freebsd-skq/sbin/dhclient/dhclient.8

.\" $OpenBSD: dhclient.8,v 1.3 2004/04/09 18:30:15 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
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.\" $FreeBSD$
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.Dd August 4, 2018
.Dt DHCLIENT 8
.Os
.Sh NAME
.Nm dhclient
.Nd "Dynamic Host Configuration Protocol (DHCP) client"
.Sh SYNOPSIS
.Nm
.Op Fl bdqu
.Op Fl c Ar file
.Op Fl l Ar file
.Op Fl p Ar file
.Ar interface
.Sh DESCRIPTION
The
.Nm
utility provides a means for configuring network interfaces using DHCP, BOOTP,
or if these protocols fail, by statically assigning an address.
.Pp
The name of the network interface that
.Nm
should attempt to
configure must be specified on the command line.
.Pp
The options are as follows:
.Bl -tag -width ".Fl c Ar file"
.It Fl b
Forces
.Nm
to immediately move to the background.
.It Fl c Ar file
Specify an alternate location,
.Ar file ,
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,
.Ar file ,
for the leases file.
.It Fl p Ar file
Specify an alternate location for the PID file.
The default is
.Pa /var/run/dhclient/dhclient. Ns Ar interface Ns Pa .pid .
.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 gets a list of all the
network interfaces that are configured in the current system.
It then attempts to configure each 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 Ar IFNAME
file.
.Ar IFNAME
represents the network interface of the DHCP client
(e.g.,
.Li 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 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.
.Sh NOTES
You must have the Berkeley Packet Filter (BPF) configured in your kernel.
The
.Nm
utility
requires at least one
.Pa /dev/bpf*
device for each broadcast network interface that is attached to your system.
See
.Xr bpf 4
for more information.
.Sh FILES
.Bl -tag -width ".Pa /var/db/dhclient.leases. Ns Ar IFNAME" -compact
.It Pa /etc/dhclient.conf
DHCP client configuration file
.It Pa /var/db/dhclient.leases. Ns Ar IFNAME
database of acquired leases
.El
.Sh SEE ALSO
.Xr dhclient.conf 5 ,
.Xr dhclient.leases 5 ,
.Xr dhclient-script 8
.Sh AUTHORS
.An -nosplit
The
.Nm
utility
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 .
.Sh BUGS
The
.Nm
utility uses
.Xr capsicum 4
to sandbox the main process.
If the requisite kernel support is not available, the main process will
attempt to run in a
.Xr chroot 2
sandbox instead.
This will fail if the process is jailed or the
.Va kern.chroot_allow_open_directories
sysctl is set to 0.