interface group "egress" and	configure it with
     DHCP.  If there is	more than one network interface	in the egress group
     dhclient will exit	with an	error.

     When configuring the interface, dhclient attempts to remove any existing
     addresses,	gateway	routes that use	the interface, and non-permanent
     arp(8) entries.  dhclient automatically exits whenever a new dhclient is
     run on the	same interface.

     Once the interface	is configured, dhclient	constructs a resolv.conf(5)
     file.  It does this only if any of	the options domain-name,
     domain-name-servers, or domain-search are present (note that these	op-
     tions may be offered by the DHCP server but suppressed by
     dhclient.conf(5)).	 If a resolv.conf is constructed, dhclient appends any
     contents of the resolv.conf.tail(5) file, which are read once at start
     up.  The constructed resolv.conf is copied	into /etc/resolv.conf whenever
     the default route goes out	the interface dhclient is running on.
     dhclient monitors the system for changes to the default route and re-
     checks whether it should write its	resolv.conf when possible changes are

     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
     /var/db/dhclient.leases.<IFNAME> file.  IFNAME represents the network in-
     terface of	the DHCP client	(e.g. em0), one	for each interface.  On
     startup, after reading the	dhclient.conf(5) file, dhclient	reads the
     leases file to refresh its	memory about what leases it has	been assigned.

     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.<IFNAME>
     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, it
     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.

     /etc/dhclient.conf			  DHCP client configuration file
     /etc/hostname.XXX			  interface-specific configuration
     /var/db/dhclient.leases.<IFNAME>	  database of acquired leases

     dhclient.conf(5), dhclient.leases(5), hostname.if(5), dhcpd(8),
     dhcrelay(8), ifconfig(8)

     R.	Droms, Interoperation Between DHCP and BOOTP, RFC 1534,	October	1993.

     R.	Droms, Dynamic Host Configuration Protocol, RFC	2131, March 1997.

     S.	Alexander and R. Droms,	DHCP Options and BOOTP Vendor Extensions, RFC
     2132, March 1997.

     T.	Lemon and S. Cheshire, Encoding	Long Options in	the Dynamic Host
     Configuration Protocol (DHCPv4), RFC 3396,	November 2002.

     T.	Lemon, S. Cheshire, and	B. Volz, The Classless Static Route Option for
     Dynamic Host Configuration	Protocol (DHCP)	version	4, RFC 3442, December

     N.	Swamy, G. Halwasia, and	P. Jhingram, Client Identifier Option in DHCP
     Server Replies, RFC 6842, January 2013.

     dhclient was written by Ted Lemon <> and Elliot Poger

     The current implementation	was reworked by	Henning	Brauer

FreeBSD	13.0		       February	24, 2021		  FreeBSD 13.0


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