Kerio Tech Firewall6 - Manual (Page 108)

Chapter 8 Configuration of network services
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Example
The local domain’s name is company.com. The host called john is configured so as to
obtain an IP address from the DHCP server. After the operating system is started the
host sends to the DHCP server a query with the information about its name (john). The
DHCP server assigns the host IP address 192.168.1.56. The DHCP server then keeps the
information that the IP address is assigned to the john host.
Another host that wants to start communication with the host will send a query on the
john.company.com name (the john host in the company.com domain). If the local do-
main name would not have been known by DNS Forwarder, the forwarder would pass the
query to another DNS server as it would not recognize that it is a local host. However, as
DNS Forwarder knows the local domain name, the company.com name will be separated
and the john host with the appropriate IP address will be easily looked up in the DHCP
table.
8.2 DHCP server
The DHCP protocol (Dynamic Host Configuration Protocol) is used for easy TCP/IP configura-
tion of hosts within the network. Upon an operation system start-up, the client host sends
a configuration request that is detected by the DHCP server. The DHCP server selects appro-
priate configuration parameters (IP address with appropriate subnet mask and other optional
parameters, such as IP address of the default gateway, addresses of DNS servers, domain
name, etc.) for the client stations. All client parameters can be set at the server only — at
individual hosts, enable the option that TCP/IP parameters are configured automatically from
the DHCP server. For most operating systems (e.g. Windows, Linux, etc.), this option is set by
default — it is not necessary to perform any additional settings at client hosts.
The DHCP server assigns clients IP addresses within a predefined scope for a certain period
(lease time). If an IP address is to be kept, the client must request an extension on the period
of time before the lease expires. If the client has not required an extension on the lease time,
the IP address is considered free and can be assigned to another client. This is performed
automatically and transparently.
So called reservations can be also defined on the DHCP server — certain clients will have their
own IP addresses reserved. Addresses can be reserved for a hardware address (MAC) or a host
name. These clients will have fixed IP address. These addresses are configured automatically.
Using DHCP brings two main benefits. First, the administration is much easier than with the
other protocols as all settings may be done at the server (it is not necessary to configure
individual workstations). Second, many network conflicts are eliminated (i.e. one IP address
cannot be assigned to more than one workstation, etc.).