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dhclient.conf(5)
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 configu‐
ration file can also be preinitialized with addresses to use on net‐
works 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 time‐
out has passed, if there are any static leases defined in the configu‐
ration 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 proto‐
col 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 ini‐
tial 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 Sec‐
tion 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 uncom‐
menting 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;
}
NOTE: For ISC DHCP release 4.1-ESV-R14 and earlier, dhclient running in
-6 mode does not discard offers as described above. This has been cor‐
rected as of release 4.1-ESV-R15 but must be enabled at compile time
(see ENFORCE_DHCPV6_CLIENT_REQUIRE in includes/site.h).
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 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 val‐
ues 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 TIM‐
ING) 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, fol‐
lowed 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 particu‐
lar 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 inter‐
face'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 infor‐
mation, 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 initializ‐
ing 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 gener‐
ally 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 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 perma‐
nent 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 declara‐
tion. A medium statement should never be included in an alias declara‐
tion.
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 mis‐
configured 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[/pre‐
fixlen], where ip-address is a dotted quad IP address, and prefixlen is
the CIDR prefix length of the subnet, counting the number of signifi‐
cant bits in the netmask starting from the leftmost end. Example con‐
figuration 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 sin‐
gle address 10.0.0.5, to be rejected.
interface "name" { declarations ... }
A client with more than one network interface may require different be‐
haviour 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. Inter‐
faces for which there is no interface declaration will use the parame‐
ters declared outside of any interface declaration, or the default set‐
tings.
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 inter‐
face 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, config‐
uring 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.
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";
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)