Firewall (Basic Introduction)

What is Firewall?
Firewall is a system designed to prevent unauthorized access to or from a private network. Firewalls can be implemented in both hardware and software, or a combination of both. Firewall are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.
The job of a firewall is use filtering to prevent unauthorized data from entering the network and restricting the data that can be sent out.
Firewalls can be hardware devices, which are dedicated single-purpose computers that run proprietary software, or they can be software-only packages that are installed on a regular PC running on top of on operating system like Windows or UNIX. Hardware firewalls tend to be more expensive (since you’re buying both hardware and software) but also usually offer better performance. Firewalls use NAT or router software to get data to the appropriate internal computer after checking it to ensure that the filtering rules allow it to go through. 

Firewall Filtering

Firewalls can filter data at different levels (different layers of the OSI networking model). The most common filtering methods are:

• Packet filtering, which works primarily at the network layer
• Circuit filtering, which works at the transport layer
• Application filtering, which works at the application layer

Packet filters examine the information in the IP packet headers of messages and make the decision as to whether the data is allowed in (or out) based on that information. Thus packet filtering allows you to designate specific IP addresses (or host or domain names) that will be specifically blocked or specifically allowed. Filters can also process information at the transport layer (TCP and UDP port numbers). Specific ports can be blocked or left open. Because particular services use specific ports (for example, POP 3 incoming email uses port 110), this allows you to prevent specific types of data from entering the network (in this case, incoming POP3 email). There are two types of filtering, static and dynamic. With dynamic filtering, the necessary ports are opened up only when a communication is actually taking place, rather than staying open all the time. As soon as the communication ends, the port is closed.

November 08, 2012 Tags : Technology