Provided by:
dhcp_2.0pl5-19.4_i386 
NAME
dhcpd - Dynamic Host Configuration Protocol Server
SYNOPSIS
dhcpd [ -p port ] [ -f ] [ -d ] [ -q ] [ -cf config-file ] [ -lf lease-
file ] [ if0 [ ...ifN ] ]
DESCRIPTION
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.
OPERATION
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.
New leases are appended to the end of the dhcpd.leases file. In order
to prevent the file from becoming arbitrarily 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
dhcpd.leases~, 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 a special crash recovery process.
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 to
simply 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. To restart dhcpd, send a SIGTERM (signal 15) to the
process ID contained in /var/run/dhcpd.pid, and then re-invoke dhcpd.
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.
Note: We get a lot of complaints about this. 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.
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
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.
If dhcpd should listen on a port other than the standard (port 67), the
-p flag may used. It should be followed by the udp port number on
which dhcpd should listen. This is mostly useful for debugging
purposes. If the -p flag is specified, the server will transmit
responses to clients at a port number that is one greater than the one
specified - i.e., if you specify -p 67, then the server will listen on
port 67 and transmit to port 68. Datagrams that must go through relay
agents are sent to the port number specified with the -p flag - if you
wish to use alternate port numbers, you must configure any relay agents
you are using to use the same alternate port numbers.
To run dhcpd as a foreground process, rather than allowing it to run as
a daemon in the background, the -f flag should be specified. This is
useful when running dhcpd under a debugger, or when running it out of
inittab on System V systems.
To have dhcpd log to the standard error descriptor, specify the -d
flag. This can be useful for debugging, and also at sites where a
complete log of all dhcp activity must be kept but syslogd 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.
Dhcpd can be made to use an alternate configuration file with the -cf
flag, or an alternate lease file with the -lf flag. Because of the
importance of using the same lease database at all times when running
dhcpd in production, these options should be used only for testing
lease files or database files in a non-production environment.
When starting dhcpd up from a system startup script (e.g., /etc/rc), it
may not be desirable to print out the entire copyright message on
startup. To avoid printing this message, the -q flag may be
specified.
CONFIGURATION
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 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";
}
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. 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 more complete description of the dhcpd.conf file syntax is provided
in dhcpd.conf(5).
FILES
/etc/dhcpd.conf, /var/lib/dhcp/dhcpd.leases, /var/run/dhcpd.pid,
/var/lib/dhcp/dhcpd.leases~.
SEE ALSO
dhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5)
AUTHOR
dhcpd(8) 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. Information about the Internet Software
Consortium can be found at http://www.isc.org/isc.
dhcpd(8)