Managing the network configurations of the hosts on even a small network can be tedious. Administrators of large networks, including ISPs, have long used the DHCP service to centrally manage network configurations. Red Hat Linux includes a DHCP server that you can install in order to facilitate the management of your network. Hosts configured with DHCP clients can load their network configurations from the DHCP server at boot time, including such configuration items as:
Hostname
Domain name
IP address
Netmask
Broadcast IP address
Gateway IP address
DNS server address
Before installing the DHCP server, you should check whether your system’s network adapter is properly configured to support DHCP. To do so, issue the ifconfig command, as follows:
[root@localhost]# ifconfig -a
eth0 Link encap:Ethernet HWaddr 00:A0:CC:25:8A:EC
inet addr:192.168.0.5 Bcast:192.168.255.255
Mask:255.255.0.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:71910 errors:0 dropped:0 overruns:0
frame:0
TX packets:108334 errors:0 dropped:0 overruns:0
carrier:0
collisions:89 txqueuelen:100
Interrupt:11 Base address:0x6000If your system’s network adapter is properly configured to
support DHCP, the output of the ifconfig command will indicate that the
adapter supports BROADCAST and
MULTICAST. If the output doesn’t include
these specifications, you must reconfigure or replace the network
adapter. Fortunately, it’s rare that an adapter lacks these
capabilities.
To set up a DHCP server, use GnoRPM to install the
dhcp package. Then, configure the service as
explained in the following section.
To
configure the DHCP service, you must create the DHCP configuration
file, /etc/dhcpd.conf. Here’s a simple
configuration that you can use as a starting point:
default lease-time 64800;
max-lease-time 64800;
option domain-name-servers 192.168.0.1;
option domain-name "oreilly.com";
subnet 192.168.0.0 netmask 255.255.255.0
{
option subnet-mask 255.255.255.0;
option broadcast-address 192.168.0.255;
option routers 192.168.0.1;
server-identifier 192.168.0.5;
host sara
{
hardware ethernet 00:50:04:d2:3f:15;
fixed-address 192.168.0.33;
default-lease-time 86400;
}
range 192.168.0.50 192.168.0.254;
}
When a DHCP client obtains a network
configuration from the server, it doesn’t generally obtain the
configuration permanently. Instead, a DHCP client is said to
lease a configuration. The two lines at the top
of the configuration file specify the default and maximum lease
duration, in seconds. The figure 64800 (seconds)
is equivalent to 18 hours. By choosing a relatively long lease time,
a client will not generally need to renew its leased network
configuration during a workday. You can choose a shorter or longer
duration, as you prefer.
The next two lines specify information transmitted to clients as part of their network configurations:
-
domain-name-servers The DNS server IP address. More than one server can be specified. Each server is separated from its neighbor by a comma.
-
domain-name The domain name.
Next comes a group of lines—delimited with paired curly braces
appearing in column 1—that define a network or subnetwork. In
this case, the network defined has the IP address
192.168.0.0 with a netmask of
255.255.255.0. This means that the range of
network addresses is from 192.168.0.0 to
192.168.0.255.
Hosts in this network share three parameters:
-
subnet-mask The network mask, which indicates by 1-bits the bit positions of the IP address associated with the network, rather than the host. Often, the network mask has the value 255.255.255.0.
-
broadcast-address The IP address of the network, with all 1-bits in the bit positions associated with the host address. Often, this means that the first three members of the dotted quad IP address appear, followed by the value 255.
-
routers The default gateway IP address.
The next set of lines define the network configuration for a
particular host, named sara:
host sara
{
hardware ethernet 00:50:04:d2:3f:15;
fixed-address 192.168.0.33;
default-lease-time 86400;
}The host’s network adapter has an Ethernet MAC address of
00:50:04:d2:3f:15. The Ethernet address is a
unique code, assigned by the adapter’s manufacturer, that
serves to identify the adapter. When it queries the DHCP server, this
adapter will be leased the IP address
192.168.0.33; the lease will have a duration of 24
hours (86400 seconds). This adapter will always
receive this IP address, which is also known as a static IP
address.
The next line defines a range of IP addresses:
range 192.168.0.50 192.168.0.254;
Hosts not assigned a static IP address will be leased an address
within the specified range. Such an IP address is termed a
dynamic IP address.
For more information about the dhcpd.conf file,
see the associated manpage.
Before starting the DHCP service for the first time, you must create the file that DHCP uses to store information on current leases. The file need not have any particular content; an empty file will do. To create the file, issue the following command:
touch /var/lib/dhcp/dhcpd.leasesNow, the DHCP service can be started. To do so, issue the command:
service dhcpd startTo verify that the DHCP service has started, issue the following command to view recent system log entries:
tail -40 /var/log/messagesYou should see something like the following:
May 5 11:57:39 localhost dhcpd: Internet Software Consortium DHCP Server 2.0pl5 May 5 11:57:39 localhost dhcpd: Copyright 1995, 1996, 1997, 1998, 1999 The Internet Software Consortium. May 5 11:57:39 localhost dhcpd: All rights reserved. May 5 11:57:39 localhost dhcpd: May 5 11:57:39 localhost dhcpd: Please contribute if you find this software useful. My 5 11:57:39 localhost dhcpd: For info, please visit http://www.isc.org/dhcp-contrib.html May 5 11:57:39 localhost dhcpd: May 5 11:57:39 localhost dhcpd: Listening on Socket/eth0/192.168.0.0 May 5 11:57:39 localhost dhcpd: Sending on Socket/eth0/192.168.0.0 May 5 11:57:39 localhost dhcpd: dhcpd startup succeeded
Now, boot a client configured to obtain its network configuration via DHCP. If you need help configuring a client to use DHCP, consult the next section. If the DHCP client and server are working, you should see system log messages that resemble the following:
May 5 11:59:40 localhost dhcpd: DHCPREQUEST for 192.168.0.4 from 00:50:04:d2:3f:15 via eth0 May 5 11:59:40 localhost dhcpd: DHCPACK on 192.168.0.4 to 00:50:04:d2:3f:15 via eth0
If you find that the DHCP server is not working, consult the file
/usr/share/doc/dhcp-*/README. Due to an error,
this HTML file lacks the file extension .html.
To view the file, first rename it, like so:
cd /usr/share/doc/dhcp-*mv README README.html
Then view it with Links, Netscape Navigator, Konqueror, or some other web browser.
Tip
What often appears to be a problem with a DHCP server is most likely a problem with the DHCP client. If you have difficulty getting the DHCP service to work properly, configure the client as explained in the next subsection. Another common problem is configuring multiple DHCP servers on the same network. In order to avoid conflicts between servers, you should generally operate only a single DHCP server on your network.
You can use the service command to control the DHCP service. To stop the server, issue this command:
service dhcpd stopTo stop and restart the DHCP service, issue this command:
service dhcpd restartThis command reports the current status of the server:
service dhcpd statusIf you want the DHCP service to start automatically when you boot your system, issue the command:
chkconfig --level 345 dhcpd onOr use Neat to configure the service to start automatically.
To configure a Windows
9x client to use DHCP, select Start →
Settings → Control Panel → Network → Configuration to
open the TCP/IP Properties dialog box. Select the TCP/IP network
component associated with the network adapter you want to configure
and click Properties. Select the IP Address tab and choose
“Obtain an IP address automatically.” Then select the DNS
tab and choose Disable DNS. This setting does not actually disable
DNS; it merely configures the system to rely on DHCP to provide the
IP address of the DNS server.
Next, select the Gateway tab and remove any installed gateways. Click OK to dismiss the TCP/IP Properties dialog box, and click OK again to dismiss the Network Properties dialog box. You can use a similar procedure to configure Windows NT/2000 clients.
Windows 9x lets you view leased network
configuration information. To do so, run the program
winipcfgand
select the proper adapter. The program shows the Ethernet address, IP
address, subnet mask, and default gateway associated with the client,
if any. Click More to view additional information, such as hostname,
DNS server IP address, and the lease expiration time. You can
manually release or renew a lease by clicking Release or Renew.
Under Windows 2000, you can view similar information describing the network configuration by issuing the command:
ipconfig /all
To configure a Linux client to use DHCP, launch Neat: select Config → Networking → Client Tasks → Basic Host Information, and set the following options for the adapter:
Enabled
Config Mode DHCP
You can view the status of a DHCP lease on a Linux client by issuing the command:
/sbin/pump -sTo release and renew a lease, issue the command:
/sbin/pump -i eth0 -RIf the lease is associated with an adapter other than
eth0, revise the command accordingly.