dhcpd - Dynamic Host Configuration Protocol Server
dhcpd [-df] [-c config-file] [-l lease-file] [if0 [... ifN]]
The Internet Software Consortium DHCP Server, dhcpd, implements the Dynamic
Host Configuration Protocol (DHCP) and the Internet
Bootstrap Protocol
(BOOTP). DHCP allows hosts on a TCP/IP network to request and be
assigned IP addresses, and also to discover information
about the network
to which they are attached. BOOTP provides similar functionality, with
certain restrictions.
The DHCP protocol allows a host which is unknown to the network administrator
to be automatically assigned a new IP address out of
a pool of IP
addresses for its network. In order for this to work, the
network administrator
allocates address pools in each subnet and enters
them into the
dhcpd.conf(5) file.
On startup, dhcpd reads the dhcpd.conf file and stores a
list of available
addresses on each subnet in memory. When a client requests an address
using the DHCP protocol, dhcpd allocates an address
for it. Each
client is assigned a lease, which expires after an amount of
time chosen
by the administrator (by default, one day). Before leases
expire, the
clients to which leases are assigned are expected to renew
them in order
to continue to use the addresses. Once a lease has expired,
the client
to which that lease was assigned is no longer permitted to
use the leased
IP address.
In order to keep track of leases across system reboots and
server
restarts, dhcpd keeps a list of leases it has assigned in
the
dhcpd.leases(5) file. Before dhcpd grants a lease to a
host, it records
the lease in this file and makes sure that the contents of
the file are
flushed to disk. This ensures that even in the event of a
system crash,
dhcpd will not forget about a lease that it has assigned.
On startup,
after reading the dhcpd.conf file, dhcpd reads the
dhcpd.leases file to
refresh its memory about what leases have been assigned.
BOOTP support is also provided by this server. Unlike DHCP,
the BOOTP
protocol does not provide a protocol for recovering dynamically-assigned
addresses once they are no longer needed. It is still possible to dynamically
assign addresses to BOOTP clients, but some administrative process
for reclaiming addresses is required. By default, leases
are granted to
BOOTP clients in perpetuity, although the network administrator may set
an earlier cutoff date or a shorter lease length for BOOTP
leases if that
makes sense.
BOOTP clients may also be served in the old standard way,
which is simply
to provide a declaration in the dhcpd.conf file for each
BOOTP client,
permanently assigning an address to each client.
Whenever changes are made to the dhcpd.conf file, dhcpd must
be restarted.
Because the DHCP server database is not as lightweight
as a BOOTP
database, dhcpd does not automatically restart itself when
it sees a
change to the dhcpd.conf file.
DHCP traffic always bypasses IPsec. Otherwise there could
be situations
when a server has an IPsec SA for the client and sends
replies over that,
which a newly booted client would not be able to grasp.
The names of the network interfaces on which dhcpd should
listen for
broadcasts may be specified on the command line. This
should be done on
systems where dhcpd is unable to identify non-broadcast interfaces, but
should not be required on other systems. If no interface
names are specified
on the command line, dhcpd will identify all network
interfaces
which are up, eliminating non-broadcast interfaces if possible, and listen
for DHCP broadcasts on each interface.
The options are as follows:
-c config-file
Use an alternate configuration file, config-file.
Because of the
importance of using the same lease database at all
times when
running dhcpd in production, this option should be
used only for
testing database files in a non-production environment.
-d Force dhcpd to log to stderr. This can be useful
for debugging,
and also at sites where a complete log of all dhcp
activity must
be kept, but syslogd(8) is not reliable or otherwise
cannot be
used. Normally, dhcpd will log all output using the
syslog(3)
function with the log facility set to LOG_DAEMON.
-f Run dhcpd as a foreground process, rather than allowing it to run
as a daemon in the background. This is useful when
running dhcpd
under a debugger, or when running it out of inittab
on System V
systems.
-l lease-file
Use an alternate lease file, lease-file. Because of
the importance
of using the same lease database at all times
when running
dhcpd in production, this option should be used only
for testing
lease files in a non-production environment.
The syntax of the dhcpd.conf(5) file is discussed separately. This section
should be used as an overview of the configuration process, and the
dhcpd.conf(5) documentation should be consulted for detailed
reference
information.
Subnets
dhcpd needs to know the subnet numbers and netmasks of
all subnets
for which it will be providing service. In addition,
in order to
dynamically allocate addresses, it must be assigned one
or more
ranges of addresses on each subnet which it can in turn
assign to
client hosts as they boot. Thus, a very simple configuration providing
DHCP support might look like this:
subnet 239.252.197.0 netmask 255.255.255.0 {
range 239.252.197.10 239.252.197.250;
}
Multiple address ranges may be specified like this:
subnet 239.252.197.0 netmask 255.255.255.0 {
range 239.252.197.10 239.252.197.107;
range 239.252.197.113 239.252.197.250;
}
If a subnet will only be provided with BOOTP service
and no dynamic
address assignment, the range clause can be left out
entirely, but
the subnet statement must appear.
Lease Lengths
DHCP leases can be assigned almost any length from zero
seconds to
infinity. What lease length makes sense for any given
subnet, or
for any given installation, will vary depending on the
kinds of
hosts being served.
For example, in an office environment where systems are
added from
time to time and removed from time to time, but move
relatively infrequently,
it might make sense to allow lease times of
a month of
more. In a final test environment on a manufacturing
floor, it may
make more sense to assign a maximum lease length of 30
minutes -
enough time to go through a simple test procedure on a
network appliance
before packaging it up for delivery.
It is possible to specify two lease lengths: the default length that
will be assigned if a client doesn't ask for any particular lease
length, and a maximum lease length. These are specified as clauses
to the subnet command:
subnet 239.252.197.0 netmask 255.255.255.0 {
range 239.252.197.10 239.252.197.107;
default-lease-time 600;
max-lease-time 7200;
}
This particular subnet declaration specifies a default
lease time of
600 seconds (ten minutes), and a maximum lease time of
7200 seconds
(two hours). Other common values would be 86400 (one
day), 604800
(one week) and 2592000 (30 days).
Each subnet need not have the same lease - in the case
of an office
environment and a manufacturing environment served by
the same DHCP
server, it might make sense to have widely disparate
values for default
and maximum lease times on each subnet.
BOOTP Support
Each BOOTP client must be explicitly declared in the
dhcpd.conf(5)
file. A very basic client declaration will specify the
client network
interface's hardware address and the IP address to
assign to
that client. If the client needs to be able to load a
boot file
from the server, that file's name must be specified. A
simple BOOTP
client declaration might look like this:
host haagen {
hardware ethernet 08:00:2b:4c:59:23;
fixed-address 239.252.197.9;
filename "haagen.boot";
}
Options
DHCP (and also BOOTP with Vendor Extensions) provides a
mechanism
whereby the server can provide the client with information about how
to configure its network interface (e.g., subnet mask),
and also how
the client can access various network services (e.g.,
DNS, IP
routers, and so on).
These options can be specified on a per-subnet basis,
and, for BOOTP
clients, also on a per-client basis. In the event that
a BOOTP
client declaration specifies options that are also
specified in its
subnet declaration, the options specified in the client
declaration
take precedence. A reasonably complete DHCP configuration might
look something like this:
subnet 239.252.197.0 netmask 255.255.255.0 {
range 239.252.197.10 239.252.197.250;
default-lease-time 600 max-lease-time 7200;
option subnet-mask 255.255.255.0;
option broadcast-address 239.252.197.255;
option routers 239.252.197.1;
option domain-name-servers 239.252.197.2,
239.252.197.3;
option domain-name "isc.org";
}
A BOOTP host on that subnet that needs to be in a different domain
and use a different name server might be declared as
follows:
host haagen {
hardware ethernet 08:00:2b:4c:59:23;
fixed-address 239.252.197.9;
filename "haagen.boot";
option domain-name-servers 192.5.5.1;
option domain-name "vix.com";
}
A more complete description of the dhcpd.conf file syntax is
provided in
dhcpd.conf(5).
/etc/dhcpd.conf DHCPD configuration file.
/var/db/dhcpd.leases DHCPD lease file.
dhcpd.conf(5), dhcpd.leases(5), dhclient(8), dhcp(8), dhcrelay(8),
pxeboot(8)
dhcpd was written by Ted Lemon <[email protected]> under a contract with
Vixie Labs.
The current implementation was reworked by
Henning Brauer <[email protected]>.
We realize that it would be nice if one could send a SIGHUP
to the server
and have it reload the database. This is not technically
impossible, but
it would require a great deal of work, our resources are extremely limited,
and they can be better spent elsewhere. So please don't
complain
about this on the mailing list unless you're prepared to
fund a project
to implement this feature, or prepared to do it yourself.
OpenBSD 3.6 January 1, 1995
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