How can you configure user access controls in your Linux OS to prevent unauthorized access?

During my stint as a system administrator, I found that the most secure method to configure user access controls on a Linux OS is by utilizing a combination of strong password policies, configuring sudo privileges for role-based access, and implementing key-based SSH logins for remote access. For instance, after a security audit, I revised the sudoers file to restrict command execution and set up SSH keys, eliminating default password logins, which significantly tightened our server's security posture against unauthorized access. These steps ensure that only authorized personnel can perform certain actions, thus fortifying the system against potential breaches.

Setting up user access controls on my Linux OS involves creating and managing user accounts and groups. Using commands like useradd, usermod, userdel, groupadd, groupmod, and groupdel, I efficiently handle permissions and privileges. The passwd command helps in setting or changing passwords. Regularly checking the /etc/passwd and /etc/group files provides insights into user and group details, ensuring a secure and organized access environment. This approach contributes to a robust user access control framework aligned with security best practices.

It is highly recommended to set up user groups - Create groups for different types of users, such as admins, developers, etc. Assign permissions to files/directories based on groups, not individual users.

You should also use filesystem permissions - Restrict access to sensitive files using chmod/chown. Set correct permissions on /etc files, /var/log files, binaries, etc. Other things to do include: (i) Enable password policies - Set requirements for password length, complexity, expiration, etc. Use tools like passwdqc to enforce password policies. (ii)Lock down root account - Don't allow direct root login. Only allow root access via sudo. Disable root SSH login.

The use of Access Control Lists (ACLs) is also highly recommended when configuring the Linux OS- ACLs allow you to set permissions for specific users or groups on a more granular level than standard Unix permissions. This is very useful for collaborative directories.

Prioritize the use of sudo - Only give sudo privileges to trusted users who need to be able to execute commands as root or other users. Restrict the commands they can run with sudo by editing the /etc/sudoers file.

You can also do these: (i)Encrypt sensitive data - Use encryption tools like GPG or dm-crypt to encrypt data at rest and prevent unauthorized access. (ii)Use two-factor authentication - Require two forms of authentication like passwords + security tokens. Especially for remote access.