1 .\" $OpenBSD: src/sbin/dhclient/dhclient.8,v 1.8 2011/03/02 07:44:42 jmc Exp $
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33 .\" This software has been written for the Internet Software Consortium
34 .\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
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43 .Nd Dynamic Host Configuration Protocol (DHCP) client
51 The Dynamic Host Configuration Protocol (DHCP) allows hosts on a TCP/IP network
52 to configure one or more network interfaces based on information collected from
54 DHCP is often used, for example, by cable modem and DSL network
55 providers to automate network configuration for their customers.
57 Information typically provided via DHCP includes
58 address and subnet mask for the interface,
60 and domain name server.
64 configure an interface using DHCP
65 (or its predecessor, BOOTP)
70 is run on the command line with the name of the
74 The options are as follows:
75 .Bl -tag -width "-p port"
77 Specify an alternate location to
78 .Pa /etc/dhclient.conf
79 for the configuration file.
83 to always run as a foreground process.
86 runs in the foreground until it has configured the interface, and then
87 will revert to running in the background.
89 Specify an alternate location to
90 .Pa /var/db/dhclient.leases. Ns Aq Ar IFNAME
95 to be less verbose on startup.
99 to reject leases with unknown options in them.
100 The default behaviour is to accept such lease offers.
103 The DHCP protocol allows a host to contact a central server which
104 maintains a list of IP addresses which may be assigned on one or more
106 A DHCP client may request an address from this pool, and
107 then use it on a temporary basis for communication on the network.
108 The DHCP protocol also provides a mechanism whereby a client can learn
109 important details about the network to which it is attached, such as
110 the location of a default router, the location of a name server, and
116 .Pa /etc/dhclient.conf
117 for configuration instructions.
118 It then attempts to configure the network interface
122 In order to keep track of leases across system reboots and server
125 keeps a list of leases it has been assigned in the
126 .Pa /var/db/dhclient.leases. Ns Aq Ar IFNAME
129 represents the network interface of the DHCP client
131 one for each interface.
132 On startup, after reading the
136 reads the leases file to refresh its memory about what leases it has been
139 Old leases are kept around in case the DHCP server is unavailable when
141 is first invoked (generally during the initial system boot
143 In that event, old leases from the
144 .Pa dhclient.leases. Ns Aq Ar IFNAME
145 file which have not yet expired are tested, and if they are determined to
146 be valid, they are used until either they expire or the DHCP server
149 A mobile host which may sometimes need to access a network on which no
150 DHCP server exists may be preloaded with a lease for a fixed
151 address on that network.
152 When all attempts to contact a DHCP server have failed,
154 will try to validate the static lease, and if it
155 succeeds, it will use that lease until it is restarted.
157 A mobile host may also travel to some networks on which DHCP is not
158 available but BOOTP is.
159 In that case, it may be advantageous to
160 arrange with the network administrator for an entry on the BOOTP
161 database, so that the host can boot quickly on that network rather
162 than cycling through the list of old leases.
165 requires at least one
167 file for each broadcast network interface.
170 for more information.
172 .Bl -tag -width "/var/db/dhclient.leases.<IFNAME>XXX" -compact
173 .It Pa /etc/dhclient.conf
174 DHCP client configuration file
175 .It Pa /var/db/dhclient.leases. Ns Aq Ar IFNAME
176 database of acquired leases
180 .Xr dhclient.conf 5 ,
181 .Xr dhclient.leases 5 ,
182 .Xr dhclient-script 8 ,
184 .Xr dhcpd 8 Pq Pa net/isc-dhcp42-server ,
185 .Xr dhcrelay 8 Pq Pa net/isc-dhcp42-relay
191 .%T Interoperation Between DHCP and BOOTP
198 .%T Dynamic Host Configuration Protocol
206 .%T DHCP Options and BOOTP Vendor Extensions
214 .%T Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4)
223 .%T The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4
233 .An Ted Lemon Aq Mt mellon@fugue.com
235 .An Elliot Poger Aq Mt elliot@poger.com .
237 The current implementation was reworked by
238 .An Henning Brauer Aq Mt henning@openbsd.org .