dhclient(8)dhclient(8)NAMEdhclient - Dynamic Host Configuration Protocol Client
SYNOPSISdhclient [ -p port ] [ -d ] [ -q ] [ -1 ] [ -r ] [ -lf
lease-file ] [ -pf pid-file ] [ -cf config-file ] [ -sf
script-file ] [ -s server ] [ -g relay ] [ -n ] [ -nw ] [
-w ] [ if0 [ ...ifN ] ]
DESCRIPTION
The Internet Software 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.
On startup, dhclient reads the dhclient.conf for configu-
ration instructions. It then gets a list of all the net-
work 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 dur-
ing 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
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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,
elimininating 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
destination port - one greater than the specified destina-
tion 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.
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.
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The DHCP client will normally run in the foreground until
it has configured an interface, and then will revert to
running in the background. 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 client normally prints a startup message and displays
the protocol sequence to the standard error descriptor
until it has acquired an address, and then only logs mes-
sages using the syslog (3) facility. The -q flag pre-
vents any messages other than errors from being printed to
the standard error descriptor.
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 -1 flag cause dhclient to try once to get a lease. If
it fails, dhclient exits with exit code two.
The DHCP client normally gets its configuration informa-
tion from /etc/dhclient.conf, its lease database from
/var/db/dhclient.leases, stores its process ID in a file
called /var/run/dhclient.pid, and configures the network
interface using /sbin/dhclient-script To specify 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 exam-
ple, /var/db or /var/run has not yet been mounted when the
DHCP client is started.
The DHCP client normally exits if it isn't able to iden-
tify any network interfaces to configure. On laptop com-
puters 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 (8) 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.
The DHCP client can be directed not to attempt to config-
ure any interfaces using the -n flag. This is most
likely to be useful in combination with the -w flag.
The client can also be instructed to become a daemon imme-
diately, rather than waiting until it has acquired an IP
address. This can be done by supplying the -nw flag.
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The syntax of the dhclient.conf(8) file is discussed
seperately.
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 automati-
cally. 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.
FILES
/sbin/dhclient-script, /etc/dhclient.conf,
/var/db/dhclient.leases, /var/run/dhclient.pid,
/var/db/dhclient.leases~.
SEE ALSOdhcpd(8), dhcrelay(8), dhclient-script (8),
dhclient.conf(5), dhclient.leases(5).
AUTHORdhclient(8) has been written for the Internet Software
Consortium by Ted Lemon <mellon@fugue.com> in cooperation
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with Vixie Enterprises. To learn more about the Internet
Software Consortium, see http://www.vix.com/isc. 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 Stanford.
The current version owes much to Elliot's Linux enhance-
ments, but was substantially reorganized and partially
rewritten by Ted Lemon so as to use the same networking
framework that the Internet Software 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.
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