Linux is considered a very secure operating system for computers of all kinds. Realistically, however, even Linux is not immune to vulnerabilities and malware, which is why you can find a large number of tools in the package sources of Linux derivatives to help scan for vulnerabilities. However, these tools tend to focus on specific weaknesses and narrowly defined attack scenarios by only analyzing a computer system for individual potential security problems.

With the checksecurity tool collection, on the other hand, you can automatically check Ubuntu and Debian systems [1] and their derivatives for multiple potential basic security flaws or anomalies.

Concept

Checksecurity consists of a number of plugins, each of which you then customize in a configuration file. The corresponding files are in text format, so a simple editor is all it takes to set up the service. When checksecurity is called, the program works through the activated plugins one by one and outputs the results of the tests in a terminal window.

To achieve the highest possible level of security through an automated process, the system automatically creates two different cron jobs during installation. The check-setuid plugin does this by checking setuid attributes (see the "setuid" box) for modifications and looking for remote filesystems that are mounted insecurely on the local system.

The Check-sockets plugin searches for and monitors open and modified ports, making it possible to detect malware that tries to enter the system through these ports. The check-passwd plugin checks the system for unsecured system accounts, while check-diskfree checks mounted filesystems for their capacity limits. The check-iptables-logs plugin takes care of possible intrusion attempts, which it finds using the iptables logfiles.

Installation

You can set up checksecurity on Debian, Ubuntu, and their derivatives conveniently using the system's package management sources. Since it runs entirely on the command line, the installer does not create a launcher in the desktop menu hierarchy. The individual scripts for running the tests can be found after installation in /usr/share/checksecurity/.

During installation, checksecurity also loads the Postfix mail transfer agent. This mail server allows the system to send mails independently. However, this does not work without a fixed IP address and complex configuration. The recommended approach is to configure Postfix so that the service routes outgoing mail via a commercial provider such as Gmail or GMX. You can find information on this by searching for "Postfix email provider" on the web.

Alternatively, you can set up Postfix to deliver mail locally. To do this, select the Local only option in Postfix Configuration. Then install the mailutils package and add your user to the mail group by running the command:

sudo adduser $USER mail

To be able to read mail sent by checksecurity to the root user via the mail program, forward the messages sent to root to your own account by typing

echo $USER | sudo tee /root/.forward

Configuration

You create the global configuration for checksecurity in the /etc/checksecurity.conf file. In the file, you can switch the individual modules on and off or set check intervals for the automated execution of the individual scripts (Figure 1).

Figure 1: Configuring the basic settings in the checksecurity.conf file. The individual plugins are set up in separate configuration files.

In the /etc/checksecurity/ directory, you will find additional configuration files for all plugins except check-iptables-logs. These files are not used for global configuration, but they do support customization of individual script flows. They are also in text format and can therefore be easily modified using your favorite editor.

You need to pay special attention to the CHECK_DISK_PERCENT option in the check-diskfree.conf file, which defaults to 70. This setting results in a system message being output as soon as one of the mounted partitions reaches a utilization level of more than 70 percent. Since this value can be reached quite quickly, especially with small system partitions that also store temporary data, it is a good idea to increase the value to 80 or even 90 percent.

In the check-passwd.conf configuration file, you can define whether or not you want the script to detect empty system password entries or duplicate passwords. By default, both routines are activated.