dhclient(8)                                                        dhclient(8)



[1mNAME[0m
       dhclient - Dynamic Host Configuration Protocol Client

[1mSYNOPSIS[0m
       [1mdhclient [22m[ [1m-p [4m[22mport[24m ] [ [1m-d [22m] [ [1m-e [4m[22mVAR=value[24m ] [ [1m-q [22m] [ [1m-1 [22m] [ [1m-r [22m] [ [1m-x[0m
       ] [ [1m-lf [4m[22mlease-file[24m ] [ [1m-pf [4m[22mpid-file[24m ] [ [1m-cf [4m[22mconfig-file[24m ] [ [1m-sf [4m[22mscript-[0m
       [4mfile[24m ] [ [1m-s [22mserver ] [ [1m-g [22mrelay ] [ [1m-n [22m] [ [1m-nw [22m] [ [1m-w [22m] [ [4mif0[24m [ [4m...ifN[0m
       ] ]

[1mDESCRIPTION[0m
       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.

[1mOPERATION[0m
       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.

       On startup, dhclient reads the [4mdhclient.conf[24m 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(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 [4mdhclient.leases~[0m
       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.

[1mCOMMAND LINE[0m
       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
       [1mdhclient.conf(5) [22mfile.   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 [1m-p [22mflag 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
       destination port - one less than the specified 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 [1m-s [22mflag, 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 [1m-g [22mflag, 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
       configured an interface, and then will revert to running in the
       background.   To run force dhclient to always run as a foreground
       process, the [1m-d [22mflag 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 dhclient daemon creates its own environment when executing the
       dhclient-script to do the grunt work of interface configuration.  To
       define extra environment variables and their values, use the [1m-e [22mflag,
       followed by the environment variable name and value assignment, just as
       one would assign a variable in a shell.  Eg: [1m-e [4m[22mIF_METRIC=1[0m

       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 [1msyslog (3) [22mfacility.
       The [1m-q [22mflag 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 [1m-r [22mflag explicitly releases the current lease, and once the lease
       has been released, the client exits.

       If the client is killed by a signal (for example at shutdown or reboot)
       it won't execute the [1mdhclient-script (8) [22mat exit. However if you shut
       the client down gracefully with [1m-r [22mor [1m-x [22mit will execute [1mdhclient-[0m
       [1mscript (8) [22mat shutdown with the specific reason for calling the script
       set.

       The [1m-1 [22mflag will 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
       [1m/etc/dhclient.conf, [22mits lease database from [1m/var/db/dhclient.leases,[0m
       stores its process ID in a file called [1m/var/run/dhclient.pid, [22mand
       configures the network interface using [1m/usr/local/sbin/dhclient-script[0m
       To specify different names and/or locations for these files, use the
       [1m-cf, -lf, -pf [22mand [1m-sf [22mflags, respectively, followed by the name of the
       file.   This can be particularly useful if, for example, [1m/var/db [22mor
       [1m/var/run [22mhas 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 [1m-w [22mflag can be used to cause the
       client not to exit when it doesn't find any such interfaces.   The
       [1momshell (1) [22mprogram 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
       interfaces using the [1m-n [22mflag.   This is most likely to be useful in
       combination with the [1m-w [22mflag.

       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 [1m-nw [22mflag.

[1mCONFIGURATION[0m
       The syntax of the dhclient.conf(5) file is discussed separately.

[1mOMAPI[0m
       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 [1mdhcpctl(3)[0m
       and [1momapi(3)[22m.   Most things you'd want to do with the client can be
       done directly using the [1momshell(1) [22mcommand, rather than having to write
       a special program.

[1mTHE CONTROL OBJECT[0m
       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.

[1mFILES[0m
       [1m/usr/local/sbin/dhclient-script, /etc/dhclient.conf,[0m
       [1m/var/db/dhclient.leases, /var/run/dhclient.pid,[0m
       [1m/var/db/dhclient.leases~.[0m

[1mSEE ALSO[0m
       dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5),
       dhclient.leases(5), dhcp-eval(5).

[1mAUTHOR[0m
       [1mdhclient(8) [22mhas been written for Internet Systems Consortium by Ted
       Lemon in cooperation with Vixie Enterprises.  To learn more about
       Internet Systems Consortium, see [1mhttps://www.isc.org [22mTo learn more
       about Vixie Enterprises, see [1mhttp://www.vix.com.[0m

       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)
