Dynamic Host Configuration Protocol (DHCP)
The Dynamic Host Configuration Protocol (DHCP) allows hosts on a TCP/IP network
to configure one or more network interfaces based on information collected
from a DHCP server. DHCP is often used, for example, by cable modem and DSL
network providers to automate network configuration for their customers.
Information typically provided via DHCP includes address and subnet mask for the
interface, default route, and domain name server.
To have OpenBSD
configure an interface using DHCP (or
its predecessor, BOOTP) the dhclient
is run on the command line with
the name of the interface
to be configured.
can also be run at boot time from
, in which
hostname files and runs dhclient
interface that is to be configured via DHCP.
The options are as follows:
- Specify an alternate location to
/etc/dhclient.conf for the configuration
- Do not daemonize. If this option is specified,
dhclient will run in the foreground and log
- dhclient will ignore any
values provided by leases for the options specified. This list will
override any ignore statements in
options must be a comma separated list of
valid option names. Invalid option names will cause the entire list to be
- Specify a file to write
the option data to. This causes dhclient to
write two pseudo-leases, “offered” and
“effective”, to the specified file. “offered”
will be the lease offered by the DHCP server; “effective”
will be the modified lease bound to the interface.
- Specify an alternate location to
for the leases file.
- Forces dhclient to be less
verbose on startup.
- Forces dhclient to reject
leases with unknown options in them. The default behaviour is to accept
such lease offers.
The DHCP protocol allows a host to contact a central server which maintains a
list of IP addresses which may be assigned on one or more subnets. A DHCP
client may request an address from this pool, and then use it on a temporary
basis for communication on the network. The DHCP protocol also provides a
mechanism whereby a client can learn important details about the network to
which it is attached, such as the location of a default router, the location
of a name server, and so on.
On startup, dhclient
instructions. It then attempts to configure the network interface
with DHCP. The special value
“egress” may be used instead of a network interface name. In
this case dhclient
will look for the network
interface currently in the interface group “egress” and
configure it with DHCP. If there is more than one network interface in the
egress group dhclient
will exit with an error.
When configuring the interface, dhclient
to remove any existing addresses, gateway routes that use the interface, and
automatically exits whenever a new
is run on the same interface.
Once the interface is configured, dhclient
constructs a resolv.conf(5)
file. It does this only when one or both of the options
are present (note that these
options may be offered by the DHCP server but suppressed by
). If a
resolv.conf is constructed, dhclient
contents of the
file, which are read once at start up. The constructed resolv.conf is copied
whenever the default route
goes out the interface dhclient
is running on.
monitors the system for changes to the
default route and re-checks whether it should write its resolv.conf when
possible changes are detected.
In order to keep track of leases across system reboots and server restarts,
keeps a list of leases it has been
assigned in the
represents the network interface
of the DHCP client (e.g. em0), one for each interface. On startup, after
reads the leases file to refresh its
memory about what leases it has been assigned.
Old leases are kept around in case the DHCP server is unavailable when
is first invoked (generally during the
initial system boot process). In that event, old leases from the
file which have not yet expired are tested, and if they are determined to be
valid, they are used until either they expire or the DHCP server becomes
A mobile host which may sometimes need to access a network on which no DHCP
server exists may be preloaded with a lease for a fixed address on that
network. When all attempts to contact a DHCP server have failed,
will try to validate the static lease,
and if it succeeds, it will use that lease until it is restarted.
A mobile host may also travel to some networks on which DHCP is not available
but BOOTP is. In that case, it may be advantageous to arrange with the network
administrator for an entry on the BOOTP database, so that the host can boot
quickly on that network rather than cycling through the list of old leases.
- On receiving
dhclient will restart itself, reading
obtaining a new lease.
- DHCP client configuration file
- interface-specific configuration files
- database of acquired leases
Interoperation Between DHCP and BOOTP,
RFC 1534, October
Dynamic Host Configuration Protocol,
RFC 2131, March 1997.
S. Alexander and
R. Droms, DHCP Options and BOOTP
Vendor Extensions, RFC 2132,
T. Lemon and
S. Cheshire, Encoding Long Options
in the Dynamic Host Configuration Protocol (DHCPv4),
RFC 3396, November
S. Cheshire, and B. Volz,
The Classless Static Route Option for Dynamic Host
Configuration Protocol (DHCP) version 4, RFC
3442, December 2002.
was written by
and Elliot Poger
The current implementation was reworked by Henning