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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.    By  default,  the  DHCP	server
       requests  the  subnet-mask,  broadcast-address,	time-offset,  routers,
       domain-name, domain-name-servers and host-name options.

       In some cases, it may be desirable to send no parameter request list at
       all.    To  do  this, simply write the request statement but specify no


       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

DYNAMIC DNS    [Toc]    [Back]

       The client now has some very limited support for doing DNS updates when
       a  lease  is  acquired.	 This is prototypical, and probably doesn't do
       what you want.	It also only works if you happen to have control  over
       your DNS server, which isn't very likely.

       To  make  it  work,  you  have to declare a key and zone as in the DHCP
       server (see dhcpd.conf(5) for details).	 You also  need  to  configure
       the fqdn option on the client, as follows:

	 send fqdn.fqdn "grosse.fugue.com.";
	 send fqdn.encoded on;
	 send fqdn.server-update off;

       The  fqdn.fqdn option MUST be a fully-qualified domain name.   You MUST
       define a zone statement for the zone to be updated.   The  fqdn.encoded
       option  may  need  to be set to on or off, depending on the DHCP server
       you are using.

       The do-forward-updates statement

	do-forward-updates [ flag ] ;

       If you want to do DNS updates in the DHCP client script (see  dhclient-
       script(8))  rather  than  having the DHCP client do the update directly
       (for example, if you want to use SIG(0) authentication,	which  is  not
       supported  directly by the DHCP client, you can instruct the client not
       to do the update using the do-forward-updates statement.   Flag	should
       be  true if you want the DHCP client to do the update, and false if you
       don't want the DHCP client to do the update.    By  default,  the  DHCP
       client will do the DNS update.

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 ] ;

       If for some option 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 ] ;

       If for some option the client should always  use  a  locally-configured
       value  or  values rather than whatever is supplied by the server, these
       values can be defined in the supersede statement.

       The prepend statement

	prepend [ 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 you ignore it, the behaviour will
       be unpredictable.

       The append statement

	append [ 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)  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 statement.

       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-script(8).

	vendor option space "name";

       The vendor option space statement is used to specify which option space
       should  be  used  for decoding the vendor-encapsulate-options option if
       one is received.  The dhcp-vendor-identifier can be used to  request  a
       specific class of vendor options from the server.   See dhcp-options(5)
       for details.

	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 the interface.

       The  dhcp  client  automatically  declares  this parameter if it uses 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.

	pseudo "name" "real-name" { declarations ...  }

       Under some circumstances it can be useful to declare a pseudo-interface
       and have the DHCP client acquire a configuration  for  that  interface.
       Each  interface	that the DHCP client is supporting normally has a DHCP
       client state machine running on it to acquire and maintain  its	lease.
       A  pseudo-interface is just another state machine running on the interface
 named real-name, with its own lease and its own  state.    If  you
       use  this  feature,  you  must provide a client identifier for both the
       pseudo-interface and the actual interface, and the two identifiers must
       be  different.	You must also provide a separate client script for the
       pseudo-interface to do what you want with the IP address.    For  example:

	    interface "ep0" {
		 send dhcp-client-identifier "my-client-ep0";
	    pseudo "secondary" "ep0" {
		 send dhcp-client-identifier "my-client-ep0-secondary";
		 script "/etc/dhclient-secondary";

       The  client  script  for  the pseudo-interface should not configure the
       interface up or down - essentially, all it  needs  to  handle  are  the
       states where a lease has been acquired or renewed, and the states where
       a lease has expired.   See dhclient-script(8) for more information.

	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 packets).	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 "/sbin/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), dhclient(8), RFC2132, RFC2131.

AUTHOR    [Toc]    [Back]

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

[ Back ]
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