dhclient.conf(5)                                              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 behavior 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 behavior of the client need not be configured by  the  user.
       If no timing configuration is provided by the user, a fairly reasonable
       timing behavior 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 behavior 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  lock-step.    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  client
       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.

       The omapi port statement

        omapi port [ port ] ;

       The omapi port statement causes the DHCP client to set up an OMAPI lis-
       tener on the specified port.   Only one such port should be  specified.
       The  OMAPI  listener  will  be  sensitive  to  connections  from any IP
       address, so it is important to also set up an OMAPI key to protect  the
       client from unauthorized changes.

       The omapi key statement

        omapi key [  key-id  ] ;

       The  omapi  key  statement causes the DHCP client to check any incoming
       OMAPI messages to make sure that they are signed by the specified  key.
       If a message is not signed by this key, it is rejected with an "invalid
       key" error.

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 behavior 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 behavior  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-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
       behavior 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  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 "CLIENTBINDIR/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),  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)

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