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 1, 2024 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 n 86Make 87.Nm 88not wait for ARP resolution. 89.It Fl p Ar file 90Specify an alternate location for the PID file. 91The default is 92.Pa /var/run/dhclient/dhclient. Ns Ar interface Ns Pa .pid . 93.It Fl q 94Forces 95.Nm 96to be less verbose on startup. 97.It Fl u 98Forces 99.Nm 100to reject leases with unknown options in them. 101The default behaviour is to accept such lease offers. 102.El 103.Pp 104The DHCP protocol allows a host to contact a central server which 105maintains a list of IP addresses which may be assigned on one or more 106subnets. 107A DHCP client may request an address from this pool, and 108then use it on a temporary basis for communication on the network. 109The DHCP protocol also provides a mechanism whereby a client can learn 110important details about the network to which it is attached, such as 111the location of a default router, the location of a name server, and 112so on. 113.Pp 114On startup, 115.Nm 116reads 117.Pa /etc/dhclient.conf 118for configuration instructions. 119It then gets a list of all the 120network interfaces that are configured in the current system. 121It then attempts to configure each interface with DHCP. 122.Pp 123In order to keep track of leases across system reboots and server 124restarts, 125.Nm 126keeps a list of leases it has been assigned in the 127.Pa /var/db/dhclient.leases. Ns Ar IFNAME 128file. 129.Ar IFNAME 130represents the network interface of the DHCP client 131(e.g., 132.Li em0 ) , 133one for each interface. 134On startup, after reading the 135.Xr dhclient.conf 5 136file, 137.Nm 138reads the leases file to refresh its memory about what leases it has been 139assigned. 140.Pp 141Old leases are kept around in case the DHCP server is unavailable when 142.Nm 143is first invoked (generally during the initial system boot 144process). 145In that event, old leases from the 146.Pa dhclient.leases. Ns Ar IFNAME 147file which have not yet expired are tested, and if they are determined to 148be valid, they are used until either they expire or the DHCP server 149becomes available. 150.Pp 151A mobile host which may sometimes need to access a network on which no 152DHCP server exists may be preloaded with a lease for a fixed 153address on that network. 154When all attempts to contact a DHCP server have failed, 155.Nm 156will try to validate the static lease, and if it 157succeeds, it will use that lease until it is restarted. 158.Pp 159A mobile host may also travel to some networks on which DHCP is not 160available but BOOTP is. 161In that case, it may be advantageous to 162arrange with the network administrator for an entry on the BOOTP 163database, so that the host can boot quickly on that network rather 164than cycling through the list of old leases. 165.Sh NOTES 166You must have the Berkeley Packet Filter (BPF) configured in your kernel. 167The 168.Nm 169utility 170requires at least one 171.Pa /dev/bpf* 172device for each broadcast network interface that is attached to your system. 173See 174.Xr bpf 4 175for more information. 176.Sh FILES 177.Bl -tag -width ".Pa /var/db/dhclient.leases. Ns Ar IFNAME" -compact 178.It Pa /etc/dhclient.conf 179DHCP client configuration file 180.It Pa /var/db/dhclient.leases. Ns Ar IFNAME 181database of acquired leases 182.El 183.Sh SEE ALSO 184.Xr dhclient.conf 5 , 185.Xr dhclient.leases 5 , 186.Xr dhclient-script 8 187.Sh AUTHORS 188.An -nosplit 189The 190.Nm 191utility 192was written by 193.An Ted Lemon Aq Mt mellon@fugue.com 194and 195.An Elliot Poger Aq Mt elliot@poger.com . 196.Pp 197The current implementation was reworked by 198.An Henning Brauer Aq Mt henning@openbsd.org . 199.Sh BUGS 200The 201.Nm 202utility uses 203.Xr capsicum 4 204to sandbox the main process. 205If the requisite kernel support is not available, the main process will 206attempt to run in a 207.Xr chroot 2 208sandbox instead. 209This will fail if the process is jailed or the 210.Va kern.chroot_allow_open_directories 211sysctl is set to 0. 212