Basic Functions of Firewalls

4. Logging: A firewall should generate and save a log of all its activities, especially on data packets it has blocked. This log will be used in the future when the user/administrator of the network would like to audit the firewall activity.
5. Speed and transparency: A firewall should perform the aforementioned activities a fast way, transparent to the user.

In the scope of this course, we will elaborate on the packet filtering responsibility of a firewall.

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As the name suggests, a packet-filtering firewall filters the packets that are entering and leaving the network. The firewall inspects each IP packet and a decision is made. Each packet is compared with a set of filter rules and based on any match, the packet is either allowed, denied, or the connection is dropped. Packet filtering works on the network layer and transport layer of the OSI model or TCP and IP layer of the TCP/IP model (see Slide 14 of Topic 8: Networks & Network Security.) It does not remember the state of a network (when the connection started, the previous states of the connection, etc.) and hence it is called a stateless firewall.