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

       dhclient - Dynamic Host Configuration Protocol Client

       dhclient [ -4 | -6 ] [ -S ] [ -N [ -N...  ] ] [ -T [ -T...  ] ] [ -P [ -P...  ] ] [ -i ] [
       -I ] [ -D LL|LLT ] [ -p port-number ] [ -d ] [ -df duid-lease-file ] [ -e VAR=value ] [ -q
       ]  [ -1 ] [ -r | -x ] [ -lf lease-file ] [ -pf pid-file ] [ --no-pid ] [ -cf config-file ]
       [ -sf script-file ] [ -s server-addr ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [ -v ] [  --ver‐
       sion ] [ if0 [ ...ifN ] ]

       The  Internet  Systems  Consortium DHCP Client, dhclient, provides a means for configuring
       one or more network interfaces using the Dynamic Host Configuration Protocol, BOOTP proto‐
       col, or if these protocols fail, by statically assigning an address.

       The  DHCP  protocol allows a host to contact a central server which maintains a list of IP
       addresses which may be assigned on one or more subnets.  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.

       There are two versions of the DHCP protocol DHCPv4 and DHCPv6.  At startup the client  may
       be started for one or the other via the -4 or -6 options.

       On startup, dhclient reads the dhclient.conf for configuration instructions.  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  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 cre‐
       ates 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

       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 names of the network interfaces that dhclient should attempt to configure may be spec‐
       ified  on  the  command  line.   If  no  interface names are specified on the command line
       dhclient will normally identify all network interfaces, eliminating  non-broadcast  inter‐
       faces if possible, and attempt to configure each interface.

       It  is  also  possible to specify interfaces by name in the dhclient.conf file.  If inter‐
       faces 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 client normally prints no output during its startup sequence.  It can be made to  emit
       verbose  messages  displaying the startup sequence events until it has acquired an address
       by supplying the -v command line argument.  In either case, the client logs messages using
       the syslog(3) facility.

       -4     Use  the  DHCPv4  protocol  to obtain an IPv4 address and configuration parameters.
              This is the default and cannot be combined with -6.

       -6     Use the DHCPv6 protocol to obtain whatever IPv6 addresses are available along  with
              configuration  parameters.   It cannot be combined with -4.  The -S -T -P -N and -D
              arguments provide more control over aspects of the DHCPv6 processing.  Note: it  is
              not  recommended  to  mix  queries of different types together or even to share the
              lease file between them.

       -1     Try to get a lease once.  On failure exit with code 2.  In  DHCPv6  this  sets  the
              maximum  duration  of  the  initial  exchange to timeout (from dhclient.conf with a
              default of sixty seconds).

       -d     Force dhclient to run as a foreground process.  Normally the DHCP client  will  run
              in the foreground until is has configured an interface at which time it will revert
              to running in the background.  This option is useful when running the client  under
              a  debugger,  or  when running it out of inittab on System V systems.  This implies

       -nw    Become a daemon immediately (nowait) rather than waiting until an  IP  address  has
              been acquired.

       -q     Be quiet at startup, this is the default.

       -v     Enable verbose log messages.

       -w     Continue  running even if no broadcast interfaces were found.  Normally DHCP client
              will exit 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.   This  flag  can  be
              used to cause the client not to exit when it doesn't find any such interfaces.  The
              omshell(1) 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.

       -n     Do not configure any interfaces.  This is most likely to be useful  in  combination
              with the -w flag.

       -e VAR=value
              Define  additional  environment variables for the environment where dhclient-script
              executes.  You may specify multiple -e options on the command line.

       -r     Release the current lease and stop the running DHCP client as  previously  recorded
              in  the  PID  file.  When shutdown via this method dhclient-script will be executed
              with the specific reason for calling the script set.  The client  normally  doesn't
              release  the  current  lease  as this is not required by the DHCP protocol but some
              cable ISPs require their clients to notify the server if they wish  to  release  an
              assigned IP address.

       -x     Stop  the  running DHCP client without releasing the current lease.  Kills existing
              dhclient process as previously recorded in the PID file.  When  shutdown  via  this
              method  dhclient-script  will  be executed with the specific reason for calling the
              script set.

       -p port-number
              The UDP port number on which the  DHCP  client  should  listen  and  transmit.   If
              unspecified,  dhclient  uses  the  default  port  of 68.  This is mostly useful for
              debugging purposes.  If a different port is specified on which  the  client  should
              listen  and  transmit,  the client will also use a different destination port - one
              less than the specified port.

       -s server-addr
              Specify the server IP address or fully qualified domain name to use as  a  destina‐
              tion  for  DHCP protocol messages before dhclient has acquired an IP address.  Nor‐
              mally, dhclient transmits these messages to (the IP limited  broad‐
              cast address).  Overriding this is mostly useful for debugging purposes.  This fea‐
              ture is not supported in DHCPv6 (-6) mode.

       -g relay
              Set the giaddr field of all packets to the relay  IP  address  simulating  a  relay
              agent.  This is for testing purposes only and should not be expected to work in any
              consistent or useful way.

       -i     Use a DUID with DHCPv4 clients.  If no DUID is available in the lease file one will
              be  constructed  and  saved.   The  DUID  will be used to construct a RFC4361 style
              client id that will be included in the client's messages.  This client  id  can  be
              overridden  by  setting  a  client  id  in the configuration file.  Overridding the
              client id in this fashion is discouraged.

       -I     Use the standard DDNS scheme from RFCs 4701 & 4702.

              Print version number and exit.

       Options available for DHCPv6 mode:

       -S     Use Information-request to get only stateless configuration parameters (i.e., with‐
              out address).  This implies -6.  It also doesn't rewrite the lease database.

       -T     Ask  for  IPv6  temporary addresses, one set per -T flag.  This implies -6 and also
              disables the normal address query.  See -N to restore it.

       -P     Enable IPv6 prefix delegation.  This  implies  -6  and  also  disables  the  normal
              address  query.   See  -N  to  restore  it.   Note  only one requested interface is

       -D LL or LLT
              Override the default when selecting the type of DUID to use.   By  default,  DHCPv6
              dhclient  creates  an identifier based on the link-layer address (DUID-LL) if it is
              running in stateless mode (with -S, not requesting an address), or  it  creates  an
              identifier  based  on  the  link-layer address plus a timestamp (DUID-LLT) if it is
              running in stateful mode (without -S, requesting an address).  When DHCPv4 is  con‐
              figured  to  use a DUID using -i option the default is to use a DUID-LLT.  -D over‐
              rides these default, with a value of either LL or LLT.

       -N     Restore normal address query for IPv6. This implies -6.  It is used to restore nor‐
              mal operation after using -T or -P.

       Modifying  default  file  locations: The following options can be used to modify the loca‐
       tions a client uses for its files.  They can be particularly useful if, for example, DBDIR
       or RUNDIR have not been mounted when the DHCP client is started.

       -cf config-file
              Path   to   the   client   configuration   file.    If   unspecified,  the  default
              ETCDIR/dhclient.conf is used.  See dhclient.conf(5) for a description of this file.

       -df duid-lease-file
              Path to a secondary lease file.  If the primary lease file doesn't contain  a  DUID
              this  file  will  be searched.  The DUID read from the secondary will be written to
              the primary.  This option can be used to allow an IPv4 instance of  the  client  to
              share a DUID with an IPv6 instance.  After starting one of the instances the second
              can be started with this option pointing to the lease file of the  first  instance.
              There  is  no default.  If no file is specified no search is made for a DUID should
              one not be found in the main lease file.

       -lf lease-file
              Path to the lease database file.  If unspecified, the default DBDIR/dhclient.leases
              is used.  See dhclient.leases(5) for a description of this file.

       -pf pid-file
              Path  to  the  process ID file.  If unspecified, the default RUNDIR/dhclient.pid is

              Option to disable writing pid files.  By default the program will write a pid file.
              If the program is invoked with this option it will not attempt to kill any existing
              client processes even if invoked with -r or -x.

       -sf script-file
              Path to the network configuration script invoked by dhclient when it gets a  lease.
              If  unspecified,  the  default CLIENTBINDIR/dhclient-script is used.  See dhclient-
              script(8) for a description of this file.

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

       The DHCP client provides some ability to control it while it is running, 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 housekeep‐
       ing  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 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  interfaces  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.

       The following environment variables may be defined to override the  builtin  defaults  for
       file  locations.  Note that use of the related command-line options will ignore the corre‐
       sponding environment variable settings.

              The dhclient.conf configuration file.

              The dhclient.leases database.

              The dhclient PID file.

              The dhclient-script file.

       CLIENTBINDIR/dhclient-script,         ETCDIR/dhclient.conf,         DBDIR/dhclient.leases,
       RUNDIR/dhclient.pid, DBDIR/dhclient.leases~.

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

       dhclient(8) To learn more about Internet Systems Consortium, see https://www.isc.org

       This client was substantially modified and enhanced by Elliot Poger for use on Linux while
       he was working on the MosquitoNet project at Stanford.

       The  current version owes much to Elliot's Linux enhancements, but was substantially reor‐
       ganized 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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