dhclient.conf
dhclient.conf(5) dhclient.conf(5)
NAME
dhclient.conf - DHCP client configuration file
DESCRIPTION
The dhclient.conf file contains configuration information for
dhclient, the Internet Systems Consortium DHCP Client.
The dhclient.conf file is a free-form ASCII text file. It is parsed
by the recursive-descent parser built into dhclient. The file may
contain extra tabs and newlines for formatting purposes. Keywords in
the file are case-insensitive. Comments may be placed anywhere
within the file (except within quotes). Comments begin with the #
character and end at the end of the line.
The dhclient.conf file can be used to configure the behaviour of the
client in a wide variety of ways: protocol timing, information
requested from the server, information required of the server,
defaults to use if the server does not provide certain information,
values with which to override information provided by the server, or
values to prepend or append to information provided by the server.
The 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 reason-
able timing behaviour will be used by default - one which results in
fairly timely updates without placing an inordinate load on the
server.
The following statements can be used to adjust the timing behaviour of
the DHCP client if required, however:
The timeout statement
timeout time ;
The timeout statement determines the amount of time that must pass
between the time that the client begins to try to determine its
address and the time that it decides that it’s not going to be able to
contact a server. By default, this timeout is sixty seconds. After
the 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 min-
utes.
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 pos-
sible 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 reac-
quire 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. It defaults to two minutes.
The initial-interval statement
initial-interval time;
The initial-interval statement sets the amount of time between the
first attempt to reach a server and the second attempt to reach a
server. Each time a message is sent, the interval between messages is
incremented by twice the current interval multiplied by a random num-
ber between zero and one. If it is greater than the backoff-cutoff
amount, it is set to that amount. It defaults to ten seconds.
LEASE REQUIREMENTS AND REQUESTS
The DHCP protocol allows the client to request that the server send it
specific information, and not send it other information that it is not
prepared to accept. The protocol also allows the client to reject
offers from servers if they don’t contain information the client
needs, or if the information provided is not satisfactory.
There is a variety of data contained in offers that DHCP servers send
to DHCP clients. The data that can be specifically requested is what
are called DHCP Options. DHCP Options are defined in
dhcp-options(5).
The request statement
request [ option ] [, ... option ];
The request statement causes the client to request that any server
responding to the client send the client its values for the specified
options. Only the option names should be specified in the request
statement - not option parameters. By default, the DHCP server
requests the subnet-mask, broadcast-address, time-offset, routers,
domain-name, domain-name-servers, host-name, nis-domain, nis-servers,
and ntp-servers options.
In some cases, it may be desirable to send no parameter request list
at all. To do this, simply write the request statement but specify
no parameters:
request;
The require statement
require [ option ] [, ... option ];
The require statement lists options that must be sent in order for an
offer to be accepted. Offers that do not contain all the listed
options will be ignored.
The send statement
send { [ option declaration ] [, ... option declaration ]}
The send statement causes the client to send the specified options to
the server with the specified values. These are full option declara-
tions as described in dhcp-options(5). Options that are always sent
in the DHCP protocol should not be specified here, except that the
client can specify a requested-lease-time option other than the
default requested lease time, which is two hours. The other obvious
use for this statement is to send information to the server that will
allow it to differentiate between this client and other clients or
kinds of clients.
DYNAMIC DNS
The client now has some very limited support for doing DNS updates
when a lease is acquired. This is prototypical, and probably doesn’t
do what you want. It also only works if you happen to have control
over your DNS server, which isn’t very likely.
To make it work, you have to declare a key and zone as in the DHCP
server (see dhcpd.conf(5) for details). You also need to configure
the fqdn option on the client, as follows:
send fqdn.fqdn "grosse.fugue.com.";
send fqdn.encoded on;
send fqdn.server-update off;
The fqdn.fqdn option MUST be a fully-qualified domain name. You MUST
define a zone statement for the zone to be updated. The fqdn.encoded
option may need to be set to on or off, depending on the DHCP server
you are using.
The do-forward-updates statement
do-forward-updates [ flag ] ;
If you want to do DNS updates in the DHCP client script (see dhclient-
script(8)) rather than having the DHCP client do the update directly
(for example, if you want to use SIG(0) authentication, which is not
supported directly by the DHCP client, you can instruct the client not
to do the update using the do-forward-updates statement. Flag should
be true if you want the DHCP client to do the update, and false if you
don’t want the DHCP client to do the update. By default, the DHCP
client will do the DNS update.
OPTION MODIFIERS
In some cases, a client may receive option data from the server which
is not really appropriate for that client, or may not receive informa-
tion that it needs, and for which a useful default value exists. It
may also receive information which is useful, but which needs to be
supplemented with local information. To handle these needs, several
option modifiers are available.
The default statement
default [ option declaration ] ;
If for some option the client should use the value supplied by the
server, but needs to use some default value if no value was supplied
by the server, these values can be defined in the default statement.
The supersede statement
supersede [ option declaration ] ;
If for some option the client should always use a locally-configured
value or values rather than whatever is supplied by the server, these
values can be defined in the supersede statement.
The prepend statement
prepend [ option declaration ] ;
If for some set of options the client should use a value you supply,
and then use the values supplied by the server, if any, these values
can be defined in the prepend statement. The prepend statement can
only be used for options which allow more than one value to be given.
This restriction is not enforced - if you ignore it, the behaviour
will be unpredictable.
The append statement
append [ option declaration ] ;
If for some set of options the client should first use the values sup-
plied by the server, if any, and then use values you supply, these
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 partic-
ular 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 sup-
plied 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 initial-
izing 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 configura-
tion.
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 pre-
defined lease whose expiry time has passed will not be used by the
DHCP client.
Dates are specified as follows:
<weekday> <year>/<month>/<day> <hour>:<minute>:<second>
The weekday is present to make it easy for a human to tell when a
lease expires - it’s specified as a number from zero to six, with zero
being Sunday. When declaring a predefined lease, it can always be
specified as zero. The year is specified with the century, so it
should 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.
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 per-
manent IP address even while roaming. The Internet Systems Consor-
tium 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 declara-
tion.
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 decla-
ration. A medium statement should never be included in an alias dec-
laration.
OTHER DECLARATIONS
reject ip-address;
The reject statement causes the DHCP client to reject offers from
servers who use the specified address as a server identifier. This
can be used to avoid being configured by rogue or misconfigured dhcp
servers, although it should be a last resort - better to track down
the bad DHCP server and fix it.
interface "name" { declarations ... }
A client with more than one network interface may require different
behaviour depending on which interface is being configured. All tim-
ing parameters and declarations other than lease and alias declara-
tions can be enclosed in an interface declaration, and those parame-
ters will then be used only for the interface that matches the speci-
fied name. Interfaces for which there is no interface declaration
will use the parameters declared outside of any interface declaration,
or the default settings.
pseudo "name" "real-name" { declarations ... }
Under some circumstances it can be useful to declare a pseudo-inter-
face and have the DHCP client acquire a configuration for that inter-
face. 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 informa-
tion.
media "media setup" [ , "media setup", ... ];
The media statement defines one or more media configuration parameters
which may be tried while attempting to acquire an IP address. The
dhcp client will cycle through each media setup string on the list,
configuring the interface using that setup and attempting to boot, and
then trying the next one. This can be used for network interfaces
which aren’t capable of sensing the media type unaided - whichever
media type succeeds in getting a request to the server and hearing the
reply is probably right (no guarantees).
The media setup is only used for the initial phase of address acquisi-
tion (the DHCPDISCOVER and DHCPOFFER packets). Once an address has
been acquired, the dhcp client will record it in its lease database
and will record the media type used to acquire the address. Whenever
the client tries to renew the lease, it will use that same media type.
The lease must expire before the client will go back to cycling
through media types.
bootp-broadcast-always;
The bootp-broadcast-always statement instructs dhclient to always set
the bootp broadcast flag in request packets, so that servers will
always broadcast replies. This is equivalent to supplying the
dhclient -B argument, and has the same effect as specifying ’always-
broadcast’ in the server’s dhcpd.conf. This option is provided as a
Red Hat extension to enable dhclient to work on IBM zSeries z/OS Linux
guests .
SAMPLE
The following configuration file is used on a laptop running NetBSD
1.3. The laptop has an IP alias of 192.5.5.213, and has one inter-
face, ep0 (a 3com 3C589C). Booting intervals have been shortened
somewhat from the default, because the client is known to spend most
of its time on networks with little DHCP activity. The laptop does
roam to multiple networks.
timeout 60;
retry 60;
reboot 10;
select-timeout 5;
initial-interval 2;
reject 192.33.137.209;
interface "ep0" {
send host-name "andare.fugue.com";
send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
send dhcp-lease-time 3600;
supersede domain-name "fugue.com rc.vix.com home.vix.com";
prepend domain-name-servers 127.0.0.1;
request subnet-mask, broadcast-address, time-offset, routers,
domain-name, domain-name-servers, host-name;
require subnet-mask, domain-name-servers;
script "/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 cre-
ate an empty dhclient.conf file - the defaults are usually fine.
SEE ALSO
dhcp-options(5), dhclient.leases(5), dhcpd(8), dhcpd.conf(5), RFC2132,
RFC2131.
AUTHOR
dhclient(8) was written by Ted Lemon under a contract with Vixie Labs.
Funding for this project was provided by Internet Systems Consortium.
Information about Internet Systems Consortium can be found at
http://www.isc.org.
dhclient.conf(5)
