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NAME    [Toc]    [Back]

       dhclient.conf - DHCP client configuration file

DESCRIPTION    [Toc]    [Back]

       The dhclient.conf file contains configuration information for dhclient,
       the Internet Software Consortium DHCP Client.

       The dhclient.conf file is a free-form ASCII text file.	It  is	parsed
       by  the	recursive-descent  parser  built into dhclient.   The file may
       contain extra tabs and newlines for formatting purposes.   Keywords  in
       the file are case-insensitive.	Comments may be placed anywhere within
       the file (except within quotes).   Comments begin with the #  character
       and end at the end of the line.

       The  dhclient.conf  file  can be used to configure the behaviour of the
       client  in  a  wide  variety  of  ways:	protocol  timing,  information
       requested from the server, information required of the server, defaults
       to use if the server does not provide certain information, values  with
       which  to  override  information  provided  by the server, or values to
       prepend or append to information provided by the server.  The  configuration
  file  can  also be preinitialized with addresses to use on networks
 that don't have DHCP servers.

PROTOCOL TIMING    [Toc]    [Back]

       The timing behaviour of the client need not be configured by the  user.
       If no timing configuration is provided by the user, a fairly reasonable
       timing behaviour will be used by default - one which results in	fairly
       timely updates without placing an inordinate load on the server.

       The  following statements can be used to adjust the timing behaviour of
       the DHCP client if required, however:

       The timeout statement

       timeout time ;

       The timeout statement determines the amount  of	time  that  must  pass
       between the time that the client begins to try to determine its address
       and the time that it decides that it's not going to be able to  contact
       a  server.    By  default,  this  timeout is sixty seconds.   After the
       timeout has passed, if there are any static leases defined in the  configuration
  file,  or  any  leases remaining in the lease database that
       have not yet  expired,  the  client  will  loop	through  these	leases
       attempting  to  validate  them,	and if it finds one that appears to be
       valid, it will use that lease's address.   If there are no valid static
       leases  or  unexpired  leases  in  the  lease database, the client will
       restart the protocol after the defined retry interval.

       The retry statement

	retry time;

       The retry statement determines the time that must pass after the client
       has  determined	that  there  is no DHCP server present before it tries
       again to contact a DHCP server.	 By default, this is five minutes.

       The select-timeout statement

	select-timeout time;

       It is possible (some might say desirable) for there to be more than one
       DHCP  server  serving any given network.   In this case, it is possible
       that a client may be sent more than one offer in response to  its  initial
  lease  discovery message.	 It may be that one of these offers is
       preferable to the other (e.g., one  offer  may  have  the  address  the
       client previously used, and the other may not).

       The  select-timeout  is the time after the client sends its first lease
       discovery request at which it stops waiting for	offers	from  servers,
       assuming  that  it has received at least one such offer.   If no offers
       have been received by the time  the  select-timeout  has  expired,  the
       client will accept the first offer that arrives.

       By  default,  the  select-timeout is zero seconds - that is, the client
       will take the first offer it sees.

       The reboot statement

	reboot time;

       When the client is restarted, it first  tries  to  reacquire  the  last
       address	it  had.    This  is  called the INIT-REBOOT state.   If it is
       still attached to the same network it was attached to when it last ran,
       this  is  the  quickest way to get started.   The reboot statement sets
       the time that must elapse after the client first tries to reacquire its
       old  address  before  it  gives up and tries to discover a new address.
       By default, the reboot timeout is ten seconds.

       The backoff-cutoff statement

	backoff-cutoff time;

       The client uses an exponential backoff algorithm with some  randomness,
       so  that  if many clients try to configure themselves at the same time,
       they will not make their requests  in  lockstep.    The	backoff-cutoff
       statement  determines  the  maximum  amount  of time that the client is
       allowed to back off.   It defaults to two minutes.

       The initial-interval statement

	initial-interval time;

       The initial-interval statement sets the	amount	of  time  between  the
       first  attempt  to  reach  a  server  and the second attempt to reach a
       server.	Each time a message is sent, the interval between messages  is
       incremented by twice the current interval multiplied by a random number
       between zero and one.  If it is greater than the backoff-cutoff amount,
       it is set to that amount.  It defaults to ten seconds.


       The  DHCP protocol allows the client to request that the server send it
       specific information, and not send it other information that it is  not
       prepared  to  accept.	The  protocol also allows the client to reject
       offers from servers if they don't contain information the client needs,
       or if the information provided is not satisfactory.

       There  is  a variety of data contained in offers that DHCP servers send
       to DHCP clients.  The data that can be specifically requested  is  what
       are called DHCP Options.  DHCP Options are defined in

       The request statement

	request [ option ] [, ... option ];

       The  request  statement	causes	the  client to request that any server
       responding to the client send the client its values for	the  specified
       options.    Only  the  option  names should be specified in the request
       statement - not option parameters.

       The require statement

	require [ option ] [, ... option ];

       The require statement lists options that must be sent in order  for  an
       offer  to  be  accepted.    Offers  that  do not contain all the listed
       options will be ignored.

       The send statement

	send { [ option declaration ] [, ... option declaration ]}

       The send statement causes the client to send the specified  options  to
       the  server  with the specified values.	These are full option declarations
 as described in dhcp-options(5).  Options that are always sent in
       the  DHCP protocol should not be specified here, except that the client
       can specify  a  requested-lease-time  option  other  than  the  default
       requested  lease  time,	which is two hours.  The other obvious use for
       this statement is to send information to the server that will allow  it
       to  differentiate  between  this  client  and other clients or kinds of

OPTION MODIFIERS    [Toc]    [Back]

       In some cases, a client may receive option data from the  server  which
       is  not really appropriate for that client, or may not receive information
 that it needs, and for which a useful default value  exists.    It
       may  also  receive  information	which is useful, but which needs to be
       supplemented with local information.   To handle these  needs,  several
       option modifiers are available.

       The default statement

	default { [ option declaration ] [, ... option declaration ]}

       If  for some set of options the client should use the value supplied by
       the server, but needs to use some default value if no  value  was  supplied
  by the server, these values can be defined in the default statement.

       The supersede statement

	supersede { [ option declaration ] [, ... option declaration ]}

       If for some set of options the client should always use its  own  value
       rather  than  any  value  supplied  by  the server, these values can be
       defined in the supersede statement.

       The prepend statement

	prepend { [ option declaration ] [, ... option declaration ]}

       If for some set of options the client should use a  value  you  supply,
       and  then  use  the values supplied by the server, if any, these values
       can be defined in the prepend statement.   The  prepend	statement  can
       only  be  used for options which allow more than one value to be given.
       This restriction is not enforced - if violated, the results are	unpredictable.

       The append statement

	append { [ option declaration ] [, ... option declaration ]}

       If  for some set of options the client should first use the values supplied
 by the server, if any, and then use values you supply, these values
  can be defined in the append statement.   The append statement can
       only be used for options which allow more than one value to  be	given.
       This restriction is not enforced - if you ignore it, the behaviour will
       be unpredictable.


       The lease declaration

	lease { lease-declaration [ ... lease-declaration ] }

       The DHCP client may decide after some period of time (see PROTOCOL TIM-
       ING)  decide  that  it  is not going to succeed in contacting a server.
       At that time, it consults its own database of old leases and tests each
       one  that  has  not yet timed out by pinging the listed router for that
       lease to see if that lease could work.	It is possible to  define  one
       or  more  fixed	leases	in  the client configuration file for networks
       where there is no DHCP or BOOTP service, so that the client  can  still
       automatically  configure  its  address.	  This	is done with the lease

       NOTE: the lease statement is also used in the dhclient.leases  file  in
       order to record leases that have been received from DHCP servers.  Some
       of the syntax for leases as described  below  is  only  needed  in  the
       dhclient.leases	file.	 Such  syntax is documented here for completeness.

       A lease statement consists of the lease keyword,  followed  by  a  left
       curly brace, followed by one or more lease declaration statements, followed
 by a right curly brace.   The following  lease  declarations  are


       The  bootp  statement  is  used to indicate that the lease was acquired
       using the BOOTP protocol rather than the DHCP protocol.	 It  is  never
       necessary  to  specify  this  in  the  client configuration file.   The
       client uses this syntax in its lease database file.

	interface "string";

       The interface lease statement is used  to  indicate  the  interface  on
       which  the lease is valid.   If set, this lease will only be tried on a
       particular interface.   When the client receives a lease from a server,
       it always records the interface number on which it received that lease.
       If predefined leases are  specified  in	the  dhclient.conf  file,  the
       interface should also be specified, although this is not required.

	fixed-address ip-address;

       The fixed-address statement is used to set the ip address of a particular
 lease.   This is  required  for  all  lease	statements.    The  IP
       address must be specified as a dotted quad (e.g.,

	filename "string";

       The  filename statement specifies the name of the boot filename to use.
       This is not used by the standard client configuration  script,  but  is
       included for completeness.

	server-name "string";

       The server-name statement specifies the name of the boot server name to
       use.   This is also not	used  by  the  standard  client  configuration

	option option-declaration;

       The option statement is used to specify the value of an option supplied
       by the server, or,  in  the  case  of  predefined  leases  declared  in
       dhclient.conf,  the value that the user wishes the client configuration
       script to use if the predefined lease is used.

	script "script-name";

       The script statement is used to specify the pathname of the dhcp client
       configuration  script.	This  script is used by the dhcp client to set
       each interface's initial configuration prior to requesting an  address,
       to  test  the  address  once it has been offered, and to set the interface's
 final configuration once a lease	has  been  acquired.	If  no
       lease  is  acquired,  the  script is used to test predefined leases, if
       any, and also called once if no valid lease can	be  identified.    For
       more information, see dhclient.leases(5) and dhclient-script(8).

	medium "media setup";

       The  medium  statement  can be used on systems where network interfaces
       cannot automatically determine the type of network to  which  they  are
       connected.   The  media	setup  string  is a system-dependent parameter
       which is passed to the dhcp client configuration script when initializing
  the  interface.   On  Unix	and Unix-like systems, the argument is
       passed on the ifconfig command line when configuring te interface.

       The dhcp client automatically declares this  parameter  if  it  used  a
       media  type (see the media statement) when configuring the interface in
       order to obtain a lease.  This statement should be used	in  predefined
       leases only if the network interface requires media type configuration.

	renew date;

	rebind date;

	expire date;

       The renew statement defines the time at which the  dhcp	client	should
       begin  trying  to contact its server to renew a lease that it is using.
       The rebind statement defines the time at which the dhcp	client	should
       begin  to  try  to contact any dhcp server in order to renew its lease.
       The expire statement defines the time at which  the  dhcp  client  must
       stop using a lease if it has not been able to contact a server in order
       to renew it.

       These declarations are automatically set in leases acquired by the DHCP
       client, but must also be configured in predefined leases - a predefined
       lease whose expiry time has passed will not be used by the DHCP client.

       Dates are specified as follows:

	<weekday> <year>/<month>/<day> <hour>:<minute>:<second>

       The weekday is present to make it easy for a human to tell when a lease
       expires - it's specified as a number from zero to six, with zero  being
       Sunday.	 When declaring a predefined lease, it can always be specified
       as zero.  The year is specified with the century, so it	should	generally
 be four digits except for really long leases.  The month is specified
 as a number starting with 1 for January.  The day of the month  is
       likewise specified starting with 1.  The hour is a number between 0 and
       23, the minute a number between 0 and 59, and the second also a	number
       between 0 and 59.


	alias {  declarations ... }

       Some  DHCP clients running TCP/IP roaming protocols may require that in
       addition to the lease they may acquire via DHCP, their  interface  also
       be configured with a predefined IP alias so that they can have a permanent
 IP address even while roaming.   The Internet Software  Consortium
       DHCP  client doesn't support roaming with fixed addresses directly, but
       in order to facilitate such experimentation, the dhcp client can be set
       up to configure an IP alias using the alias declaration.

       The  alias  declaration	resembles  a  lease  declaration,  except that
       options other than the subnet-mask option are ignored by  the  standard
       client  configuration  script, and expiry times are ignored.  A typical
       alias declaration includes an interface	declaration,  a  fixed-address
       declaration for the IP alias address, and a subnet-mask option declaration.
   A medium statement should never be included in an alias	declaration.


	reject ip-address;

       The  reject  statement  causes  the  DHCP  client to reject offers from
       servers who use the specified address as a  server  identifier.	  This
       can  be	used  to avoid being configured by rogue or misconfigured dhcp
       servers, although it should be a last resort - better to track down the
       bad DHCP server and fix it.

	interface "name" { declarations ...  }

       A client with more than one network interface may require different behaviour
 depending on which interface is being configured.   All	timing
       parameters and declarations other than lease and alias declarations can
       be enclosed in an interface declaration, and those parameters will then
       be  used  only  for  the  interface  that  matches  the specified name.
       Interfaces for which there is no interface  declaration	will  use  the
       parameters  declared  outside  of  any  interface  declaration,	or the
       default settings.

	media "media setup" [ , "media setup", ... ];

       The media statement defines one or more media configuration  parameters
       which  may  be  tried  while attempting to acquire an IP address.   The
       dhcp client will cycle through each media setup	string	on  the  list,
       configuring  the interface using that setup and attempting to boot, and
       then trying the next one.   This can be	used  for  network  interfaces
       which  aren't  capable  of  sensing  the media type unaided - whichever
       media type succeeds in getting a request to the server and hearing  the
       reply is probably right (no guarantees).

       The  media setup is only used for the initial phase of address acquisition
 (the DHCPDISCOVER and DHCPOFFER packtes).	Once  an  address  has
       been acquired, the dhcp client will record it in its lease database and
       will record the media type used to acquire the address.	 Whenever  the
       client  tries  to  renew  the  lease, it will use that same media type.
       The lease must expire before the client will go back to cycling through
       media types.

SAMPLE    [Toc]    [Back]

       The  following  configuration  file  is used on a laptop running NetBSD
       1.3.   The laptop has an IP alias of, and  has  one	interface,
  ep0  (a  3com  3C589C).	 Booting intervals have been shortened
       somewhat from the default, because the client is known to spend most of
       its  time on networks with little DHCP activity.   The laptop does roam
       to multiple networks.

       timeout 60;
       retry 60;
       reboot 10;
       select-timeout 5;
       initial-interval 2;

       interface "ep0" {
	   send host-name "andare.fugue.com";
	   send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
	   send dhcp-lease-time 3600;
	   supersede domain-name "fugue.com rc.vix.com home.vix.com";
	   prepend domain-name-servers;
	   request subnet-mask, broadcast-address, time-offset, routers,
		domain-name, domain-name-servers, host-name;
	   require subnet-mask, domain-name-servers;
	   script "/etc/dhclient-script";
	   media "media 10baseT/UTP", "media 10base2/BNC";

       alias {
	 interface "ep0";
	 option subnet-mask;
       This is a very complicated  dhclient.conf  file	-  in  general,  yours
       should be much simpler.	 In many cases, it's sufficient to just create
       an empty dhclient.conf file - the defaults are usually fine.

SEE ALSO    [Toc]    [Back]

       dhcp-options(5), dhclient.leases(5), dhcpd(8), dhcpd.conf(5),  RFC2132,

AUTHOR    [Toc]    [Back]

       dhclient(8)  was written by Ted Lemon <mellon@vix.com> under a contract
       with Vixie Labs.   Funding for this project was provided by the	Internet
 Software Corporation.  Information about the Internet Software Consortium
 can be found at http://www.isc.org/isc.

[ Back ]
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