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Assigning
Dynamic Addresses
The simplest configuration for a DHCP server
is to assign dynamic IP addresses. In this case, the DHCP server
provides whatever IP address is convenient to a computer when that computer
connects. No IP addresses are reserved for particular computers, so any given
client might receive one IP address one day and another address the next day. A
client may request a specific address, and most clients request the last
address they received, but the server may ignore this request. In practice,
computers are likely to receive the same addresses time after time if they're
seldom turned off or if they're powered down only for short periods, but this
isn't guaranteed. Dynamic configurations may be useful if your network sports a
large number of clients, but it has a drawback: Because client IP addresses may
change at unpredictable times, it's difficult to tie them to specific
hostnames, because the DNS servers that tie hostnames to IP addresses are
typically configured with static IP address mappings. This makes running
servers on dynamic DHCP clients problematic. The "
later in this chapter describes one way around this problem, but another is to
assign fixed IP addresses to servers, as described in the section "
Setting
Global Options
addresses. This file is fairly simple, but it may be suitable for a small
network, and it illustrates the basic structure of the file.
Listing
5.1 A sample dhcpd.conf file
default-lease-time 7200; max-lease-time 10800; option subnet-mask 255.255.255.0; option routers 192.168.1.1; option domain-name-servers 192.168.1.1, 172.17.102.200; option domain-name "threeroomco.com"; subnet 192.168.1.0 netmask 255.255.255.0 { range 192.168.1.50 192.168.1.150; }
The first six lines of
are parameters that define critical aspects of the network or of the server's
configuration, and that apply to all the clients served by dhcpd , unless the global option is
overridden in a more specific declaration. Specifically, the first two items ( default-lease-time and max-lease-time ) set the length of leases
in seconds. Like a tenant and landlord negotiating the lease to an apartment,
the DHCP client and server must negotiate an appropriate lease time for an IP
address. The default-lease-time
value sets the lease time that the DHCP server prefers. In
minutes. If the client asks for a longer lease, the max-lease-time value sets the maximum that dhcpd will permit10,800 seconds (180 minutes)
in the case of
down as you see fit. Setting lease times low increases network bandwidth
consumption, as clients must renew their leases regularly, and makes it more
likely that the network will be disrupted if the DHCP server crashes. Setting
lease times too high makes it more likely that you'll run out of IP addresses
if computers connect briefly and then disconnect, because these systems will
still have leases on IP addresses they're no longer using. If you want to test a
DHCP server, you might set lease times quite lowsay, to 60 secondsso that you
can make changes and observe the effect of these changes without undue delay.
If a network sees a lot of changes (say, laptops being connected and
disconnected regularly), you might set leases at a moderately low value, such
as those in
dominated by systems that remain connected for days at a time, you might set
leases in the hundreds of thousands of seconds (that is, a period of days).The next four lines in
passed on to DHCP clientsspecifically, the network's netmask, gateway/router
address, DNS server addresses, and domain name. These are the same values you'd
enter when configuring a computer to use a static IP address, as described in
specify hostnames if name resolution works correctly on the DHCP server. If you
do this, dhcpd resolves the
hostnames you provide and passes IP addresses on to the clients. The DHCP
server or client automatically derives or uses defaults for some values not
shown in
defaults. When setting any of these parameters, you must including a trailing
semicolon ( ; ) on the line, as
shown in
to set include the following: filename "filename"
The dhcpd server can function as
a boot server for computers that boot from an OS stored on a network server.
When so configured, the DHCP server must be able to provide the name of an
initial boot file for the client, and this is done through the filename parameter. The client will then
retrieve this file with some other file transfer protocol. next-server "hostname"
This parameter sets the name of the computer on which the boot file (specified
by filename ) can be found. If
you don't include this parameter, the default is the DHCP server itself. server-name "hostname"
This is another network-boot option. It's used to tell the client the name of
the server that holds the boot files, aside from the initial boot file
specified by filename and next-server . boot-unknown-clients
flag Normally, flag defaults to true , which makes dhcpd issue an IP address to any computer
that requests one. If flag
is false , the server issues IP
addresses only to computers that have explicit host
declarations, as described in the section "
ip-address If
your network uses an unusual broadcast address, you can set it with this
parameter. Normally, clients can compute this value correctly on their own. get-lease-hostnames
flag If flag is set to true , dhcpd
looks up a hostname via DNS before issuing an IP address, and passes the
returned value to the client as its hostname. This results in clients whose
hostnames match their IP addresses, which may be important if the clients run
programs that incorporate their hostnames in outgoing network traffic (as do
some mail servers, for instance). This parameter's default value is false . use-host-decl-names
flag This
parameter is similar to get-lease-hostnames ,
but if it's set to true , it
causes dhcpd to pass the
hostname as set in a host
declaration (described shortly) to the client, rather than the hostname
associated with the IP address obtained from a DNS lookup. The default value is
false .NOTE
align=left border=0> The get-lease-hostnames
and use-host-decl-names
parameters both determine whether or how the DHCP server will set the
client's hostname on the client. These
options do not affect hostname lookups
performed through DNS from other systems, although get-lease-hostnames sets the hostname on
the client to match the DNS name for the IP address. If you want to reliably
reach a DHCP client by hostname from other computers, consult the upcoming
sections, "
There are a wide variety of additional parameters, many of
which are options (they use the option
keyword followed by the name of the option). Many of these options set the
locations of various servers on a network, such as X font servers, time
servers, and print servers. Others set low-level network interface options like
whether the client should enable IP forwarding. Consult the dhcpd.conf man page if you need to set any
of these options. Many of these options require additional tools on the client
to do any good; standard DHCP clients don't automatically reconfigure features
such as X font servers.For the most part, these parameters may be set globally at the
start of the file; or as part of a definition for just one set of IP addresses,
by placing the parameters within the declaration for an individual subnet or
group as described shortly.
Defining
a Subnet Range
addresses is assigned. This is done through the subnet declaration in
that listing: subnet 192.168.1.0 netmask 255.255.255.0 { range 192.168.1.50 192.168.1.150; }
This declaration specifies that the server
operates on the 192.168.1.0/24 network. Consequently, the computer must have an
active interface on that network. The DHCP server binds to that network
interface and offers DHCP leases to clients that broadcast asking for an
address on that network. The range declaration defines the range of IP addresses that the server will
offer192.168.1.50 to 192.168.1.150 in this example. You may use other
addresses within the 192.168.1.0/24 network but outside of the specified range
for computers that require static IP addresses, including the DHCP server
itself;do not include the DHCP server's IP
address within the values specified by the range declaration.A single dhcpd.conf file may
contain multiple subnet declarations. You might do this if the server has two network
interfaces, in which case you'd use one subnet declaration per
interface. You might also do it if there were just one physical interface, but
that interface linked to multiple logical subnets. Versions of dhcpd prior to
3.0 require a separate subnet declaration
for each physical interface, whether or not the server is to operate on the
interface. For instance, if the computer is to function as a DHCP server on the
192.168.1.0/24 network but not on the 172.20.30.0/24 network, you'd still need
a subnet declaration for the latter interface. This declaration could be
empty, thus: subnet 172.20.30.0 netmask 255.255.255.0 { }
This empty declaration causes the server to
not respond to DHCP requests on that interface. Version 3.0 of dhcpd allows
you to omit this empty declaration to achieve the same end.WARNING
align=left border=0> If you're configuring a computer as a DHCP
server on one network and another computer functions as the DHCP server on a
second network to which the computer is connected, be positive you configure your server to not respond to DHCP requests on the second network.
If not properly configured to work together, two DHCP servers on a single
network can be a recipe for disaster. In fact, you may want to block incoming
DHCP traffic on the second network interface using firewall rules, as
discussed in
Configuring iptables. Better yet, move the DHCP server function to a computer
that connects only to the first network.
