*BASH User Commands Ubuntu 10.04.4 LTS Server coreutils
GNU/Linux
dhclient(8)                                                        dhclient(8)



NAME
       dhclient - Dynamic Host Configuration Protocol Client

SYNOPSIS
       dhclient  [ -p port ] [ -d ] [ -e VAR=value ] [ -q ] [ -1 ] [ -r ] [ -x
       ] [ -lf lease-file ] [ -pf pid-file ] [ -cf config-file ] [ -sf script-
       file  ] [ -s server ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [ 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.

       On startup, dhclient reads the dhclient.conf for configuration instruc-
       tions.   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(5) 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  data-
       base,  so  that  the  host can boot quickly on that network rather than
       cycling through the list of old leases.

COMMAND LINE
       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(5)  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 interfaces.

       If the DHCP client should listen and transmit on a port other than  the
       standard (port 68), the -p flag may used.  It should be followed by the
       udp port number that dhclient should use.  This is  mostly  useful  for
       debugging purposes.  If a different port is specified for the client to
       listen on and transmit on, the client will also use a different  desti-
       nation port - one less than the specified port.

       The  DHCP  client  normally  transmits  any  protocol messages it sends
       before acquiring an IP address  to,  255.255.255.255,  the  IP  limited
       broadcast  address.    For debugging purposes, it may be useful to have
       the server transmit these messages to some other address.   This can be
       specified  with  the -s flag, followed by the IP address or domain name
       of the destination.

       For testing purposes, the giaddr field of all packets that  the  client
       sends can be set using the -g flag, followed by the IP address to send.
       This is only useful for testing, and should not be expected to work  in
       any consistent or useful way.

       The  DHCP  client will normally run in the foreground until it has con-
       figured an interface, and then will revert  to  running  in  the  back-
       ground.    To run force dhclient to always run as a foreground process,
       the -d flag should be specified.   This  is  useful  when  running  the
       client  under a debugger, or when running it out of inittab on System V
       systems.

       The dhclient daemon creates its  own  environment  when  executing  the
       dhclient-script  to  do  the grunt work of interface configuration.  To
       define extra environment variables and their values, use the  -e  flag,
       followed by the environment variable name and value assignment, just as
       one would assign a variable in a shell.  Eg: -e IF_METRIC=1

       The client normally prints a startup message and displays the  protocol
       sequence  to  the  standard  error  descriptor until it has acquired an
       address, and then only logs messages using  the  syslog  (3)  facility.
       The  -q flag prevents any messages other than errors from being printed
       to the standard error descriptor.

       The client normally doesn't release the current  lease  as  it  is  not
       required  by  the DHCP protocol.  Some cable ISPs require their clients
       to notify the server if they wish to release an  assigned  IP  address.
       The  -r  flag explicitly releases the current lease, and once the lease
       has been released, the client exits.

       If the client is killed by a signal (for example at shutdown or reboot)
       it  won't  execute the dhclient-script (8) at exit. However if you shut
       the client down gracefully with -r or  -x  it  will  execute  dhclient-
       script  (8) at shutdown with the specific reason for calling the script
       set.

       The -1 flag will cause dhclient to try once to  get  a  lease.   If  it
       fails, dhclient exits with exit code two.

       The  DHCP  client  normally  gets  its  configuration  information from
       /etc/dhcp3/dhclient.conf,      its      lease       database       from
       /var/lib/dhcp3/dhclient.leases,  stores its process ID in a file called
       /var/run/dhclient.pid,  and  configures  the  network  interface  using
       /sbin/dhclient-script  To  specify different names and/or locations for
       these files, use the -cf, -lf, -pf and -sf  flags,  respectively,  fol-
       lowed  by  the  name of the file.   This can be particularly useful if,
       for example, /var/lib/dhcp3 or /var/run has not yet been  mounted  when
       the DHCP client is started.

       The DHCP client normally exits 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.   The -w 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  net-
       work  interface  has  been  added  or  removed,  so that the client can
       attempt to configure an IP address on that interface.

       The DHCP client can be directed not to attempt to configure any  inter-
       faces using the -n flag.   This is most likely to be useful in combina-
       tion with the -w flag.

       The client can also be  instructed  to  become  a  daemon  immediately,
       rather  than waiting until it has acquired an IP address.   This can be
       done by supplying the -nw flag.

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
       resume it, set its state attribute to 4.

FILES
       /sbin/dhclient-script,                        /etc/dhcp3/dhclient.conf,
       /var/lib/dhcp3/dhclient.leases,                  /var/run/dhclient.pid,
       /var/lib/dhcp3/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)