dhclient(8)



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 con-
       fig-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 main-
       tains  a list of IP addresses which may be assigned on one or more sub-
       nets.  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 instruc-
       tions.  It then gets a list of all the network interfaces that are con-
       figured in the current system.  For each interface, it attempts to con-
       figure 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 arbi-
       trarily 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  be-
       comes 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  ar-
       range  with  the  network administrator for an entry on the BOOTP data-
       base, so that the host can boot quickly on that network rather than cy-
       cling 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 configura-
       tion 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 sys-
       log(3) facility.

OPTIONS
       -4     Use the DHCPv4 protocol to obtain an IPv4 address and configura-
              tion  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.  Com-
              munication 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  sys-
              tems.  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 use-
              ful 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 op-
              tions 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 trans-
              mit, 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 sim-
              ulating  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  in-
              cluded in the client's messages.  This client id can be overrid-
              den by setting a client id in the configuration file.   Overrid-
              ing 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  de-
              fault  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  pa-
              rameters  (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 im-
              plies  -6 and also disables the normal address query.  See -N to
              restore it.

       -P     Enable IPv6 prefix delegation.  This implies -6  and  also  dis-
              ables 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  in-
              cludes  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 ac-
              cept 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 identi-
              fier based on the link-layer address plus a timestamp (DUID-LLT)
              if it is running in stateful mode (without -S, requesting an ad-
              dress).  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  de-
              fault value is 128.  Alternatively you may change the default at
              compile  time  by  setting  DHCLIENT_DEFAULT_PREFIX_LEN  in  in-
              cludes/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  in-
              terface.  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  com-
              plete.  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 re-
              questing 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 particu-
       larly 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 de-
              fault /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 op-
              tion 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 de-
              scription of this file.

PORTS
       During operations the client may use multiple UDP ports to provide dif-
       ferent  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  run-
       ning, 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  inter-
       faces 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 re-
       sume 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 com-
       mand-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  Stan-
       ford.

       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)

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