We can understand firewalls as networked security devices. The central purpose of a firewall device is to inspect both incoming and outgoing traffic and decide which to block according to a particular set of rules or policies.
Firewalls can operate in a stateless or stateful manner. Stateless firewalls do not keep any information regarding the connections between clients and servers, inspecting each packet independently. Stateful firewalls, in turn, keep track of the connections, thus monitoring their stages and identifying unusual and potentially malicious behavior on them.
3Types of Firewalls
Packet filter firewall
The firewalls employ a set of rules applied to the incoming network traffic. If a packet does not violate any rule, the firewall forwards it to the next destination; otherwise, the firewall discards it immediately.
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Most of the rules are based on the protocols’ headers encapsulating the packets and on the information provided by them. For example, we can define specific rules for blocking particular IP addresses (source or destination), ports (in the transport layer), or even protocols (ICMP, IP, TCP, UDP, among others).
Aplication-level firewall
Circuit-level firewall