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dhclient.conf(5)              File Formats Manual             dhclient.conf(5)

       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  configu‐
       ration  file  can  also be preinitialized with addresses to use on net‐
       works 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.

       If  required  the following statements can be used to adjust the timing
       behaviour of the DHCPv4 client.  The DHCPv6 protocol provides values to
       use and they are not currently configurable.

       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

       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.

       In the DHCPv6 protocol the client will wait a small amount of  time  to
       allow ADVERTISE messages from multiple servers to arrive.  It will then
       need to choose from all of the messages that may  have  arrived  before
       proceeding to making a request of the selected server.

       The first selection criteria is the set of options and addresses in the
       message.  Messages that don't include an option specified  as  required
       will  be given a score of 0 and not used.  If the -R option is given on
       the command line then messages that don't include the correct number of
       bindings (IA-NA, IA-TA or IA-PD) will be discarded.

       The  next  criteria is the preference value from the message.  With the
       highest  preference  value  being  used  even  if  leases  with  better
       addresses or options are available.

       Finally  the  lease  is  scored and the lease with the highest score is
       selected.  A lease's score is based on the number of  bindings,  number
       of addresses and number of options it contains:
            bindings * X + addresses * Y + options
       By  default X = 10000 and Y = 100, this will cause the client to select
       a lease with more bindings over a lease with  less  bindings  but  more
       addresses.   The  weightings  were  changed as part of implementing RFC
       7550.  Previously they were X = 50 and Y = 100 meaning  more  addresses
       were  preferred  over more bindings.  If you wish to continue using the
       old style you may do so by editing the file includes/site.h and  uncom‐
       menting the define for USE_ORIGINAL_CLIENT_LEASE_WEIGHTS.

       The  DHCP protocol allows the client to request that the server send it
       specific information, and not send it other information that it is  not
       prepared  to  accept.   The  protocol  also allows the client to reject
       offers from servers if they don't contain information the client needs,
       or if the information provided is not satisfactory.

       There  is  a variety of data contained in offers that DHCP servers send
       to DHCP clients.  The data that can be specifically requested  is  what
       are called DHCP Options.  DHCP Options are defined in

       The request statement

        [ also ] request [ [ option-space . ] option ] [, ... ];

       The  request  statement  causes  the  client to request that any server
       responding to the client send the client its values for  the  specified
       options.   Only  the  option  names  should be specified in the request
       statement - not option  parameters.   By  default,  the  DHCPv4  client
       requests  the  subnet-mask,  broadcast-address,  time-offset,  routers,
       domain-name, domain-name-servers and host-name options while the DHCPv6
       client requests the dhcp6 name-servers and domain-search options.  Note
       that if you enter a ´request´ statement, you over-ride  these  defaults
       and these options will not be requested.

       In some cases, it may be desirable to send no parameter request list at
       all.  To do this, simply write the request  statement  but  specify  no


       In  most cases, it is desirable to simply add one option to the request
       list which is of interest to the client in question.  In this case,  it
       is best to ´also request´ the additional options:

            also request domain-search, dhcp6.sip-servers-addresses;

       The require statement

        [ also ] require [ [ option-space . ] option ] [, ... ];

       The  require  statement lists options that must be sent in order for an
       offer to be accepted.  Offers  that  do  not  contain  all  the  listed
       options will be ignored.  There is no default require list.

            require name-servers;

            interface eth0 {
                 also require domain-search;

       NOTE: For ISC DHCP release 4.1-ESV-R14 and earlier, dhclient running in
       -6 mode does not discard offers as described above. This has been  cor‐
       rected  as  of  release 4.1-ESV-R15 but must be enabled at compile time
       (see ENFORCE_DHCPV6_CLIENT_REQUIRE in includes/site.h).

       The send statement

        send [ option declaration ] ;

       The send statement causes the client to send the  specified  option  to
       the server with the specified value.  This is a full option declaration
       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

       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.example.com.";
         send fqdn.encoded on;
         send fqdn.server-update off;
         also request fqdn, dhcp6.fqdn;

       The fqdn.fqdn option MUST be a fully-qualified domain name.   You  MUST
       define  a  zone statement for the zone to be updated.  The fqdn.encoded
       option may need to be set to on or off, depending on  the  DHCP  server
       you are using.

       The do-forward-updates statement

        do-forward-updates [ flag ] ;

       If  you want to do DNS updates in the DHCP client script (see dhclient-
       script(8)) rather than having the DHCP client do  the  update  directly
       (for  example,  if  you want to use SIG(0) authentication, which is not
       supported directly by the DHCP client, you can instruct the client  not
       to  do  the update using the do-forward-updates statement.  Flag should
       be true if you want the DHCP client to do the update, and false if  you
       don't  want  the  DHCP  client  to do the update.  By default, the DHCP
       client will do the DNS update.

       In some cases, a client may receive option data from the  server  which
       is  not really appropriate for that client, or may not receive 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.

       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


       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.,

        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  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 ... }

       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‐

        db-time-format [ default | local ] ;

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

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

       The reject statement causes the  DHCP  client  to  reject  offers  from
       servers  whose  server identifier matches any of the specified hosts or
       subnets.  This can be used to avoid being configured by rogue  or  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:


       The above example would cause offers from any server identifier in  the
       entire  RFC 1918 "Class C" network, or the specific sin‐
       gle address, 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‐

       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.

       The following configuration file was used on a  laptop  running  NetBSD
       1.3, though the domains have been modified.  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 "andare.example.com";
           send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
           send dhcp-lease-time 3600;
           supersede domain-search "example.com", "rc.isc.org", "home.isc.org";
           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 "CLIENTBINDIR/dhclient-script";
           media "media 10baseT/UTP", "media 10base2/BNC";

       alias {
         interface "ep0";
         option subnet-mask;
       This  is  a  very  complicated  dhclient.conf  file - in general, yours
       should be much simpler.  In many cases, it's sufficient to just  create
       an empty dhclient.conf file - the defaults are usually fine.

       dhcp-options(5),     dhcp-eval(5),     dhclient.leases(5),    dhcpd(8),
       dhcpd.conf(5), RFC2132, RFC2131.

       dhclient(8) Information about Internet Systems Consortium can be  found
       at https://www.isc.org.

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