dhclient(8)dhclient(8)NAMEdhclient - Dynamic Host Configuration Protocol Client
SYNOPSISdhclient [ -4 | -6 ] [ -S ] [ -N [ -N... ] ] [ -T [ -T... ] ] [ -P [
-P... ] ] [ -p port ] [ -d ] [ -e VAR=value ] [ -q ] [ -1 ] [ -r | -x
] [ -lf lease-file ] [ -pf pid-file ] [ -cf config-file ] [ -sf script-
file ] [ -s server ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [ -v ] [ --ver‐
sion ] [ 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 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.
If given the -4 command line argument (default), dhclient will use the
DHCPv4 protocol to obtain an IPv4 address and configuration parameters.
If given the -6 command line argument, dhclient will use the DHCPv6
protocol to obtain whatever IPv6 addresses are available along with
configuration parameters. But with -S it uses Information-request to
get only (i.e., without address) stateless configuration parameters.
The default DHCPv6 behavior is modified too with -T which asks for IPv6
temporary addresses, one set per -T flag. -P enables the IPv6 prefix
delegation. As temporary addresses or prefix delegation disables the
normal address query, -N restores it. Note it is not recommended to mix
queries of different types together, or even to share the lease file
between them.
If given the --version command line argument, dhclient will print its
version number and exit.
On startup, dhclient reads the dhclient.conf for configuration instruc‐
tions. 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(5) 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 data‐
base, 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(5) 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.
If the DHCP client should listen and transmit on a port other than the
standard (port 68), the -p flag may used. It should be followed by the
udp port number that dhclient should use. This is mostly useful for
debugging purposes. If a different port is specified for the client to
listen on and transmit on, the client will also use a different desti‐
nation port - one less than the specified port.
The DHCP client normally transmits any protocol messages it sends
before acquiring an IP address to, 255.255.255.255, the IP limited
broadcast address. For debugging purposes, it may be useful to have
the server transmit these messages to some other address. This can be
specified with the -s flag, followed by the IP address or domain name
of the destination. This feature is not supported by DHCPv6.
For testing purposes, the giaddr field of all packets that the client
sends can be set using the -g flag, followed by the IP address to send.
This is only useful for testing, and should not be expected to work in
any consistent or useful way.
The DHCP client will normally run in the foreground until it has con‐
figured an interface, and then will revert to running in the back‐
ground. To run force dhclient to always run as a foreground process,
the -d flag should be specified. This is useful when running the
client under a debugger, or when running it out of inittab on System V
systems.
The dhclient daemon creates its own environment when executing the
dhclient-script to do the grunt work of interface configuration. To
define extra environment variables and their values, use the -e flag,
followed by the environment variable name and value assignment, just as
one would assign a variable in a shell. Eg: -e IF_METRIC=1
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‐
log (3) facility. A -q command line argument is provided for backwards
compatibility, but since dhclient is quiet by default, it has no
effect.
The client normally doesn't release the current lease as it is not
required by the DHCP protocol. Some cable ISPs require their clients
to notify the server if they wish to release an assigned IP address.
The -r flag explicitly releases the current lease, and once the lease
has been released, the client exits.
The -x flag tells any currently running client to exit gracefully with‐
out releasing leases first.
If the client is killed by a signal (for example at shutdown or reboot)
it won't execute the dhclient-script (8) at exit. However if you shut
the client down gracefully with -r or -x it will execute dhclient-
script (8) at shutdown with the specific reason for calling the script
set.
The -1 flag will cause dhclient to try once to get a lease. If it
fails, dhclient exits with exit code two. In DHCPv6 the -1 flag sets
the max duration of the initial exchange to timeout (from
dhclient.conf, default sixty seconds).
The DHCP client normally gets its configuration information from
ETCDIR/dhclient.conf, its lease database from DBDIR/dhclient.leases,
stores its process ID in a file called RUNDIR/dhclient.pid, and config‐
ures the network interface using CLIENTBINDIR/dhclient-script To spec‐
ify different names and/or locations for these files, use the -cf, -lf,
-pf and -sf flags, respectively, followed by the name of the file.
This can be particularly useful if, for example, DBDIR or RUNDIR has
not yet been mounted when the DHCP client is started.
The DHCP client normally exits 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. The -w 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 net‐
work interface has been added or removed, so that the client can
attempt to configure an IP address on that interface.
The DHCP client can be directed not to attempt to configure any inter‐
faces using the -n flag. This is most likely to be useful in combina‐
tion with the -w flag.
The client can also be instructed to become a daemon immediately,
rather than waiting until it has acquired an IP address. This can be
done by supplying the -nw flag.
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 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 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 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.
FILES
CLIENTBINDIR/dhclient-script, ETCDIR/dhclient.conf,
DBDIR/dhclient.leases, RUNDIR/dhclient.pid, DBDIR/dhclient.leases~.
SEE ALSOdhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5),
dhclient.leases(5), dhcp-eval(5).
AUTHORdhclient(8) has been written for Internet Systems Consortium by Ted
Lemon in cooperation with Vixie Enterprises. To learn more about
Internet Systems Consortium, see https://www.isc.org To learn more
about Vixie Enterprises, see http://www.vix.com.
This client was substantially modified and enhanced by Elliot Poger for
use on Linux while he was working on the MosquitoNet project at Stan‐
ford.
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)
