dhclient.conf(5)

NAME dhclient.conf - DHCP client configuration file

DESCRIPTION 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.

PROTOCOL TIMING 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
   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.

LEASE REQUIREMENTS AND REQUESTS 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
    dhcp-options(5).

   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
   parameters:

        request;

   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 declara?
   tions 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
   clients.

DYNAMIC DNS 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 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.

LEASE DECLARATIONS 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?
   ness.

   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
   possible:

    bootp;

   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., 12.34.56.78).

    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
   script.

    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.leases(5) and 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 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.

ALIAS DECLARATIONS 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?
   ration.

OTHER DECLARATIONS 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 tim?
   ing 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 inter?
   face 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  exam?
   ple:

        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 acquisi?
   tion (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 The following configuration file is used on a laptop running NetBSD 1.3. The laptop has an IP alias of 192.5.5.213, 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;
   reject 192.33.137.209;

   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 127.0.0.1;
       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";
     fixed-address 192.5.5.213;
     option subnet-mask 255.255.255.255;
   }
   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 dhcp-options(5), dhcp-eval(5), dhclient.leases(5), dhcpd(8), dhcpd.conf(5), RFC2132, RFC2131.

AUTHOR 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 http://www.isc.org.

                                                          dhclient.conf(5)