dhclient

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...  ] ] [ -R ] [ -i ] [ -I ] [ -4o6 port ] [ -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 ] [ --dad-wait-time seconds ] [ --prefix-len-hint length ] [
       --decline-wait-time seconds ] [ -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.

       -4o6 port
              Participate in the DHCPv4 over DHCPv6 protocol specified by RFC
              7341.  This associates a DHCPv4 and a DHCPv6 client to allow the
              v4 client to send v4 requests encapsulated in a v6 packet.
              Communication between the two clients is done on a pair of UDP
              sockets bound to ::1 port and port + 1. Both clients must be
              launched using the same port argument.

       -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.

       -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.  Overriding the
              client id in this fashion is discouraged.

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


       --decline-wait-time seconds
              Specify the time (in seconds) that an IPv4 client should wait
              after declining an address before issuing a discover.  The default
              is 10 seconds as recommended by RFC 2131, Section 3.1.5.  A value
              of zero equates to no wait at all.

       --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.  Multiple
              prefixes can be requested with multiple -P flags.  Note only one
              requested interface is allowed.

       -R     Require that responses include all of the items requested by any
              -N, -T, or -P options.  Normally even if the command line includes
              a number of these the client will be willing to accept the best
              lease it can even if the lease doesn't include all of the
              requested items.  This option causes the client to only accept
              leases that include all of the requested items.

              Note well: enabling this may prevent the client from using any
              leases it receives if the servers aren't configured to supply all
              of the items.

       -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.  Multiple
              addresses can be requested with multiple -N flags.

       --address-prefix-len length
              Specify the length of the prefix for IPv6 addresses. This value is
              passed by dhclient into the client script via the environment
              variable, ip6_prefixlen, when binding IPv6 addresses.  The default
              value is 128.  Alternatively you may change the default at compile
              time by setting DHCLIENT_DEFAULT_PREFIX_LEN in includes/site.h.

       --dad-wait-time seconds
              Specify maximum time (in seconds) that the client should wait for
              the duplicate address detection (DAD) to complete on an interface.
              This value is propagated to the dhclient script in a dad_wait_time
              environment variable. If any of the IPv6 addresses on the
              interface are tentative (DAD is in progress), the script will wait
              for the specified number of seconds for DAD to complete. If the
              script ignores this variable the parameter has no effect.

       --prefix-len-hint length
              When used in conjunction with -P, it directs the client to use the
              given length to use a prefix hint of, "::/length", when requesting
              new prefixes.

       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.

       -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 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.

PORTS
       During operations the client may use multiple UDP ports to provide
       different functions.  Which ports are opened depends on both the way you
       compiled your code and the configuration you supply.  The following
       should provide you an idea of what ports may be in use.

       Normally a DHCPv4 client will open a raw UDP socket to receive and send
       most DHCPv4 packets.  It also opens a fallback UDP socket for use in
       sending unicast packets.  Normally these will both use the well known
       port number for BOOTPC.

       For DHCPv6 the client opens a UDP socket on the well known client port
       and a fallback UDP socket on a random port for use in sending unicast
       messages.  Unlike DHCPv4 the well known socket doesn't need to be opened
       in raw mode.

       If you have included an omapi port statement in your configuration file
       then the client will open a TCP socket on that port to listen for OMPAI
       connections.  When something connects another port will be used for the
       established connection.

       When DDNS is enabled at compile time (see includes/site.h) the client
       will open both a v4 and a v6 UDP socket on random ports.  These ports are
       not opened unless/until the client first attempts to do an update.  If
       the client is not configured to do updates, the ports will never be
       opened.

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)