The following is a compilation of various settings and techniques you can employ to harden the security of your vulnerable Linux systems. While I have tried to put them in order of the most important features first I would recommend all of these options be used on your critical production servers.
TIP #1 – Strong Passwords
Always create long passwords that contain upper and lower case letters, numbers and non alpha-numeric characters. Enforce password ageing so users need to change their passwords regularly. Lock user accounts after a certain number of failed login attempts.
TIP #2 – Use Public/Private Keys
Make use of Public/Private SSH keys for login of remote users instead of passwords, this provides the benefit of turning off password authentication in SSH so that your server can’t be Brute-Force cracked. However this does introduce a new problem whereby a malicious person could compromise a user’s computer or steal their laptop and then have access to the server. This can be overcome by using a password on the client certificate which must be entered before connecting, a kind of two factor authentication.
TIP #3 – Disable Root Login
Disable the Root user from being able to login either via the console or remote SSH connections. Instead have users use Sudo to run programs that require root privileges, or use sudo su to change to the Root user once logged in. This provides an audit path to show which user installed a piece of software or ran a program.
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