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dhclient


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 ] [ 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.
       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
       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/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.
       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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