How to Identify the Implicit Deny Policy in FortiGate Diagnose Output
An admin runs the command 'diagnose firewall iprope list 100000' and sees the following output: id=2000000000 action=deny flag=0x0 src-interface=any dst-interface=any proto=0 src-addr=0.0.0.0-255.255.255.255 dst-addr=0.0.0.0-255.255.255.255 What does this entry represent?
Quick Answer
The answer is the implicit deny policy at the end of the policy list. This entry is identified by its reserved ID of 2000000000, which FortiGate uses specifically for the default deny rule that matches all traffic from any source to any destination, as shown by the src-addr and dst-addr ranges of 0.0.0.0-255.255.255.255. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this question tests your ability to interpret diagnose firewall iprope output and distinguish the implicit deny from user-created policies; a common trap is confusing the ID 2000000000 with a normal policy ID, but remember that only the implicit deny uses this specific high-numbered identifier. To lock it in, use the mnemonic: "Two billion denies all, the last rule to fall."
⚠ Common exam trap
Many candidates confuse the high ID (2000000000) with a user-created policy or think it's a NAT rule, but FortiGate reserves this ID specifically for the implicit deny, which is automatically generated and cannot be manually created or removed.
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 implicit deny policy at the end of the policy list
The output shows an entry with id=2000000000, action=deny, and source/destination addresses covering all possible IPs (0.0.0.0-255.255.255.255). In FortiGate, the implicit deny policy is automatically inserted at the end of the policy list with a high ID (typically 2000000000) and matches any traffic that hasn't been permitted by earlier policies. This is not a user-created policy but the built-in default deny rule.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A loopback interface policy
Why it's wrong here
The entry matches all interfaces and addresses, so it cannot be a loopback interface policy, which would reference the loopback interface specifically. It is tempting because loopback policies also use deny actions, but a loopback policy would be correct when traffic to or from the loopback interface needs explicit control.
- ✓
The implicit deny policy at the end of the policy list
Why this is correct
The all-zero source and destination with proto=0 and deny action is the default drop rule FortiGate applies after all configured policies are evaluated. Its id=2000000000 confirms it is the final implicit deny, not an admin-created policy.
- ✗
A user-created deny policy that blocks all traffic
Why it's wrong here
This is the implicit deny rule automatically present at the end of the IP rope, not a user-created policy; user policies carry distinct IDs and appear above it. It is tempting because the action is deny across all addresses, but a user-created deny would be correct when an administrator deliberately blocks specific traffic.
- ✗
A NAT policy that translates all addresses
Why it's wrong here
The entry is an implicit deny rule, not a NAT policy; NAT is configured under firewall policy with translation objects, and this output shows no translation fields. It is tempting because the all-address range resembles a wildcard translation, but NAT would be correct when source or destination addresses must be rewritten.
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Same concept, more angles
1 more way this is tested on NSE4
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. An administrator runs 'diagnose firewall iprope list 100000' and sees 'action=deny' entries for traffic that should be allowed. The policy list shows an allow policy with ID 1 for that traffic. What is the most likely cause of the deny?
medium- A.The traffic is being blocked by a local-in policy
- B.The implicit deny rule is being triggered because the policy is disabled
- ✓ C.The firewall policy is not installed in the kernel due to an error
- D.A security profile is dropping the traffic after the policy matches
Why C: The 'diagnose firewall iprope list 100000' command displays the kernel-level firewall policy list. If the policy list shows an allow policy (ID 1) but the kernel entries show 'action=deny', it indicates that the policy was not successfully installed into the kernel's connection tracking or firewall engine. This typically occurs due to a policy installation error, such as a configuration inconsistency or a failure during the commit process, causing the kernel to fall back to a default deny action for that traffic.
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.