sbin/dhclient/dhclient.8

.\" $OpenBSD: src/sbin/dhclient/dhclient.8,v 1.8 2011/03/02 07:44:42 jmc Exp $
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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''.
.Dd July 23, 2013
.Dt DHCLIENT 8
.Os
.Sh NAME
.Nm dhclient
.Nd Dynamic Host Configuration Protocol (DHCP) client
.Sh SYNOPSIS
.Nm
.Op Fl dqu
.Op Fl c Ar file
.Op Fl l Ar file
.Ar interface
.Sh DESCRIPTION
The Dynamic Host Configuration Protocol (DHCP) allows hosts on a TCP/IP network
to configure one or more network interfaces based on information collected from
a DHCP server.
DHCP is often used, for example, by cable modem and DSL network
providers to automate network configuration for their customers.
.Pp
Information typically provided via DHCP includes
address and subnet mask for the interface,
default route,
and domain name server.
.Pp
To have
.Dx
configure an interface using DHCP
(or its predecessor, BOOTP)
the
.Nm
utility is used.
.Nm
is run on the command line with the name of the
.Ar interface
to be configured.
.Pp
The options are as follows:
.Bl -tag -width "-p port"
.It Fl c Ar file
Specify an alternate location to
.Pa /etc/dhclient.conf
for the configuration file.
.It Fl d
Forces
.Nm
to always run as a foreground process.
By default,
.Nm
runs in the foreground until it has configured the interface, and then
will revert to running in the background.
.It Fl l Ar file
Specify an alternate location to
.Pa /var/db/dhclient.leases. Ns Aq Ar IFNAME
for the leases file.
.It Fl q
Forces
.Nm
to be less verbose on startup.
.It Fl u
Forces
.Nm
to reject leases with unknown options in them.
The default behaviour is to accept such lease offers.
.El
.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 the 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,
.Nm
reads
.Pa /etc/dhclient.conf
for configuration instructions.
It then attempts to configure the network interface
.Ar interface
with DHCP.
.Pp
In order to keep track of leases across system reboots and server
restarts,
.Nm
keeps a list of leases it has been assigned in the
.Pa /var/db/dhclient.leases. Ns Aq Ar IFNAME
file.
.Ar IFNAME
represents the network interface of the DHCP client
.Pq e.g. em0 ,
one for each interface.
On startup, after reading the
.Xr dhclient.conf 5
file,
.Nm
reads the leases file to refresh its memory about what leases it has been
assigned.
.Pp
Old leases are kept around in case the DHCP server is unavailable when
.Nm
is first invoked (generally during the initial system boot
process).
In that event, old leases from the
.Pa dhclient.leases. Ns Aq Ar IFNAME
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,
.Nm
will try to validate the static lease, and if it
succeeds, it 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
.Nm
requires at least one
.Pa /dev/bpf*
file for each broadcast network interface.
See
.Xr bpf 4
for more information.
.Sh FILES
.Bl -tag -width "/var/db/dhclient.leases.<IFNAME>XXX" -compact
.It Pa /etc/dhclient.conf
DHCP client configuration file
.It Pa /var/db/dhclient.leases. Ns Aq Ar IFNAME
database of acquired leases
.El
.Sh SEE ALSO
.Xr bpf 4 ,
.Xr dhclient.conf 5 ,
.Xr dhclient.leases 5 ,
.Xr dhclient-script 8 ,
.Xr dhcp 8 ,
.Xr dhcpd 8 Pq Pa net/isc-dhcp42-server ,
.Xr dhcrelay 8 Pq Pa net/isc-dhcp42-relay
.Sh STANDARDS
.Rs
.%A R. Droms
.%D October 1993
.%R RFC 1534
.%T Interoperation Between DHCP and BOOTP
.Re
.Pp
.Rs
.%A R. Droms
.%D March 1997
.%R RFC 2131
.%T Dynamic Host Configuration Protocol
.Re
.Pp
.Rs
.%A S. Alexander
.%A R. Droms
.%D March 1997
.%R RFC 2132
.%T DHCP Options and BOOTP Vendor Extensions
.Re
.Pp
.Rs
.%A T. Lemon
.%A S. Cheshire
.%D November 2002
.%R RFC 3396
.%T Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4)
.Re
.Pp
.Rs
.%A T. Lemon
.%A S. Cheshire
.%A B. Volz
.%D December 2002
.%R RFC 3442
.%T The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4
.Re
.Sh HISTORY
.Nm
was imported into
.Dx 2.1 .
.Sh AUTHORS
.An -nosplit
.Nm
was written by
.An Ted Lemon Aq Mt mellon@fugue.com
and
.An Elliot Poger Aq Mt elliot@poger.com .
.Pp
The current implementation was reworked by
.An Henning Brauer Aq Mt henning@openbsd.org .