Provided by: isc-dhcp-client_4.3.3-5ubuntu11_i386 bug


       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
       maintains a list of IP addresses which may be assigned on one  or  more
       subnets.  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
       instructions.  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 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
       database, 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
       configuration  file  or  on the command line, and will ignore all other

       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
       syslog(3) facility.


       -4     Use  the  DHCPv4  protocol  to  obtain  an  IPv4   address   and
              configuration  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
              systems.  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
              useful 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
              transmit,  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
              simulating 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
              overridden  by  setting  a  client id in the configuration file.
              Overridding 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
              disables  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
              identifier 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
       particularly 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/ 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
       different 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
       running, 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
       interfaces 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
       command-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/,


       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

       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.