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 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 time-
       out has passed, if there are any static leases defined in the  configu-
       ration  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 unex-
       pired  leases in the lease database, the client will restart the proto-
       col 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 fifteen
       seconds.

       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  Sec-
       tion  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.

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

        [ 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
       parameters:

            request;

       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;
            }

       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 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.

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.

       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.

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 completeness.

       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 infor-
       mation, see dhclient-script(8).

        vendor option space "name";

       The vendor option space statement is used to specify which option space
       should  be  used  for decoding the vendor-encapsulate-options option if
       one is received.  The dhcp-vendor-identifier can be used to  request  a
       specific  class of vendor options from the server.  See dhcp-options(5)
       for details.

        medium "media setup";

       The medium statement can be used on systems  where  network  interfaces
       cannot  automatically  determine  the type of network to which they are
       connected.  The media setup  string  is  a  system-dependent  parameter
       which is passed to the dhcp client configuration script when initializ-
       ing the interface.  On Unix and  Unix-like  systems,  the  argument  is
       passed on the ifconfig command line when configuring the interface.

       The  dhcp  client  automatically  declares  this parameter if it uses a
       media type (see the media statement) when configuring the interface  in
       order  to  obtain a lease.  This statement should be used in predefined
       leases only if the network interface requires media type configuration.

        renew date;

        rebind date;

        expire date;

       The  renew  statement  defines the time at which the dhcp client should
       begin trying to contact its server to renew a lease that it  is  using.
       The  rebind  statement defines the time at which the dhcp client should
       begin to try to contact any dhcp server in order to  renew  its  lease.
       The  expire  statement  defines  the time at which the dhcp client must
       stop using a lease if it has not been able to contact a server in order
       to renew it.

       These declarations are automatically set in leases acquired by the DHCP
       client, but must also be configured in predefined leases - a predefined
       lease whose expiry time has passed will not be used by the DHCP client.

       Dates are specified in one of two ways.  The software will output times
       in  these  two formats depending on if the db-time-format configuration
       parameter has been set to default or local.

       If it is set to default, then date values appear as follows:

        <weekday> <year>/<month>/<day> <hour>:<minute>:<second>

       The weekday is present to make it easy for a human to tell when a lease
       expires  - it's specified as a number from zero to six, with zero being
       Sunday.  When declaring a predefined lease, it can always be  specified
       as  zero.   The year is specified with the century, so it should gener-
       ally be four digits except for really long leases.  The month is speci-
       fied  as a number starting with 1 for January.  The day of the month is
       likewise specified starting with 1.  The hour is a number between 0 and
       23,  the minute a number between 0 and 59, and the second also a number
       between 0 and 59.

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

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

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

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

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

ALIAS DECLARATIONS
        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 declara-
       tion.

OTHER DECLARATIONS
        db-time-format [ default | local ] ;

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

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

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

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

       reject 192.168.0.0/16, 10.0.0.5;

       The above example would cause offers from any server identifier in  the
       entire  RFC 1918 "Class C" network 192.168.0.0/16, or the specific sin-
       gle address 10.0.0.5, to be rejected.

        interface "name" { declarations ...  }

       A client with more than one network interface may require different be-
       haviour  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.  Inter-
       faces  for which there is no interface declaration will use the parame-
       ters declared outside of any interface declaration, or the default set-
       tings.

       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 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,  config-
       uring  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.

        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 lim-
       ited 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 param-
       eters are configured in a similar manner to the hardware statement.


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 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  multi-
       ple 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";
           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 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),     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
       https://www.isc.org.



                                                              dhclient.conf(5)

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