1.\" $OpenBSD: dhclient.8,v 1.3 2004/04/09 18:30:15 jmc Exp $ 2.\" 3.\" Copyright (c) 1997 The Internet Software Consortium. 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of The Internet Software Consortium nor the names 16.\" of its contributors may be used to endorse or promote products derived 17.\" from this software without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM AND 20.\" CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, 21.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF 22.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 23.\" DISCLAIMED. IN NO EVENT SHALL THE INTERNET SOFTWARE CONSORTIUM OR 24.\" CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 25.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 26.\" LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF 27.\" USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND 28.\" ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, 29.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT 30.\" OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 31.\" SUCH DAMAGE. 32.\" 33.\" This software has been written for the Internet Software Consortium 34.\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie 35.\" Enterprises. To learn more about the Internet Software Consortium, 36.\" see ``http://www.isc.org/isc''. To learn more about Vixie 37.\" Enterprises, see ``http://www.vix.com''. 38.\" 39.\" $FreeBSD$ 40.\" 41.Dd October 13, 2011 42.Dt DHCLIENT 8 43.Os 44.Sh NAME 45.Nm dhclient 46.Nd "Dynamic Host Configuration Protocol (DHCP) client" 47.Sh SYNOPSIS 48.Nm 49.Op Fl bdqu 50.Op Fl c Ar file 51.Op Fl l Ar file 52.Op Fl p Ar file 53.Ar interface 54.Sh DESCRIPTION 55The 56.Nm 57utility provides a means for configuring network interfaces using DHCP, BOOTP, 58or if these protocols fail, by statically assigning an address. 59.Pp 60The name of the network interface that 61.Nm 62should attempt to 63configure must be specified on the command line. 64.Pp 65The options are as follows: 66.Bl -tag -width ".Fl c Ar file" 67.It Fl b 68Forces 69.Nm 70to immediately move to the background. 71.It Fl c Ar file 72Specify an alternate location, 73.Ar file , 74for the configuration file. 75.It Fl d 76Forces 77.Nm 78to always run as a foreground process. 79By default, 80.Nm 81runs in the foreground until it has configured the interface, and then 82will revert to running in the background. 83.It Fl l Ar file 84Specify an alternate location, 85.Ar file , 86for the leases file. 87.It Fl p Ar file 88Specify an alternate location for the PID file. 89The default is 90.Pa /var/run/dhclient. Ns Ar interface Ns Pa .pid . 91.It Fl q 92Forces 93.Nm 94to be less verbose on startup. 95.It Fl u 96Forces 97.Nm 98to reject leases with unknown options in them. 99The default behaviour is to accept such lease offers. 100.El 101.Pp 102The DHCP protocol allows a host to contact a central server which 103maintains a list of IP addresses which may be assigned on one or more 104subnets. 105A DHCP client may request an address from this pool, and 106then use it on a temporary basis for communication on the network. 107The DHCP protocol also provides a mechanism whereby a client can learn 108important details about the network to which it is attached, such as 109the location of a default router, the location of a name server, and 110so on. 111.Pp 112On startup, 113.Nm 114reads 115.Pa /etc/dhclient.conf 116for configuration instructions. 117It then gets a list of all the 118network interfaces that are configured in the current system. 119It then attempts to configure each interface with DHCP. 120.Pp 121In order to keep track of leases across system reboots and server 122restarts, 123.Nm 124keeps a list of leases it has been assigned in the 125.Pa /var/db/dhclient.leases. Ns Ar IFNAME 126file. 127.Ar IFNAME 128represents the network interface of the DHCP client 129(e.g., 130.Li em0 ) , 131one for each interface. 132On startup, after reading the 133.Xr dhclient.conf 5 134file, 135.Nm 136reads the leases file to refresh its memory about what leases it has been 137assigned. 138.Pp 139Old leases are kept around in case the DHCP server is unavailable when 140.Nm 141is first invoked (generally during the initial system boot 142process). 143In that event, old leases from the 144.Pa dhclient.leases. Ns Ar IFNAME 145file which have not yet expired are tested, and if they are determined to 146be valid, they are used until either they expire or the DHCP server 147becomes available. 148.Pp 149A mobile host which may sometimes need to access a network on which no 150DHCP server exists may be preloaded with a lease for a fixed 151address on that network. 152When all attempts to contact a DHCP server have failed, 153.Nm 154will try to validate the static lease, and if it 155succeeds, it will use that lease until it is restarted. 156.Pp 157A mobile host may also travel to some networks on which DHCP is not 158available but BOOTP is. 159In that case, it may be advantageous to 160arrange with the network administrator for an entry on the BOOTP 161database, so that the host can boot quickly on that network rather 162than cycling through the list of old leases. 163.Sh NOTES 164You must have the Berkeley Packet Filter (BPF) configured in your kernel. 165The 166.Nm 167utility 168requires at least one 169.Pa /dev/bpf* 170device for each broadcast network interface that is attached to your system. 171See 172.Xr bpf 4 173for more information. 174.Sh FILES 175.Bl -tag -width ".Pa /var/db/dhclient.leases. Ns Ar IFNAME" -compact 176.It Pa /etc/dhclient.conf 177DHCP client configuration file 178.It Pa /var/db/dhclient.leases. Ns Ar IFNAME 179database of acquired leases 180.El 181.Sh SEE ALSO 182.Xr dhclient.conf 5 , 183.Xr dhclient.leases 5 , 184.Xr dhclient-script 8 185.Sh AUTHORS 186.An -nosplit 187The 188.Nm 189utility 190was written by 191.An Ted Lemon Aq Mt mellon@fugue.com 192and 193.An Elliot Poger Aq Mt elliot@poger.com . 194.Pp 195The current implementation was reworked by 196.An Henning Brauer Aq Mt henning@openbsd.org . 197