Provided by: isc-dhcp-client_4.4.1-2.3ubuntu2.4_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)