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dhcpd(8) dhcpd(8)
NNAAMMEE
dhcpd - Dynamic Host Configuration Protocol server
SSYYNNOOPPSSIISS
ddhhccppdd [ --pp _p_o_r_t ]
DDEESSCCRRIIPPTTIIOONN
dhcpd(8) implements the Dynamic Host Configuration Proto-
col (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 pro-
vides similar but much more limited functionality.
OOPPEERRAATTIIOONN
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 allo-
cates address pools in each subnet and enters them into
the dhcpd.conf(5) file.
On startup, dhcpd reads the _d_h_c_p_d_._c_o_n_f file and keeps the
list of available 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, 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.
New leases are appended to the end of the dhcpd.leases
file. In order to prevent the file from becoming arbi-
trarily large, from time to time dhcpd creates a new
dhcpd.leases file from its in-core lease database. Once
this file has been written to disk, the old file is
renamed _d_h_c_p_d_._l_e_a_s_e_s_~, and the new file is renamed
dhcpd.leases. If the system crashes in the middle of
this process, whichever dhcpd.leases file remains will
contain all the lease information, so there is no need for
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dhcpd(8) dhcpd(8)
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 address, allo-
cate an IP address for the client, and enter that informa-
tion into the dhcpd.conf file.
Whenever changes are made to the dhcpd.conf file, dhcpd
must be restarted. To restart dhcpd, send a SIGTERM
(signal 15) to the process ID contained in _/_d_h_c_p_d_._p_i_d, and
then re-invoke dhcpd.
CCOONNFFIIGGUURRAATTIIOONN
The syntax of the dhcpd.conf(8) file is discussed seper-
ately. This section 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 needs to know the subnet numbers and netmasks of all
subnets for which it will be providing service. In addi-
tion, 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.
LLeeaassee LLeennggtthhss
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
2
dhcpd(8) dhcpd(8)
lease times of a month of more. In a final test environ-
ment 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 appli-
ance 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.
BBOOOOTTPP SSuuppppoorrtt
Each BOOTP client must be explicitly declared in the
dhcpd.conf 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 declaration, the
options specified in the client declaration take
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dhcpd(8) dhcpd(8)
precedence. 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 differ-
ent 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 pro-
vided in dhcpd.conf(5).
FFIILLEESS
//eettcc//ddhhccppdd..ccoonnff,, //vvaarr//ddbb//ddhhccppdd..lleeaasseess,, //vvaarr//rruunn//ddhhccppdd..ppiidd,,
//vvaarr//ddbb//ddhhccppdd..lleeaasseess~~..
SSEEEE AALLSSOO
dhcpd.conf(5), dhcpd.leases(5)
AAUUTTHHOORR
ddhhccppdd((88)) was written by Ted Lemon <mellon@vix.com> under a
contract with Vixie Labs. Funding for this project was
provided by the Internet Software Corporation. Informa-
tion about the Internet Software Consortium can be found
at hhttttpp::////wwwwww..iisscc..oorrgg//iisscc..
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