- A
Configure NAT traversal to translate the dynamic ports to static ones.
Why wrong: NAT traversal deals with IPsec or NAT, not application-layer dynamic port negotiation; it won't resolve the issue.
- B
Create a firewall rule allowing all traffic from the application server to the client subnets.
Why wrong: This bypasses micro-segmentation entirely, reintroducing lateral movement risk.
- C
Disable the host-based firewall on the application server to eliminate connectivity issues.
Why wrong: This removes all protection from the server, making it vulnerable to compromise.
- D
Deploy a reverse proxy in front of the application that performs deep packet inspection to negotiate and manage dynamic port allocations.
The proxy can understand the proprietary protocol and adjust firewall rules dynamically or relay traffic securely, maintaining segmentation.
Quick Answer
The answer is deploying a reverse proxy with deep packet inspection (DPI) to negotiate and manage dynamic port allocations. This is correct because the reverse proxy intercepts the legacy application’s proprietary protocol, maps the ephemeral dynamic ports (49152-65535) to static, predictable ports, and transparently manages connections without modifying application code or host firewall rules, thereby preserving zero-trust micro-segmentation. On the CISSP exam, this scenario tests your understanding of how to handle legacy applications with dynamic ports in a zero-trust network, often appearing as a trick where candidates mistakenly choose to widen firewall ranges—which violates zero-trust principles—or attempt protocol rewriting. The key trap is remembering that a reverse proxy acts as a protocol-aware intermediary, not just a traffic forwarder. Memory tip: “Proxy parses ports” — the proxy parses the dynamic port negotiation to keep zero-trust strict.
CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and network security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 financial institution is implementing a zero-trust network architecture (ZTNA) using micro-segmentation. They have a legacy accounting application that runs on a Windows Server and communicates with multiple client workstations using both TCP and UDP dynamic ports (49152-65535) for various features. After deploying strict host-based firewall rules that only allow specific ports, users report that the application frequently loses connection and fails to authenticate. The security team verified that the application's required ports are allowed, but the dynamic port negotiation fails because the application uses a proprietary protocol that includes ephemeral ports outside the allowed range. The application vendor is no longer supporting it. The organization cannot replace the application immediately. What is the MOST effective short-term solution?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"immediately / without restart"Why it matters: Time or reboot constraint — the correct answer must take effect right away without requiring a reboot or reload.
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
Deploy a reverse proxy in front of the application that performs deep packet inspection to negotiate and manage dynamic port allocations.
Option D is correct because deploying a reverse proxy with deep packet inspection (DPI) can intercept the proprietary protocol's dynamic port negotiation, map the ephemeral ports (49152-65535) to static, predictable ports, and manage the connections transparently. This allows the legacy application to function without modifying its code or the host firewall rules, providing an immediate workaround while maintaining zero-trust micro-segmentation principles.
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 NAT traversal to translate the dynamic ports to static ones.
- ✗
Create a firewall rule allowing all traffic from the application server to the client subnets.
Why it's wrong here
This bypasses micro-segmentation entirely, reintroducing lateral movement risk.
- ✗
Disable the host-based firewall on the application server to eliminate connectivity issues.
Why it's wrong here
This removes all protection from the server, making it vulnerable to compromise.
- ✓
Deploy a reverse proxy in front of the application that performs deep packet inspection to negotiate and manage dynamic port allocations.
Why this is correct
The proxy can understand the proprietary protocol and adjust firewall rules dynamically or relay traffic securely, maintaining segmentation.
Clue confirmation
The clue word "immediately / without restart" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume disabling the firewall (Option C) or opening broad rules (Option B) is acceptable for legacy compatibility, but the CISSP exam emphasizes that security controls must be preserved even during workarounds, and a reverse proxy with DPI is the correct architectural solution to handle dynamic port protocols.
Detailed technical explanation
How to think about this question
In zero-trust micro-segmentation, host-based firewalls enforce allowlists for specific ports and protocols. The legacy application's proprietary protocol uses dynamic port negotiation (similar to FTP's PASV mode but with ephemeral ports in the 49152–65535 range), which the firewall cannot predict. A reverse proxy with DPI can parse the application-layer negotiation, rewrite the port assignments, and maintain a stateful mapping, effectively acting as an application-layer gateway (ALG) without requiring vendor support.
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 developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach 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.
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Communication and Network Security — study guide chapter
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FAQ
Questions learners often ask
What does this CISSP question test?
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Deploy a reverse proxy in front of the application that performs deep packet inspection to negotiate and manage dynamic port allocations. — Option D is correct because deploying a reverse proxy with deep packet inspection (DPI) can intercept the proprietary protocol's dynamic port negotiation, map the ephemeral ports (49152-65535) to static, predictable ports, and manage the connections transparently. This allows the legacy application to function without modifying its code or the host firewall rules, providing an immediate workaround while maintaining zero-trust micro-segmentation principles.
What should I do if I get this CISSP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "immediately / without restart". Time or reboot constraint — the correct answer must take effect right away without requiring a reboot or reload.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This CISSP practice question is part of Courseiva's free ISC2 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 CISSP exam.
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