Provided by: isc-dhcp-client_4.3.5-3ubuntu7_amd64 bug

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 ] [ -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.  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.  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.

       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,
       /var/lib/dhcp or /var/run have not been mounted when the DHCP client is started.

       -cf config-file
              Path   to   the   client   configuration   file.    If   unspecified,  the  default
              /etc/dhcp/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
              /var/lib/dhcp/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 /var/run/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 /sbin/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 opened even if DDNS is  disabled  in
       the configuration 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

       /sbin/dhclient-script,       /etc/dhcp/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), 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)