The access control list (ACL) is a concept in computer security, used to enforce privilege separation. It is a means of determining the appropriate access rights to a given object given certain aspects of the user process that is requesting them, principally the process's user identity (in POSIX, effective UID).

The list is a data structure, usually a table, containing entries that specify individual user or group rights to specific system objects, such as a program, a process, or a file. These entries are known as access control entries (ACE) in the Microsoft Windows and OpenVMS operating systems. Each accessible object contains an identifier to its ACL. The privileges or permissions determine specific access rights, such as whether a user can read from, write to or execute an object. In some implementations an ACE can control whether or not a user, or group of users, may alter the ACL on an object.

The ACL is a concept, with several different implementations in various operating systems, although there is a POSIX standard.

ACL implementations can be quite complex. ACLs can apply to objects, directories and other containers, and for the objects and the containers created within this container. ACLs cannot implement all of the security measures that one might wish to have on all systems, and a fine-grained capability-based operating system may be a better approach, with the authority transferred from the objects being accessed to the objects seeking access — allowing for much finer-grained control.

In networking, the term Access Control List (ACL) refers to a list of the computing services available on a server, each with a list of hosts permitted to use the service.

On a router an access list specifies which addresses are allowed to access services. Access lists are used to control both inbound and outbound traffic on a router.

External links

• Online ACL Wildcard Mask Calculator for Access Control Lists
• C2-Wiki Discussion and Relational Implementation