Which type of attack can a traditional firewall protect a system against?

A traditional firewall is designed to monitor and control incoming and outgoing network traffic based on predetermined security rules. It primarily acts as a barrier between a trusted internal network and untrusted external networks, such as the Internet. Firewalls are particularly effective at mitigating specific types of attacks that target network traffic.

Denial of Service (DoS) attacks aim to make a system or service unavailable to its intended users by overwhelming it with excessive requests. A traditional firewall can help protect against DoS attacks by filtering out malicious traffic and blocking requests that exceed certain thresholds. In this way, it can ensure that legitimate traffic still has access to the server or network service, thereby maintaining availability.

In contrast, the other types of attacks mentioned do not typically involve direct network traffic manipulation in a way that a traditional firewall can effectively mitigate. Dumpster diving and shoulder surfing are physical security threats, targeting human behavior and physical access to sensitive information. Phishing is a social engineering attack that typically occurs through email or web-based communications and requires user awareness and training to combat, rather than network-level filtering. Therefore, the ability of a traditional firewall to protect against Denial of Service attacks underscores its role in defending network availability, making it the correct answer in this context.