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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...  ] ] -R ] [ -i ] [ -I ] [ -4o6 port ] [ -D LL|LLT ]  [  -p	 port-
       number  ] [ -d ] [ -df duid-lease-file ] [ -e VAR=value ] [ -q ] [ -1 ]
       [ -r | -x ] [ -lf lease-file ] [ -pf pid-file ] [ --no-pid ] [ -cf con‐
       fig-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 con‐
       figured in the current system.  For each interface, it attempts to con‐
       figure 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 arbi‐
       trarily 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.

       -4o6 port
	      Participate in the DHCPv4 over DHCPv6 protocol specified by  RFC
	      7341.  This associates a DHCPv4 and a DHCPv6 client to allow the
	      v4 client to send v4 requests encapsulated in a v6 packet.  Com‐
	      munication  between  the	two  clients  is done on a pair of UDP
	      sockets bound to ::1 port and port + 1.  Both  clients  must  be
	      launched using the same port argument.

       -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 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 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=value
	      Define  additional  environment  variables  for  the environment
	      where dhclient-script 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 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
	      will be executed with the specific reason for calling the script
	      set.

       -p port-number
	      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  purposes  only  and
	      should not be expected to work in any consistent or useful way.

       -i     Use  a DUID with DHCPv4 clients.	If no DUID is available in the
	      lease file one will be constructed and saved.  The DUID will  be
	      used  to	construct  a  RFC4361  style  client  id  that will be
	      included in the client's messages.  This client id can be	 over‐
	      ridden  by setting a client id in the configuration file.	 Over‐
	      ridding the client id in this fashion is discouraged.

       -I     Use the standard DDNS scheme from RFCs 4701 & 4702.

       --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.  Multiple
	      prefixes can be requested with multiple -P flags.	 Note only one
	      requested interface is allowed.

       -R     Require that responses include all of the items requested by any
	      -N, -T, or -P  options.	Normally  even	if  the	 command  line
	      includes	a number of these the client will be willing to accept
	      the best lease it can even if the lease doesn't include  all  of
	      the  requested  items.   This  option  causes the client to only
	      accept leases that include all of the requested items.

	      Note well: enabling this may prevent the client from  using  any
	      leases  it  receives  if the servers aren't configured to supply
	      all of the items.

       -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).	 When DHCPv4 is configured to  use  a  DUID  using  -i
	      option  the  default  is	to use a DUID-LLT.  -D overrides these
	      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.  Multiple
	      addresses can be requested with multiple -N flags.

       Modifying default file locations: The following options can be used  to
       modify the locations a client uses for its files.  They can be particu‐
       larly useful if, for example, /var/lib/dhcp or /var/run have  not  been
       mounted when the DHCP client is started.

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

       -df duid-lease-file
	      Path  to	a  secondary  lease  file.   If the primary lease file
	      doesn't contain a DUID this file will  be	 searched.   The  DUID
	      read  from  the  secondary will be written to the primary.  This
	      option can be used to allow an IPv4 instance of  the  client  to
	      share  a	DUID with an IPv6 instance.  After starting one of the
	      instances the second can be started with this option pointing to
	      the  lease file of the first instance.  There is no default.  If
	      no file is specified no search is made for a DUID should one not
	      be found in the main lease file.

       -lf lease-file
	      Path  to	the  lease database file.  If unspecified, the default
	      /var/lib/dhcp/dhclient.leases is used.   See  dhclient.leases(5)
	      for a description of this file.

       -pf pid-file
	      Path  to	the  process  ID  file.	  If  unspecified, the default
	      /var/run/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
	      /sbin/dhclient-script  is	 used.	 See  dhclient-script(8) for a
	      description of this file.

PORTS
       During operations the client may use multiple UDP ports to provide dif‐
       ferent  functions.   Which ports are opened depends on both the way you
       compiled your code and the configuration	 you  supply.	The  following
       should provide you an idea of what ports may be in use.

       Normally a DHCPv4 client will open a raw UDP socket to receive and send
       most DHCPv4 packets.  It also opens a fallback UDP socket  for  use  in
       sending	unicast	 packets.  Normally these will both use the well known
       port number for BOOTPC.

       For DHCPv6 the client opens a UDP socket on the well known client  port
       and  a  fallback UDP socket on a random port for use in sending unicast
       messages.  Unlike DHCPv4 the well  known	 socket	 doesn't  need	to  be
       opened in raw mode.

       If you have included an omapi port statement in your configuration file
       then the client will open a TCP socket on that port to listen for OMPAI
       connections.  When something connects another port will be used for the
       established connection.

       When DDNS is enabled at compile time (see includes/site.h)  the	client
       will  open  both a v4 and a v6 UDP socket on random ports.  These ports
       are opened even if DDNS is disabled in the configuration 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
       /sbin/dhclient-script,			      /etc/dhcp/dhclient.conf,
       /var/lib/dhcp/dhclient.leases,			/var/run/dhclient.pid,
       /var/lib/dhcp/dhclient.leases~.

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

AUTHOR
       dhclient(8) To  learn  more  about  Internet  Systems  Consortium,  see
       https://www.isc.org

       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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