dhclient(8) System Manager's Manual dhclient(8)
dhclient - Dynamic Host Configuration Protocol Client
dhclient [ -4 | -6 ] [ -S ] [ -N [ -N... ] ] [ -T [ -T... ] ] [ -P [
-P... ] ] [ -i ] [ -I ] [ -D LL|LLT ] [ -p port-number ] [ -d ] [ -df
duid-lease-file ] [ -e VAR=value ] [ -q ] [ -1 ] [ -r | -x ] [ -lf
lease-file ] [ -pf pid-file ] [ --no-pid ] [ -cf config-file ] [ -sf
script-file ] [ -s server-addr ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [
-v ] [ --version ] [ if0 [ ...ifN ] ]
The Internet Systems Consortium DHCP Client, dhclient, provides a means
for configuring one or more network interfaces using the Dynamic Host
Configuration Protocol, BOOTP protocol, or if these protocols fail, by
statically assigning an address.
The DHCP protocol allows a host to contact a central server which main-
tains a list of IP addresses which may be assigned on one or more sub-
nets. A DHCP client may request an address from this pool, and then
use it on a temporary basis for communication on network. The DHCP
protocol also provides a mechanism whereby a client can learn important
details about the network to which it is attached, such as the location
of a default router, the location of a name server, and so on.
There are two versions of the DHCP protocol DHCPv4 and DHCPv6. At
startup the client may be started for one or the other via the -4 or -6
On startup, dhclient reads the dhclient.conf for configuration instruc-
tions. It then gets a list of all the network interfaces that are con-
figured in the current system. For each interface, it attempts to con-
figure the interface using the DHCP protocol.
In order to keep track of leases across system reboots and server
restarts, dhclient keeps a list of leases it has been assigned in the
dhclient.leases file. On startup, after reading the dhclient.conf
file, dhclient reads the dhclient.leases file to refresh its memory
about what leases it has been assigned.
When a new lease is acquired, it is appended to the end of the
dhclient.leases file. In order to prevent the file from becoming arbi-
trarily large, from time to time dhclient creates a new dhclient.leases
file from its in-core lease database. The old version of the
dhclient.leases file is retained under the name dhclient.leases~ until
the next time dhclient rewrites the database.
Old leases are kept around in case the DHCP server is unavailable when
dhclient is first invoked (generally during the initial system boot
process). In that event, old leases from the dhclient.leases file
which have not yet expired are tested, and if they are determined to be
valid, they are used until either they expire or the DHCP server
A mobile host which may sometimes need to access a network on which no
DHCP server exists may be preloaded with a lease for a fixed address on
that network. When all attempts to contact a DHCP server have failed,
dhclient will try to validate the static lease, and if it succeeds,
will use that lease until it is restarted.
A mobile host may also travel to some networks on which DHCP is not
available but BOOTP is. In that case, it may be advantageous to
arrange with the network administrator for an entry on the BOOTP data-
base, so that the host can boot quickly on that network rather than
cycling through the list of old leases.
The names of the network interfaces that dhclient should attempt to
configure may be specified on the command line. If no interface names
are specified on the command line dhclient will normally identify all
network interfaces, eliminating non-broadcast interfaces if possible,
and attempt to configure each interface.
It is also possible to specify interfaces by name in the dhclient.conf
file. If interfaces are specified in this way, then the client will
only configure interfaces that are either specified in the configura-
tion file or on the command line, and will ignore all other interfaces.
The client normally prints no output during its startup sequence. It
can be made to emit verbose messages displaying the startup sequence
events until it has acquired an address by supplying the -v command
line argument. In either case, the client logs messages using the sys-
-4 Use the DHCPv4 protocol to obtain an IPv4 address and configura-
tion parameters. This is the default and cannot be combined
-6 Use the DHCPv6 protocol to obtain whatever IPv6 addresses are
available along with configuration parameters. It cannot be
combined with -4. The -S -T -P -N and -D arguments provide more
control over aspects of the DHCPv6 processing. Note: it is not
recommended to mix queries of different types together or even
to share the lease file between them.
-1 Try to get a lease once. On failure exit with code 2. In
DHCPv6 this sets the maximum duration of the initial exchange to
timeout (from dhclient.conf with a default of sixty seconds).
-d Force dhclient to run as a foreground process. Normally the
DHCP client will run in the foreground until is has configured
an interface at which time it will revert to running in the
background. This option is useful when running the client under
a debugger, or when running it out of inittab on System V sys-
tems. This implies -v.
-nw Become a daemon immediately (nowait) rather than waiting until
an IP address has been acquired.
-q Be quiet at startup, this is the default.
-v Enable verbose log messages.
-w Continue running even if no broadcast interfaces were found.
Normally DHCP client will exit if it isn't able to identify any
network interfaces to configure. On laptop computers and other
computers with hot-swappable I/O buses, it is possible that a
broadcast interface may be added after system startup. This
flag can be used to cause the client not to exit when it doesn't
find any such interfaces. The omshell(1) program can then be
used to notify the client when a network interface has been
added or removed, so that the client can attempt to configure an
IP address on that interface.
-n Do not configure any interfaces. This is most likely to be use-
ful in combination with the -w flag.
Define additional environment variables for the environment
where dhclient-script executes. You may specify multiple -e
options on the command line.
-r Release the current lease and stop the running DHCP client as
previously recorded in the PID file. When shutdown via this
method dhclient-script will be executed with the specific reason
for calling the script set. The client normally doesn't release
the current lease as this is not required by the DHCP protocol
but some cable ISPs require their clients to notify the server
if they wish to release an assigned IP address.
-x Stop the running DHCP client without releasing the current
lease. Kills existing dhclient process as previously recorded
in the PID file. When shutdown via this method dhclient-script
will be executed with the specific reason for calling the script
The UDP port number on which the DHCP client should listen and
transmit. If unspecified, dhclient uses the default port of 68.
This is mostly useful for debugging purposes. If a different
port is specified on which the client should listen and trans-
mit, the client will also use a different destination port - one
less than the specified port.
Specify the server IP address or fully qualified domain name to
use as a destination for DHCP protocol messages before dhclient
has acquired an IP address. Normally, dhclient transmits these
messages to 255.255.255.255 (the IP limited broadcast address).
Overriding this is mostly useful for debugging purposes. This
feature is not supported in DHCPv6 (-6) mode.
Set the giaddr field of all packets to the relay IP address sim-
ulating a relay agent. This is for testing purposes only and
should not be expected to work in any consistent or useful way.
-i Use a DUID with DHCPv4 clients. If no DUID is available in the
lease file one will be constructed and saved. The DUID will be
used to construct a RFC4361 style client id that will be
included in the client's messages. This client id can be over-
ridden by setting a client id in the configuration file. Over-
ridding the client id in this fashion is discouraged.
-I Use the standard DDNS scheme from RFCs 4701 & 4702.
Print version number and exit.
Options available for DHCPv6 mode:
-S Use Information-request to get only stateless configuration
parameters (i.e., without address). This implies -6. It also
doesn't rewrite the lease database.
-T Ask for IPv6 temporary addresses, one set per -T flag. This
implies -6 and also disables the normal address query. See -N
to restore it.
-P Enable IPv6 prefix delegation. This implies -6 and also dis-
ables the normal address query. See -N to restore it. Note
only one requested interface is allowed.
-D LL or LLT
Override the default when selecting the type of DUID to use. By
default, DHCPv6 dhclient creates an identifier based on the
link-layer address (DUID-LL) if it is running in stateless mode
(with -S, not requesting an address), or it creates an identi-
fier based on the link-layer address plus a timestamp (DUID-LLT)
if it is running in stateful mode (without -S, requesting an
address). When DHCPv4 is configured to use a DUID using -i
option the default is to use a DUID-LLT. -D overrides these
default, with a value of either LL or LLT.
-N Restore normal address query for IPv6. This implies -6. It is
used to restore normal operation after using -T or -P.
Modifying default file locations: The following options can be used to
modify the locations a client uses for its files. They can be particu-
larly useful if, for example, /var/lib/dhcp or /var/run have not been
mounted when the DHCP client is started.
Path to the client configuration file. If unspecified, the
default /etc/dhcp/dhclient.conf is used. See dhclient.conf(5)
for a description of this file.
Path to a secondary lease file. If the primary lease file
doesn't contain a DUID this file will be searched. The DUID
read from the secondary will be written to the primary. This
option can be used to allow an IPv4 instance of the client to
share a DUID with an IPv6 instance. After starting one of the
instances the second can be started with this option pointing to
the lease file of the first instance. There is no default. If
no file is specified no search is made for a DUID should one not
be found in the main lease file.
Path to the lease database file. If unspecified, the default
/var/lib/dhcp/dhclient.leases is used. See dhclient.leases(5)
for a description of this file.
Path to the process ID file. If unspecified, the default
/var/run/dhclient.pid is used.
Option to disable writing pid files. By default the program
will write a pid file. If the program is invoked with this
option it will not attempt to kill any existing client processes
even if invoked with -r or -x.
Path to the network configuration script invoked by dhclient
when it gets a lease. If unspecified, the default
/sbin/dhclient-script is used. See dhclient-script(8) for a
description of this file.
During operations the client may use multiple UDP ports to provide dif-
ferent functions. Which ports are opened depends on both the way you
compiled your code and the configuration you supply. The following
should provide you an idea of what ports may be in use.
Normally a DHCPv4 client will open a raw UDP socket to receive and send
most DHCPv4 packets. It also opens a fallback UDP socket for use in
sending unicast packets. Normally these will both use the well known
port number for BOOTPC.
For DHCPv6 the client opens a UDP socket on the well known client port
and a fallback UDP socket on a random port for use in sending unicast
messages. Unlike DHCPv4 the well known socket doesn't need to be
opened in raw mode.
If you have included an omapi port statement in your configuration file
then the client will open a TCP socket on that port to listen for OMPAI
connections. When something connects another port will be used for the
When DDNS is enabled at compile time (see includes/site.h) the client
will open both a v4 and a v6 UDP socket on random ports. These ports
are opened even if DDNS is disabled in the configuration file.
The syntax of the dhclient.conf(5) file is discussed separately.
The DHCP client provides some ability to control it while it is run-
ning, without stopping it. This capability is provided using OMAPI, an
API for manipulating remote objects. OMAPI clients connect to the
client using TCP/IP, authenticate, and can then examine the client's
current status and make changes to it.
Rather than implementing the underlying OMAPI protocol directly, user
programs should use the dhcpctl API or OMAPI itself. Dhcpctl is a
wrapper that handles some of the housekeeping chores that OMAPI does
not do automatically. Dhcpctl and OMAPI are documented in dhcpctl(3)
and omapi(3). Most things you'd want to do with the client can be done
directly using the omshell(1) command, rather than having to write a
THE CONTROL OBJECT
The control object allows you to shut the client down, releasing all
leases that it holds and deleting any DNS records it may have added.
It also allows you to pause the client - this unconfigures any inter-
faces the client is using. You can then restart it, which causes it to
reconfigure those interfaces. You would normally pause the client
prior to going into hibernation or sleep on a laptop computer. You
would then resume it after the power comes back. This allows PC cards
to be shut down while the computer is hibernating or sleeping, and then
reinitialized to their previous state once the computer comes out of
hibernation or sleep.
The control object has one attribute - the state attribute. To shut
the client down, set its state attribute to 2. It will automatically
do a DHCPRELEASE. To pause it, set its state attribute to 3. To
resume it, set its state attribute to 4.
The following environment variables may be defined to override the
builtin defaults for file locations. Note that use of the related com-
mand-line options will ignore the corresponding environment variable
The dhclient.conf configuration file.
The dhclient.leases database.
The dhclient PID file.
The dhclient-script file.
dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5),
dhclient(8) To learn more about Internet Systems Consortium, see
This client was substantially modified and enhanced by Elliot Poger for
use on Linux while he was working on the MosquitoNet project at Stan-
The current version owes much to Elliot's Linux enhancements, but was
substantially reorganized and partially rewritten by Ted Lemon so as to
use the same networking framework that the Internet Systems Consortium
DHCP server uses. Much system-specific configuration code was moved
into a shell script so that as support for more operating systems is
added, it will not be necessary to port and maintain system-specific
configuration code to these operating systems - instead, the shell
script can invoke the native tools to accomplish the same purpose.
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