The answer is to ensure the policy allows both control and data connections by using the predefined FTP service and to verify that the FTP ALG is enabled on the FortiGate firewall policy. This is required because FTP uses separate control and data channels; the control channel on port 21 negotiates a dynamic data port, which the FTP ALG must inspect to dynamically open the necessary data connection through the firewall. Without the ALG, the data channel would be blocked, breaking the transfer. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your understanding of how application layer gateways handle multi-channel protocols, especially when combined with a VIP that translates the public IP 203.0.113.10 to the private server. A common trap is assuming the standard TCP/21 service alone suffices, but the ALG is essential for the data channel. Memory tip: think "FTP ALG opens both doors—control and data—so your file transfer doesn't get stuck in the lobby."
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.
An admin is configuring a firewall policy to allow FTP traffic from a client to a server. The server is behind a VIP that translates public IP 203.0.113.10 port 21 to private IP 10.0.0.10 port 21. The admin wants to ensure the FTP data channel works correctly. Which TWO additional configurations are required? (Choose two.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable FTP ALG on the firewall policy
FTP uses separate control and data connections. FortiGate's FTP ALG (application layer gateway) or session helper is needed to inspect FTP traffic and handle dynamic data ports. Additionally, for VIP, the policy must allow the appropriate services, and the ALG must be enabled.
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.
✓
Enable FTP ALG on the firewall policy
Why this is correct
FTP ALG inspects FTP traffic and manages data connections.
Related concept
Static NAT maps one inside address to one outside address.
With ALG, one policy is sufficient; the ALG dynamically opens data connections.
✗
Configure a service object for FTP data port (TCP 20)
Why it's wrong here
FTP ALG handles dynamic ports automatically; explicit data port service is not needed.
✓
Ensure the policy allows both control and data connections by using the predefined FTP service
Why this is correct
The predefined FTP service includes both port 21 (control) and port 20 (data) but more importantly triggers the ALG.
Related concept
Static NAT maps one inside address to one outside address.
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: Enable FTP ALG on the firewall policy — FTP uses separate control and data connections. FortiGate's FTP ALG (application layer gateway) or session helper is needed to inspect FTP traffic and handle dynamic data ports. Additionally, for VIP, the policy must allow the appropriate services, and the ALG must be enabled.
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.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Question Discussion
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