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Host-based intrusion detection system (HIDS)


  • Proficiency with a command-line text editor (using vi in this example)
  • A heavy comfort level with issuing commands from the command-line, viewing logs, and other general systems administrator duties
  • An understanding that installing this tool also requires monitoring of actions and tuning to your environment
  • The root user runs all commands or a regular user with sudo


ossec-hids is a host intrusion detection system that offers automatic action-response steps to help mitigate host intrusion attacks. It is just one possible piece of a hardened Apache web server setup. You can use it with or without other tools.

If you want to use this and other hardening tools, refer back to the Apache Hardened Web Server document. This document also uses all of the assumptions and conventions outlined in that original document. It is a good idea to review it before continuing.

Installing Atomicorp's repository

To install ossec-hids, we need a third-party repository from Atomicorp. Atomicorp also offers a reasonably priced fee-based supported version for those who want professional support if they run into trouble.

If you prefer support, and have the budget for it, check out Atomicorp's paid ossec-hids version. You need just a few packages from Atomicorp's no cost repository. You are going to change the repository after downloading.

Downloading the repository requires wget. Install that first, and install the EPEL repository if you do not have it installed already, with:

dnf install wget epel-release

Download and enable Atomicorp's no cost repository:

wget -q -O - | sh

This script will ask you to agree to the terms. Enter "yes" or Enter to accept the default.

Next, it will ask you if you want to enable the repository by default, and again you want to accept the default or enter "yes".

Configuring the Atomicorp repository

You only need the atomic repository for a couple of packages. For this reason, you are going to change the repository and specify only those packages needed:

vi /etc/yum.repos.d/atomic.repo

Add this line beneath the "enabled = 1" in the top section:

includepkgs = ossec* GeoIP* inotify-tools

That is the only change you need. Save your changes and get out of the repository (in vi that is esc to enter command mode, then SHIFT+:+wq to save and quit).

This restricts the Atomicorp repository only to install and update these packages.

Installing ossec-hids

With the repository configured, you need to install the packages:

dnf install ossec-hids-server ossec-hids inotify-tools

Configuring ossec-hids

The default configuration is in a state requiring many changes. Most of these have to do with server administrator notification and log locations.

ossec-hids looks at the logs to try and decide if an attack is underway, and whether to apply mitigation. It also sends reports to the server administrator with a notification, or a message regarding a mitigation procedure launched based on what ossec-hids has seen.

To edit the configuration file, enter:

vi /var/ossec/etc/ossec.conf

The author will break apart this configuration showing the changes in line and explaining them:


Email notifications are off by default and the <global> configuration is almost empty. You want to turn on email notification and identify the people who will receive the email reports by their email address.

The <smtp_server> section currently shows localhost, however you can specify an email server relay if you prefer, or setup the postfix email settings for the local host by following this guide.

You need to set the "from" email address. You need this to deal with SPAM filters on your email server which might see this email as SPAM. To avoid getting inundated with email, set the email reporting to 1 per hour. You can expand or remark out this command while starting with ossec-hids.

The <white_list> sections deal with the server's localhost IP and with the "public" IP address (remember our substitution of a private IP address) of the firewall, from which all connections on the trusted network will show. You can add many <white_list> entries.

  <!-- Frequency that syscheck is executed -- default every 22 hours -->

The <syscheck> section reviews a list of directories to include and exclude when looking for compromised files. Think of this as yet another tool for watching and protecting the file system against vulnerabilities. You need to review the list of directories and add others you want to the <syscheck> section.

The <rootcheck> section just beneath the <syscheck> section is yet another protection layer. The locations that <syscheck> and <rootcheck> watch are editable, but you probably will not need to make any changes to them.

Changing the <frequency> for the running of <rootcheck> to once every 24 hours (86400 seconds) from the default of 22 hours is an optional change shown.


The <localfile> section deals with the locations of the logs you want to watch. Entries are already in place for the syslog and secure logs that you just need to verify the path to, but everything else can remain.

You need to add in the Apache log locations, and you want to add these in as wild cards because you could have a bunch of logs for many different web customers.



Finally, toward the end of the file, you need to add the active response section. This has two parts: the <command> section, and the <active-response> section.

The "firewall-drop" script already exists within the ossec-hids path. It tells ossec-hids that if a level of 7 occurs, add a firewall rule to block the IP address.

Enable and start the service when all the configuration changes are complete. If everything starts correctly, you are ready to move on:

systemctl enable ossec-hids
systemctl start ossec-hids

The ossec-hids configuration file. You can find out about these options by visiting the official documentation site.


ossec-hids is just one element of an Apache hardened web server. You can gain better security by selecting it with other tools.

While the installation and configuration are relatively straightforward, you will find that this is not an 'install it and forget it' application. You will need to tune it to your environment to gain the most security with the least number of false-positive responses.

Dernière mise à jour: September 14, 2023

Author: Steven Spencer

Contributors: Ezequiel Bruni, Ganna Zhyrnova