Logs, Syslog, and Alerts

A log is a time-stamped record of an event: a login, an interface change, a dropped packet, or an error. Logs are the network's memory. When something breaks, logs let you reconstruct what happened and when. Without logging, troubleshooting becomes guesswork. With it, you have evidence to follow the trail back to a root cause.

Syslog is the long-standing standard for generating and sending event messages on network and Unix-like systems. Devices produce syslog messages and can forward them to a central syslog server. This centralization is powerful: instead of logging into 50 devices, you search one place. Syslog typically travels over UDP port 514.

Centralized logging collects logs from many devices into one server or platform. This helps in three ways: you correlate events across devices, you preserve logs even if a device is wiped or fails, and you search everything at once. A central log store is also vital for security, since attackers often try to erase local logs to hide their tracks.

Syslog ranks messages by severity, from 0 to 7. Lower numbers are more urgent:

• 0 Emergency, 1 Alert, 2 Critical
• 3 Error, 4 Warning
• 5 Notice, 6 Informational, 7 Debug

You usually filter so noisy debug messages do not bury the critical ones. Knowing that 0 is most severe helps you prioritize.

Each syslog message also carries a facility code indicating where it came from, such as the kernel, mail system, or authentication subsystem. Combined with severity, the facility helps you route and filter messages, for example sending all authentication failures to a security team. Together facility and severity describe both the "what" and the "from where" of an event.

A typical log line includes a timestamp, the source host or device, the facility and severity, and a human-readable message. Accurate timestamps are essential, which is why devices sync clocks with NTP. If two devices disagree on the time, correlating their logs during an incident becomes nearly impossible.

Logs are passive records, but you can build alerts on top of them. A monitoring system watches incoming logs and reacts to patterns, such as five failed logins in a minute or any "critical" message. The alert can email, text, or page staff. This turns a quiet log stream into active, timely warnings that demand attention.

A SIEM (Security Information and Event Management) system gathers logs from across the network, normalizes them, and correlates events to detect threats. For example, a SIEM might link a firewall block, a failed login, and a new admin account into one suspicious story. SIEMs combine logging, alerting, and analysis, and are central to modern security operations.

Retention is how long you keep logs. Too short and you lose history needed for an investigation; too long and storage costs climb. Many organizations and regulations require keeping logs for months or years. Retention policy balances investigative value, compliance rules, and storage capacity, and is decided up front rather than after an incident.

Because correlating logs depends on consistent time, every device should use NTP (Network Time Protocol) to sync its clock to a common source. With synchronized time, a log on the firewall and a log on the server line up to the second, letting you trace an event's path. Mismatched clocks are a classic, frustrating cause of confusing investigations.

As with all monitoring, alerts must be tuned. Too many alerts cause alert fatigue and important warnings get ignored; too few mean real incidents slip by. Good practice is to alert on high-severity events and meaningful patterns, suppress known-noisy messages, and review the alert rules regularly as the network changes.

Test your logging knowledge.

You learned about logging and alerts. Key points:

• Logs are time-stamped event records; syslog standardizes them (UDP 514).
• Centralized logging aids correlation, preservation, and search.
• Severity runs 0 (Emergency) to 7 (Debug); lower is more urgent.
• A SIEM correlates logs to detect threats.
• NTP keeps clocks in sync so logs line up.