The term “FIRE WALL” originally meant, and still means, a fireproof wall intended to prevent the spread of fire from one room or area of a building to another. The Internet is a volatile and unsafe environment when viewed from a computer-security perspective, therefore firewall is an excellent metaphor for network security.
A firewall is a secure and trusted machine that sits between a private network and a public network.The firewall machine is configured with a set of rules that determines, which all network traffics will be allowed to pass and which all network traffics will be blocked or refused. In some large organizations, you may even find a firewall located inside their corporate network to segregate sensitive areas of the organization from other employees.
Firewalls can be implemented in either hardware or software, or a combination of both. Firewalls 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 firewall uses the following techniques to control the inflow and the outflow of the traffic in a particular network..
- Packet Filtering
- Circuit Relay / Circuit Level Gateway
- Application Level Gateway
A packet is a quantity of data of limited size, kept small for easy handling. All Internet traffic travels in the form of packets. All your file downloads, Web page retrievals, emails…etc.. all these Internet communications always occur in packets.
Packet filtering inspects each packet passing through the network and acception or rejection it based on user-defined rules.
In packet filtering, only the protocol and the address information of each packet is examined. Its contents and context (its relation to other packets and to the intended application) are ignored. The firewall pays no attention to applications on the host or local network and it knows nothing about the sources of incoming data.
Circuit Relay / Circuit Level Gateway
Application Level Gateway
The logic is simple..
A firewall must be positioned to control all incoming and outgoing traffic. If some other program has that control, there is no firewall..!!