Source NAT Requirement — Why Sessions Created but No Response
A firewall is configured with a destination NAT rule to translate public IP 203.0.113.10 to internal server 10.0.0.5 on port 443. Internal users from 10.0.0.0/24 can access the server using its private IP, but cannot access using the public IP. What should be configured to allow internal users to reach the server using the public IP?
⚠ Common exam trap
Many candidates think a security policy or route is sufficient, but they miss the fundamental requirement for symmetric routing in stateful firewalls, where the return traffic must traverse the same firewall that performed the NAT.
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 source NAT rule that translates the internal source IP to the firewall's interface IP when the destination is the public IP.
When internal users send traffic to the public IP (203.0.113.10), the firewall performs destination NAT, translating the destination to 10.0.0.5. However, the return traffic from the server is sent directly to the internal user's IP (since they are on the same subnet), bypassing the firewall and causing asymmetric routing. A source NAT rule (often called NAT hairpin or NAT reflection) translates the internal source IP to the firewall's interface IP, forcing return traffic to go through the firewall and maintain session state.
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 that translates the internal source IP to the firewall's interface IP when the destination is the public IP.
Why this is correct
Internal clients hitting the public IP create a flow where the server replies directly to the private source, bypassing the firewall and breaking the session. Source NAT rewrites the source to the firewall interface IP, forcing return traffic back through the firewall for correct translation.
- ✗
Create a policy-based forwarding (PBF) rule to send the traffic to the server.
Why it's wrong here
PBF alters routing based on policy match conditions; it does not perform the source NAT required so that return traffic from 10.0.0.5 reaches internal clients. The correct fix is a U-turn NAT rule translating the internal clients' source addresses. PBF would be right for steering traffic by application or user criteria.
- ✗
Add a security policy allowing traffic from internal zone to the public IP.
Why it's wrong here
The security policy already permits the flow; the failure is that internal traffic never matches the destination NAT rule, so no translation occurs. A security policy is the right fix when traffic is dropped by an implicit deny, not when NAT itself is bypassed.
- ✗
Add a static route on the firewall for the public IP pointing to the internal server.
Why it's wrong here
A static route cannot rewrite the destination address; the packet still carries 203.0.113.10 and the firewall forwards it untranslated, so the server never sees it. Static routes are for reaching subnets, not for triggering destination NAT on traffic originating internally.
Visual reference
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCNSE 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 PCNSE exam.