Packet Filtering Firewalls

Packet Filtering Firewalls

Packet filtering firewalls represent the most basic form of network protection, examining individual packets based on predetermined rules. These firewalls operate at the network and transport layers (OSI Layers 3 and 4), making decisions based on packet header information such as source and destination IP addresses, ports, and protocols. Despite their simplicity, packet filters remain valuable components in many security architectures due to their speed and efficiency.

The operation of packet filtering firewalls is straightforward yet effective for basic security needs. Each incoming or outgoing packet is compared against a list of rules that specify which combinations of addresses, ports, and protocols to allow or deny. For example, a rule might allow incoming traffic on port 443 (HTTPS) from any source while blocking all other ports. This simplicity translates to minimal processing overhead, making packet filters ideal for high-traffic environments where performance is critical.

However, packet filtering firewalls have significant limitations when protecting modern web servers. They cannot inspect packet contents, making them blind to application-layer attacks hidden within seemingly legitimate traffic. They also lack state awareness, treating each packet independently without understanding the context of the connection. This limitation makes them vulnerable to certain attacks, such as TCP sequence number spoofing or fragmentation attacks. Additionally, managing complex rule sets can become challenging as security requirements grow, potentially leading to configuration errors that create security gaps.