Explaining application firewalls

A packet filter is a type of firewall that operates at the network layer (Layer 3) of the OSI model. It examines each packet of data that passes through the network and makes decisions based on predetermined rules. These rules typically specify criteria such as source and destination IP addresses, source and destination ports, and protocols.

To create a rule for a packet filter firewall to block a port or protocol, you would typically access the firewall's configuration interface or command-line interface and define a rule that matches the specific criteria you want to block. For example, if you want to block all incoming traffic on port 80 (HTTP), you would create a rule that denies any packets with a destination port of 80.
Once the rule is configured, you can test it by attempting to establish a connection using the blocked port or protocol. For example, you could use a network scanning tool like Nmap to scan the firewall's external interface and check if the specified port is blocked. If the rule is working correctly, the scan should indicate that the port is closed or filtered.
Testing a stateful firewall's rules involves verifying that the firewall correctly tracks the state of connections and applies the appropriate rules to each packet. This can be done by generating network traffic that traverses multiple connections (e.g., initiating a web browsing session) and observing if the firewall allows the traffic to pass according to the established stateful inspection rules.

Tools commonly used for testing firewall rules include network scanning tools like Nmap, packet sniffers like Wireshark, and firewall testing frameworks like Firewalk.
Application firewalls, also known as layer 7 firewalls, operate at the application layer (Layer 7) of the OSI model. Unlike packet filter firewalls, which primarily examine network traffic based on IP addresses and ports, application firewalls analyze the content of the data packets and make decisions based on the specific application protocols being used (e.g., HTTP, FTP, SMTP).