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dhclient(8) dhclient(8)
NAME
dhclient - Dynamic Host Configuration Protocol Client
SYNOPSIS
dhclient [ -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 Systems Consortium DHCP Client, dhclient, provides a means for configuring
one or more network interfaces using the Dynamic Host Configuration Protocol, BOOTP proto-
col, 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 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(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 cre-
ates 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 spec-
ified on the command line. If no interface names are specified on the command line
dhclient will normally identify all network interfaces, eliminating non-broadcast inter-
faces 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 destination 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 desti-
nation.
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 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 messages
using the syslog (3) facility. The -q flag prevents 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 information from /etc/dhclient.conf, its
lease database from /var/lib/dhcp/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 example, /var/lib/dhcp 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 identify any network interfaces to con-
figure. On laptop computers and other computers with hot-swappable I/O buses, it is pos-
sible 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 configure 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 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 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 house-
keeping 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.
FILES
/sbin/dhclient-script, /etc/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).
AUTHOR
dhclient(8) has been written for Internet Systems Consortium by Ted Lemon in cooperation
with Vixie Enterprises. To learn more about Internet Systems Consortium, see
http://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 Stanford.
The current version owes much to Elliot’s Linux enhancements, but was substantially reor-
ganized 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 configura-
tion 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 accom-
plish the same purpose.
dhclient(8)
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Under GNU General Public License
2008-08-28 13:10 @38.103.63.61 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)