Harden your systems with Lynis

Starting Your Security Scan

After confirming by pressing the Enter key, Lynis starts its tests and writes the results to your standard output. Results marked in green are the successfully completed tests; red warnings point to problems or vulnerabilities (Figure 2). Administrators simply need to make a note of gray result messages. They show that, for instance, the service is not installed on the system. Messages highlighted in yellow do not point to critical problems, but Lynis does see some scope for improvement. The tool might even suggest some remedies, which appear right at the end of the test report (Figure 3).

Figure 2: Test results marked in yellow or red definitely need to be checked by an administrator. In this case, Lynis is complaining about problems with password security.
Figure 3: Lynis returns a list of proposed improvements that is fairly long on a freshly installed Ubuntu 14.10.

After each test, Lynis stops, thus giving the user the opportunity to inspect the output. The test continues after you press the Enter key. If you want Lynis to simply perform all the tests in succession, just add the --quick or -Q parameter to the command line. Depending on your terminal, the output from Lynis may use problematic color schemes, or it might be difficult to read. You can stipulate the additional --reverse-colors parameter to tell Lynis to adapt the colors for a light background. If this doesn't help, you can also completely disable colors by stipulating the --no-colors parameter.

Lynis consists of a collection of shell scripts, each of which checks precisely one system component. For their work, the scripts harness the power of typical system commands. For example, one of the scripts tests the firewall rules by running iptables. You might find that programs cannot be executed – either because of a program error or because the required access privileges for executing the program are missing. Lynis then deems the test to have failed and issues a warning. You can see an example of this behavior in Figure 2, where access to D-Bus failed. Administrators thus always need to check whether Lynis really has found a problem or whether the test was canceled. It makes sense to check out the error message that appears below the test.

When finished, Lynis delivers some statistics, as shown in Figure 4. You will find a somewhat more exhaustive test report in the /var/log/lynis.log file. Its counterpart, /var/log/lynis-report.dat, contains a summary of the test run and the improvements suggested by Lynis. But caution is advisable: as soon as you launch Lynis the next time, it will overwrite the existing logfile and report file.

Figure 4: Finally, Lynis outputs a "Hardening index." The higher the score, the better your system is protected against attacks – at least in the tool's opinion.

Automated Scans

To identify problems that have crept in the back door, administrators will want to run Lynis regularly. The --cronjob option will help you execute the program on a regular schedule: If you run the tool with this parameter, it does not wait for user input and also removes all critical characters (special chars) from the input. In this way, you can run Lynis as a Cron job. Of course, the tool would overwrite the logfile and report file, /var/log/lynis.log and /var/log/lynis-report.dat, in each run. You can prevent Lynis from overwriting the logfiles by running Cron using a self-programmed wrapper script. Listing 1 contains a simple example. In the extras/systemd subdirectory of the Lynis archive, you will also find two files for Systemd: You can use the service file to enable Lynis automatically on system startup; The timer file launches the tool once a day. In both cases, it makes sense to copy Lynis to a directory such as /usr/local/lynis.

Listing 1

Integrating Lynis with Cron

DATE=$(date +%Y%m%d)
# Launch Lynis:
/usr/local/lynis/lynis -c --cronjob > /var/log/lynis-output.${DATE}.txt
# Back up generated log files:
mv /var/log/lynis.log /var/log/lynis.${DATE}.log
mv /var/log/lynis-report.dat /var/log/lynis-report.${DATE}.dat

Using Individual Scans

If so desired, Lynis will also perform predefined tests. The approach is somewhat convoluted, however: Each test has a unique, cryptic ID. For example, Lynis refers to the test against the iptables kernel module as FIRE-4511. To find out which ID belongs to which test, let Lynis perform all the tests once, and then take a look at the /var/log/lynis.log file. It lists the IDs to match the tests.

Another source of information are the shell scripts in the include subdirectory whose filenames start with test_. The scripts contain the tests themselves; for example the tests_firewall script groups all the tests relating to the firewall. The test IDs are included as comments in the script. Once you have determined the IDs for the desired tests, add them as a blank-separated list following the --tests parameter. As an example, the following command would execute the FIRE-4511 and AUTH-9226 tests:

./lynis -Q --tests "FIRE-4511 AUTH-9226"

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