DHCLIENT.CONF(5) BSD File Formats Manual DHCLIENT.CONF(5)NAMEdhclient.conf — DHCP client configuration file
DESCRIPTION
The dhclient.conf file contains configuration information for
dhclient(8), the Internet Software 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(8). The file may con‐
tain 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 do not have
DHCP servers.
PROTOCOL TIMING
The timing behaviour of the client need not be configured by the user.
If no timing configuration is provided by the user, a fairly reasonable
timing behaviour will be used by default - one which results in fairly
timely updates without placing an inordinate load on the server.
The following statements can be used to adjust the timing behaviour of
the DHCP client if required, however:
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 is 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.
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.
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.
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.
backoff-cutoff time;
The client uses an exponential backoff algorithm with some ran‐
domness, 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.
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.
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 do not 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).
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.
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.
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 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.
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.
default { [option declaration] [, ... option declaration] }
If for some set of options the client should use the value sup‐
plied 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.
supersede { [option declaration] [, ... option declaration] }
If for some set of options the client should always use its own
value rather than any value supplied by the server, these values
can be defined in the supersede statement.
prepend { [option declaration] [, ... option declaration] }
If for some set of options the client should use a value you sup‐
ply, 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 violated, the results are unpredictable.
append { [option declaration] [, ... 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 config‐
ure 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 speci‐
fied, 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 con‐
figuration 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.leases(5).
medium "media setup";
The medium statement can be used on systems where network inter‐
faces 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(8) command line when con‐
figuring the interface.
The DHCP client automatically declares this parameter if it used
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 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 is 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 permanent
IP address even while roaming. The Internet Software Consortium DHCP
client does not 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 con‐
figuration script, and expiry times are ignored. A typical alias decla‐
ration 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
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 misconfig‐
ured 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 differ‐
ent 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.
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 are not 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.
EXAMPLES
The following configuration file is used on a laptop which has an IP
alias of 192.5.5.213, and has one interface, ep0 (a 3Com 3C589C). Boot‐
ing 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 "/etc/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 is sufficient to just create an empty
dhclient.conf file - the defaults are usually fine.
SEE ALSOdhclient.leases(5), dhcpd.conf(5), dhcp-options(5), dhclient(8), dhcpd(8)
RFC 2132, RFC 2131.
AUTHORS
The dhclient(8) utility was written by Ted Lemon ⟨mellon@vix.com⟩ under a
contract with Vixie Labs.
The current implementation was reworked by Henning Brauer
⟨henning@openbsd.org⟩.
BSD January 1, 1997 BSD
