dhclient - Dynamic Host Configuration Protocol Client
] [ -S
] [ -N
] ] [ -T
] ] [ -P
] [ -i
] [ -I
] [ -4o6 port
] [ -p port-number
] [ -d
] [ -e VAR=value
] [ -q
] [ -1
] [ -r
] [ -lf lease-file
] [ --no-pid
] [ -cf
] [ -sf script-file
] [ -s
] [ -g relay
] [ -n
] [ -nw
] [ -v
] [ --version
] [ if0
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 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
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 protocol.
In order to keep track of leases across system reboots and server restarts,
keeps a list of leases it has been assigned in the
dhclient.leases file. On startup, after reading the dhclient.conf file,
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 dhclient.leases~
until the next time dhclient
Old leases are kept around in case the DHCP server is unavailable when
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 becomes available.
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,
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 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 syslog(3)
- 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
- -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
- 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).
- 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 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 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
- Do not configure any interfaces. This is most likely to be useful in
combination with the -w flag.
- -e VAR=value
- Define additional environment variables for the environment where
dhclient-script executes. You may specify multiple -e
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
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.
- 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 set.
- -p port-number
- 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 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 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.
- -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. Overridding the client id in this fashion is
- Use the standard DDNS scheme from RFCs 4701 & 4702.
Options available for DHCPv6 mode:
- Print version number and exit.
- 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
- 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.
Modifying default file locations:
- 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.
The following options can be used to
modify the locations a client uses for its files. They can be particularly
useful if, for example, /var/lib/dhcp
have not been
mounted when the DHCP client is started.
- -cf config-file
- 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.
- -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 lease file.
- -lf 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.
- -pf pid-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
- -sf script-file
- 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
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
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 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 dhcpctl(3)
. Most things
you'd want to do with the client can be done directly using the
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
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.
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 Stanford.
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.