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

       dhclient - Dynamic Host Configuration Protocol Client

       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 ] ]

       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.

       -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

       -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.  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.

              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.

       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.

       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.

       /sbin/dhclient-script,                         /etc/dhcp/dhclient.conf,
       /var/lib/dhcp/dhclient.leases,                   /var/run/dhclient.pid,

       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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