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dhclient(8)                 System Manager's Manual                dhclient(8)



NAME
       dhclient - Dynamic Host Configuration Protocol Client

SYNOPSIS
       dhclient [ -p port ] [ -e ] [ -d ] [ 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 dhcpd.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 identify all network
       interfaces, elimininating non-broadcast  interfaces  if  possible,  and
       attempt to configure each interface.

       If  dhclient  should listen and transmit on a port other than the stan-
       dard (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 debug-
       ging purposes.  If the -p flag is specified, the client  will  transmit
       responses  to  servers  at  a port number that is one less than the one
       specified - i.e., if you specify -p 68, then the client will listen  on
       port  68 and transmit to port 67.  Datagrams that must go through relay
       agents are sent to the port number specified with the -p flag - if  you
       wish to use alternate port numbers, you must configure any relay agents
       you are using to use the same alternate port numbers.

       Dhclient 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 dhclient under a
       debugger, or when running it out of inittab on System V systems.

       The -e flag will cause dhclient to exit with an error if the  interface
       cannot  be  configured  after  a certain amount of time. This is useful
       when dhclient is used in scripts or other systems when  a  failed  dhcp
       attempt needs to be reported.

CONFIGURATION
       The syntax of the dhclient.conf(8) file is discussed seperately.

FILES
       /etc/dhclient.conf,                      /var/lib/dhcp/dhclient.leases,
       /var/run/dhclient.pid, /var/lib/dhcp/dhclient.leases~.

SEE ALSO
       dhcpd(8), dhcrelay(8), dhclient.conf(5), dhclient.leases(5)

AUTHOR
       dhclient(8) has been written for the Internet  Software  Consortium  by
       Ted Lemon <mellonATfugue.com> in cooperation with Vixie Enterprises.  To
       learn   more   about   the   Internet    Software    Consortium,    see
       http://www.vix.com/isc.   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)