Provided by: dhcp3-client_3.1.1-1ubuntu2_i386 bug

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
       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.

       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(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
       database, 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
       destination 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
       configured  an  interface,  and  then  will  revert  to  running in the
       background.   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,
       followed 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
       network 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
       interfaces using the -n flag.   This is most likely  to  be  useful  in
       combination 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
       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

       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.

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 http://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
       Stanford.

       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)