Essential Guide to Log Files: Types of Logs, Sources, What to Capture

Logs are an essential tool for managing and securing IT environments. In this article, we’ll explain what a logs file is, outline the key types of logs, which sources to collect from, and what should be logged to support effective incident detection, investigation, and IT compliance.

What Is a Log File?

Log file (log) is a structured or semi-structured digital record of system states and events generated by every component in an IT environment, from operating systems and applications to networking and security infrastructure. Logs enable IT administrators to monitor system behavior in detail, identify anomalies, and detect security or operational issues in real time.

Fig 1: Example of a raw log file capturing event data from a system.

What is stored in a log file can vary by source and purpose, but most log entries contain several common elements such as:

• Timestamp indicates when the event occurred
• Source what generated the event
• User or process ID who or what initiated the event.
• Status code for services like web servers or APIs 
• Event type or ID – numeric or categorical code
• IP address / hostname especially for access or connection logs
• Log severity indicates the importance or urgency of the event (info, warning, error, etc.) 

What Are the Types of Log Files?

Commonly collected types of logs include:

System logs capture OS-level activity such as service starts/stops, system errors, configuration changes, and updates.

Application logs provide detailed insights into application behavior, including errors, warnings, successful and failed connection attempts, and specific application events.

Security log files contain data on user login and logout activities, unauthorized access attempts, permission changes, malware detection, and unusual user behavior.

Network logs track communication between devices on the network and may include information about intrusion attempts, firewall configuration changes, or VPN activity.

Change logs refers to a specific type of a log file that records modifications made to systems, configurations, software, or infrastructure. It helps track who made what change, when, and why, which is critical for auditing, troubleshooting, compliance, and security.

Logs from firewalls, IDS/IPS, and other security appliances capture failed connection attempts, exploit attempts, and monitor suspicious activity.

Fig. 2: Example of different logging formats used by firewalls (Sophos above, Fortinet below)

Why is Log Management Important?

Effective log management is not just about storage. It’s about making data available for security, compliance, and operational efficiency. Typically, log management plays an important role in the following areas:

Troubleshooting – Log files provide insights, helping IT teams quickly identify and resolve issues such as application crashes, slow response times, and hardware failures.

Cybersecurity – Log files provide security administrators and analysts with critical insights needed to detect and investigate security incidents. They support early remediation, help contain the impact, and enable security teams to prevent threats from spreading across the IT environment. For this purpose, specialized solutions are used, ranging from lightweight tools such as firewall log analyzer to robust SIEM platforms.

IT compliance – Proper log management helps organizations comply with regulations and industry standards (GDPR, NIS2, HIPAA, PCI-DSS, ISO 27001) by ensuring effective monitoring and long-term log retention. We cover this topic in a dedicated blog post on IT compliance.

Business continuity and forensics – In the event of a system breach or failure, logs act as a forensic trail that helps teams reconstruct events and pinpoint vulnerabilities.

What Should Be Logged?

Effective log management isn’t about collecting everything, it’s about logging the right data. What you choose to log determines how quickly IT teams can detect incidents, troubleshoot issues, or trace unauthorized changes.

Thus, it is important to focus on events with real impact on security, availability, or operational integrity.

Some of the most important events include user activity, configuration changes, error states, and network communication. These logs form the basis for audit trails, threat detection, and proactive system maintenance.

To give you a clearer picture, here are specific types of events that should be included in any properly configured logging setup:

Security & Threat Detection

• User access: Log who logged in and when, including IP address, method of authentication, and accessed resources, especially for admin or privileged accounts.
• Failed login attempts: Record unsuccessful login attempts with timestamps, usernames, and source IPs to detect brute-force attacks or credential stuffing.
• Privilege escalations: Capture events where user roles or permissions are changed, or when users assume elevated privileges.
• Blocked connections: Capture firewall, WAF, or endpoint blocks for traffic to/from known malicious IPs or domains.
• Anomalous data access: Record attempts to access restricted files, databases, or APIs, especially outside normal patterns.

Fig 3: Suspicious MS365 admin action log: An IT admin modified mailbox forwarding rules which might indicate misconfiguration or worse, a malicious insider or compromised account – potential data exfiltration risk. (Simplified log file example)

Fig 4: Successful VPN login log: A user successfully connected to the VPN using FortiClient. This information is critical for auditing secure remote access and detecting unusual login origins or behavior. (Simplified log file example)

IT Operations & Infrastructure Monitoring

• System health metrics: Log system status like CPU, memory, disk usage, and hardware errors (e.g., RAID failures, overheating).
• Service status: Track when services start, stop, crash, or restart, and capture exit codes or error messages.
• Device connectivity: Monitor devices joining or leaving the network, along with MAC/IP addresses and DHCP lease events.
• Performance issues like slow database queries, long response times, or queue backlogs in background job systems.
• Unusual network traffic: Log connections to rare or foreign IPs, unusual ports, or unexpected protocols.

Fig. 5: Log record showing that a Cisco switch port went down unexpectedly, possibly due to flapping or cable damage. Such a record helps IT operations quickly identify physical layer issues, hardware instability, or unplugged devices. (Simplified log file example)

Compliance & Audit

• Access to sensitive data: Log who viewed, modified, or deleted personal, financial, or regulated data.
• Policy violations: Capture events where access rules were broken, such as off-hours access or login from restricted regions.
• Audit trail for changes: Track who changed system configurations, security settings, or user permissions, including timestamps and original vs. new values.
• Log integrity: Record events related to log tampering attempts, deletion of log files, or failure of the logging service itself.

Fig 6: MS365 File Deletion. User h.bauer deleted a sensitive file from SharePoint Online. Such a log file tracks sensitive file deletions in Microsoft 365 which is important for data loss prevention and access auditing. (Simplified log file example)

Alerting for Administrators and Security Managers

Timely and relevant alerts about suspicious or unusual activity are essential for maintaining a secure and stable IT environment. Alerting enables administrators and security managers to respond immediately to unwanted events.

A properly configured alerting system should automatically detect and report incidents such as unauthorized access attempts, changes to security settings, signs of malware, or data exfiltration attempts.

Automated alerts should be configured for critical events such as:

• Unauthorized access attempts or configuration changes
• A high number of failed login attempts within a short time frame
• Detection of malware or unknown executable files
• Changes to firewall settings or security rules
• Evidence of attempted exfiltration of sensitive data

Just as important as detection itself is defining the severity levels of alerts. Alerting logic must be properly configured so administrators receive only relevant notifications and are not overwhelmed by false positives or non-escalating alerts.

Fig. 7: Example of an alert notifying an administrator about a potential port scan attack

Testing and Validation of Logging

Regular testing to ensure that all critical devices and applications are properly configured is essential for effective logging. It’s important to verify that the collected log files are sufficiently detailed, relevant, and properly parsed (so they can be searched and filtered).

It’s also recommended to run periodic simulations and tests to assess how logging and alerting systems would respond in the event of an attack or security breach.

Protecting Log Files from Tampering

As part of your log management strategy, it’s essential to ensure that log files are stored in secure locations to prevent unauthorized access, tampering, or deletion by attackers. Stored log data should be protected with strict access controls and regular audits.

Log Sources

In the following section, we’ll look at common sources of log files along with examples of what should be logged. The most typical log sources include:

• Operating systems
• Applications
• Network devices and infrastructure
• Security appliances
• Backup systems
• IoT and industrial devices

1. Operating Systems (OS)

Windows

• Event logs: Access attempts, failed logins, configuration changes, software installations, error states, and system events.
• Audit logs: Records of user access, permission changes, and group modifications.

Linux/Unix
Dependent on the specific distribution or version:

• /var/log/auth.log: User logins and logouts, SSH access, failed login attempts.
• /var/log/syslog: System events, errors, warnings, configuration changes, and service status.
• /var/log/messages: Error messages and general system events.
• /var/log/secure: Security events, including attempts to access system resources.

2. Application Logs

Web servers
Apache/Nginx

• access.log: Records of HTTP requests (IP addresses, URLs, timestamps, status codes, user agents).
• error.log: Errors, warnings, failed requests, and configuration issues.

Databases
MySQL/PostgreSQL

• error.log: Connection errors, SQL query issues, and performance warnings.
• general.log: Records of all SQL queries (if enabled), useful for auditing and detecting SQL injection attempts.

Mail servers
Sendmail, Postfix

• mail.log: Logs of received and sent emails, delivery attempts, and delivery errors.

Application logs

• Error and exception logs: Records of runtime errors, unexpected exceptions, and abnormal application behavior.
• Audit logs: Logs of access to sensitive application functions or data.

3. Network Devices and Infrastructure

Firewalls (e.g., Fortigate, Cisco ASA, pfSense)

• Connection logs: Connection attempts (successful and failed), blocked and allowed ports, firewall configuration changes.
• Attack logs: Detection of DDoS attempts, port scans, and suspicious activity.

Intrusion Detection/Prevention systems (IDS/IPS)
Snort, Suricata

• Attack Detection Alerts: Attempts to exploit vulnerabilities (e.g., SQL injection, cross-site scripting), and other exploitation attempts.

Load balancers and reverse proxy servers
HAProxy, F5

• Traffic distribution logs: Records of traffic routing between servers, server outages, and latency metrics.

4. Security Appliances

VPN servers

• Connection logs: User logins and logouts, failed connection attempts, IP addresses, timestamps, and encryption details (e.g., OpenVPN, Cisco AnyConnect).

Antivirus and antimalware systems

• Threat detection logs: Detection of viruses, malware, unusual activity, and threat remediation.
• Update logs: Information about the status and currency of virus definitions and antivirus databases.

5. Backup Systems

Backup software (e.g., Veeam, Bacula):

• Backup logs: Success and failure of backup operations, backup errors, and timestamps of completed backups.

Cloud Backups:

• Backup status logs: Information on synchronization and the success of cloud-based backups (e.g., Amazon S3, Google Drive).

6. Security Incidents and Monitoring

Vulnerabilities and patch management

• Patch logs: Records of applied patches, updates, and remediation of known vulnerabilities.

Security incidents

• Suspicious activity logs: Log file should include records of unusual behavior (e.g., multiple login attempts from different locations), changes in user permissions, configuration changes, malware detection, and other security-related incidents.

7. IoT and Industrial Devices

Device logs

• IoT Devices (cameras, sensors, building management systems): Logs of connectivity, configuration changes, errors, and security events.
• SCADA Systems: Monitoring and logging of industrial equipment operations and security incidents.

Centralize Your Log Management

Log files play an essential role in the internal IT operations of any bigger organization. However, accessing each infrastructure component separately and sifting through fragmented logs slows down response times and increases the risk of missing critical connections.

This is why organizations adopt centralized tools for collecting, managing, and analyzing log files, such as Logmanager, administrators gain better visibility into their IT environment, respond more quickly and effectively to incidents, and ultimately improve overall IT management and security.

→ If you’d like to learn more about Logmanager, you can try it free for 7 days as a virtual appliance or book a personalized product demo with one of our experts.