PCNSA Securing Traffic Practice Question
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?
⚠ Common exam trap
A common mix-up: candidates 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.
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
✓
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.
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.
Why it's wrong here
Source NAT rewrites the mail server's private address to the public IP on outbound flows, which does nothing for unsolicited inbound SMTP arriving at the firewall's public address. It is the right choice when internal DMZ hosts must initiate outbound connections appearing as the public IP.
- ✓
Configure a Destination NAT rule and a security policy rule allowing SMTP from external to DMZ.
Why this is correct
The mail server holds a private DMZ address, so inbound SMTP to the firewall's public IP requires destination NAT to translate the public address to the server, plus a security policy permitting SMTP from the external zone to the DMZ zone.
- ✗
Configure a security policy rule with source NAT to translate the public IP to the private IP.
Why it's wrong here
Destination NAT, not source NAT, rewrites the inbound packet's destination from the public IP to the mail server's private address; source NAT alters the sender and cannot deliver inbound SMTP. Source NAT is correct for outbound internet access from internal hosts, where hiding private addresses is the requirement.
- ✗
Configure a security policy rule allowing SMTP from external to DMZ without NAT.
Why it's wrong here
Without destination NAT, the inbound packet still carries the firewall's public IP as its destination, so the DMZ mail server never receives it; a permit rule alone cannot redirect traffic. A no-NAT policy is correct when the mail server already holds a routable public address.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCNSA practice question is part of Courseiva's free Palo Alto Networks 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 PCNSA exam.