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



NAME
       dhclient - Dynamic Host Configuration Protocol Client

SYNOPSIS
       dhclient	 [ -4 | -6 ] [ -S ] [ -N [ -N...  ] ] [ -T [ -T...  ] ] [ -P [
       -P...  ] ] [ -p port ] [ -d ] [ -e VAR=value ] [ -q ] [ -1 ] [ -r |  -x
       ] [ -lf lease-file ] [ -pf pid-file ] [ -cf config-file ] [ -sf script-
       file ] [ -s server ] [ -g relay ] [ -n ] [ -nc ] [ -nw ] [ -w ] [ -B  ]
       [  -I  dhcp-client-identifier  ] [ -H host-name ] [ -F fqdn.fqdn ] [ -V
       vendor-class-identifier ] [ -R request-option-list ] [ -timeout timeout
       ] [ -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.

       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.

OPTIONS
       -4     Use the DHCPv4 protocol to obtain an IPv4 address and configura-
	      tion parameters (default).


       -6     Use the DHCPv6 protocol to obtain whatever  IPv6	addresses  are
	      available	 along with configuration parameters.  The functional-
	      ity of DHCPv6 mode may be modified with the -S ,	-T  ,  and  -N
	      options.


       -S     Perform an information-only request over DHCPv6 to get stateless
	      configuration parameters.	 It is not recommended to combine this
	      option  with the -N , -P , or -T options or to share lease files
	      between different modes of operation.  Only valid	 with  the  -6
	      option.


       -N     Perform  a  normal (IA_NA) address query over DHCPv6.  It is not
	      recommended to combine this option with the -P  ,	 -S  ,	or  -T
	      options or to share lease files between different modes of oper-
	      ation.  Only valid with the -6 option.


       -T     Perform a temporary (IA_TA) address query over DHCPv6  (disables
	      normal  address  query).	 It is not recommended to combine this
	      option with the -N , -P , or -S options or to share lease	 files
	      between  different  modes	 of operation.	Only valid with the -6
	      option.


       -P     Enable IPv6 prefix delegation (disables normal  address  query).
	      It  is  not not recommended to combine this option with the -N ,
	      -S , or -T options or to share  lease  files  between  different
	      modes of operation.  Only valid with the -6 option.


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


       -d     Force dhclient to run as a foreground process.  This  is	useful
	      when running the client under a debugger, or when running it out
	      of inittab on System V systems.


       -e VAR=value
	      Define additional	 environment  variables	 for  the  environment
	      where  dhclient-script  executes.	 You may specify multiplate -e
	      options on the command line.  For example: -e IF_METRIC=1


       -q     Suppress all terminal and log output except error messages.


       -1     Try once to get a lease.	One failure, exit with code 2.


       -r     Tell dhclient to release the  current  lease  it	has  from  the
	      server.	This  is  not  required by the DHCP protocol, but some
	      ISPs require their clients to notify the server if they wish  to
	      release an assigned IP address.


       -lf <lease-file>
	      Path  to	the  lease database file.  If unspecified, the default
	      /var/lib/dhclient/dhclient.leases is used.


       -pf <pid-file>
	      Path to the  process  ID	file.	If  unspecified,  the  default
	      /var/run/dhclient.pid is used.


       -cf <config-file>
	      Path  to	the  client  configuration  file.  If unspecified, the
	      default /etc/dhcp/dhclient.conf is used.


       -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.


       -s <server>
	      Specifiy the server IP address or fully qualified domain name to
	      transmit	DHCP  protocol messages to.  Normally, dhclient trans-
	      mits these messages to 255.255.255.255 (the IP limited broadcast
	      address).	  Overriding  this is mostly useful for debugging pur-
	      poses.


       -g <relay>
	      Only for debugging.  Set the giaddr field	 of  all  packets  the
	      client  sends  to	 the IP address specified.  This should not be
	      expected to work in any consistent or useful way.


       -n     Do not configure any interfaces.	Most useful combined with  the
	      -w option.


       -nc    Do not drop capabilities.

	      Normally,	 if  dhclient  was  compiled  with  libcap-ng support,
	      dhclient	drops  most  capabilities  immediately	upon  startup.
	      While more secure, this greatly restricts the additional actions
	      that hooks in dhclient-script (8) can take.  (For	 example,  any
	      daemons that dhclient-script (8) starts or restarts will inherit
	      the restricted capabilities as well, which  may  interfere  with
	      their  correct  operation.)  Thus, the -nc option can be used to
	      prevent dhclient from dropping capabilities.

	      The -nc option is ignored if dhclient was not compiled with lib-
	      cap-ng support.


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


       -w     Keep running even if  no	network	 interfaces  are  found.   The
	      omshell  program can be used to notify the client when a network
	      interface has been added or removed so it can attempt to config-
	      ure an IP address on that interface.


       -B     Set  the BOOTP broadcast flag in request packets so servers will
	      always broadcast replies.


       -I <dhcp-client-identifier>
	      Specify the dhcp-client-identifier option to send	 to  the  DHCP
	      server.


       -H <host-name>
	      Specify  the  host-name  option to send to the DHCP server.  The
	      host-name string only contains the client's hostname prefix,  to
	      which  the server will append the ddns-domainname or domain-name
	      options, if any, to derive the fully qualified  domain  name  of
	      the client.  The -H option cannot be used with the -F option.


       -F <fqdn.fqdn>
	      Specify  the  fqdn.fqdn option to send to the DHCP server.  This
	      option cannot be used with the -H option.	 The fqdn.fqdn	option
	      must  specify the complete domain name of the client host, which
	      the server may use for dynamic DNS updates.


       -V <vendor-class-identifier>
	      Specify the vendor-class-identifier option to send to  the  DHCP
	      server.


       -R <option>[,<option>...]
	      Specify  the  list  of options the client is to request from the
	      server.  The option list must be a single string	consisting  of
	      option  names  separated	by  at	least one command and optional
	      space characters.	 The default option list is:


		  subnet-mask, broadcast-address, time-offset, routers,
		  domain-name, domain-name-servers, host-name,
		  nis-domain, nis-servers, ntp-servers, interface-mtu

	      The -R option does not append options to the default request, it
	      overrides	 the  default  request list.  Keep this in mind if you
	      want to request an additional option besides the default request
	      list.   You  will	 have  to  specify all option names for the -R
	      parameter.


       -timeout <timeout>
	      Specify the time after which dhclient will decide that  no  DHCP
	      servers can be contacted when no responses have been received.


       -v     Enable verbose log messages.


       If  the	client	is  killed  by	a  signale (for example at shutdown or
       reboot), it will not execute the dhclient-script (8) at exit.  However,
       if  you	shut  the client down gracefully with -r or -x it will execute
       dhclient-script (8) at shutdown with the specific  reason  for  calling
       the script set in the environment table.

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/dhcp/dhclient.conf,
       /var/lib/dhclient/dhclient.leases,		/var/run/dhclient.pid,
       /var/lib/dhclient/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)