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.Dd August 4, 2018 40.Dt DHCLIENT 8 41.Os 42.Sh NAME 43.Nm dhclient 44.Nd "Dynamic Host Configuration Protocol (DHCP) client" 45.Sh SYNOPSIS 46.Nm 47.Op Fl bdqu 48.Op Fl c Ar file 49.Op Fl l Ar file 50.Op Fl p Ar file 51.Ar interface 52.Sh DESCRIPTION 53The 54.Nm 55utility provides a means for configuring network interfaces using DHCP, BOOTP, 56or if these protocols fail, by statically assigning an address. 57.Pp 58The name of the network interface that 59.Nm 60should attempt to 61configure must be specified on the command line. 62.Pp 63The options are as follows: 64.Bl -tag -width ".Fl c Ar file" 65.It Fl b 66Forces 67.Nm 68to immediately move to the background. 69.It Fl c Ar file 70Specify an alternate location, 71.Ar file , 72for the configuration file. 73.It Fl d 74Forces 75.Nm 76to always run as a foreground process. 77By default, 78.Nm 79runs in the foreground until it has configured the interface, and then 80will revert to running in the background. 81.It Fl l Ar file 82Specify an alternate location, 83.Ar file , 84for the leases file. 85.It Fl p Ar file 86Specify an alternate location for the PID file. 87The default is 88.Pa /var/run/dhclient/dhclient. Ns Ar interface Ns Pa .pid . 89.It Fl q 90Forces 91.Nm 92to be less verbose on startup. 93.It Fl u 94Forces 95.Nm 96to reject leases with unknown options in them. 97The default behaviour is to accept such lease offers. 98.El 99.Pp 100The DHCP protocol allows a host to contact a central server which 101maintains a list of IP addresses which may be assigned on one or more 102subnets. 103A DHCP client may request an address from this pool, and 104then use it on a temporary basis for communication on the network. 105The DHCP protocol also provides a mechanism whereby a client can learn 106important details about the network to which it is attached, such as 107the location of a default router, the location of a name server, and 108so on. 109.Pp 110On startup, 111.Nm 112reads 113.Pa /etc/dhclient.conf 114for configuration instructions. 115It then gets a list of all the 116network interfaces that are configured in the current system. 117It then attempts to configure each interface with DHCP. 118.Pp 119In order to keep track of leases across system reboots and server 120restarts, 121.Nm 122keeps a list of leases it has been assigned in the 123.Pa /var/db/dhclient.leases. Ns Ar IFNAME 124file. 125.Ar IFNAME 126represents the network interface of the DHCP client 127(e.g., 128.Li em0 ) , 129one for each interface. 130On startup, after reading the 131.Xr dhclient.conf 5 132file, 133.Nm 134reads the leases file to refresh its memory about what leases it has been 135assigned. 136.Pp 137Old leases are kept around in case the DHCP server is unavailable when 138.Nm 139is first invoked (generally during the initial system boot 140process). 141In that event, old leases from the 142.Pa dhclient.leases. Ns Ar IFNAME 143file which have not yet expired are tested, and if they are determined to 144be valid, they are used until either they expire or the DHCP server 145becomes available. 146.Pp 147A mobile host which may sometimes need to access a network on which no 148DHCP server exists may be preloaded with a lease for a fixed 149address on that network. 150When all attempts to contact a DHCP server have failed, 151.Nm 152will try to validate the static lease, and if it 153succeeds, it will use that lease until it is restarted. 154.Pp 155A mobile host may also travel to some networks on which DHCP is not 156available but BOOTP is. 157In that case, it may be advantageous to 158arrange with the network administrator for an entry on the BOOTP 159database, so that the host can boot quickly on that network rather 160than cycling through the list of old leases. 161.Sh NOTES 162You must have the Berkeley Packet Filter (BPF) configured in your kernel. 163The 164.Nm 165utility 166requires at least one 167.Pa /dev/bpf* 168device for each broadcast network interface that is attached to your system. 169See 170.Xr bpf 4 171for more information. 172.Sh FILES 173.Bl -tag -width ".Pa /var/db/dhclient.leases. Ns Ar IFNAME" -compact 174.It Pa /etc/dhclient.conf 175DHCP client configuration file 176.It Pa /var/db/dhclient.leases. Ns Ar IFNAME 177database of acquired leases 178.El 179.Sh SEE ALSO 180.Xr dhclient.conf 5 , 181.Xr dhclient.leases 5 , 182.Xr dhclient-script 8 183.Sh AUTHORS 184.An -nosplit 185The 186.Nm 187utility 188was written by 189.An Ted Lemon Aq Mt mellon@fugue.com 190and 191.An Elliot Poger Aq Mt elliot@poger.com . 192.Pp 193The current implementation was reworked by 194.An Henning Brauer Aq Mt henning@openbsd.org . 195.Sh BUGS 196The 197.Nm 198utility uses 199.Xr capsicum 4 200to sandbox the main process. 201If the requisite kernel support is not available, the main process will 202attempt to run in a 203.Xr chroot 2 204sandbox instead. 205This will fail if the process is jailed or the 206.Va kern.chroot_allow_open_directories 207sysctl is set to 0. 208