Troubleshooting Mindset Series #002: How I Read Firewall Logs Before Touching Any Policy

One lesson I've learned over the years is this:

Firewall logs should guide your investigation—not your assumptions.

When users report that an application is inaccessible, many administrators immediately start creating new firewall policies or modifying existing ones.

I prefer a different approach.

Before making any policy changes, I always read the firewall logs first.

Step 1 – Confirm the Reported Issue

Before opening the firewall console, I ask a few simple questions:

• What is the source IP?
  
  
• What is the destination IP or FQDN?
  
  
• Which application or service is affected?
  
  
• Which port or protocol is being used?
  
  
• When did the issue start?
  
  

Having accurate information makes log analysis much more efficient.

Step 2 – Search for Matching Traffic

Rather than reviewing every log entry, I filter by:

• Source IP
  
  
• Destination IP
  
  
• Service/Port
  
  
• Time Range
  
  

This helps isolate the exact session instead of searching through thousands of records.

Step 3 – Look at the Action

This is the first field I pay attention to.

Is the traffic:

• Allowed?
  
  
• Denied?
  
  
• Dropped?
  
  
• Reset?
  
  

Understanding the action immediately narrows down the investigation.

A denied session suggests a policy issue.

An allowed session often means the problem exists elsewhere.

Step 4 – Identify Which Policy Was Matched

If the traffic is allowed, I verify:

• Which security policy matched?
  
  
• Is it the expected rule?
  
  
• Is NAT being applied correctly?
  
  
• Are security profiles affecting the session?
  
  

This helps determine whether the traffic followed the intended path.

Step 5 – Don't Stop at "Allowed"

One mistake I often see is assuming that an Allowed log means everything is working.

An allowed session only confirms that the firewall forwarded the traffic.

The issue could still be:

• DNS resolution
  
  
• Server availability
  
  
• Routing
  
  
• Application failure
  
  
• ISP connectivity
  
  

The firewall may simply be doing exactly what it was configured to do.

My Rule

I never modify a firewall policy unless I can answer these two questions:

✔ Which log entry proves the issue?

✔ What evidence supports the configuration change?

If I can't answer those questions, I continue investigating.

Lessons Learned

The firewall log is one of the most valuable troubleshooting tools available.

Reading the logs carefully before making changes not only reduces troubleshooting time but also prevents unnecessary configuration changes that could introduce new issues.

Logs provide facts.

Assumptions create risks.

Discussion

When investigating a firewall issue, which log field do you check first?

• Action
  
  
• Source IP
  
  
• Destination IP
  
  
• Matched Policy
  
  
• NAT Information
  
  
• Security Event
  
  

I'm interested to learn how other engineers analyze firewall logs during troubleshooting.

Troubleshooting Mindset

Good engineers don't start by changing configurations.

They start by collecting evidence.

Engineering Takeaway

Every firewall log tells a story.

Your job is to understand the story before changing the ending.

Hi,

The best troubleshooting is based on evidence. Logs will tell you what happened before deciding why. The best firewall engineers don't start by changing policies—they start by understanding the traffic.