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

NAME
       dhclient.conf - DHCP client configuration file

DESCRIPTION
       The  dhclient.conf file contains configuration information for dhclient, the Internet Sys‐
       tems 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 for‐
       matting purposes.  Keywords in the file are case-insensitive.  Comments may be placed any‐
       where 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  informa‐
       tion,  values  with  which  to  override  information provided by the server, or values to
       prepend or append to information provided by the server.  The configuration file can  also
       be preinitialized with addresses to use on networks that don't have DHCP servers.

PROTOCOL TIMING
       The  timing behaviour of the client need not be configured by the user.  If no timing con‐
       figuration 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 300 seconds.  After
       the timeout has passed, if there are any static leases defined in the configuration  file,
       or  any  leases remaining in the lease database that have not yet expired, the client will
       loop through these leases attempting to validate them, and if it finds one that appears to
       be  valid, it will use that lease's address.  If there are no valid static leases or unex‐
       pired 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 serv‐
       ing any given network.  In this case, it is possible that a client may be sent  more  than
       one offer in response to its initial lease discovery message.  It may be that one of these
       offers is preferable to the other (e.g., one offer may have the address the client  previ‐
       ously 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 state‐
       ment 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  commenc‐
       ing  first  transmission.  According to RFC2131 Section 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 informa‐
       tion, 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  addi‐
       tional 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 state‐
       ment 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 appro‐
       priate for that client, or may not receive information that it needs, and for which a use‐
       ful  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  super‐
       sede 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 val‐
       ues 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  unpre‐
       dictable.

       The append statement

        append [ option declaration ] ;

       If  for some set of options the client should first use the values supplied by the server,
       if any, and then use values you supply, these values can be defined in the  append  state‐
       ment.   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 TIMING) that it is not
       going to succeed in contacting a server.  At that time, it consults its  own  database  of
       old  leases and tests each one that has not yet timed out by pinging the listed router for
       that lease to see if that lease could work.  It is possible to define one  or  more  fixed
       leases  in the client configuration file for networks where there is no DHCP or BOOTP ser‐
       vice, 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, followed by a right curly brace.  The follow‐
       ing lease declarations are possible:

        bootp;

       The bootp statement is used to indicate that the lease was acquired using the BOOTP proto‐
       col  rather  than  the DHCP protocol.  It is never necessary to specify this in the client
       configuration file.  The client uses this syntax in its lease database file.

        interface "string";

       The interface lease statement is used to indicate the interface  on  which  the  lease  is
       valid.   If set, this lease will only be tried on a particular interface.  When the client
       receives a lease from a server, it  always  records  the  interface  number  on  which  it
       received  that  lease.   If predefined leases are specified in the dhclient.conf file, the
       interface should also be specified, although this is not required.

        fixed-address ip-address;

       The fixed-address statement is used to set the ip address of a particular lease.  This  is
       required  for  all  lease  statements.   The IP address must be specified as a dotted quad
       (e.g., 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 configura‐
       tion prior to requesting an address, to test the address once it has been offered, and  to
       set  the  interface's  final configuration once a lease has been acquired.  If no lease is
       acquired, the script is used to test predefined leases, if any, and also called once if no
       valid lease can be identified.  For more information, see dhclient-script(8).

        vendor option space "name";

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

        medium "media setup";

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

         // ::

       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  prede‐
       fined  lease, it can always be specified as zero.  The year is specified with the century,
       so it should generally be four digits except for really long leases.  The month is  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 fol‐
       lows:

        epoch  ;  #          :: 

       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 permanent IP address even while roaming.  The Internet  Sys‐
       tems  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  declaration.
       A medium statement should never be included in an alias declaration.

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,
       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 misconfigured dhcp servers, although it should be a last resort -
       better to track down the bad DHCP server and fix it.

       The cidr-ip-address configuration type is of the form  ip-address[/prefixlen],  where  ip-
       address  is  a dotted quad IP address, and prefixlen is the CIDR prefix length of the sub‐
       net, counting the number of significant bits in the netmask  starting  from  the  leftmost
       end.  Example configuration 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 single address 10.0.0.5, to be rejected.

        interface "name" { declarations ...  }

       A client with more than one network interface may require different behaviour 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 param‐
       eters  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 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 interface
       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 infor‐
       mation.

        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 acquisition (the DHCPDIS‐
       COVER 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 limited broadcast IPv4 address.  This con‐
       figuration  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 parameters 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 multiple net‐
       works.

       timeout 300;
       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 "/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)   Information   about   Internet   Systems   Consortium   can   be   found  at
       https://www.isc.org.

                                                                                 dhclient.conf(5)

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