AIDE

AIDE
AIDE stands for Advanced Intrusion Detection Environment and is an application that scans files and other resources and stores information about these files in a database. This information can be hash information, file size, ownership and more. The application can then, once this database is available, rescan the system and compare the results with the previously stored values. If values differ, then the file is changed and this change is reported.

Installation and configuration
Within Gentoo, it is easy to install aide after setting the USE flags accordingly. The supported USE flags at the time of writing are:

Then it is a matter of installing the software:

The configuration file for aide is not as daunting as it might seem at first sight. The default file is stored at but administrators can easily create multiple separate configuration files if necessary. Besides a few variables, the configuration file contains a few short-hand notations for what aspects of files to scan for (only hashes, or also inode information, etc.) and then which files to scan.

Let's first look at the variables.

These parameters define where the database is stored that contains the known values and where to store a new database when a new one  is created. It is generally recommended to not have these variables point to the same, instead manually copying over the generated database from one location to the other.

For now, leave those variables as-is, we'll get back to them later.

These are short-hand notations for what to measure. The letters are described in the default file, but the next table gives an overview of the most common ones.

Also, it is pretty obvious that  and   mean that the MD5 and SHA-1 checksums are taken.

These short-hand notations are then used to identify what to scan for which files.

This is the overview of which directories to scan, and what to scan for. In the above three lines example, we tell AIDE to scan the and  locations and take the measures identified earlier in the   short-hand notation. The location should use the   scan measures.

AIDE supports regular expressions and users are allowed to "remove" matches. For instance, to scan but not  then include an exclusion set like so:

Initialization and frequent scanning
First we need to initialize the database once.

Once initialized, we can copy over the database file.

With the database now available, we can scan the entries again for potential modifications:

When a file modification occurred, a notification will be sent out:

Be clear with what to scan
The default AIDE configuration is useful, but it needs to be fine-tuned to suit the users' needs. It is important to know which files to scan and why.

For instance, to scan for all authentication-related files but not for other files, use a configuration like so:

Keep the database offline and read-only
A second important aspect is that the result database should be stored off-line when not needed, and use it in read-only modus when the database is needed. This gives some protection against a malicious user, that might already have compromised the machine, to also modify the results database. For instance, provide the result database on a read-only NFS mount (for servers) or read-only medium (when physical access to the machine is possible) such as CD/DVD or read-only USB sticks.

After storing the database on such location, update the file to have   point to this new location.

Do offline scanning
If applicable, try using offline scanning methods for the system. In case of virtual platforms, it might be possible to take a snapshot of the system, mount this snapshot (read-only) and then run the aide scan on the mounted file system.

The above approach uses a chroot. This is only needed when the initial file system has been scanned from the live system and the administrator wants to perform an offline validation. If the initial scan was done offline, then the file will point to the mount point already and the database will use these paths immediately, so then there is no need for chrooting.