DHCPD(8) OpenBSD System Manager's Manual DHCPD(8)
NAME
dhcpd - Dynamic Host Configuration Protocol Server
SYNOPSIS
dhcpd [-dfn] [-c config-file] [-l lease-file] [if0 [... ifN]]
DESCRIPTION
The Internet Software Consortium DHCP Server, dhcpd, implements the Dy-
namic Host Configuration Protocol (DHCP) and the Internet Bootstrap Pro-
tocol (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 adminis-
trator 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 admin-
istrator 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 avail-
able addresses on each subnet in memory. When a client requests an ad-
dress 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 dynam-
ically 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 restart-
ed. 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.
COMMAND LINE
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 spec-
ified on the command line, dhcpd will identify all network interfaces
which are up, eliminating non-broadcast interfaces if possible, and lis-
ten 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 impor-
tance 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.
-n Only test configuration, do not run dhcpd.
CONFIGURATION
The syntax of the dhcpd.conf(5) file is discussed separately. This sec-
tion 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 pro-
viding 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 in-
frequently, 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 ap-
pliance 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 de-
fault 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 net-
work 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).
FILES
/etc/dhcpd.conf DHCPD configuration file.
/var/db/dhcpd.leases DHCPD lease file.
SEE ALSO
dhcpd.conf(5), dhcpd.leases(5), dhclient(8), dhcp(8), dhcrelay(8),
pxeboot(8)
AUTHORS
dhcpd was written by Ted Lemon <mellon@vix.com> under a contract with
Vixie Labs.
The current implementation was reworked by Henning Brauer
<henning@openbsd.org>.
BUGS
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 limit-
ed, 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 3.9 January 1, 1995 4