NSE4 Firewall Policies and NAT Practice Question
What is the purpose of the 'implicit deny' policy on a FortiGate?
⚠ Common exam trap
A common mix-up: candidates confuse the implicit deny with a logging or allow action, or assume it behaves like a default permit, when in fact it silently drops all unmatched traffic without logging unless explicitly configured.
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
✓
It denies all traffic that does not match any explicit policy
The 'implicit deny' policy on a FortiGate is a default, last-resort rule that denies all traffic not matching any explicit firewall policy. It ensures that any packet that does not meet the source, destination, service, or schedule criteria of a configured policy is dropped, enforcing a default-deny security posture. This behavior is fundamental to stateful firewall operation and prevents unauthorized traffic from traversing the device.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It allows traffic from trusted internal networks
Why it's wrong here
Trusted internal networks are permitted by explicit firewall policies that specify source, destination, and service parameters. The implicit deny rule sits at the very end of the FortiGate policy table and never allows traffic based on trust level or interface zone. Without a matching explicit allow policy, even traffic from a trusted internal source is discarded, so this option misdescribes the implicit deny's function.
- ✓
It denies all traffic that does not match any explicit policy
Why this is correct
The implicit deny is a built-in fallback rule that FortiGate automatically applies to any session that does not match an earlier explicit firewall policy. It performs a drop/deny action, ensuring default-deny security so unknown traffic cannot traverse the device. Because it is the last rule, it is never explicitly shown in the policy list but is conceptually always present.
- ✗
It logs all traffic that is denied
Why it's wrong here
Although the implicit deny drops packets, logging is a separate feature that must be explicitly enabled, either on a custom deny policy or via global session log settings. By default, FortiGate does not write a log entry for packets rejected solely by the implicit deny, so it cannot be described as logging all denied traffic. The rule's primary purpose is to block, not to audit.
- ✗
It allows all traffic that matches no other policy
Why it's wrong here
This is the inverse of the actual behavior: the implicit deny is a catch-all that denies, not allows, any traffic lacking a matching explicit policy. Permitting unmatched traffic would make the firewall default-allow, defeating the security boundary and exposing protected networks to unknown threats. FortiGate's policy evaluation ends with this rule, which forces strict enforcement rather than open access.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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