dhclient.conf

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 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 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 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 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 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 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 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 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.

DHCPv6 LEASE SELECTION
       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 uncommenting
       the define for USE_ORIGINAL_CLIENT_LEASE_WEIGHTS.

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 ] ;

       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 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.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.

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 information
       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
       supplied by the server, if any, and then use values you supply, these
       values 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
       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 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, followed 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 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 configuration 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-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 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 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 generally be
       four digits except for really long leases.  The month is specified 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 permanent
       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
       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.

       The lease-id-format parameter

         lease-id-format format;

         The format parameter must be either octal or hex.  This parameter
         governs the format used to write certain values to lease files. With
         the default format, octal, values are written as quoted strings in
         which non-printable characters are represented as octal escapes - a
         backslash character followed by three octal digits.  When the hex
         format is specified, values are written as an unquoted series of
         hexadecimal digit pairs, separated by colons.

         Currently, the values written out based on lease-id-format are the
         default-duid and the IAID value (DHCPv6 only).  The client
         automatically reads the values in either format.  Note that when the
         format is octal, rather than as an octal string, IAID is output as hex
         if it contains no printable characters or as a string if contains only
         printable characters. This is done to maintain backward compatibility.

          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 subnet, 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 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
         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 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
         acquisition (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
         limited 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
         parameters are configured in a similar manner to the hardware
         statement.

SAMPLE
       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
       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 networks.


       timeout 60;
       retry 60;
       reboot 10;
       select-timeout 5;
       initial-interval 2;
       reject 192.33.137.209;

       interface "ep0" {
           send host-name "andare.example.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 "example.com", "rc.isc.org", "home.isc.org";
           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) Information about Internet Systems Consortium can be found at
       https://www.isc.org.



                                                                dhclient.conf(5)