Provided by: isc-dhcp-client_4.1.ESV-R4-0ubuntu5_i386 bug

NAME

       dhclient - Dynamic Host Configuration Protocol Client

SYNOPSIS

       dhclient  [ -4 | -6 ] [ -S ] [ -N [ -N...  ] ] [ -T [ -T...  ] ] [ -P [
       -P...  ] ] [ -p port ] [ -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
       ] [ -v ] [ --version ] [ 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.

       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
       options.

       On   startup,   dhclient  reads  the  dhclient.conf  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.

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
       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.

       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.

OPTIONS

       -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 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(5) 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 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=val
              Define additional  environment  variables  for  the  environment
              where  dhclient-script(8) 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(8) 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(8)  will be executed with the specific reason for calling
              the script set.

       -p port
              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 255.255.255.255 (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 pruposes only and
              should not be expected to work in any consistent or useful way.

       --version
              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.

       -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  it's  files.   They  can  be
       particularly  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
              descriptionof this file.

       -pf pid-file
              Path to the  process  ID  file.   If  unspecified,  the  default
              RUNDIR/dhclient.pid is used.

       --no-pid
              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
              CLIENTBINDIR/dhclient-script is  used.   See  dhclient-script(8)
              for a description of this file.

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.

ENVIRONMENT VARIABLES

       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
       settings.

       PATH_DHCLIENT_CONF
              The dhclient.conf configuration file.

       PATH_DHCLIENT_DB
              The dhclient.leases database.

       PATH_DHCLIENT_PID
              The dhclient PID file.

       PATH_DHCLIENT_SCRIPT
              The dhclient-script file.

FILES

       CLIENTBINDIR/dhclient-script,                     ETCDIR/dhclient.conf,
       DBDIR/dhclient.leases, RUNDIR/dhclient.pid, DBDIR/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
       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)