Getting started with OSSEC (Intrusion Detection System)

OSSEC markets itself as the world’s most widely used Intrusion Detection System. An Intrusion Detection System (commonly called IDS) is a software which helps us to monitor our network for anomalies, incidents  or any event we determine to be reported. Intrusion Detection Systems are customizable like a firewall, they can be configured to send alarm messages upon a rule’s instruction, to apply a security measure or to automatically answer to the threat or warning as convenient for your network or device.

An Intrusion Detection System can warn us against DDOS, brute force, exploits, data leak, and more, it monitors our network in real time and interacts with us and with our system as we decide.

At LinuxHint we previously dedicated Snort  two tutorials, Snort is one of the leading Intrusion Detection Systems in the market and probably the first one. The articles were Installing and Using Snort Intrusion Detection System to Protect Servers and Networks and Configure Snort IDS and Create Rules.

This time I’ll show how to setup OSSEC. The server is the core of the software, it contains the rules, event entries and policies while agents are installed on the devices to monitor. Agents deliver logs and inform on incidents to the server. In this tutorial we will only install the server side to monitor the device in use, the server already contains agent’s functions to the device it is installed in.

OSSEC Installation:

First of all run:

apt install libmariadb2

For Debian and Ubuntu packages you can download OSSEC Server at

For this tutorial I will download the current version by typing in the console:


Then run:

dpkg -i ossec-hids-server_3.3.0.6515stretch_amd64.deb

Start OSSEC by executing:

/var/ossec/bin/ossec-control start

By default our installation didn’t enable mail notification, to edit it  type

nano /var/ossec/etc/ossec.conf
And add:
<smtp_server>SMTP SERVER</smtp_server>

Press ctrl+x and Y to save and exit and start OSSEC again:

/var/ossec/bin/ossec-control start

Note: if you want to install  OSSEC’s agent on a different device type:


dpkg -i ossec-hids-agent_3.3.0.6515stretch_amd64.deb

Again lets check the configuration file for OSSEC

nano /var/ossec/etc/ossec.conf

Scroll down to reach Syscheck section

Here you can determine the directories checked by OSSEC and the revision  intervals. We can also define directories and files to be ignored.

To set OSSEC to report events in real time edit the lines

<directories check_all="yes">/etc,/usr/bin,/usr/sbin</directories>
<directories check_all="yes">/bin,/sbin</directories>


<directories report_changes="yes" realtime="yes" check_all="yes">/etc,/usr/bin,
<directories report_changes="yes" realtime="yes" check_all="yes">/bin,/sbin</directories>

To add a new directory for OSSEC to check add a line:

<directories report_changes="yes" realtime="yes" check_all="yes">/DIR1,/DIR2</directories>

Close nano by pressing CTRL+X and Y and type:

nano /var/ossec/rules/ossec_rules.xml

This file contains OSSEC’s rules, the rule level will determine the system’s response. For example, by default OSSEC only reports on level 7 warnings, if there is any rule with level lower than 7 and you want to get informed when OSSEC identifies the incident edit the level number for 7 or higher. For example if you want to get informed when a host gets unblocked by OSSEC’s Active Response edit the following rule:

<rule id="602" level="3">
<description>Host Unblocked by Active Response</description>


<rule id="602" level="7">
<description>Host Unblocked by Active Response</description>

A safer alternative may be to add a new rule at the end of the file rewritting the previous one:

<rule id="602" level="7" overwrite="yes">
<description>Host Unblocked by Active Response</description>

Now we have OSSEC installed at local level, on a next tutorial we will learn more about OSSEC rules and configuration.

I hope you found this tutorial useful to get started with OSSEC, keep following for more tips and updates on Linux.

About the author

David Adams

David Adams is a System Admin and writer that is focused on open source technologies, security software, and computer systems.