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       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 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,  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

        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


       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 "";
         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.


       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

       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,
       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
       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:

 interface "name" { declarations ...  }
Note well: ISC dhclient only maintains one list of interfaces, which is either
 pseudo "name" "real-name" { declarations ...  }
is  just  another state machine running on the interface named real-name, with
     interface "ep0" {
          send dhcp-client-identifier "my-client-ep0";
     pseudo "secondary" "ep0" {
          send dhcp-client-identifier "my-client-ep0-secondary";
          script "/etc/dhclient-secondary";
dhclient-script(8) for more information.

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

The  media  statement defines one or more media configuration parameters which


       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 "";
           send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
           send dhcp-lease-time 3600;
           supersede domain-name "";
           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) 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