NSE4 Firewall Policies and NAT Practice Question
You run the following CLI command on a FortiGate:
# diagnose debug flow filter saddr 192.168.1.10 # diagnose debug flow show function enable # diagnose debug enable
You then initiate a ping from 192.168.1.10 to 8.8.8.8. The output shows 'no matching policy'. What does this indicate?
⚠ Common exam trap
A common mix-up: candidates confuse 'no matching policy' with a routing problem or NAT misconfiguration, but the debug flow output explicitly pinpoints the firewall policy layer as the point of failure, not routing or NAT.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The traffic is dropped by the implicit deny rule
The 'no matching policy' output from the debug flow indicates that the FortiGate evaluated the packet against its firewall policy table and found no explicit policy permitting the traffic from source 192.168.1.10 to destination 8.8.8.8. Since no matching policy exists, the packet is implicitly denied by the default deny-all rule at the end of the policy table, which drops the traffic without logging unless explicitly configured. This is the expected behavior when no permit policy is configured for the traffic flow.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The traffic is being NAT'd but not logged
Why it's wrong here
NAT is applied only after a policy match is found in the FortiGate session table. Since the traffic does not match any firewall policy, it never reaches the NAT stage, so it cannot have been NAT'd. Furthermore, NAT itself does not generate log entries; logging is tied to policy settings, so this option incorrectly attributes the lack of logs to NAT rather than to the absence of a matching policy.
- ✗
The debug filter is incorrectly configured
Why it's wrong here
The debug filter appears correct because it is capturing the traffic of interest; if it were misconfigured, the filter would match no packets at all, and the debug output would be empty. The observed symptom is that traffic is seen but no policy hit is recorded, indicating the filter is functioning as intended. The root cause is not a filter syntax or netmask error, but rather that the traffic does not satisfy any explicit policy's matching criteria, leading to implicit deny.
- ✗
There is a routing issue preventing the traffic
Why it's wrong here
Routing lookups in FortiGate are performed after a policy is matched, not before. If the traffic were dropped due to a routing issue, it would first have to be permitted by a policy; only then would the router check for a valid next hop. Since no policy matches this traffic, it is discarded by the implicit deny before any routing decision is attempted, making a routing problem irrelevant in this scenario.
- ✓
The traffic is dropped by the implicit deny rule
Why this is correct
The implicit deny rule is the final entry in the FortiGate policy table, and it drops any traffic that does not match an explicit allow or deny policy. By default, the implicit deny rule does not generate log entries, which explains the absence of logs in the output. The diagnose flow would show the packet count incrementing at this rule, confirming that the traffic is silently dropped here. This is the correct and most common reason for traffic failing to pass through a FortiGate when policies appear to be missing.
Visual reference
Go deeper
Related to this question
About these practice questions
Courseiva writes every NSE4 question from scratch — 773 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.