Security Handbook/Intrusion detection

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Security Handbook
General Guidance
Boot Path Security
Information Security
Mounting partitions
User and group limitations
File permissions
Kernel security
Firewalls and Network Security
Securing services
Chrooting and virtual servers
Intrusion detection
Staying up-to-date

This section is on detecting changes made to your system indicating possible intrusions.


The Q applets program qcheck can be used to check the existence, modification times and MD5 sums of all files of packages installed by portage. It is a fast program that requires no manual configuration in order to check the integrity of your host's installed files. qcheck is provided through the app-portage/portage-utils package.

To use qcheck, type in a console:

user $qcheck package-name

Replace package-name in the example above with the desired package.

To check the integrity of all packages installed, enter:

user $qcheck


AIDE is a Host-Based Intrusion Detection System (HIDS), a free alternative to Tripwire. HIDS are used to detect changes to important system configuration files and binaries, generally by making a unique cryptographic hash for the files to be checked and storing it in a secure place. On a regular basis (such as once a day), the stored "known-good" hash is compared to the one generated from the current copy of each file, to determine if that file has changed. HIDS are a great way to detect disallowed changes to your system, but they take a little work to implement properly and make good use of.

The AIDE ebuild now comes with a working default configuration file, a helper script and a crontab script. The helper script does a number of tasks for you and provides an interface that is a little more script friendly. To see all available options, try aide --help. To get started, all that needs to be done is aide -i and the crontab script should detect the database and send mails as appropriate every day. We recommend that you review the /etc/aide/aide.conf file and ensure that the configuration accurately reflects what is in place on the machine.

Please see AIDE for more details on configuration and usage.

Let's watch a full blown example:

FILE /etc/aide/aide.conf
@@ifndef TOPDIR
@@define TOPDIR /

@@ifndef AIDEDIR
@@define AIDEDIR /etc/aide

@@ifhost smbserv
@@define smbactive

# The location of the database to be read.

# The location of the database to be written.


# Rule definition

@@{TOPDIR} Norm
@@ifdef smbactive
=@@{TOPDIR}home Norm

In the above example we specify with some macros where the topdir starts and where the AIDE directory is. AIDE checks the /etc/aide/aide.db file when checking for file integrity. But when updating or creating a new file it stores the information in /etc/aide/ This is done so it won't automatically overwrite the old db file. The option report_URL is not yet implemented, but the author's intention was that it should be able to e-mail or maybe even execute scripts.

Depending on your CPU, disk access speed, and the flags you have set on files, this can take some time.
Remember to set an alias so you get root's mail. Otherwise you will never know what AIDE reports.

Now there is some risk inherent with storing the db files locally, since the attacker will (if they know that AIDE is installed) most certainly try to alter the db file, update the db file or modify /usr/bin/aide. So you should create a CD or other media and put on it a copy of the .db file and the AIDE binaries.

You can find information at the AIDE project page.


Snort is a Network Intrusion Detection System (NIDS). To install and configure it use the following examples.

FILE /etc/conf.d/snort
SNORT_OPTS="-q -D -u snort -d -l $LOGDIR -h $NETWORK -c $SNORT_CONF"

Copy /etc/snort/snort.conf.distrib to /etc/snort/snort.conf.

root #cd /etc/snort && cp snort.conf.distrib snort.conf

You might need to comment out the blacklist and whitelist entries if no lists are created.

More information is at the Snort website.

Detecting malware with chkrootkit

HIDS like AIDE are a great way to detect changes to your system, but it never hurts to have another line of defense. chkrootkit is a utility that scans common system files for the presence of rootkits-software designed to hide an intruder's actions and allow him to retain his access-and scans your system for likely traces of key loggers and other "malware". While chkrootkit (and alternatives like rkhunter) are useful tools, both for system maintenance and for tracking an intruder after an attack has occurred, they cannot guarantee your system is secure.

The best way to use chkrootkit to detect an intrusion is to run it routinely from cron. To start, emerge app-forensics/chkrootkit:

root #emerge --ask app-forensics/chkrootkit

chkrootkit can be run from the command line by the command of the same name, or from cron with an entry such as this:

0 3 * * * /usr/sbin/chkrootkit