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 August 17, 2006 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 bdpqu 50.Op Fl c Ar file 51.Op Fl l Ar file 52.Ar interface 53.Sh DESCRIPTION 54The 55.Nm 56utility provides a means for configuring network interfaces using DHCP, BOOTP, 57or if these protocols fail, by statically assigning an address. 58.Pp 59The name of the network interface that 60.Nm 61should attempt to 62configure must be specified on the command line. 63.Pp 64The options are as follows: 65.Bl -tag -width ".Fl c Ar file" 66.It Fl b 67Forces 68.Nm 69to immediately move to the background. 70.It Fl c Ar file 71Specify an alternate location, 72.Ar file , 73for the configuration file. 74.It Fl d 75Forces 76.Nm 77to always run as a foreground process. 78By default, 79.Nm 80runs in the foreground until it has configured the interface, and then 81will revert to running in the background. 82.It Fl l Ar file 83Specify an alternate location, 84.Ar file , 85for the leases file. 86.It Fl p 87Tells 88.Nm 89to persist in trying to configure the interface, despite 90an inability to gain carrier. 91This is used to survive switch outages and when 92.Nm 93is required as soon as the cable is connected. 94.It Fl q 95Forces 96.Nm 97to be less verbose on startup. 98.It Fl u 99Forces 100.Nm 101to reject leases with unknown options in them. 102The default behaviour is to accept such lease offers. 103.El 104.Pp 105The DHCP protocol allows a host to contact a central server which 106maintains a list of IP addresses which may be assigned on one or more 107subnets. 108A DHCP client may request an address from this pool, and 109then use it on a temporary basis for communication on the network. 110The DHCP protocol also provides a mechanism whereby a client can learn 111important details about the network to which it is attached, such as 112the location of a default router, the location of a name server, and 113so on. 114.Pp 115On startup, 116.Nm 117reads 118.Pa /etc/dhclient.conf 119for configuration instructions. 120It then gets a list of all the 121network interfaces that are configured in the current system. 122It then attempts to configure each interface with DHCP. 123.Pp 124In order to keep track of leases across system reboots and server 125restarts, 126.Nm 127keeps a list of leases it has been assigned in the 128.Pa /var/db/dhclient.leases. Ns Ar IFNAME 129file. 130.Ar IFNAME 131represents the network interface of the DHCP client 132(e.g., 133.Li em0 ) , 134one for each interface. 135On startup, after reading the 136.Xr dhclient.conf 5 137file, 138.Nm 139reads the leases file to refresh its memory about what leases it has been 140assigned. 141.Pp 142Old leases are kept around in case the DHCP server is unavailable when 143.Nm 144is first invoked (generally during the initial system boot 145process). 146In that event, old leases from the 147.Pa dhclient.leases. Ns Ar IFNAME 148file which have not yet expired are tested, and if they are determined to 149be valid, they are used until either they expire or the DHCP server 150becomes available. 151.Pp 152A mobile host which may sometimes need to access a network on which no 153DHCP server exists may be preloaded with a lease for a fixed 154address on that network. 155When all attempts to contact a DHCP server have failed, 156.Nm 157will try to validate the static lease, and if it 158succeeds, it will use that lease until it is restarted. 159.Pp 160A mobile host may also travel to some networks on which DHCP is not 161available but BOOTP is. 162In that case, it may be advantageous to 163arrange with the network administrator for an entry on the BOOTP 164database, so that the host can boot quickly on that network rather 165than cycling through the list of old leases. 166.Sh NOTES 167You must have the Berkeley Packet Filter (BPF) configured in your kernel. 168The 169.Nm 170utility 171requires at least one 172.Pa /dev/bpf* 173device for each broadcast network interface that is attached to your system. 174See 175.Xr bpf 4 176for more information. 177.Sh FILES 178.Bl -tag -width ".Pa /var/db/dhclient.leases. Ns Ar IFNAME" -compact 179.It Pa /etc/dhclient.conf 180DHCP client configuration file 181.It Pa /var/db/dhclient.leases. Ns Ar IFNAME 182database of acquired leases 183.El 184.Sh SEE ALSO 185.Xr dhclient.conf 5 , 186.Xr dhclient.leases 5 , 187.Xr dhclient-script 8 , 188.Xr dhcp 8 , 189.Xr dhcpd 8 , 190.Xr dhcrelay 8 191.Sh AUTHORS 192.An -nosplit 193The 194.Nm 195utility 196was written by 197.An Ted Lemon Aq mellon@fugue.com 198and 199.An Elliot Poger Aq elliot@poger.com . 200.Pp 201The current implementation was reworked by 202.An Henning Brauer Aq henning@openbsd.org . 203