Section: Maintenance Commands (8)
dhclient - Dynamic Host Configuration Protocol Client
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
maintains a list of IP addresses which may be assigned on one or more
subnets. 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
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
On startup, dhclient reads the dhclient.conf
for configuration instructions. It then gets a list of all the
network interfaces that are configured in the current system. For
each interface, it attempts to configure the interface using the DHCP
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
arbitrarily 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
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
database, 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
configuration file or on the command line, and will ignore all other
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
command line argument. In either case, the client logs messages using
Use the DHCPv4 protocol to obtain an IPv4 address and configuration
parameters. This is the default and cannot be combined with
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.
- -4o6 port
Participate in the DHCPv4 over DHCPv6 protocol specified by RFC 7341.
This associates a DHCPv4 and a DHCPv6 client to allow the v4 client to
send v4 requests encapsulated in a v6 packet. Communication
between the two clients is done on a pair of UDP sockets bound
to ::1 port and port + 1. Both clients must
be launched using the same port argument.
Try to get a lease once. On failure exit with code 2. In DHCPv6 this
sets the maximum duration of the initial exchange to
(from dhclient.conf with a default of sixty seconds).
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 systems. This implies -v.
Become a daemon immediately (nowait) rather than waiting until an
IP address has been acquired.
Be quiet at startup, this is the default.
Enable verbose log messages.
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
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.
Do not drop capabilities.
was compiled with libcap-ng support,
drops most capabilities immediately upon startup. While more secure,
this greatly restricts the additional actions that hooks in
can take. (For example, any daemons that
starts or restarts will inherit the restricted capabilities as well,
which may interfere with their correct operation.) Thus, the
option can be used to prevent
from dropping capabilities.
option is ignored if
was not compiled with libcap-ng support.
Do not configure any interfaces. This is most likely to be useful in
combination with the
- -e VAR=value
Define additional environment variables for the environment where
executes. You may specify multiple
options on the command line.
Release the current lease and stop the running DHCP client as previously
recorded in the PID file. When shutdown via this method
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.
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
will be executed with the specific reason for calling the script set.
- -p port-number
The UDP port number on which the DHCP client should listen and transmit.
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
transmit, the client will also use a different destination port -
one less than the specified port.
- -s server-addr
Specify the server IP address or fully qualified domain name to use as
a destination for DHCP protocol messages before
has acquired an IP address. Normally,
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.
- -g relay
Set the giaddr field of all packets to the relay IP address
simulating a relay agent. This is for testing purposes only and
should not be expected to work in any consistent or useful way.
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 overridden by
setting a client id in the configuration file. Overriding the
client id in this fashion is discouraged.
Use the standard DDNS scheme from RFCs 4701 & 4702.
- --decline-wait-time seconds
Specify the time (in seconds) that an IPv4 client should wait after
declining an address before issuing a discover. The default is
10 seconds as recommended by RFC 2131, Section 3.1.5. A value of
zero equates to no wait at all.
Print version number and exit.
Options available for DHCPv6 mode:
Use Information-request to get only stateless configuration parameters
(i.e., without address). This implies -6. It also doesn't
rewrite the lease database.
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.
Enable IPv6 prefix delegation. This implies -6 and also
disables the normal address query. See -N to restore it.
Multiple prefixes can be requested with multiple -P flags.
Note only one requested interface is allowed.
Require that responses include all of the items requested by any
-N, -T, or -P options. Normally even if
the command line includes a number of these the client will be willing
to accept the best lease it can even if the lease doesn't include all
of the requested items. This option causes the client to only
accept leases that include all of the requested items.
Note well: enabling this may prevent the client from using any
leases it receives if the servers aren't configured to supply
all of the items.
- -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 identifier 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.
Restore normal address query for IPv6. This implies -6.
It is used to restore normal operation after using -T or -P.
Multiple addresses can be requested with multiple -N flags.
- --address-prefix-len length
Specify the length of the prefix for IPv6 addresses. This value is passed by
dhclient into the client script via the environment variable, ip6_prefixlen,
when binding IPv6 addresses. The default value is 128. Alternatively you may
change the default at compile time by setting DHCLIENT_DEFAULT_PREFIX_LEN in
- --dad-wait-time seconds
Specify maximum time (in seconds) that the client should wait for the
duplicate address detection (DAD) to complete on an interface. This
value is propagated to the dhclient script in a dad_wait_time environment
variable. If any of the IPv6 addresses on the interface are tentative
(DAD is in progress), the script will wait for the specified number of
seconds for DAD to complete. If the script ignores this variable the
parameter has no effect.
- --prefix-len-hint length
When used in conjunction with -P, it directs the client to use the given
length to use a prefix hint of, "::/length", when requesting new prefixes.
Modifying default file locations:
The following options can be used to modify the locations a client uses
for its files. They can be particularly useful if, for example,
have not been mounted when the DHCP client is started.
- -cf config-file
Path to the client configuration file. If unspecified, the default
is used. See dhclient.conf(5) for a description of this file.
- -df duid-lease-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
- -lf lease-file
Path to the lease database file. If unspecified, the default
is used. See dhclient.leases(5) for a description of this file.
- -pf pid-file
Path to the process ID file. If unspecified, the default
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.
- -sf script-file
Path to the network configuration script invoked by
when it gets a lease. If unspecified, the default
is used. See dhclient-script(8) for a description of this file.
Always set the bootp broadcast flag in request packets, so that
servers will always broadcast replies. This option is provided as
an extension to enable dhclient to work on IBM s390 Linux guests.
During operations the client may use multiple UDP ports
to provide different 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 established connection.
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 not opened unless/until the
client first attempts to do an update. If the client is not
configured to do updates, the ports will never be opened.
The syntax of the dhclient.conf(5)
file is discussed separately.
The DHCP client provides some ability to control it while it is
running, 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
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 special program.
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
interfaces 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 command-line options
will ignore the corresponding environment variable settings.
The dhclient.conf configuration file.
The dhclient.leases database.
The dhclient PID file.
The dhclient-script file.
/etc/dhcp/dhclient.conf, /var/lib/dhclient/dhclient.leases, /var/run/dhclient.pid,
To learn more about Internet Systems Consortium,
This client was substantially modified and enhanced by Elliot Poger
for use on Linux while he was working on the MosquitoNet project at
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