The answer is TCP SYN flood protection dropping the incomplete session. This is the most likely cause because the debug flow output shows the session stuck in a SYN_RECV state, never transitioning to ESTABLISHED, which is the hallmark of SYN flood mitigation in action. Even with an explicit allow policy permitting HTTP traffic from 10.0.1.5 to 10.0.2.100, FortiGate’s SYN flood protection will preemptively drop incomplete sessions once the configured threshold is exceeded, preventing the three-way handshake from completing. On the Fortinet NSE 7 Advanced Security NSE7 exam, this scenario tests your ability to distinguish between policy-based blocks and DoS protection mechanisms; a common trap is assuming a misconfigured policy when the real culprit is a security profile threshold. Remember the mnemonic “SYN stuck, check the flood clock”—if sessions linger in SYN_RECV, look to your SYN flood settings before touching the firewall rules.
NSE7 Troubleshooting and Diagnostics Practice Question
This NSE7 practice question tests your understanding of troubleshooting and diagnostics. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Exhibit
Refer to the exhibit.
config firewall policy
edit 1
set name "Allow-Web"
set srcintf "port1"
set dstintf "port2"
set srcaddr "10.0.1.0/24"
set dstaddr "10.0.2.100"
set action accept
set schedule "always"
set service "HTTP"
set logtraffic all
next
end
diag debug flow show function-name show-verbose
--- flow debug output ---
proton_state=0, reason=session-denied
id=20085 trace_id=155 func=print_pkt_detail line=4945 msg="vd-root:0 received a packet from port1: 10.0.1.5:45231 -> 10.0.2.100:80, proto 6."
id=20085 trace_id=155 func=resolve_ip_tuple line=4125 msg="Find an existing session, id 00001234, original direction"
id=20085 trace_id=155 func=__ip_session_match_tuple line=2818 msg="Session state: not ready"
id=20085 trace_id=155 func=__ip_session_find_by_session_id line=2773 msg="session session_deny because state proto is not ready"
A FortiGate is blocking HTTP traffic from 10.0.1.5 to 10.0.2.100, despite an explicit allow policy. The exhibit shows the configuration and debug flow output. What is the most likely cause?
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.
Refer to the exhibit.
config firewall policy
edit 1
set name "Allow-Web"
set srcintf "port1"
set dstintf "port2"
set srcaddr "10.0.1.0/24"
set dstaddr "10.0.2.100"
set action accept
set schedule "always"
set service "HTTP"
set logtraffic all
next
end
diag debug flow show function-name show-verbose
--- flow debug output ---
proton_state=0, reason=session-denied
id=20085 trace_id=155 func=print_pkt_detail line=4945 msg="vd-root:0 received a packet from port1: 10.0.1.5:45231 -> 10.0.2.100:80, proto 6."
id=20085 trace_id=155 func=resolve_ip_tuple line=4125 msg="Find an existing session, id 00001234, original direction"
id=20085 trace_id=155 func=__ip_session_match_tuple line=2818 msg="Session state: not ready"
id=20085 trace_id=155 func=__ip_session_find_by_session_id line=2773 msg="session session_deny because state proto is not ready"
A
The policy is applied to the wrong source interface.
Why wrong: The debug shows the packet received on port1, which matches the policy.
B
The policy action is set to deny.
Why wrong: Exhibit shows action accept.
C
TCP SYN flood protection is dropping the incomplete session.
The 'state proto not ready' indicates TCP handshake incomplete; often due to DoS protection thresholds.
D
The source address object does not include 10.0.1.5.
Why wrong: The srcaddr is 10.0.1.0/24 which includes 10.0.1.5.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
TCP SYN flood protection is dropping the incomplete session.
The debug flow output shows the session is in a 'SYN_RECV' state and never transitions to 'ESTABLISHED', which is characteristic of TCP SYN flood protection. When the FortiGate's SYN flood protection threshold is exceeded, it drops incomplete sessions before they can be fully established, even if an explicit allow policy exists. This explains why HTTP traffic from 10.0.1.5 to 10.0.2.100 is blocked despite the policy being correctly configured.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 policy is applied to the wrong source interface.
Why it's wrong here
The debug shows the packet received on port1, which matches the policy.
✗
The policy action is set to deny.
Why it's wrong here
Exhibit shows action accept.
✓
TCP SYN flood protection is dropping the incomplete session.
Why this is correct
The 'state proto not ready' indicates TCP handshake incomplete; often due to DoS protection thresholds.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The source address object does not include 10.0.1.5.
Why it's wrong here
The srcaddr is 10.0.1.0/24 which includes 10.0.1.5.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a policy issue (wrong interface, wrong action, or wrong address object) when the debug flow shows a session being created but not completing, but the real cause is a DoS protection mechanism that drops the session after the initial SYN.
Trap categories for this question
Command / output trap
The debug shows the packet received on port1, which matches the policy.
Detailed technical explanation
How to think about this question
FortiGate's DoS policy (config firewall dos-policy) can apply TCP SYN flood protection at the interface level, using a threshold (default 2000 sessions per second) and an action (proxy or block). When the threshold is exceeded, the FortiGate sends a SYN-ACK on behalf of the server and waits for the client's ACK; if the ACK is not received within the timeout (default 10 seconds), the session is dropped. In real-world scenarios, this can occur during a legitimate traffic spike or a slow SYN flood attack, and the debug flow output will show the session in SYN_RECV without progressing to ESTABLISHED.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Troubleshooting and Diagnostics — This question tests Troubleshooting and Diagnostics — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: TCP SYN flood protection is dropping the incomplete session. — The debug flow output shows the session is in a 'SYN_RECV' state and never transitions to 'ESTABLISHED', which is characteristic of TCP SYN flood protection. When the FortiGate's SYN flood protection threshold is exceeded, it drops incomplete sessions before they can be fully established, even if an explicit allow policy exists. This explains why HTTP traffic from 10.0.1.5 to 10.0.2.100 is blocked despite the policy being correctly configured.
What should I do if I get this NSE7 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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