dhclient


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 ] [ --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 dhclient.conf for configu-
       ration  instructions.    It  then gets a list of all the network inter-
       faces 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
       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 and -N  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
              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.

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

              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
              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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