The
file contains configuration information for
dhclient(8),
the Internet Software Consortium DHCP Client.
The
file is a free-form ASCII text file.
It is parsed by the recursive-descent parser built into
dhclient(8).
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
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 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.
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-options5.
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.
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-options5.
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.
If for some set of options 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.
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.
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 violated, the results are unpredictable.
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:
The DHCP client may decide after some period of time (see
Sx 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 Qq 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
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 Qq 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 Qq 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
,
the value that the user wishes the client configuration script to use if the
predefined lease is used.
script Qq 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.leases5.
medium Qq 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
systems, the argument is passed on the
ifconfig(8)
command line
when configuring 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.
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 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
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 Qo name Qc { 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.
media Qo media setup Qc [, Qo media setup Qc , ... ]
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).
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.
This is a very complicated
file - in general, yours should be much simpler.
In many cases, it is sufficient to just create an empty
file - the defaults are usually fine.
