Firewalls and Monitoring

Firewalls make thousands of decisions per second—allow this, block that, inspect this packet, drop that connection. Every decision generates a log entry. Somewhere in those logs is the story of your network's security: the attacks that failed, the threats that were blocked, and possibly the breach you haven't discovered yet.

Firewall monitoring is the practice of reading that story before it's too late.

What Firewalls Know

Every packet a firewall processes is a decision point, and modern firewalls remember their decisions in remarkable detail.

Allowed connections show what traffic successfully reached your systems. This reveals normal traffic patterns, confirms that firewall rules work as intended, and creates audit trails for compliance. But it also shows what you're permitting—and sometimes what you're permitting is more than you realized.

Blocked attempts reveal what the firewall is protecting you from. Attack attempts. Misconfigured systems trying to communicate. Potentially unauthorized access attempts from inside your network. Every block is a small story: someone or something tried to reach you and couldn't.

Connection statistics track traffic volumes, active connections, bandwidth usage, and which rules match most frequently. This information seems mundane until you need it—capacity planning, troubleshooting, or understanding why your network suddenly slowed down.

Security events highlight threats requiring investigation: detected attacks, suspicious patterns, rate limit violations, connections from unexpected geographic locations.

The Log Taxonomy

Different log types serve different purposes. Understanding what each contains helps you know where to look.

Traffic logs record individual connection attempts. Each entry typically includes source and destination addresses, port numbers, protocol, timestamp, action taken, and which rule matched. These logs are verbose—a busy firewall generates millions of entries daily. Most organizations filter to focus on events of interest rather than logging every routine connection.

Security logs track specifically security-relevant events: suspected attacks, intrusion detection alerts, malware detection, authentication failures, configuration changes. Lower volume than traffic logs, higher signal-to-noise ratio.

Performance logs track firewall health: CPU and memory utilization, connection table usage, bandwidth throughput, dropped packets, error conditions. These ensure the firewall itself isn't becoming your bottleneck.

Audit logs record administrative actions: rule modifications, policy changes, administrator logins, configuration backups. When something breaks or gets breached, these logs answer who changed what and when.

The Volume Problem

A busy firewall might generate gigabytes of logs daily. Terabytes over time. Somewhere in those gigabytes is the one connection attempt that matters—the attacker who found a way in, the malware phoning home, the insider exfiltrating data.

The challenge isn't collecting logs. It's finding the signal before the damage is done.

Simply storing this volume requires significant infrastructure. Organizations typically use centralized log management systems or SIEM (Security Information and Event Management) platforms to aggregate, store, and analyze firewall logs alongside other security data.

Retention policies balance historical value against storage costs. Regulatory requirements often mandate minimum retention periods—you can't delete logs you might need for an investigation or audit.

Manual review of millions of log entries isn't feasible. You need tools that surface anomalies and patterns, that know what "normal" looks like and alert when reality diverges.

Two Kinds of Watching

Effective firewall monitoring involves both real-time awareness and historical analysis.

Real-time monitoring provides immediate visibility. Dashboards show current connection counts, bandwidth utilization, blocked attacks, active alerts. This helps you respond to ongoing issues—you see the attack as it happens, not hours later in a report.

Real-time alerts notify you of conditions requiring immediate attention: suspected attacks, unusual traffic spikes, firewall resource exhaustion, critical configuration changes.

Historical analysis reveals patterns invisible in real-time data. Gradually increasing blocked attempts from a specific country. Cyclical traffic patterns indicating legitimate business processes. Slow degradation in firewall performance suggesting approaching capacity limits.

Historical analysis also powers incident investigation. When you discover a breach, examining historical logs helps determine when it started, how the attacker got in, and what other systems might be affected. The firewall was watching the whole time—you just need to read its memory.

Metrics That Matter

Blocked connection attempts indicate what threats the firewall stops. Sudden spikes suggest active attacks. Patterns in source addresses, targeted ports, or timing reveal attack characteristics.

Rule hit counts show how frequently each rule matches. Rules that never match might be obsolete. Rules matching unexpectedly often might indicate misconfiguration or changed traffic patterns.

Connection table utilization tracks how full the firewall's state table is. Approaching capacity causes performance degradation or prevents new legitimate connections.

Bandwidth utilization helps identify DDoS attacks, bandwidth-heavy applications, or capacity planning needs.

Geographic distribution reveals anomalies. If you don't do business in certain countries, connection attempts from those locations warrant attention.

Top talkers shows which hosts generate the most traffic—both legitimate heavy users and potentially compromised systems.

Finding the Anomalies

Some of the most valuable insights come from detecting what's different.

Statistical anomaly detection establishes baselines for normal patterns and alerts when behavior diverges. If a server typically receives 100 connections per hour but suddenly receives 10,000, something changed. Maybe it's legitimate. Maybe it's not. Either way, you should know.

Behavioral anomaly detection looks for unusual connection patterns: an internal workstation suddenly attempting outbound connections on unusual ports (possible malware), external hosts probing many ports sequentially (scanning), connections from geographic locations you've never seen before.

The paradox of firewall monitoring: the most important events often look like nothing. A single outbound connection on port 443 is perfectly normal—unless it's ransomware encrypting your files and sending the key to an attacker. Context is everything.

Correlation and Context

Firewalls don't operate in isolation, and their logs become more valuable when combined with other data.

SIEM platforms aggregate firewall logs with intrusion detection data, antivirus alerts, authentication logs, and application events. This correlation reveals patterns invisible in any single source.

Consider: firewall logs show blocked connection attempts from a specific IP. By themselves, that's routine—firewalls block things constantly. But combined with web server logs showing successful authentication from that same IP address? Now you might have a compromised account being used for reconnaissance. The firewall saw the blocked probes. The web server saw the successful login. Neither told the whole story alone.

Threat intelligence feeds provide context about IP addresses and domains appearing in logs. When your firewall blocks a connection to an IP address known to be malware command-and-control infrastructure, that's not routine blocking—that's a machine on your network trying to phone home to its controller.

Automated Response

Advanced implementations go beyond watching to acting.

Dynamic rule updates can automatically block IP addresses showing attack patterns, adjust rate limits during potential DDoS attacks, or enable additional scrutiny for suspicious traffic.

Security orchestration integration allows firewall monitoring to trigger workflows: creating incident tickets, notifying teams, isolating compromised systems, initiating forensic collection.

Automation helps teams respond faster and more consistently. But it requires careful tuning—false positives triggering automated blocks cause their own kind of damage.

Key Takeaways

• Firewalls log every decision they make—allowed connections, blocked attempts, security events, administrative changes.
• A busy firewall generates gigabytes of logs daily. The challenge is finding the signal that matters.
• Real-time monitoring catches active attacks. Historical analysis reveals patterns and powers investigations.
• Anomaly detection identifies deviations from normal—but the most dangerous events often look routine without context.
• Correlation with other security data transforms firewall logs from noise into narrative.
• The firewall is always watching. The question is whether you're reading what it sees.