The correct answer is to configure a Destination NAT rule and a security policy rule allowing SMTP from external to DMZ. Destination NAT is required because the mail server in the DMZ uses a private IP address, so the firewall must translate the public IP on its external interface to that private address to route inbound SMTP traffic correctly. Without this translation, the firewall has no way to know which internal server should receive the connection. The separate security policy rule is equally critical, as it explicitly permits SMTP (TCP port 25) from the external zone to the DMZ zone; even with NAT configured, the firewall’s default deny policy would block the traffic otherwise. On the PCNSA exam, this scenario tests your understanding that NAT and security rules are independent but must work together—a common trap is thinking NAT alone handles access. Remember the mnemonic: “NAT navigates, policy permits.”
PCNSA Securing Traffic Practice Question
This PCNSA practice question tests your understanding of securing traffic. 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 administrator needs to allow inbound SMTP traffic to a mail server located in the DMZ. The firewall has a public IP address on the external interface. Which configuration is necessary to ensure the mail server receives the traffic?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure a Destination NAT rule and a security policy rule allowing SMTP from external to DMZ.
To allow inbound SMTP traffic from the internet to a mail server in the DMZ, the firewall must perform Destination NAT (DNAT) to translate the public IP address on the external interface to the private IP address of the mail server. A corresponding security policy rule must permit SMTP (TCP port 25) traffic from the external zone to the DMZ zone. Without DNAT, the firewall would not know which internal server should receive the traffic, and without the security rule, the traffic would be blocked.
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.
✗
Configure a Source NAT rule to translate the mail server's IP to the public IP.
Configure a Destination NAT rule and a security policy rule allowing SMTP from external to DMZ.
Why this is correct
Destination NAT translates the public IP to the private IP, and the policy allows the traffic.
Related concept
Read the scenario before looking for a memorised answer.
✗
Configure a security policy rule with source NAT to translate the public IP to the private IP.
Why it's wrong here
Source NAT is not used for inbound traffic; it changes source IP.
✗
Configure a security policy rule allowing SMTP from external to DMZ without NAT.
Why it's wrong here
Without NAT, the destination IP is the public IP, but the mail server has a private IP; traffic won't reach it.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse Source NAT with Destination NAT, assuming any NAT rule will work, or they think a security policy alone is sufficient without understanding that NAT is required to route the traffic to the internal server.
Detailed technical explanation
How to think about this question
Destination NAT (also called port forwarding or server publishing) works by rewriting the destination IP address (and optionally port) of incoming packets at the firewall. For SMTP, the firewall typically inspects TCP port 25 and maps the public IP to the mail server's private IP. A common subtlety is that the security policy rule must reference the original destination (public IP) or the post-NAT destination (private IP) depending on the firewall's rule order; in Palo Alto Networks firewalls, the security rule is evaluated after NAT, so the destination zone and IP should reflect the post-NAT state (DMZ zone and private IP).
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 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Securing Traffic — This question tests Securing Traffic — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure a Destination NAT rule and a security policy rule allowing SMTP from external to DMZ. — To allow inbound SMTP traffic from the internet to a mail server in the DMZ, the firewall must perform Destination NAT (DNAT) to translate the public IP address on the external interface to the private IP address of the mail server. A corresponding security policy rule must permit SMTP (TCP port 25) traffic from the external zone to the DMZ zone. Without DNAT, the firewall would not know which internal server should receive the traffic, and without the security rule, the traffic would be blocked.
What should I do if I get this PCNSA question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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