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Definition: What Is auditd?

auditd or Linux Audit Daemon is a user-space component of the Linux Auditing System, responsible for collecting and writing audit log file records to the disk. It is, however, not responsible for viewing the logs, which can be done through ausearch or aureport utilities.

Why Use Linux Audit Daemon?

The audit system is an important part of the production system as it maintains a log of events happening in the system. These logs can be crucial in monitoring security breaches or security incidents.

While applications and databases related to the product can implement various precautionary measures to prevent these incidents, those measures may not be sufficient in cases such as a remote shell running on the machine or threats arising from malefactors who have access to the machine. To address this issue, auditd was introduced.

auditd is akin to a black box in an airplane; it allows a system administrator to log different system events such as executed commands, system calls, file access information, and network statistics. System Administrators can use these logs to discover unusual activities and track back to learn how the system was compromised. This post-analysis can help respond to the incident and increase security to avoid similar occurrences in the future.

Installing auditd

You can install auditd in several distributions including Red Hat Enterprises (RHE), openSUSE, Arch and Fedora. However the command and configuration for each distribution can be different from the other.

In Red Hat Enterprise Linux distributions, the auditd package is installed by default in version 7 and above. If it is not installed by default, the following command will do it:

sudo dnf install audit

auditd is not always operating on an openSUSE Leap system by default. However, the following command will enable it:

sudo systemctl enable auditd

Similarly, in Debian-based Linux distributions, you can install the latest version of auditd along with its relevant plugins using:

sudo apt-get install auditd audispd-plugins

After the installation is successfully complete, you can perform a list of operations. Once you’ve finished installing, you will want to start auditd with the service auditd start command. If auditd is already running but you want to restart it, use service auditd condrestart.

To check the status of the auditd service, run service auditd status. But to reload auditd service use service auditd reload.

Similarly, to stop auditd service, run service auditd stop; to restart it, run service auditd restart

Use the service auditd rotate comand, if you want to rotate the log files in the/var/log/audit/ directory.

Set Up Rules

In RHE, you must use the auditctl utility to set up auditd rules. This utility gets the audit status and adds or deletes the rules into the system.

The rules for auditd are added to /etc/audit/rules.d/audit.rules file by default.When your system starts up, these rules are read by auditctl utility. It is recommended that you use the same path to maintain the rules for auditd to ensure uninterrupted access to the rules across the reboots.

Here is a list of some common options available to the auditctlutility.

  • -w : adds a watch to the file. auditd will record the user activities of that particular file.
  • -k : on a specific auditd rule, sets an optional string or key, which can be used for identifying the rule (or a set of rules) that created a specific log entry,
  • -F : builds a rule field using a name, arithmetic and/or logical operator, and a value.
  • -l : lists all currently loaded auditd rules in multiple lines, each line representing a rule.
  • -t : trims the subtrees that appear after a mount command.
  • -S : assigns system call name or number.
  • -a : appends rule to the end of a comma-separated catalog of list and action pairs
    • Valid list names – task, exit, user, exclude
    • Valid action names – never, always

    The pairs can be in either of the following order:

    • list, action
    • action, list

In OpenSuse, in addition to building a set of rules in /etc/audit/audit.rules, which is processed each time the audit daemon starts, audit rules can also be provided to the audit daemon via the auditctlprogram. Edit /etc/audit/audit.rules directly if you want to modify it.

Rules provided on the program must be entered once more when the audit daemon is restarted because they do not persist.

The following would be a straightforward rule configuration for basic audits on a few significant files and directories:

# basic audit system parameters
-b 8192
-f 1
-e 1

# some file and directory watches with keys
-w /var/log/audit/ -k LOG_audit
-w /etc/audit/auditd.conf -k CFG_audit_conf -p rxwa
-w /etc/audit/audit.rules -k CFG_audit_rules -p rxwa

-w /etc/passwd -k CFG_passwd -p rwxa
-w /etc/sysconfig/ -k CFG_sysconfig
# an example system call rule
-a entry,always -S umask

Try these parameters with your audit rule set while configuring the fundamental audit system parameters to see if the backlog size is suitable for the volume of logging activity your audit rule set generates. Your system may not be capable of handling the audit load and may raise the failure flag (-f) whenever the backlog threshold is reached (if the backlog size you’ve specified is too tiny).

How to Work with Linux auditd Data: Common Utilities

To understand how auditd works, you first need to understand how a Linux daemon works.

A daemon is a background process that is not dependent on the interaction of an active user, i.e., a general user can’t control the periodic execution of a daemon. It is launched when Linux boots, and an init process acts as its parent process. To start and stop the daemon, /etc/init.d scripts on the OS should be accessed initially.

In Linux, a daemon process has the suffix d. Using this suffix, you can differentiate whether a process is a daemon or a system, or a user process. By that definition, auditd is also a daemon process. Let’s look closer into the components and utilities of auditd utility.

auditd File Location

auditd.conf is located in the /etc/audit/directory and is the configuration file for auditd. It contains the specific configuration information of the audit daemon and includes multiple lines of configuration, where each line has a keyword, an equal sign, and the appropriate configuration value. Some of the keywords available in the file are:

  • log_file
  • log_format
  • flush
  • freq
  • num_logs
  • max_ log_file
  • max_log_ file_action

You can check the configuration file by running:

$ sudo /etc/audit/auditd.conf

A auditd.conf file could look something like this:

log_file = /var/log/audit/audit.log 
log_format = RAW
freq = 20
num_logs = 5
max_log_file = 8
max_log_file_action = ROTATE

Search auditd Events

As mentioned above, auditd doesn’t let you view the logs. To view the logs, you must use the ausearch utility. It is used to query audit daemon logs for events based on different search criteria.

Here are some of the most common options used to query the logs

  • -p : Search for events with the given process ID.
  • -m : Search for events with the given message type.
  • -sv : Search for events with a given success value.
  • -ua : Search for an event using user ID, effective user ID, or login user ID or auid.
  • -ts : Search for events with time stamps equal to or after the given end time.

Create auditd Reports

aureport is a tool that produces summary reports of the audit system logs. The reports contain a column label at the top to help interpret various fields. All reports have an audit event number except the main summary report. Some of the aureport keywords are:

  • -k : Report about audit rule keys.
  • -i : Interpret numeric entities into text e.g., uid is converted to the account name.
  • -au : Report about authentication attempts.
  • -l : Report about logins.

Other Useful Utilities

On top of the utilities described above, there are additional utilities that can be advantageous to you.

For instance, if you need to define Audit rules that are permanent across reboots, there are two ways. You must either explicitly include Audit rules in the /etc/audit/rules.d/audit.rules file or run theaugenrules program, which reads rules from the /etc/audit/rules.d/ directory.

But, say you want to add the audit rules to trace a process and save the audit information in audit logs when the audit daemon is running, autrace will do that for you.autrace runs a program until it exists. As a safety precaution, however, this command will not run unless all the rules are deleted withauditcl as autrace deletes all audit rules before executing the target program and after executing it.

If you want to analyze the logs events captured by auditd, use autdisp. It takes audit events and distributes them to child programs to analyze them in real-time. It is an audit event multiplexor that has to be started by the audit daemon.

Finally, to return information needed for analysis, aulast can be used to print a list of the last logged-in users. Itsearches through the audit logs and displays a list of all users logged in (and out). Meanwhile, there is also aulastlog thatreturns the latest login details of all machine users by printing the login name, port, and last login time. The port and time fields will show “Never logged in” if a user has never logged in.

Sematext and auditd

Serious usage of auditd – even on just a few hosts – can generate lots of events. With standard tools like ausearch, it might be difficult to sift through all this data and even more complicated to set up alerts and automate responses. This is whereSematext Logs comes in: with a lightweight tool such as AuditBeat or alog shipper, you can aggregate audit logs across all hosts of your infrastructure. Once data is in, you can slice and dice it through customizable dashboards or the API, create alerts or rely on anomaly detection – so you can react to anything suspicious in a timely manner.

Watch the video below to learn more about Sematext Logs or try the 14-day free trial to test it ou yourself.

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