path: root/usr.sbin/dhcp/dhclient/dhclient.8

.\"	dhclient.8
.\"
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.\" This software has been written for the Internet Software Consortium
.\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
.\" Enterprises.  To learn more about the Internet Software Consortium,
.\" see ``http://www.isc.org/isc''.  To learn more about Vixie
.\" Enterprises, see ``http://www.vix.com''.
.TH dhclient 8
.SH NAME
dhclient - Dynamic Host Configuration Protocol Client
.SH SYNOPSIS
.B dhclient
[
.B -p
.I port
]
[
.B -d
]
[
.I if0
[
.I ...ifN
]
]
.SH 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.
.SH OPERATION
.PP
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.
.PP
On startup, dhclient reads the
.IR 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.
.PP
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.
.PP
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
.IR dhcpd.leases~
until the next time dhclient rewrites the database.
.PP
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.
.PP
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.
.PP
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.
.PP
DHCP traffic always bypass IPsec, otherwise there can come up situations
when a server has an IPsec SA for the client, and sends replies over that,
which a potentially newly booted client cannot grasp.
.SH COMMAND LINE
.PP
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.
.PP
If dhclient should listen and transmit on a port other than the
standard (port 68), the
.B -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 the
.B -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
.B -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
.B -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.
.PP
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
.B -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.
.PP
.SH CONFIGURATION
The syntax of the dhclient.conf(8) file is discussed seperately.
.SH FILES
.B ETCDIR/dhclient.conf, DBDIR/dhclient.leases, RUNDIR/dhclient.pid,
.B DBDIR/dhclient.leases~.
.SH SEE ALSO
dhcpd(8), dhcrelay(8), dhclient.conf(5), dhclient.leases(5)
.SH AUTHOR
.B dhclient(8)
has been written for the Internet Software Consortium
by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
Enterprises.  To learn more about the Internet Software Consortium,
see
.B http://www.vix.com/isc.
To learn more about Vixie
Enterprises, see
.B http://www.vix.com.
.PP
This client was substantially modified and enhanced by Elliot Poger
for use on Linux while he was working on the MosquitoNet project at
Stanford.
.PP
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.
.PP