dhclient.conf man page on Scientific

Man page or keyword search:  
man Server   26626 pages
apropos Keyword Search (all sections)
Output format
Scientific logo
[printable version]

dhclient.conf(5)					      dhclient.conf(5)

       dhclient.conf - DHCP client configuration file

       The dhclient.conf file contains configuration information for dhclient,
       the Internet Systems 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  configu‐
       ration  file  can  also be preinitialized with addresses to use on net‐
       works that don't have DHCP servers.

       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  con‐
       figuration  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  ini‐
       tial  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,  the  actual  value	 will  be  evaluated  randomly
       between	1/2  to	 1  1/2 times the time specified.   It defaults to two

       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

	[ also ] request [ [ option-space . ] 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	DHCPv4	client
       requests	 the  subnet-mask,  broadcast-address,	time-offset,  routers,
       domain-name, domain-search, domain-name-servers, host-name, nis-domain,
       nis-servers,  ntp-servers  and interface-mtu options. The DHCPv6 client
       requests by default name-servers and domain-search. Note	 that  if  you
       enter  a	 'request'  statement,	you  over-ride	this default and these
       options will not be requested.

       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


       In most cases, it is desirable to simply add one option to the  request
       list  which is of interest to the client in question.  In this case, it
       is best to 'also request' the additional options:

	    also request domain-search, dhcp6.sip-servers-addresses;

       The require statement

	[ also ] require [ [ option-space . ] 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.	 There is no default require list.

	    require name-servers;

	    interface eth0 {
		 also require domain-search;


	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 dhcp-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 clients.

       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.

       Note  that  everything  in  this	 section is true whether you are using
       DHCPv4 or DHCPv6.  The exact same syntax is used for both.

       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;
	 also request fqdn, dhcp6.fqdn;

       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.

       In some cases, a client may receive option data from the	 server	 which
       is  not really appropriate for that client, or may not receive informa‐
       tion 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  sup‐
       plied by the server, if any, and then use values you supply, these val‐
       ues 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 complete‐

       A lease statement consists of the lease keyword,	 followed  by  a  left
       curly brace, followed by one or more lease declaration statements, fol‐
       lowed 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 particu‐
       lar 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 inter‐
       face'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 initializ‐
       ing  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 in one of two ways.	The software will output times
       in these two formats depending on if the	 db-time-format	 configuration
       parameter has been set to default or local.

       If it is set to default, then date values appear 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	gener‐
       ally be four digits except for really long leases.  The month is speci‐
       fied 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.

       If  the	db-time-format	configuration  was set to local, then the date
       values appear as follows:

	epoch <seconds-since-epoch>; #	<day-name>  <month-name>  <day-number>
       <hours>:<minutes>:<seconds> <year>

       The  seconds-since-epoch	 is  as	 according to the system's local clock
       (often referred to as "unix time").  The # symbol  supplies  a  comment
       that  describes	what  actual time this is as according to the system's
       configured timezone, at the time the value was written.	It is provided
       only for human inspection, the epoch time is the only recommended value
       for machine inspection.

       Note that when defining a static lease, one may use either time	format
       one wishes, and need not include the comment or values after it.

       If  the time is infinite in duration, then the date is never instead of
       an actual date.

	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 perma‐
       nent  IP	 address even while roaming.   The Internet Systems 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 declara‐
       tion.	A medium statement should never be included in an alias decla‐

	db-time-format [ default | local ] ;

       The db-time-format option determines which of two  output  methods  are
       used  for  printing times in leases files.  The default format provides
       day-and-time in UTC, whereas local uses a seconds-since-epoch to	 store
       the time value, and helpfully places a local timezone time in a comment
       on the same line.  The formats are described in detail in this manpage,
       whithin the LEASE DECLARATIONS section.

	reject cidr-ip-address [, ... cidr-ip-address ] ;

       The  reject  statement  causes  the  DHCP  client to reject offers from
       servers whose server identifier matches any of the specified  hosts  or
       subnets.	  This	can be used to avoid being configured by rogue or mis‐
       configured dhcp servers, although it should be a last resort  -	better
       to track down the bad DHCP server and fix it.

       The  cidr-ip-address configuration type is of the form ip-address[/pre‐
       fixlen], where ip-address is a dotted quad IP address, and prefixlen is
       the  CIDR  prefix length of the subnet, counting the number of signifi‐
       cant bits in the netmask starting from the leftmost end.	 Example  con‐
       figuration syntax:


The  above example would cause offers from any server identifier in the entire
RFC 1918 "Class C" network,  or	the  specific  single  address, to be rejected.

 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	inter‐
face declaration, or the default settings.

Note well: ISC dhclient only maintains one list of interfaces, which is either
determined at startup from command line arguments,  or	otherwise  is  autode‐
tected.	 If you supplied the list of interfaces on the command line, this con‐
figuration clause will add the named interface to the list in such a way  that
will  cause  it to be configured by DHCP.  Which may not be the result you had
intended.  This is an undesirable side effect that  will  be  addressed	 in  a
future release.

 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	inter‐
face  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.


The bootp-broadcast-always statement instructs	dhclient  to  always  set  the
bootp broadcast flag in request packets, so that servers will always broadcast
replies.  This is equivalent to supplying the dhclient -B  argument,  and  has
the  same  effect as specifying 'always-broadcast' in the server's dhcpd.conf.
This option is provided as an extension to enable dhclient to work on IBM s390
Linux guests.

       The  following  configuration  file  is used on a laptop running NetBSD
       1.3.   The laptop has an IP alias of, and  has  one	inter‐
       face,  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-search "fugue.com", "rc.vix.com", "home.vix.com";
	   prepend domain-name-servers;
	   request subnet-mask, broadcast-address, time-offset, routers,
		domain-search, 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.

       dhcp-options(5),	   dhcp-eval(5),     dhclient.leases(5),     dhcpd(8),
       dhcpd.conf(5), RFC2132, RFC2131.

       dhclient(8)  was written by Ted Lemon under a contract with Vixie Labs.
       Funding for this project was provided by Internet  Systems  Consortium.
       Information   about   Internet  Systems	Consortium  can	 be  found  at


List of man pages available for Scientific

Copyright (c) for man pages and the logo by the respective OS vendor.

For those who want to learn more, the polarhome community provides shell access and support.

[legal] [privacy] [GNU] [policy] [cookies] [netiquette] [sponsors] [FAQ]
Polarhome, production since 1999.
Member of Polarhome portal.
Based on Fawad Halim's script.
Vote for polarhome
Free Shell Accounts :: the biggest list on the net