Uses the Linux Audit framework (auditd) with ausearch and aureport utilities to detect intrusion attempts, unauthorized access, privilege escalation, and suspicious system activity. Covers audit rule configuration, log querying, timeline reconstruction, and integration with SIEM platforms. Activates

Analyzing Linux Audit Logs for Intrusion When to Use Investigating suspected unauthorized access or privilege escalation on Linux hosts Hunting for evidence of exploitation, backdoor installation, or persistence mechanisms Auditing compliance with security baselines (CIS, STIG, PCI DSS) that require system call monitoring Reconstructing a timeline of attacker actions during incident response Detecting file tampering on critical system files such as /etc/passwd , /etc/shadow , or SSH keys Do not use for network level intrusion detection; use Suricata or Zeek for network traffic analysis. Auditd operates at the kernel level on individual hosts. Prerequisites Linux system with auditd package installed and the audit daemon running ( systemctl status auditd ) Root or sudo access to configure audit rules and query logs Audit rules deployed via /etc/audit/rules.d/ .rules or loaded with auditctl Recommended: Neo23x0/auditd ruleset from GitHub for comprehensive baseline coverage Familiarity with Linux syscalls ( execve , open , connect , ptrace , etc.) Log storage with sufficient retention (default location: /var/log/audit/audit.log ) Workflow Step 1: Verify Audit Daemon Status and Configuration Confirm the audit system is running and check the current rule set: If the backlog limit is being reached, increase it: Step 2: Deploy Intrusion Focused Audit Rules Add rules that target common intrusion indicators. Place these in /etc/audit/rules.d/intrusion.rules : Reload rules after editing: Step 3: Search for Intrusion Indicators with ausearch Use ausearch to query the audit log for specific events: Step 4: Generate Summary Reports with aureport Use aureport to produce aggregate summaries for triage: Step 5: Reconstruct the Attack Timeline Combine ausearch queries to build a chronological narrative: Step 6: Forward Audit Logs to SIEM Configure audisp remote or auditbeat to ship logs to a central SIEM for correlation: Key Concepts Term Definition auditd The Linux Audit daemon that receives audit events from the kernel and writes them to /var/log/audit/audit.log auditctl Command line utility to control the audit system: add/remove rules, check status, set backlog size ausearch Query tool that searches audit logs by message type, user, file, key, time range, or event ID aureport Reporting tool that generates aggregate summaries of audit events for triage and compliance audit rule key ( k) A user defined label attached to an audit rule, enabling fast filtering of related events with ausearch and aureport syscall auditing Kernel level monitoring of system calls (execve, open, connect, ptrace) that captures process and file activity augenrules Utility that merges all files in /etc/audit/rules.d/ into /etc/audit/audit.rules and loads them into the kernel Verification [ ] auditd is running and rules are loaded ( auditctl l returns expected rule count) [ ] No audit backlog overflow ( auditctl s shows backlog: 0 or low value, lost: 0) [ ] ausearch returns events for each custom key ( ausearch k <key ts today returns results) [ ] aureport generates non empty summaries for authentication, executable, and file events [ ] Timeline reconstruction produces a coherent chronological sequence of attacker actions [ ] Critical file watches trigger alerts on test modifications ( touch /etc/shadow generates an event) [ ] Logs are forwarding to central SIEM (verify with a test event and confirm receipt) [ ] Audit rules persist across reboot (rules in /etc/audit/rules.d/ , not only via auditctl )

analyzing-linux-audit-logs-for-intrusion