The answer is to review the firewall policies that apply to the traffic and modify them as needed. This is correct because the "Deny by policy" message in a FortiGate diagnose debug flow output explicitly indicates that the packet matched an implicit or explicit firewall policy with an action set to deny, rather than being dropped due to a routing issue or session table problem. On the Fortinet NSE 4 Network Security Professional exam, this scenario tests your ability to interpret debug flow traces and correlate them to policy configuration, a common troubleshooting skill where candidates mistakenly look at routing or NAT first. A frequent trap is assuming "deny" means a network error, but the debug flow is policy-driven, so the immediate next step is identifying the specific policy ID in the trace and checking its source, destination, and schedule. Memory tip: "Deny by policy" means check the policy, not the packet—focus on the rule, not the route.
NSE4 High Availability and Diagnostics Practice Question
This NSE4 practice question tests your understanding of high availability and diagnostics. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An administrator executes 'diagnose debug flow' for a specific session and sees the output: 'id=20085 trace_id=10 func=print_pkt_detail line=5567 msg="vd-root:0 received packet via port1".' Later, the trace shows 'msg="Deny by policy"'. What is the most likely next step the administrator should take?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Review the firewall policies that apply to the traffic and modify as needed
The debug flow indicates the packet is denied by a firewall policy. The admin should identify which policy is blocking it and adjust accordingly.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Check the routing table for the destination
Why it's wrong here
The packet was already received and denied by policy, not routing.
✓
Review the firewall policies that apply to the traffic and modify as needed
Why this is correct
The debug clearly states 'Deny by policy', so the solution is to adjust policy.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
Restart the FortiGate to clear session table
Why it's wrong here
Restarting is drastic and won't fix the policy configuration.
✗
Enable session helper for the protocol
Why it's wrong here
Session helpers are for ALG handling; denial by policy is not a session helper issue.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
High Availability and Diagnostics — This question tests High Availability and Diagnostics — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Review the firewall policies that apply to the traffic and modify as needed — The debug flow indicates the packet is denied by a firewall policy. The admin should identify which policy is blocking it and adjust accordingly.
What should I do if I get this NSE4 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A FortiGate administrator runs 'diagnose debug flow' with a filter for a specific source IP. The output shows 'no policy matched' for the traffic. The administrator verifies that a firewall policy exists with that source IP. What is the most likely reason for the 'no policy matched' message?
hard
A.The firewall policy is disabled
B.The debug flow filter is not configured correctly
C.The source IP is not in the routing table
✓ D.The traffic is being blocked by an implicit deny or a different policy before reaching the expected policy
Why D: Option B is correct. Debug flow may show 'no policy matched' when traffic hits an implicit deny before reaching the explicit policy, e.g., due to inter-VDOM routing or policy order where an earlier policy with a broader match blocks it.
Variation 2. A FortiGate administrator runs 'diagnose debug flow' and sees the output 'FW-6: packet is allowed by policy' but the packet is still dropped. What additional debug information should the administrator check to determine why the packet is dropped after being allowed?
hard
A.Check the traffic log for the session
✓ B.Enable 'diagnose debug flow show function-name' to see more detailed stages
C.Check the session table for the packet
D.Run 'diagnose sniffer packet' to capture the packet
Why B: After policy lookup, further processing like security profiles, NAT, or routing may drop the packet. The 'function-name' parameter in debug flow shows deeper inspection stages.
Variation 3. An administrator runs 'diagnose debug flow' and sees the output 'no matching policy'. What does this indicate?
medium
A.The packet is being processed by a policy with the correct source/destination
✓ B.There is no firewall policy that allows the traffic from the source to the destination
C.The packet was dropped due to an antivirus signature match
D.The packet is being routed through a blackhole route
Why B: The debug flow trace shows packets flowing through the firewall policy evaluation. 'No matching policy' means the packet did not match any firewall policy.
Variation 4. An administrator runs 'diagnose debug flow' for a specific policy and sees the following output:
id=20085 trace_id=10 func=vf_ip_route_in msg='No matching interface to route packet'
What does this indicate?
hard
A.The packet is being blocked by a firewall policy
B.The source interface is down
✓ C.The destination IP address has no matching route in the routing table
D.The session table is full
Why C: The trace indicates that FortiGate cannot find a route to forward the packet, meaning the destination is unreachable.
Last reviewed: Jun 21, 2026
Question Discussion
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This NSE4 practice question is part of Courseiva's free Fortinet certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the NSE4 exam.
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