dhclient - Dynamic Host Configuration Protocol Client


   dhclient  [ -4 | -6 ] [ -S ] [ -N [ -N...  ] ] [ -T [ -T...  ] ] [ -P [
   -P...  ] ] [ -i ] [ -I ] [ -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 ] ]


   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.


   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

   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.


   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

   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.


   -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.

   -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

   -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 (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.

          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, /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.

          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.


   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.


   The syntax of the dhclient.conf(5) file is discussed separately.


   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  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.


   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

          The dhclient.conf configuration file.

          The dhclient.leases database.

          The dhclient PID file.

          The dhclient-script file.


   /sbin/dhclient-script,                         /etc/dhcp/dhclient.conf,
   /var/lib/dhcp/dhclient.leases,                   /var/run/dhclient.pid,


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


   dhclient(8)  To  learn  more  about  Internet  Systems  Consortium, see

   This client was substantially modified and enhanced by Elliot Poger for
   use  on  Linux  while  he  was  working  on  the MosquitoNet project at

   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.



Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.