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dhclient(8)                                                        dhclient(8)

NAME
       dhclient - Dynamic Host Configuration Protocol Client

SYNOPSIS
       dhclient  [  -p  port ] [ -D ] [ -d ] [ -q ] [ -1 ] [ -r ] [ -lf lease-
       file ] [ -pf pid-file ] [ -cf config-file ] [ -sf script-file  ]  [  -s
       server ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [ if0 [ ...ifN ] ]

DESCRIPTION
       The  Internet  Software  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.

       The  -D  flag  causes dhclient to save the script it creates for use in
       conjunction with dhclient-script in /tmp.

       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 greater than the specified destination 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 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.

       The -1 flag 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/dhclient.conf, its lease  database  from  /var/db/dhclient.leases,
       stores  its process ID in a file called /var/run/dhclient.pid, and con-
       figures 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/db 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  (8)  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/dhclient.conf,   /var/db/dhclient.leases,
       /var/run/dhclient.pid, /var/db/dhclient.leases~.

SEE ALSO
       dhclient.conf(5), dhclient.leases(5), dhclient-script(8).

AUTHOR
       dhclient(8) has been written for the Internet  Software  Consortium  by
       Ted  Lemon  in cooperation with Vixie Enterprises.  To learn more about
       the Internet Software 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  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 Software 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)

NAME | SYNOPSIS | DESCRIPTION | OPERATION | COMMAND LINE | CONFIGURATION | OMAPI | THE CONTROL OBJECT | FILES | SEE ALSO | AUTHOR

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