dhclient



dhclient(8)                                                        dhclient(8)




NAME

       dhclient - Dynamic Host Configuration Protocol Client


SYNOPSIS

       dhclient  [  -p port ] [ -d ] [ -q ] [ -1 ] [ -r ] [ -lf lease-file ] [
       -pf pid-file ] [ -cf config-file ] [ -sf script-file ] [ -s server ]  [
       -g relay ] [ -n ] [ -nw ] [ -w ] [ -x ] [ 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 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
       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 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/lib/dhcp/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, fol-
       lowed by the name of the file.   This can be  particularly  useful  if,
       for  example,  /var/lib/dhcp  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
       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 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.

       The  -x  argument  enables eXtended option information to be created in
       the -s dhclient-script environment, which would allow applications run-
       ning  in  that  environment to handle options they do not know about in
       advance.


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/lib/dhcp/dhclient.leases,                   /var/run/dhclient.pid,
       /var/lib/dhcp/dhclient.leases~.


SEE ALSO

       dhcpd(8),    dhcrelay(8),     dhclient-script(8),     dhclient.conf(5),
       dhclient.leases(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 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.




                                                                   dhclient(8)

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