Copyright (c) 1997 The Internet Software Consortium.
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. Neither the name of The Internet Software Consortium nor the names
of its contributors may be used to endorse or promote products derived
from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM AND
CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE INTERNET SOFTWARE CONSORTIUM OR
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
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 $Mdocdate: March 31 2022 $ .Dt DHCLIENT 8 .Os .Sh NAME .Nm dhclient .Nd Dynamic Host Configuration Protocol (DHCP) client .Sh SYNOPSIS .Nm .Op Fl dnrv .Op Fl c Ar file .Op Fl i Ar options .Ar interface .Sh DESCRIPTION .Nm uses the Dynamic Host Configuration Protocol (DHCP), or its predecessor BOOTP, to configure a network interface. Information typically provided via DHCP includes IPv4 address and subnet mask, default route, and domain name server.
p The options are as follows: l -tag -width Ds t Fl c Ar file Specify an alternate location to
a /etc/dhclient.conf for the configuration file. If .Ar file is the empty string then no configuration file is read. t Fl d Do not daemonize. If this option is specified, .Nm will run in the foreground and log to .Em stderr . t Fl i Ar options .Nm will ignore values provided by leases for the options specified. This list will supplement ignore statements in .Xr dhclient.conf 5 . .Ar options must be a comma separated list of valid option names. t Fl n Configtest mode. Only check the configuration file for validity. t Fl r Release the current lease back to the server it came from. .Nm exits after removing the active lease from
a /var/db/dhclient.leases . Ns Aq Ar IFNAME , deleting the address the lease caused to be added to the interface, and sending a DHCPRELEASE packet to the server that supplied the lease.
p If there is no .Nm controlling the specified interface, or .Nm has no active lease configured, no action is performed. t Fl v Causes .Nm to show more information about interactions with the DHCP server and what network configuration changes are attempted after accepting a lease. .Fl v is implied if either .Fl d or .Fl n is present. .El
p 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.
p On startup, .Nm reads
a /etc/dhclient.conf for configuration instructions. It then attempts to configure the network interface .Ar interface with DHCP. The special value .Dq egress may be used instead of a network interface name. In this case .Nm will look for the network interface currently in the interface group .Dq egress and configure it with DHCP. If there is more than one network interface in the egress group, .Nm will exit with an error.
p When configuring the interface, .Nm attempts to remove any existing addresses, gateway routes that use the interface, and non-permanent .Xr arp 8 entries. .Nm automatically exits whenever a new .Nm is run on the same interface.
p Once the interface is configured, .Nm constructs a .Xr resolv.conf 5 file. It does this only if any of the options .Cm domain-name , .Cm domain-name-servers , or .Cm domain-search are present (note that these options may be offered by the DHCP server but suppressed by .Xr dhclient.conf 5 ) . If a resolv.conf is constructed, .Nm appends any contents of the
a /etc/resolv.conf.tail file, which are read once at start up. The constructed resolv.conf is copied into
a /etc/resolv.conf whenever the default route goes out the interface .Nm is running on. .Nm monitors the system for changes to the default route and re-checks whether it should write its resolv.conf when possible changes are detected.
p 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
a /var/db/dhclient.leases . Ns Aq Ar IFNAME file. .Ar IFNAME represents the network interface of the DHCP client
q 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.
p 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
a 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.
p 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.
p 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 FILES l -tag -width "/var/db/dhclient.leases.<IFNAME>XXX" -compact t Pa /etc/dhclient.conf DHCP client configuration file t Pa /etc/hostname.XXX interface-specific configuration files t Pa /var/db/dhclient.leases . Ns Aq Ar IFNAME database of acquired leases .El .Sh SEE ALSO .Xr dhclient.conf 5 , .Xr dhclient.leases 5 , .Xr hostname.if 5 , .Xr dhcpd 8 , .Xr dhcrelay 8 , .Xr ifconfig 8 .Sh STANDARDS .Rs .%A R. Droms .%D October 1993 .%R RFC 1534 .%T Interoperation Between DHCP and BOOTP .Re
p .Rs .%A R. Droms .%D March 1997 .%R RFC 2131 .%T Dynamic Host Configuration Protocol .Re
p .Rs .%A S. Alexander .%A R. Droms .%D March 1997 .%R RFC 2132 .%T DHCP Options and BOOTP Vendor Extensions .Re
p .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
p .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
p .Rs .%A N. Swamy .%A G. Halwasia .%A P. Jhingram .%D January 2013 .%R RFC 6842 .%T Client Identifier Option in DHCP Server Replies .Re .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 .
p The current implementation was reworked by .An Henning Brauer Aq Mt henning@openbsd.org .