path: root/client/dhclient.8

.\"	dhclient.8
.\"
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.\" This file is part of the ISC DHCP distribution.   The documentation
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.\"
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.TH dhclient 8
.SH NAME
dhclient - Dynamic Host Configuration Protocol Client
.SH SYNOPSIS
.B dhclient
[
.B -p
.I port
]
[
.B -d
]
[
.B -D
]
[
.B -q
]
[
.B -c
]
[
.B -lf
.I lease-file
]
[
.B -pf
.I pid-file
]
[
.B -cf
.I config-file
]
[
.B -s
server
]
[
.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 dhclient.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.
.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 normally identify all
network interfaces, elimininating non-broadcast interfaces if
possible, and attempt to configure each interface.
.PP
It is also possible to specify interfaces by name in the
.B 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.
.PP
If the DHCP client 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 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.
.PP
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 
.B -s
flag, followed by the IP address or domain name of the destination.
.PP
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
.B -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.
.PP
The client writes a temporary shell script whenever it invokes
dhclient-script.   This script is normally deleted after the client
runs, but it can be helpful when debugging the client script to see
what the client wrote.   The client can be configured not to delete
these scripts by specifying the
.B -D
flag.
.PP
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
.B syslog (3)
facility.   The
.B -q
flag prevents any messages other than errors from being printed to the
standard error descriptor.
.PP
The DHCP client normally gets its configuration information from
.B ETCDIR/dhclient.conf,
its lease database from
.B DBDIR/dhclient.leases
and stores its process ID in a file called
.B RUNDIR/dhclient.pid.
To specify different names and/or locations for these files, use the
.B -cf,
.B -lf
and
.B -pf
flags, respectively, followed by the name of the file.   This can be
particularly useful if, for example,
.B DBDIR
or
.B RUNDIR
has not yet been mounted when the DHCP client is started.
.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