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

       dhclient - Dynamic Host Configuration Protocol Client

       dhclient  [ -4 | -6 ] [ -S ] [ -N [ -N...  ] ] [ -T [ -T...  ] ] [ -P [
       -P...  ] ] [ -i ] [ -I ] [ -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 config-file  ]  [  -sf
       script-file  ]  [ -s server-addr ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [
       -v ] [ --version ] [ if0 [ ...ifN ] ]

       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.

       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

       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.

       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.

       -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 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

       -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 (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.

              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).   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.

       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, 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.

       -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
              DBDIR/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
              RUNDIR/ is used.

              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.

       The syntax of the dhclient.conf(5) file is discussed separately.

       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 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.

       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

              The dhclient.conf configuration file.

              The dhclient.leases database.

              The dhclient PID file.

              The dhclient-script file.

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

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

       dhclient(8)  To  learn  more  about  Internet  Systems  Consortium, see

       This client was substantially modified and enhanced by Elliot Poger for
       use  on  Linux while he was working on the MosquitoNet project at Stan-

       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.


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