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dhclient(8)							   dhclient(8)

NAME
       dhclient - Dynamic Host Configuration Protocol Client

SYNOPSIS
       dhclient	 [ -4 | -6 ] [ -S ] [ -N [ -N...  ] ] [ -T [ -T...  ] ] [ -P [
       -P...  ] ] [ -D LL|LLT ] [ -p port ] [ -d ] [ -e VAR=value ] [ -q  ]  [
       -1 ] [ -r | -x ] [ -lf lease-file ] [ -pf pid-file ] [ --no-pid ] [ -cf
       config-file ] [ -sf script-file ] [ -s server-addr ] [ -g relay ] [  -n
       ] [ -nw ] [ -w ] [ -v ] [ --version ] [ 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.

       There are two versions of the DHCP  protocol  DHCPv4  and  DHCPv6.   At
       startup the client may be started for one or the other via the -4 or -6
       options.

       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	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
       file.   If interfaces are specified in this way, then the  client  will
       only  configure	interfaces that are either specified in the configura‐
       tion file or on the command line, and will ignore all other interfaces.

       The client normally prints no output during its startup	sequence.   It
       can  be	made  to emit verbose messages displaying the startup sequence
       events until it has acquired an address by  supplying  the  -v  command
       line argument.  In either case, the client logs messages using the sys‐
       log(3) facility.

OPTIONS
       -4     Use the DHCPv4 protocol to obtain an IPv4 address and configura‐
	      tion  parameters.	  This	is  the default and cannot be combined
	      with -6.

       -6     Use the DHCPv6 protocol to obtain whatever  IPv6	addresses  are
	      available	 along	with  configuration  parameters.  It cannot be
	      combined with -4.	 The -S -T -P -N and -D arguments provide more
	      control  over aspects of the DHCPv6 processing.  Note: it is not
	      recommended to mix queries of different types together  or  even
	      to share the lease file between them.

       -1     Try  to  get  a  lease  once.   On failure exit with code 2.  In
	      DHCPv6 this sets the maximum duration of the initial exchange to
	      timeout (from dhclient.conf(5) with a default of sixty seconds).

       -d     Force  dhclient  to  run	as a foreground process.  Normally the
	      DHCP client will run in the foreground until is  has  configured
	      an  interface  at	 which	time  it will revert to running in the
	      background.  This option is useful when running the client under
	      a	 debugger,  or when running it out of inittab on System V sys‐
	      tems.  This implies -v.

       -nw    Become a daemon immediately (nowait) rather than	waiting	 until
	      an an IP address has been acquired.

       -q     Be quiet at startup, this is the default.

       -v     Enable verbose log messages.

       -w     Continue	running	 even  if  no broadcast interfaces were found.
	      Normally DHCP client will exit 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.  This
	      flag can be used to cause the client not to exit when it doesn't
	      find  any	 such interfaces.   The omshell(1) 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 interface.

       -n     Do not configure any interfaces.	This is most likely to be use‐
	      ful in combination with the -w flag.

       -e VAR=val
	      Define  additional  environment  variables  for  the environment
	      where dhclient-script(8) executes.  You may specify multiple  -e
	      options on the command line.

       -r     Release  the  current  lease and stop the running DHCP client as
	      previously recorded in the PID file.   When  shutdown  via  this
	      method  dhclient-script(8)  will	be  executed with the specific
	      reason for calling the script set.  The client normally  doesn't
	      release  the  current  lease as this is not required by the DHCP
	      protocol but some cable ISPs require their clients to notify the
	      server if they wish to release an assigned IP address.

       -x     Stop  the	 running  DHCP	client	without	 releasing the current
	      lease.  Kills existing dhclient process as  previously  recorded
	      in  the  PID  file.   When  shutdown  via	 this method dhclient-
	      script(8) will be executed with the specific reason for  calling
	      the script set.

       -p port
	      The  UDP	port number on which the DHCP client should listen and
	      transmit.	 If unspecified, dhclient uses the default port of 68.
	      This  is	mostly	useful for debugging purposes.	If a different
	      port is specified on which the client should listen  and	trans‐
	      mit, the client will also use a different destination port - one
	      less than the specified port.

       -s server-addr
	      Specify the server IP address or fully qualified domain name  to
	      use  as a destination for DHCP protocol messages before dhclient
	      has acquired an IP address.  Normally, dhclient transmits	 these
	      messages	to 255.255.255.255 (the IP limited broadcast address).
	      Overriding this is mostly useful for debugging  purposes.	  This
	      feature is not supported in DHCPv6 (-6) mode.

       -g relay
	      Set the giaddr field of all packets to the relay IP address sim‐
	      ulating a relay agent.  This is for testing  pruposes  only  and
	      should not be expected to work in any consistent or useful way.

       --version
	      Print version number and exit.

       Options available for DHCPv6 mode:

       -S     Use  Information-request	to  get	 only  stateless configuration
	      parameters (i.e., without address).  This implies -6.   It  also
	      doesn't rewrite the lease database.

       -T     Ask  for	IPv6  temporary	 addresses, one set per -T flag.  This
	      implies -6 and also disables the normal address query.   See  -N
	      to restore it.

       -P     Enable  IPv6  prefix  delegation.	 This implies -6 and also dis‐
	      ables the normal address query.  See -N  to  restore  it.	  Note
	      only one requested interface is allowed.

       -D LL or LLT
	      Override the default when selecting the type of DUID to use.  By
	      default, DHCPv6 dhclient creates	an  identifier	based  on  the
	      link-layer  address (DUID-LL) if it is running in stateless mode
	      (with -S, not requesting an address), or it creates  an  identi‐
	      fier based on the link-layer address plus a timestamp (DUID-LLT)
	      if it is running in stateful mode	 (without  -S,	requesting  an
	      address).	  -D overrides this default, with a value of either LL
	      or LLT.

       -N     Restore normal address query for IPv6. This implies -6.	It  is
	      used to restore normal operation after using -T or -P.

       Modifying  default file locations: The following options can be used to
       modify the locations a client uses for it's files.  They can be partic‐
       ularly  useful  if,  for example, DBDIR or RUNDIR have not been mounted
       when the DHCP client is started.

       -cf config-file
	      Path to the client  configuration	 file.	 If  unspecified,  the
	      default  ETCDIR/dhclient.conf is used.  See dhclient.conf(5) for
	      a description of this file.

       -lf lease-file
	      Path to the lease database file.	If  unspecified,  the  default
	      DBDIR/dhclient.leases  is	 used.	 See  dhclient.leases(5) for a
	      descriptionof this file.

       -pf pid-file
	      Path to the  process  ID	file.	If  unspecified,  the  default
	      RUNDIR/dhclient.pid is used.

       --no-pid
	      Option  to  disable  writing  pid files.	By default the program
	      will write a pid file.  If the  program  is  invoked  with  this
	      option it will not attempt to kill any existing client processes
	      even if invoked with -r or -x.

       -sf script-file
	      Path to the network configuration	 script	 invoked  by  dhclient
	      when  it	gets  a	 lease.	  If  unspecified, the default CLIENT‐
	      BINDIR/dhclient-script is used.  See  dhclient-script(8)	for  a
	      description of this file.

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.

ENVIRONMENT VARIABLES
       The  following  environment  variables  may  be defined to override the
       builtin defaults for file locations.  Note that use of the related com‐
       mand-line  options  will	 ignore the corresponding environment variable
       settings.

       PATH_DHCLIENT_CONF
	      The dhclient.conf configuration file.

       PATH_DHCLIENT_DB
	      The dhclient.leases database.

       PATH_DHCLIENT_PID
	      The dhclient PID file.

       PATH_DHCLIENT_SCRIPT
	      The dhclient-script file.

FILES
       CLIENTBINDIR/dhclient-script,			 ETCDIR/dhclient.conf,
       DBDIR/dhclient.leases, RUNDIR/dhclient.pid, DBDIR/dhclient.leases~.

SEE ALSO
       dhcpd(8),     dhcrelay(8),     dhclient-script(8),    dhclient.conf(5),
       dhclient.leases(5), dhcp-eval(5).

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  https://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)
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