File security permissions

A computer server is a source of many services, resources, and facilities. Some of these are open for public use while others may be protected. The system that permits an authority to specify and control access of individuals to areas and resources in a server is called the Access Control System (ACS). In Linux almost every resource available in the system is defined as a file-system object. Therefore a crucial part of Linux security is file system permission. The following sections describe the several types of ACS available today.

Discretionary Access Control—DAC

This is the basic type of access protection based on the identity of the subjects and/or groups to which they may or may not belong. It is discretionary because a subject with certain permissions is capable of passing that permission on to any other subject. Linux implements DAC using the concept of owner. Every object in the file-system must have an owner and a group specified. Owner controls permissions...