The answer is that IAP TCP forwarding is not enabled for the project or VPC. Even when a custom IAM role correctly includes the `iap.tunnel.dest` permission and the developers hold the `iap.tunnelUser` role, the IAP TCP forwarding service itself must be activated at the infrastructure level; without enabling the IAP API on the project and the service for the VPC, the proxy cannot intercept TCP traffic, causing the permission check to fail before IAM evaluation. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that IAP TCP forwarding is a two-layer configuration—IAM permissions alone are insufficient if the underlying service is disabled, a common trap where candidates focus only on roles. Remember the memory tip: "IAM says yes, but the service says no"—always verify the IAP API is enabled before troubleshooting permissions.
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing 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 large organization uses Shared VPC with hundreds of projects. They want to implement fine-grained access control for SSH access to Compute Engine instances using IAP TCP forwarding. They have created a custom IAM role with the necessary permissions (iap.tunnel.dest, iap.tunnel.getIamPolicy, compute.instances.use) and granted it to a group of developers. The developers have also been granted the iap.tunnelUser role on the project. However, when they try to use `gcloud compute ssh --tunnel-through-iap instance-name`, they get a permission error: "Permission 'iap.tunnel.dest' denied on resource 'projects/project/zones/zone/instances/instance'". The network admin has verified that the custom role includes the required permissions and that the developers are members of the group with the role. What is the most likely missing configuration?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
IAP TCP forwarding is not enabled for the project or VPC.
IAP TCP forwarding requires the IAP API to be enabled on the project and the service to be activated for the VPC. Without this, the IAP proxy cannot intercept and forward TCP traffic to the instance, even if the IAM permissions are correctly assigned. The error 'Permission 'iap.tunnel.dest' denied' typically occurs when the IAP TCP forwarding service itself is not enabled, as the permission check fails at the service level before evaluating the IAM role.
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.
✗
The developers need the `iap.tunnel` resource-level permission on the IAP TCP forwarding resource.
Why it's wrong here
The correct permission is `iap.tunnel.dest`, which they already have.
✓
IAP TCP forwarding is not enabled for the project or VPC.
Why this is correct
IAP TCP forwarding must be explicitly enabled at the project level. If not enabled, the permission check fails.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The instances do not have the required network tags for IAP.
Why it's wrong here
IAP does not rely on network tags; it uses IAM permissions.
✗
The developers need the `compute.instances.osLogin` permission.
Why it's wrong here
OS Login is unrelated to IAP TCP forwarding permissions.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume the error is due to missing IAM permissions or instance configuration, when in fact the IAP TCP forwarding service must be explicitly enabled at the project level, which is a prerequisite often forgotten in Shared VPC environments.
Detailed technical explanation
How to think about this question
IAP TCP forwarding works by creating a tunnel through the IAP service, which acts as a proxy between the client and the instance. The IAP API must be enabled in the project, and the 'gcloud compute ssh --tunnel-through-iap' command relies on the IAP service to validate the 'iap.tunnel.dest' permission on the instance resource. If the API is disabled, the permission check fails immediately because the IAP service cannot process the request, even if the IAM role is correctly assigned. In real-world scenarios, this is often overlooked when setting up Shared VPC, as the IAP API must be enabled in the service project where the instances reside.
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 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
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.
Implementing network security — This question tests Implementing network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: IAP TCP forwarding is not enabled for the project or VPC. — IAP TCP forwarding requires the IAP API to be enabled on the project and the service to be activated for the VPC. Without this, the IAP proxy cannot intercept and forward TCP traffic to the instance, even if the IAM permissions are correctly assigned. The error 'Permission 'iap.tunnel.dest' denied' typically occurs when the IAP TCP forwarding service itself is not enabled, as the permission check fails at the service level before evaluating the IAM role.
What should I do if I get this PCNE 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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Refer to the exhibit. A network engineer is unable to SSH to instance-1 using IAP TCP forwarding. What is the most likely reason?
easy
A.IAP TCP forwarding is not enabled for the project.
✓ B.The instance does not have the 'ssh-iap' tag that the firewall rule applies to.
C.The instance's service account does not have the necessary IAP permissions.
D.SSH keys are not configured on the instance.
Why B: The exhibit shows a firewall rule that allows IAP TCP forwarding traffic from the IAP source range (35.235.240.0/20) to instances with the network tag 'ssh-iap'. Since instance-1 lacks this tag, the firewall rule does not apply, and SSH traffic from IAP is blocked. Without the tag, the instance's firewall policy drops the forwarded TCP connections, preventing SSH access.
Last reviewed: Jun 25, 2026
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This PCNE practice question is part of Courseiva's free Google Cloud 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 PCNE exam.
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