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dhcpd(8) NetBSD System Manager's Manual dhcpd(8)
NNAAMMEE
ddhhccppdd - Dynamic Host Configuration Protocol server
SSYYNNOOPPSSIISS
ddhhccppdd [--pp --ppoorrtt]
DDEESSCCRRIIPPTTIIOONN
dhcpd(8) 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 in-
formation about the network to which they are attached. BOOTP provides
similar but much more limited functionality.
OOPPEERRAATTIIOONN
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 ad-
ministrator allocates address pools in each subnet and enters them into
the dhcpd.conf(5) file.
On startup, dhcpd reads the ddhhccppdd..ccoonnff file and keeps the list of avail-
able addresses on each subnet in memory. When a host requests an address
using the DHCP protocol, dhcpd allocates an address for it. Each such
host is assigned a lease, which expires after an amount of time chosen by
the administrator (by default, one day). As leases expire, the hosts to
which they are assigned are expected to renew the leases if they wish to
continue to use the addresses. Once a lease has expired, the host to
which that lease is assigned is no longer permitted to use the IP address
assigned to it.
In order to keep track of leases across system reboots and server
restarts, ddhhccppdd 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,
ddhhccppdd will not forget about a lease that it has assigned. On startup,
after reading the ddhhccppdd..ccoonnff file, ddhhccppdd reads the ddhhccppdd..lleeaasseess file to
refresh its memory about what leases have been assigned.
New leases are appended to the end of the ddhhccppdd..lleeaasseess file. In order
to prevent the file from becoming arbitrarily large, from time to time
ddhhccppdd creates a new ddhhccppdd..lleeaasseess file from its in-core lease database.
Once this file has been written to disk, the old file is renamed
ddhhccppdd..lleeaasseess~~, and the new file is renamed ddhhccppdd..lleeaasseess. If the system
crashes in the middle of this process, whichever ddhhccppdd..lleeaasseess file re-
mains will contain all the lease information, so there is no need for a
special crash recovery process.
BOOTP support is also provided by this server. Unlike DHCP, the BOOTP
protocol requires that the server know the hardware address of the client
that is to be booted. The network administrator must determine that ad-
dress, allocate an IP address for the client, and enter that information
into the ddhhccppdd..ccoonnff file.
Whenever changes are made to the ddhhccppdd..ccoonnff file, ddhhccppdd must be restart-
ed. To restart ddhhccppdd, send a SIGTERM (signal 15) to the process ID con-
tained in //vvaarr//rruunn//ddhhccppdd..ppiidd, and then re-invoke ddhhccppdd.
CCOONNFFIIGGUURRAATTIIOONN
The syntax of the dhcpd.conf(8) file is discussed seperately. This sec-
tion should be used as an overview of the configuration process, and the
dhcpd.conf(8) documentation should be consulted for detailed reference
information.
SSuubbnneettss
dhcpd(8) needs to know the subnet numbers and netmasks of all subnets for
which it will be providing service. In addition, in order to dynamical-
ly allocate addresses, it must be assigned one or more ranges of address-
es 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 ad-
dress assignment, the range clause can be left out entirely, but the sub-
net statement must appear.
LLeeaassee LLeennggtthhss
DHCP leases can be assigned almost any length from zero seconds to infin-
ity. 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 as-
sign 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 envi-
ronment and a manufacturing environment served by the same DHCP server,
it might make sense to have widely disparate values for default and maxi-
mum lease times on each subnet.
BBOOOOTTPP SSuuppppoorrtt
Each BOOTP client must be explicitly declared in the ddhhccppdd..ccoonnff 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 "/tftpboot/haagen.boot";
OOppttiioonnss
DHCP (and also BOOTP with Vendor Extensions) provide 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 dec-
laration, the options specified in the client declaration take prece-
dence. An 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 "/tftpboot/haagen.boot"
option domain-name-servers 192.5.5.1
option domain-name "vix.com";
A complete list of DHCP Options and their syntaxes is provided in
dhcpd.conf(5).
FFIILLEESS
//eettcc//ddhhccppdd..ccoonnff, //eettcc//ddhhccppdd..lleeaasseess, //vvaarr//rruunn//ddhhccppdd..ppiidd,
//eettcc//ddhhccppdd..lleeaasseess~~.
SSEEEE AALLSSOO
dhcpd.conf(5), dhcpd.leases(5)
AAUUTTHHOORR
dhcpd(8) was written by Ted Lemon <<mmeelllloonn@@vviixx..ccoomm>> under a contract with
Vixie Labs. Funding for this project was provided by the Internet Soft-
ware Corporation. Information about the Internet Software Consortium can
be found at hhttttpp::////wwwwww..iisscc..oorrgg//iisscc.
March 16, 1996 3
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