Applies to SUSE Linux Enterprise Server 15 SP3

20 Intrusion detection with AIDE #

Securing your systems is a mandatory task for any mission-critical system administrator. Because it is impossible to always guarantee that the system is not compromised, it is very important to do extra checks regularly (for example with cron) to ensure that the system is still under your control. This is where AIDE, the Advanced Intrusion Detection Environment, comes into play.

20.1 Why use AIDE? #

An easy check that often can reveal unwanted changes can be done by means of RPM. The package manager has a built-in verify function that checks all the managed files in the system for changes. To verify of all files, run the command rpm -Va. However, this command will also display changes in configuration files and you will need to do some filtering to detect important changes.

An additional problem to the method with RPM is that an intelligent attacker will modify rpm itself to hide any changes that might have been done by some kind of root-kit which allows the attacker to mask its intrusion and gain root privilege. To solve this, you should implement a secondary check that can also be run completely independent of the installed system.

20.2 Setting up an AIDE database #

Important: Initialize AIDE database after installation

Before you install your system, verify the checksum of your medium (see Abschnitt 12.1, „Überprüfen von Medien“) to make sure you do not use a compromised source. After you have installed the system, initialize the AIDE database. To make sure that all went well during and after the installation, do an installation directly on the console, without any network attached to the computer. Do not leave the computer unattended or connected to any network before AIDE creates its database.

AIDE is not installed by default on SUSE Linux Enterprise Server. To install it, either use Computer › Install Software, or enter zypper install aide on the command line as root.

To tell AIDE which attributes of which files should be checked, use the /etc/aide.conf configuration file. It must be modified to become the actual configuration. The first section handles general parameters like the location of the AIDE database file. More relevant for local configurations are the Custom Rules and the Directories and Files sections. A typical rule looks like the following:

Binlib     = p+i+n+u+g+s+b+m+c+md5+sha1

After defining the variable Binlib, the respective check boxes are used in the files section. Important options include the following:

Table 20.1: Important AIDE check boxes #

Option	Description
p	Check for the file permissions of the selected files or directories.
i	Check for the inode number. Every file name has a unique inode number that should not change.
n	Check for the number of links pointing to the relevant file.
u	Check if the owner of the file has changed.
g	Check if the group of the file has changed.
s	Check if the file size has changed.
b	Check if the block count used by the file has changed.
m	Check if the modification time of the file has changed.
c	Check if the files access time has changed.
S	Check for a changed file size.
I	Ignore changes of the file name.
md5	Check if the md5 checksum of the file has changed. We recommend to use sha256 or sha512.
sha1	Check if the sha1 (160 Bit) checksum of the file has changed. We recommend to use sha256 or sha512.
sha256	Check if the sha256 checksum of the file has changed.
sha512	Check if the sha512 checksum of the file has changed.

This is a configuration that checks for all files in /sbin with the options defined in Binlib but omits the /sbin/conf.d/ directory:

/sbin  Binlib
!/sbin/conf.d

To create the AIDE database, proceed as follows:

Open /etc/aide.conf.
Define which files should be checked with which check boxes. For a complete list of available check boxes, see /usr/share/doc/packages/aide/manual.html. The definition of the file selection needs some knowledge about regular expressions. Save your modifications.
To check whether the configuration file is valid, run:
```
# aide --config-check
```
Any output of this command is a hint that the configuration is not valid. For example, if you get the following output:
```
# aide --config-check
35:syntax error:!
35:Error while reading configuration:!
Configuration error
```
The error is to be expected in line 36 of /etc/aide.conf. Note that the error message contains the last successfully read line of the configuration file.
Initialize the AIDE database. Run the command:
```
# aide -i
```
Copy the generated database to a save location like a CD-R or DVD-R, a remote server or a flash disk for later use.
Important
This step is essential as it avoids compromising your database. It is recommended to use a medium which can be written only once to prevent the database being modified. Never leave the database on the computer which you want to monitor.

20.3 Local AIDE checks #

To perform a file system check, proceed as follows:

Rename the database:

# mv /var/lib/aide/aide.db.new /var/lib/aide/aide.db

After any configuration change, you always need to re-initialize the AIDE database and subsequently move the newly generated database. It is also a good idea to make a backup of this database. See Section 20.2, “Setting up an AIDE database” for more information.
Perform the check with the following command:
```
# aide --check
```

If the output is empty, everything is fine. If AIDE found changes, it displays a summary of changes, for example:

# aide --check
AIDE found differences between database and filesystem!!

Summary:
  Total number of files:        1992
  Added files:                  0
  Removed files:                0
  Changed files:                1

To learn about the actual changes, increase the verbose level of the check with the parameter -V. For the previous example, this could look like the following:

# aide --check -V
AIDE found differences between database and filesystem!!
Start timestamp: 2009-02-18 15:14:10

Summary:
  Total number of files:        1992
  Added files:                  0
  Removed files:                0
  Changed files:                1


---------------------------------------------------
Changed files:
---------------------------------------------------

changed: /etc/passwd

--------------------------------------------------
Detailed information about changes:
---------------------------------------------------


File: /etc/passwd
  Mtime    : 2009-02-18 15:11:02              , 2009-02-18 15:11:47
  Ctime    : 2009-02-18 15:11:02              , 2009-02-18 15:11:47

In this example, the file /etc/passwd was touched to demonstrate the effect.

20.4 System independent checking #

To avoid risk, it is advisable to also run the AIDE binary from a trusted source. This excludes the risk that some attacker also modified the aide binary to hide its traces.

To accomplish this task, AIDE must be run from a rescue system that is independent of the installed system. With SUSE Linux Enterprise Server it is relatively easy to extend the rescue system with arbitrary programs, and thus add the needed functionality.

Before you can start using the rescue system, you need to provide two packages to the system. These are included with the same syntax as you would add a driver update disk to the system. For a detailed description about the possibilities of linuxrc that are used for this purpose, see https://en.opensuse.org/SDB:Linuxrc. In the following, one possible way to accomplish this task is discussed.

Procedure 20.1: Starting a rescue system with AIDE #

Provide an FTP server as a second machine.
Copy the packages aide and mhash to the FTP server directory, in our case /srv/ftp/. Replace the placeholders ARCH and VERSION with the corresponding values:
```
# cp DVD1/suse/ARCH/aideVERSION.ARCH.rpm /srv/ftp
# cp DVD1/suse/ARCH/mhashVERSION.ARCH.rpm /srv/ftp
```
Create an info file /srv/ftp/info.txt that provides the needed boot parameters for the rescue system:
```
dud:ftp://ftp.example.com/aideVERSION.ARCH.rpm
dud:ftp://ftp.example.com/mhashVERSION.ARCH.rpm
```
Replace your FTP domain name, VERSION and ARCH with the values used on your system.
Restart the server that needs to go through an AIDE check with the Rescue system from your DVD. Add the following string to the boot parameters:
```
info=ftp://ftp.example.com/info.txt
```
This parameter tells linuxrc to also read in all information from the info.txt file.

After the rescue system has booted, the AIDE program is ready for use.

20.5 More information #

Information about AIDE is available at the following places:

The home page of AIDE: https://aide.github.io/
In the documented template configuration /etc/aide.conf.
In several files below /usr/share/doc/packages/aide after installing the aide package.
On the AIDE user mailing list at https://www.ipi.fi/mailman/listinfo/aide.