dhclient(8) System Manager's Manual dhclient(8)
dhclient - Dynamic Host Configuration Protocol Client
dhclient [ -p port ] [ -e ] [ -d ] [ if0 [ ...ifN ] ]
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.
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.
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 dhcpd.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
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.
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 identify all network
interfaces, elimininating non-broadcast interfaces if possible, and
attempt to configure each interface.
If dhclient should listen and transmit on a port other than the stan-
dard (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 debug-
ging purposes. If the -p flag is specified, the client will transmit
responses to servers at a port number that is one less than the one
specified - i.e., if you specify -p 68, then the client will listen on
port 68 and transmit to port 67. Datagrams that must go through relay
agents are sent to the port number specified with the -p flag - if you
wish to use alternate port numbers, you must configure any relay agents
you are using to use the same alternate port numbers.
Dhclient 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 dhclient under a
debugger, or when running it out of inittab on System V systems.
The -e flag will cause dhclient to exit with an error if the interface
cannot be configured after a certain amount of time. This is useful
when dhclient is used in scripts or other systems when a failed dhcp
attempt needs to be reported.
The syntax of the dhclient.conf(8) file is discussed seperately.
dhcpd(8), dhcrelay(8), dhclient.conf(5), dhclient.leases(5)
dhclient(8) has been written for the Internet Software Consortium by
Ted Lemon <mellonATfugue.com> in cooperation with Vixie Enterprises. To
learn more about the Internet Software Consortium, see
http://www.vix.com/isc. To learn more about Vixie Enterprises, see
This client was substantially modified and enhanced by Elliot Poger for
use on Linux while he was working on the MosquitoNet project at Stan-
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 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.