What is the term for a rule that blocks all traffic that does not match specific conditions?

The term that describes a rule designed to block all traffic that does not match specific conditions is known as "implicit deny." This concept is fundamental in network security, particularly in firewall configurations and access control lists (ACLs). An implicit deny rule serves as a safeguard, ensuring that any network traffic not explicitly permitted by prior allow rules is automatically blocked. This helps minimize the risk of unauthorized access and potential threats, as it defaults to denying access unless stated otherwise.

In many security frameworks, after allowing particular types of traffic through explicit allow rules, any traffic that falls outside these parameters is not given permission, effectively denying it. This approach aligns with the principle of least privilege, adhering to the notion that users and systems should only have access to the resources necessary for their roles and tasks.

Understanding this principle is crucial for configuring secure networks, as it ensures that unaccounted-for traffic is not inadvertently permitted, which could lead to vulnerabilities and security breaches.