Configure Linux audit system with auditd for security compliance and file monitoring

Update system packages

Start by updating your package manager to ensure you get the latest security patches and audit tools.

sudo apt update && sudo apt upgrade -y

sudo dnf update -y

Install audit packages

Install the audit daemon and utilities for managing audit rules and analyzing logs.

sudo apt install -y auditd audispd-plugins

sudo dnf install -y audit audit-libs

Enable and start auditd service

Start the audit daemon and enable it to start automatically on boot. The auditd service runs with high priority to ensure audit events are captured.

sudo systemctl enable auditd
sudo systemctl start auditd
sudo systemctl status auditd

Configure audit buffer and log retention

Set audit buffer size and log retention policies in the main configuration file. This prevents audit events from being dropped during high activity periods.

# Log file location and rotation
log_file = /var/log/audit/audit.log
num_logs = 5
max_log_file = 50
max_log_file_action = ROTATE

# Buffer settings to prevent event loss
space_left = 100
space_left_action = SYSLOG
admin_space_left = 50
admin_space_left_action = SUSPEND

# Disk full actions
disk_full_action = SUSPEND
disk_error_action = SUSPEND

# Kernel audit buffer
freq = 20

Create file and directory monitoring rules

Set up audit rules to monitor sensitive files and directories. These rules track who accesses critical system files and configuration directories.

# Monitor sensitive system files
-w /etc/passwd -p wa -k user_accounts
-w /etc/group -p wa -k user_accounts
-w /etc/shadow -p wa -k user_accounts
-w /etc/sudoers -p wa -k privilege_escalation
-w /etc/sudoers.d/ -p wa -k privilege_escalation

# Monitor SSH configuration
-w /etc/ssh/sshd_config -p wa -k ssh_config
-w /root/.ssh -p wa -k root_ssh_access

# Monitor system configuration
-w /etc/hosts -p wa -k network_config
-w /etc/hostname -p wa -k system_config
-w /etc/fstab -p wa -k filesystem_config

# Monitor critical directories
-w /home -p wa -k user_home_access
-w /tmp -p wa -k temporary_files
-w /var/tmp -p wa -k temporary_files

Configure user activity and privilege monitoring

Create rules to monitor user authentication, privilege escalations, and administrative actions. This tracks sudo usage and user account changes.

# Monitor authentication events
-w /var/log/auth.log -p wa -k authentication
-w /var/log/secure -p wa -k authentication

# Monitor sudo usage
-a always,exit -F arch=b64 -S execve -F euid=0 -F auid!=0 -k privilege_escalation
-a always,exit -F arch=b32 -S execve -F euid=0 -F auid!=0 -k privilege_escalation

# Monitor user and group modifications
-w /usr/sbin/useradd -p x -k user_modification
-w /usr/sbin/usermod -p x -k user_modification
-w /usr/sbin/userdel -p x -k user_modification
-w /usr/sbin/groupadd -p x -k group_modification
-w /usr/sbin/groupmod -p x -k group_modification
-w /usr/sbin/groupdel -p x -k group_modification

# Monitor password changes
-w /usr/bin/passwd -p x -k password_modification
-w /usr/bin/chsh -p x -k user_shell_modification

Set up system call monitoring

Configure audit rules for security-relevant system calls like file permissions changes, network connections, and process execution.

# Monitor file permission changes
-a always,exit -F arch=b64 -S chmod,fchmod,fchmodat -k file_permissions
-a always,exit -F arch=b32 -S chmod,fchmod,fchmodat -k file_permissions
-a always,exit -F arch=b64 -S chown,fchown,lchown,fchownat -k file_ownership
-a always,exit -F arch=b32 -S chown,fchown,lchown,fchownat -k file_ownership

# Monitor file deletions
-a always,exit -F arch=b64 -S unlink,unlinkat,rename,renameat -k file_deletion
-a always,exit -F arch=b32 -S unlink,unlinkat,rename,renameat -k file_deletion

# Monitor network connections
-a always,exit -F arch=b64 -S socket,connect,bind -k network_activity
-a always,exit -F arch=b32 -S socket,connect,bind -k network_activity

# Monitor process execution
-a always,exit -F arch=b64 -S execve -k process_execution
-a always,exit -F arch=b32 -S execve -k process_execution

Configure log immutability

Make the audit configuration immutable to prevent tampering. This rule must be added last and prevents further rule modifications without a reboot.

# Make audit configuration immutable
-e 2

Load audit rules and restart service

Apply the new audit rules by restarting the auditd service. The immutable rule requires a service restart to take effect.

sudo auditctl -R /etc/audit/rules.d/file-monitoring.rules
sudo auditctl -R /etc/audit/rules.d/user-activity.rules
sudo auditctl -R /etc/audit/rules.d/system-calls.rules
sudo systemctl restart auditd

Configure log rotation

Set up proper log rotation for audit logs to prevent disk space issues while maintaining compliance retention requirements.

/var/log/audit/audit.log {
    weekly
    rotate 52
    compress
    delaycompress
    missingok
    notifempty
    create 0640 root root
    postrotate
        /sbin/service auditd restart 2> /dev/null || true
    endscript
}

Generate audit reports

Use aureport to generate summary reports of audit activity. These reports help identify patterns and compliance violations.

# Generate daily summary report
sudo aureport --start today

# Authentication report
sudo aureport --auth --summary

# File access report
sudo aureport --file --summary

# User activity report
sudo aureport --user --summary

# Failed events report
sudo aureport --failed

Search audit logs

Use ausearch to find specific audit events by time, user, file, or keyword. This helps investigate security incidents.

# Search for events by user
sudo ausearch -ua root --start today

# Search for privilege escalation events
sudo ausearch -k privilege_escalation --start today

# Search for file access events
sudo ausearch -k user_accounts --start today

# Search for failed authentication
sudo ausearch --message USER_AUTH --success no

# Search by process ID
sudo ausearch -p 1234

Set up automated monitoring alerts

Create scripts to monitor audit logs for suspicious activity and send alerts. This provides real-time security monitoring.

#!/bin/bash

# Check for privilege escalation in last hour
if sudo ausearch -k privilege_escalation --start recent | grep -q "type=SYSCALL"; then
    echo "ALERT: Privilege escalation detected" | logger -p auth.warning
fi

# Check for failed authentication attempts
if sudo ausearch --message USER_AUTH --success no --start recent | grep -q "type=USER_AUTH"; then
    echo "ALERT: Failed authentication attempts detected" | logger -p auth.warning
fi

# Check for file permission changes
if sudo ausearch -k file_permissions --start recent | grep -q "type=SYSCALL"; then
    echo "INFO: File permissions changed" | logger -p auth.info
fi

sudo chmod +x /usr/local/bin/audit-monitor.sh

Create systemd timer for monitoring

Set up automatic execution of the audit monitoring script using systemd timers.

[Unit]
Description=Audit Log Monitor

[Service]
Type=oneshot
ExecStart=/usr/local/bin/audit-monitor.sh
User=root

[Unit]
Description=Run audit monitor every 15 minutes
Requires=audit-monitor.service

[Timer]
OnCalendar=*:0/15
Persistent=true

[Install]
WantedBy=timers.target

sudo systemctl daemon-reload
sudo systemctl enable --now audit-monitor.timer
sudo systemctl status audit-monitor.timer