Basic Functions of Firewalls

A firewall in the networking world should examine the traffic that is entering into the network and pass the "wall" based on some rules defined by the network and its resources. It acts as a security guard, who normally sits at the main gate, and checks your identity and access privileges and lets you in.

The main responsibilities of a firewall include:

1. (Stateless) Packet filtering: A firewall filters IP packets. The IP headers of all the packets that enter or exit the network firewall are inspected (to check their source, destination address, etc.) Firewall makes an explicit decision on each packet that enters as to whether to allow the packet or deny the packet.
2. Stateful Packet Filtering: Here the packet filtering goes beyond basic packet filtering. This keeps track of state of connection flows for all the packets, in both directions. It also keeps track of all the IP addresses currently connected at any point of time.
3. Application Level Gateways (Proxy): A firewall is also capable of inspecting application level protocols. This requires the firewall to understand certain specific application protocols.