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
   configuration  file can also be preinitialized with addresses to use on
   networks 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 300 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,  the  actual  value  will  be  evaluated  randomly
   between  1/2 to 1 1/2 times the time specified.  It defaults to fifteen

   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 initial-delay statement

    initial-delay time;

   initial-delay  parameter  sets  the  maximum time client can wait after
   start before  commencing  first  transmission.   According  to  RFC2131
   Section 4.4.1, client should wait a random time between startup and the
   actual first transmission. Previous versions of ISC DHCP client used to
   wait  random  time up to 5 seconds, but that was unwanted due to impact
   on startup time. As such, new versions have the default  initial  delay
   set to 0. To restore old behavior, please set initial-delay to 5.


   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-name-servers and host-name options while the DHCPv6
   client requests the dhcp6 name-servers and domain-search options.  Note
   that if you enter a request statement, you over-ride  these  defaults
   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
   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
   TIMING)  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-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

   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
   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.

   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
   permanent   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

   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


    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,
   within 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
   misconfigured  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[/prefixlen],  where ip-address is a dotted quad IP address, and
   prefixlen is the CIDR prefix length of the subnet, counting the  number
   of  significant  bits  in  the  netmask starting from the leftmost end.
   Example configuration 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  interface  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 autodetected.  If you supplied the list of interfaces on the command
   line, this configuration 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  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.

    hardware link-type mac-address;

   The hardware statement defines the hardware MAC address to use for this
   interface,  for  DHCP  servers  or  relays  to  direct  their  replies.
   dhclient will determine the interface's MAC address  automatically,  so
   use of this parameter is not recommended.  The link-type corresponds to
   the interface's link layer type (example: ethernet), while  the  mac-
   address is a string of colon-separated hexadecimal values for octets.

    anycast-mac link-type mac-address;

   The anycast-mac statement over-rides the all-ones broadcast MAC address
   dhclient will use when it  is  transmitting  packets  to  the  all-ones
   limited broadcast IPv4 address.  This configuration parameter is useful
   to reduce the number of broadcast packets transmitted by DHCP  clients,
   but  is only useful if you know the DHCP service(s) anycast MAC address
   prior to  configuring  your  client.   The  link-type  and  mac-address
   parameters   are  configured  in  a  similar  manner  to  the  hardware


   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 300;
   retry 60;
   reboot 10;
   select-timeout 5;
   initial-interval 2;

   interface "ep0" {
       send host-name "andare.fugue.com";
       hardware ethernet 00:a0:24:ab:fb:9c;
       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-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) Information about Internet Systems Consortium can be  found
   at https://www.isc.org.



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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.