Access Control List (ACL)

An Access Control List (ACL) is a set of rules that determines who or what can access specific resources in a computer system or network. These rules can control access to files, directories, or network resources like routers and switches. Each rule in an ACL specifies what actions are allowed or denied for users or systems when they try to access a resource.

An ACL works like a list of permissions tied to an object. For example, when applied to a file, an ACL can state that a particular user is allowed to read and write the file, another user can only read the file, while all others are denied any access. This level of detailed control helps to protect sensitive data from unauthorized access.

There are two main types of ACLs: discretionary ACLs (DACL) and system ACLs (SACL). A DACL allows the owner of a resource to decide who can access it and what they can do with it. This is commonly used in operating systems like Windows and Unix. A SACL, on the other hand, is used to log attempts to access a resource. It is typically used for auditing and tracking purposes to detect any unauthorized attempts to access the resource.

In network security, ACLs are often used to control incoming and outgoing traffic in network devices like routers and switches. Network ACLs can block or allow specific types of traffic based on criteria such as IP address, port number, and protocol type. For example, an ACL can be set up to allow traffic only from trusted IP addresses while blocking all other sources.

Creating an ACL involves specifying a sequence of rules that gets checked one by one until a match is found. If a rule matches the access request, the action specified by the rule is applied, whether it is to permit or deny the access. If no rules match, a default action, often to deny access, is applied.

Overall, ACLs are essential tools in managing how resources are accessed and ensuring that only authorized users or systems can interact with sensitive or critical data and services.