Page 192 - DCAP106_OPERATING_SYSTEM_TOOLS

Unit 10: Servers

Notes
Example: Let us consider a statement: host ns { hardware ethernet 12:34:56:78:AB:CD;
fixed_address 192.168.1.5;}
This statement defines the host name, Ethernet address, and IP address of the client. Using this
statement, any time the client with the matching Ethernet address connects to the server, the
server will return the defined host name and the defined static IP address.

Example: Let us consider a subnet statement: subnet 192.168.1.0 netmask
255.255.255.0 {range 192.168.1.10 192.168.1.200;}
This statement describes the network we are providing DHCP service for, in this case 192.168.1.0.
It also says that the IP addresses it is allowed to lease are from 192.168.1.10 to 192.168.1.200.
The range clause defines the low and high values of the IP addresses the DHCP server is allowed
to lease.
!
Caution Range clause must always be used within a subnet statement, and the range
defined must be within the address space of the defined subnet.
Usually if you have a cable modem or DSL, you get your home PC’s IP address dynamically
assigned from your service provider. If you install a home cable/DSL router between your
modem and home network, your PC will most likely get its IP address at boot time from the
home router instead. You can choose to disable the DHCP server feature on your home router
and set up a Linux box as the DHCP server.

Configuration Problems

There are a few uncertain blocks you may encounter after your DHCP server is configured
and running. Primarily, if you provide DHCP services to Microsoft Windows clients, you may
encounter problems with the limited broadcast address. If Windows clients do not see DHCP
messages from the server while other clients, such as Linux clients, do, you may need to define a
specific route for the limited broadcast address on your Linux server. To do so, add the following
to your /etc/hosts file:
255.255.255.255 lim_broad
Then, add a route for the limited broadcast address by using this command:
route add _host lim_broad dev eth0
To make this a more permanent change, add the preceding command to your /etc/rc.d/rc.local
file. For Red Hat and Caldera users, the above commands have been added for you in the stock
DHCP init script, /etc/rc.d/init.d/dhcpd.
If you have problems running dhcpd on numerous network interfaces, you most likely have
an older kernel. Be sure you are using a kernel version of 2.0.31 or higher. If you get a Protocol
Not Configured error, be sure that your kernel was compiled with the CONFIG_PACKET and
CONF_FILTER options turned on.

DHCP Configuration Options

There are a lot of configuration options available to you in the /etc/dhcpd.conf file. All of these
options can be specified within a host or subnet statement or within a group statement. The group
statement can be used to supply configuration options to a group of host or subnet statements.
Also, you can use options outside of group, subnet, or host statements if you wish to apply
those options to every single system and network you define. Because of this flexibility, you can
completely tailor your configuration to your specific needs.

LOVELY PROFESSIONAL UNIVERSITY 185