The answer is a missing destination NAT (VIP) configuration for the web server. When you see SYN_SENT sessions in a session list, it means the FortiGate has sent a SYN packet to the server but never received a SYN-ACK in return, which directly indicates the server is unreachable or not responding. In this scenario, the firewall policy permits TCP/443, so the issue isn’t a blocked policy; instead, the FortiGate is likely forwarding the packet with the original destination IP, which never reaches the actual web server because no DNAT/VIP translates the public IP to the private server IP. On the Fortinet NSE 4 exam, this tests your understanding of how stateful inspection tracks session states and the critical role of VIP objects in inbound traffic flows. A common trap is to assume a policy issue when the real culprit is a missing NAT rule. Memory tip: “SYN_SENT = Sent, no answer; check your VIP translator.”
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. 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.
A network admin runs 'diag sys session filter proto 6' and 'diag sys session list' and sees many sessions with state 'SYN_SENT' to a public web server. The firewall policy allows TCP/443. 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.
The web server is overloaded and dropping connections
Why wrong: Could be, but the question asks for the most likely cause in the FortiGate context; a missing DNAT is more specific to firewall configuration.
B
The policy is in proxy mode but should be flow mode
Why wrong: Proxy vs flow mode affects inspection depth, not basic session establishment.
C
The destination NAT (VIP) for the web server is not configured
Without a VIP, the firewall does not translate the destination IP to the internal server, so the server never receives the request.
D
The firewall policy has session TTL set too low
Why wrong: Session TTL would affect established sessions, not initial SYN_SENT.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The destination NAT (VIP) for the web server is not configured
SYN_SENT indicates that the FortiGate has sent a SYN but not received a SYN-ACK, suggesting the server is not responding, possibly due to a missing DNAT or VIP configuration for inbound traffic.
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.
✗
The web server is overloaded and dropping connections
Why it's wrong here
Could be, but the question asks for the most likely cause in the FortiGate context; a missing DNAT is more specific to firewall configuration.
✗
The policy is in proxy mode but should be flow mode
Why it's wrong here
Proxy vs flow mode affects inspection depth, not basic session establishment.
✓
The destination NAT (VIP) for the web server is not configured
Why this is correct
Without a VIP, the firewall does not translate the destination IP to the internal server, so the server never receives the request.
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.
✗
The firewall policy has session TTL set too low
Why it's wrong here
Session TTL would affect established sessions, not initial SYN_SENT.
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.
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The destination NAT (VIP) for the web server is not configured — SYN_SENT indicates that the FortiGate has sent a SYN but not received a SYN-ACK, suggesting the server is not responding, possibly due to a missing DNAT or VIP configuration for inbound traffic.
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.
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