Quick Answer
The correct answer is to set firewall rules that allow only traffic from IAP proxy IP ranges. This works because Identity-Aware Proxy uses a fixed set of Google-owned IP ranges to forward authenticated and authorized requests to your backend Compute Engine instances; by restricting ingress in your VPC firewall to those specific ranges, you ensure that any traffic not originating from IAP is blocked at the network level. On the Google Professional Cloud Security Engineer exam, this scenario tests your understanding of how IAP integrates with VPC firewall controls to enforce zero-trust access—a common trap is confusing IAP’s TCP forwarding with load balancer source IPs, but remember that IAP uses its own dedicated proxy ranges, not the load balancer’s. A helpful memory tip: think of IAP as a bouncer at a private club—only guests arriving from the bouncer’s approved limo service (the IAP proxy IPs) get past the door, while all other traffic is turned away.
This PCSE practice question tests your understanding of configuring access within a cloud solution environment. 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.
A security administrator wants to ensure that only requests coming through Identity-Aware Proxy (IAP) can access a backend service running on Compute Engine. Which configuration is required?
Use a VPN between IAP and backend.
Why wrong: IAP does not require a VPN.
Configure Cloud Armor to block non-IAP traffic.
Why wrong: Cloud Armor can filter traffic but firewall rules are simpler.
Configure the backend service to require IAP credentials.
Why wrong: IAP credentials are validated at the proxy, not the backend.
Set firewall rules to allow only traffic from IAP proxy IP ranges.
This restricts access to only IAP requests.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Set firewall rules to allow only traffic from IAP proxy IP ranges.
Option D is correct because Identity-Aware Proxy (IAP) uses a fixed set of Google-owned IP ranges to forward authenticated and authorized requests to backend services. By configuring VPC firewall rules to allow ingress only from these IAP proxy IP ranges, the administrator ensures that any traffic not originating from IAP is blocked, effectively restricting access to IAP-authorized requests only.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Use a VPN between IAP and backend.
Why it's wrong here
IAP does not require a VPN.
Configure Cloud Armor to block non-IAP traffic.
Why it's wrong here
Cloud Armor can filter traffic but firewall rules are simpler.
Configure the backend service to require IAP credentials.
Why it's wrong here
IAP credentials are validated at the proxy, not the backend.
Set firewall rules to allow only traffic from IAP proxy IP ranges.
Why this is correct
This restricts access to only IAP requests.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates often confuse application-layer authentication (IAP JWT validation) with network-layer access control (firewall rules), leading them to choose option C, but the question specifically asks for the configuration to ensure only requests coming through IAP can access the backend, which at the network level is achieved by restricting source IPs to IAP proxy ranges.
Detailed technical explanation
IAP forwards requests to backends with an additional `X-Goog-Iap-Jwt-Assertion` header containing a signed JWT. The backend must validate this JWT to confirm the request came through IAP, but the firewall rule (option D) is the network-level gate that blocks non-IAP traffic before it reaches the backend. In practice, a defense-in-depth approach combines firewall rules restricting source IPs to the IAP proxy ranges (35.235.240.0/20 for TCP forwarding) with JWT validation in the backend application.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
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FAQ
Configuring access within a cloud solution environment — This question tests Configuring access within a cloud solution environment — Read the scenario before looking for a memorised answer..
The correct answer is: Set firewall rules to allow only traffic from IAP proxy IP ranges. — Option D is correct because Identity-Aware Proxy (IAP) uses a fixed set of Google-owned IP ranges to forward authenticated and authorized requests to backend services. By configuring VPC firewall rules to allow ingress only from these IAP proxy IP ranges, the administrator ensures that any traffic not originating from IAP is blocked, effectively restricting access to IAP-authorized requests only.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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