1 .\" $OpenBSD: src/sbin/dhclient/dhclient.8,v 1.8 2011/03/02 07:44:42 jmc Exp $
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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 stays around permanently and will attempt to reconnect even if
104 the interface does not exist, is down, or goes down.
105 This is the default when interfaces are configured DHCP in
106 .Pa /etc/dhclient.conf
110 on the specified interface.
113 The DHCP protocol allows a host to contact a central server which
114 maintains a list of IP addresses which may be assigned on one or more
116 A DHCP client may request an address from this pool, and
117 then use it on a temporary basis for communication on the network.
118 The DHCP protocol also provides a mechanism whereby a client can learn
119 important details about the network to which it is attached, such as
120 the location of a default router, the location of a name server, and
126 .Pa /etc/dhclient.conf
127 for configuration instructions.
128 It then attempts to configure the network interface
132 In order to keep track of leases across system reboots and server
135 keeps a list of leases it has been assigned in the
136 .Pa /var/db/dhclient.leases. Ns Aq Ar IFNAME
139 represents the network interface of the DHCP client
141 one for each interface.
142 On startup, after reading the
146 reads the leases file to refresh its memory about what leases it has been
149 Old leases are kept around in case the DHCP server is unavailable when
151 is first invoked (generally during the initial system boot
153 In that event, old leases from the
154 .Pa dhclient.leases. Ns Aq Ar IFNAME
155 file which have not yet expired are tested, and if they are determined to
156 be valid, they are used until either they expire or the DHCP server
159 A mobile host which may sometimes need to access a network on which no
160 DHCP server exists may be preloaded with a lease for a fixed
161 address on that network.
162 When all attempts to contact a DHCP server have failed,
164 will try to validate the static lease, and if it
165 succeeds, it will use that lease until it is restarted.
167 A mobile host may also travel to some networks on which DHCP is not
168 available but BOOTP is.
169 In that case, it may be advantageous to
170 arrange with the network administrator for an entry on the BOOTP
171 database, so that the host can boot quickly on that network rather
172 than cycling through the list of old leases.
175 requires at least one
177 file for each broadcast network interface.
180 for more information.
182 .Bl -tag -width "/var/db/dhclient.leases.<IFNAME>XXX" -compact
183 .It Pa /etc/dhclient.conf
184 DHCP client configuration file
185 .It Pa /var/db/dhclient.leases. Ns Aq Ar IFNAME
186 database of acquired leases
190 .Xr dhclient.conf 5 ,
191 .Xr dhclient.leases 5 ,
192 .Xr dhclient-script 8 ,
194 .Xr dhcpd 8 Pq Pa net/isc-dhcp42-server ,
195 .Xr dhcrelay 8 Pq Pa net/isc-dhcp42-relay
201 .%T Interoperation Between DHCP and BOOTP
208 .%T Dynamic Host Configuration Protocol
216 .%T DHCP Options and BOOTP Vendor Extensions
224 .%T Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4)
233 .%T The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4
243 .An Ted Lemon Aq Mt mellon@fugue.com
245 .An Elliot Poger Aq Mt elliot@poger.com .
247 The current implementation was reworked by
248 .An Henning Brauer Aq Mt henning@openbsd.org .