From Linux Man Pages
dhclient - Dynamic Host Configuration Protocol Client
SYNOPSIS
dhclient [ -p port ] [ -H hostname ] [ -d ] [ -e VAR=value ] [ -q ] [ -1 ] [ -r ] [ -lf lease-file ] [ -pf pid-
file ] [ -cf config-file ] [ -sf script-file ] [ -s server ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [ -x ] [ 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 stati-
cally 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 tem-
porary 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 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 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 inter-
faces, 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 pur-
poses. 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 -H flag may be used to specify a client hostname that should be sent to the DHCP server.
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, fol-
lowed 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 dhclient daemon creates its own environment when executing the dhclient-script to do the grunt work of inter-
face 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_MET-
RIC=1
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 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 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 use-
ful 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 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(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.
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.
The -x argument enables eXtended option information to be created in the -s dhclient-script environment, which
would allow applications running in that environment to handle options they do not know about in advance.
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 automat-
ically. 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~.
RELATED
dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5), dhclient.leases(5).
CATEGORY