What is a Rootkit?

A rootkit is a type of computer virus that conduct maliciously activities. It gains access to the privileged root account on Unix operating systems and replaces the typical standard commands (rm, ps, cd, chmod, etc).

Protection against Rootkits - keep the operating system up to date

Naturally Internet server systems are particularly vulnerable and therefore particularly at risk.
It is of importance to upgrade the operating system on a regular basis to newly discovered security vulnerabilities are closed quickly. On a Debian or Ubuntu installation the following commands to load these updates:
  sudo apt-get update
  sudo apt-check-sigs
  sudo apt-get dist-upgrade

Backdoor and Rootkit Scanner

Another possibility to increase security of a Linux installation is a Backdoor- and Rootkit scanner.
Such Backdoor and Rootkit Scanner takes a fingerprint from the main system files, stores them in a database. Takes a cryptographic fingerprint from each important file and compares this fingerprint with the entry in it's database and so would expose an intruder. The fingerprint is taken in the form of a cryptographic hashes , for example, like this:

Furthermore, he searches for clues for a compromised system such as e.g.:

Rkhunter Tutorial

rkhunter is such a Backdoor and Rootkit scanner which is part of a Debian or Ubuntu distribution.


On a Debian or Ubuntu setups the following commands load these scanner:
  sudo apt-get install rkhunter

Configuration of whitelist of suspicious files

/usr/bin/unhide.rb is one of the files that is ok but is pointed out as suspicious
The configuration files of 'rkhunter' is located here:

Here various aspects can be set, including the place where the database, the Logiles etc are.
In the Debian distribution the '/usr/bin/unhide.rb' is flagged as it is a script. To prevent this unauthorized warning we are use the following line in 'rkhunter.conf' a:
 SCRIPT WHITE LIST = "/usr/bin/unhide.rb"

The signature algorithm

The hash algorithm that is preset is called sha1sum. The Secure hash algorithm number one was developed by the American 'National Security Agency' and published in 1995. He is still considered safe and there are no known successful attacks.

The rkhunter database

The database is by default located in /var/lib/rkhunter/db/.
The file rkhunter.dat contains the database with the hashes and the file rkhunter_prop_list.dat a list of files to prove.

Verify your system - rkhunter --check

In order to check your system do the command
   rkhunter --check

This system will be reviewed and any warnings and attacks displayed.
A log file with verbose error messages is located under

Update the fingerprints

Status: Published Date: 2014-10-15 00:23:19 +0200 (Mi, 15 Okt 2014) Revision: 27

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