


dhclient(8)                                           dhclient(8)


NNAAMMEE
       dhclient - Dynamic Host Configuration Protocol Client

SSYYNNOOPPSSIISS
       ddhhcclliieenntt  [ --pp _p_o_r_t ] [ --dd ] [ --ee _V_A_R_=_v_a_l_u_e ] [ --qq ] [ --11 ] [ --rr ] [ --xx
       ] [ --llff _l_e_a_s_e_-_f_i_l_e ] [ --ppff _p_i_d_-_f_i_l_e ] [ --ccff _c_o_n_f_i_g_-_f_i_l_e ] [ --ssff _s_c_r_i_p_t_-
       _f_i_l_e  ] [ --ss server ] [ --gg relay ] [ --nn ] [ --nnww ] [ --ww ] [ _i_f_0 [ _._._._i_f_N
       ] ]

DDEESSCCRRIIPPTTIIOONN
       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.

OOPPEERRAATTIIOONN
       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 _d_h_c_l_i_e_n_t_._c_o_n_f 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 _d_h_c_l_i_e_n_t_._l_e_a_s_e_s_~
       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.



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dhclient(8)                                           dhclient(8)


       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.

CCOOMMMMAANNDD LLIINNEE
       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
       ddhhcclliieenntt..ccoonnff((55))  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 --pp 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 --ss 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 --gg 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 --dd 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  --ee  flag,
       followed by the environment variable name and value assignment, just as
       one would assign a variable in a shell.  Eg: --ee _I_F___M_E_T_R_I_C_=_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  ssyysslloogg  ((33))  facility.



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dhclient(8)                                           dhclient(8)


       The  --qq 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  --rr  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 ddhhcclliieenntt--ssccrriipptt ((88)) at exit. However if you shut
       the client down gracefully with --rr or  --xx  it  will  execute  ddhhcclliieenntt--
       ssccrriipptt  ((88)) at shutdown with the specific reason for calling the script
       set.

       The --11 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
       //eettcc//ddhhcclliieenntt..ccoonnff,, its lease  database  from  //vvaarr//ddbb//ddhhcclliieenntt..lleeaasseess,,
       stores  its process ID in a file called //vvaarr//rruunn//ddhhcclliieenntt..ppiidd,, and con-
       figures the network interface using //uussrr//llooccaall//ssbbiinn//ddhhcclliieenntt--ssccrriipptt  To
       specify  different names and/or locations for these files, use the --ccff,,
       --llff,, --ppff and --ssff flags, respectively, followed by the name of the file.
       This  can  be  particularly useful if, for example, //vvaarr//ddbb or //vvaarr//rruunn
       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 --ww flag can be used to cause the
       client not to exit when it doesn't  find  any  such  interfaces.    The
       oommsshheellll  ((11))  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 --nn flag.   This is most likely to be useful in combina-
       tion with the --ww 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 --nnww flag.

CCOONNFFIIGGUURRAATTIIOONN
       The syntax of the dhclient.conf(5) file is discussed separately.

OOMMAAPPII
       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.




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dhclient(8)                                           dhclient(8)


       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  ddhhccppccttll((33))
       and  oommaappii((33)).    Most  things  you'd want to do with the client can be
       done directly using the oommsshheellll((11)) command, rather than having to write
       a special program.

TTHHEE CCOONNTTRROOLL OOBBJJEECCTT
       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.


FFIILLEESS
       //uussrr//llooccaall//ssbbiinn//ddhhcclliieenntt--ssccrriipptt,,                    //eettcc//ddhhcclliieenntt..ccoonnff,,
       //vvaarr//ddbb//ddhhcclliieenntt..lleeaasseess,,                         //vvaarr//rruunn//ddhhcclliieenntt..ppiidd,,
       //vvaarr//ddbb//ddhhcclliieenntt..lleeaasseess~~..

SSEEEE AALLSSOO
       dhcpd(8),     dhcrelay(8),     dhclient-script(8),    dhclient.conf(5),
       dhclient.leases(5), dhcp-eval(5).

AAUUTTHHOORR
       ddhhcclliieenntt((88)) has been written for Internet  Systems  Consortium  by  Ted
       Lemon  in  cooperation  with  Vixie  Enterprises.   To learn more about
       Internet Systems Consortium,  see  hhttttppss::////wwwwww..iisscc..oorrgg  To  learn  more
       about Vixie Enterprises, see hhttttpp::////wwwwww..vviixx..ccoomm..

       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.




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dhclient(8)                                           dhclient(8)



























































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