There are multiple tasks involved to harden the security, a few general steps to follow:

1. Update the system with the latest security patches and updates

2. Making sure the firewall is on. A lot of turn it off for testing and forget to switch it on

3. Configure the SSH daemon using key-based authentication and disabling root login

4. Enforce strict password policies

5. Enable SELinux and configure it to enforce policies that restrict access to system resources

6. Install and configure intrusion detection tools like fail2ban or tripwire to detect and prevent intrusion attempts

7. Configure system logging to track and monitor events. And remember to look back into the log from time to time
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