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



NAME
       dhclient - Dynamic Host Configuration Protocol Client

SYNOPSIS
       dhclient [ -4 | -6 ] [ -S ] [ -N [ -N...  ] ] [ -T [ -T...  ] ] [ -P [
       -P...  ] ] [ -i ] [ -I ] [ -D LL|LLT ] [ -p port-number ] [ -d ] [ -e
       VAR=value ] [ -m ] [ -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 ] [ --version ] [
       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
       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 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
       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.

       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.

OPTIONS
       -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 -v.

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

       -m     Don't require that the responding ethernet address of the dhcp
              server matches the one we expect.

       -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 destination for DHCP protocol messages before dhclient
              has acquired an IP address.  Normally, dhclient transmits these
              messages to 255.255.255.255 (the IP limited broadcast address).
              Overriding this is mostly useful for debugging purposes.  This
              feature 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.

       --version
              Print version number and exit.

       Options available for DHCPv6 mode:

       -S     Use Information-request to get only stateless configuration
              parameters (i.e., without 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 allowed.

       -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 configured to use a DUID
              using -i option the default is to use a DUID-LLT.  -D overrides
              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 normal operation after using -T or -P.

       Modifying default file locations: The following options can be used to
       modify the locations 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.

       -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 used.

       --no-pid
              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.



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
       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 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
       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.

ENVIRONMENT VARIABLES
       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 corresponding environment variable
       settings.

       PATH_DHCLIENT_CONF
              The dhclient.conf configuration file.

       PATH_DHCLIENT_DB
              The dhclient.leases database.

       PATH_DHCLIENT_PID
              The dhclient PID file.

       PATH_DHCLIENT_SCRIPT
              The dhclient-script file.

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

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

AUTHOR
       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 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)