Allowdeny

Allowdeny refers to a core mechanism in access control that governs who may or may not perform actions or access resources. It combines two related concepts: allowing certain principals (such as users, IP addresses, or processes) and denying others. The approach is widely used across computing disciplines, including operating systems, networks, web servers, and application programming, to enforce security policies and limit exposure to authorized entities only.

In practice, allowdeny systems are built on access-control lists or policy rules. A typical model evaluates a set of allow rules and deny rules in a defined order, often with a default outcome when no rule applies. Some platforms apply deny rules first, ensuring that any explicit denial takes precedence, while others apply allow rules first and fall back to a default deny. A well-known historical example is the Apache web server’s use of the Order directive with Allow and Deny directives, though modern Apache versions use different mechanisms.

Common contexts for allowdeny include firewall configurations, network routers, cloud IAM policies, database access controls, and application-level security. Related concepts include allowlists (whitelists) and denylists (blacklists), which are sometimes used synonymously with allow/deny controls but may be implemented with different tooling or semantics.

Key considerations involve explicit rule definition to avoid unintended access, consistent evaluation order, thorough logging, and testing changes in a controlled environment. Properly managed allowdeny configurations help reduce risk by making access policies transparent and auditable.