HTTPS SSH

OSSEC

OSSEC is an open source Host-Based Intrusion Detection System (HIDS).
It has a powerful correlation and analysis engine that integrates log analysis,
file integrity monitoring, centralized policy enforcement, rootkit detection,
real-time alerting, and active response.

It provides a pretty complete coverage if you are looking for endpoint (server)
monitoring.

Installation

If you have not used OSSEC before, I recommend reading this guide to get started:

http://dcid.me/texts/my-ossec-setup-manual.html

More details here:

http://dcid.me/ossec