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Troubleshooting Mindset Series #022: When Firewall Policy Looks Correct but Traffic Is Still Blocked
  

rizzuan Lv2  Posted 2026-Oct-15 16:29

Last edited by rizzuan 2026-Oct-15 17:37.


1. Scenario / Issue Description
A user reports that they cannot access a server or application through the firewall.
The administrator checks the firewall policy and confirms that:
  • The source IP looks correct.
  • The destination IP is correct.
  • The required service/port is allowed.
  • The policy appears to be enabled.


Yet, the connection still fails.
The first reaction might be to modify the existing policy or create another allow rule. But before making changes, I would ask:
Is the traffic matching the expected policy, and can the complete connection path work in both directions?

2. Troubleshooting Approach

Step 1: Verify the Actual Traffic
Confirm the source IP, destination IP, protocol, destination port, and the exact time of the failed connection.
Don't rely only on the information provided by the user. Check the actual traffic details where possible.
Step 2: Check Policy Matching
Review the relevant security policy and verify:
  • Source and destination objects
  • Incoming and outgoing interfaces or zones
  • Service and protocol
  • Policy order and rule precedence
  • Policy hit counters and traffic logs, where available


A policy can look correct but still fail to match the actual traffic.

Step 3: Check the Routing Path
Verify that the firewall has a valid route to the destination.
Also check the return path. The request may reach the destination, but the response might use a different gateway or fail to return through the expected path.

Step 4: Verify NAT and Address Translation
If NAT is involved, confirm that the correct translation rule applies to the traffic.
Check whether the translated source or destination address matches what the downstream server or network expects.

Step 5: Follow the Logs and Session
Check the firewall logs for the specific connection attempt.
Look for evidence of:
  • Policy denial
  • Unexpected policy matching
  • Routing or forwarding problems
  • NAT mismatch
  • Session or connection failure

If the logs do not clearly identify the cause, use packet captures at appropriate points along the path to determine where the traffic stops.

3. Common Mistake
Adding another broad allow rule without confirming why the existing policy failed.
This may introduce unnecessary security exposure while leaving the actual problem unresolved.

4. Troubleshooting Mindset
When a firewall policy looks correct but traffic is still blocked, don't immediately assume the policy itself is wrong.
Follow the evidence:
Actual Traffic → Policy Match → Routing → NAT → Return Path
Each step should help narrow down the possible cause before changing the configuration.

5. Key TakeawayA correct-looking firewall policy does not guarantee a successful connection.
The objective is not simply to make the traffic pass. It is to identify exactly where the connection fails and fix the underlying cause without weakening the security policy.
#Sangfor #AthenaNGFW #Firewall #Networking #Troubleshooting #TroubleshootingMindset

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Humayun Ahmed Lv4  Posted 2026-Oct-15 17:51
  
Thanks to share!