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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...  ] ] -R ] [ -p port-number ] [ -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 ] [ --dad-wait-time seconds ] [ -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 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 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 pruposes only and
              should not be expected to work in any consistent or useful way.

              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.

       -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.

       --dad-wait-time seconds
              Specify maximum time (in seconds) that the  client  should  wait
              for  the  duplicate  address  detection  (DAD) to complete on an
              interface. This value is propagated to the dhclient script in  a
              dad_wait_time environment variable. If any of the IPv6 addresses
              on the interface are tentative (DAD is in progress), the  script
              will  wait  for  the specified number of seconds for DAD to com‐
              plete. If the script ignores this variable the parameter has  no

       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.

       -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/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 CLIENT‐
              BINDIR/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.

       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/dhclient.pid, 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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