Provided by: isc-dhcp-client_4.4.1-2.1ubuntu5.20.04.5_amd64 
NAME
dhclient - Dynamic Host Configuration Protocol Client
SYNOPSIS
dhclient [ -4 | -6 ] [ -S ] [ -N [ -N... ] ] [ -T [ -T... ] ] [ -P [ -P... ] ] -R ] [
-i ] [ -I ] [ -4o6 port ] [ -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 ] [ --dad-wait-time seconds ] [ --prefix-len-hint length ] [ --decline-wait-time
seconds ] [ -v ] [ --version ] [ if0 [ ...ifN ] ]
DESCRIPTION
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.
OPERATION
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 or -6 options.
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, 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 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 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, 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.
COMMAND LINE
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) facility.
OPTIONS
-4 Use the DHCPv4 protocol to obtain an IPv4 address and configuration parameters.
This is the default and cannot be combined with -6.
-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.
-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.
-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 systems. 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 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.
-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 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.
-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
overridden by setting a client id in the configuration file. Overriding the client
id in this fashion is discouraged.
-I 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.
--version 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 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.
-R 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.
-N 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
includes/site.h.
--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,
/var/lib/dhcp or /var/run 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.
--no-pid
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 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.
PORTS
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.
CONFIGURATION
The syntax of the dhclient.conf(5) file is discussed separately.
OMAPI
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) 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.
ENVIRONMENT VARIABLES
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.
PATH_DHCLIENT_CONF
The dhclient.conf configuration file.
PATH_DHCLIENT_DB
The dhclient.leases database.
PATH_DHCLIENT_PID
The dhclient PID file.
PATH_DHCLIENT_SCRIPT
The dhclient-script file.
FILES
/sbin/dhclient-script, /etc/dhcp/dhclient.conf, /var/lib/dhcp/dhclient.leases,
/var/run/dhclient.pid, /var/lib/dhcp/dhclient.leases~.
SEE ALSO
dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5), dhclient.leases(5), dhcp-
eval(5).
AUTHOR
dhclient(8) To learn more about Internet Systems Consortium, see https://www.isc.org
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.
dhclient(8)