FirewallsA firewall in an information security program that prevents specific types of information from moving between the outside world and the inside world. The firewall may be a separate computer system; a software service running on existing router or server; or a separate network having supporting devicesFirewall Categorization MethodsFirewalls are categorized by processing mode, development era, or intended structure. The 5 processing modes in which firewalls can be categorized by are:-Packet filtering-Application gateways-Circuit gateways-MAC layer firewalls-HybridsFirewalls which are categorized by intended structure are residential or commercial grade, hardware based, software based, or appliance based devices.Packet FilteringA Packet filtering firewalls examine header information of data packets which come into a network for compliance with or violation of rules of the firewall’s database.A packet filtering firewall installed on TCP/IP determines whether to deny or forward to next network connection. If a device finds a packet which matches a restriction, it stops the packet from traveling .The restrictions implemented are often based on combination of-Internet Protocol (IP) source and destination address-Direction (inbound/outbound)-Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) source and destination port requests
Simple firewall models enforce rules designed to forbid packets with certain addresses or partial addresses. The 3 subsets of packet filtering firewalls are:-
-Static filtering requires that filtering rules governing how the firewall decides which packets are allowed and which are denied are developed and installed-Dynamic filtering permits firewall to react to emergent event and update or create rules to deal with event. It only permits a particular packet with a source, destination and port address to enter through firewall.-Stateful inspection called as stateful firewalls which keep track of each network connection between internal and external systems by using a state table. A state table tracks the state and context of each pocket. Stateful firewalls block incoming packets which are not responses to internal requests. Dynamic stateful filtering firewalls keep dynamic state table to make changes to the filtering rules. Figure given below shows how packets are filtered by using the Packet Filtering Router and Table given below shows an example of Firewall rules and formats.