The answer is to create a single firewall rule allowing SSH only from the bastion host's IP and rely on the implicit deny. This works because Google Cloud VPC firewall rules are evaluated in order of priority, and any traffic not matching an allow rule is automatically denied by the implicit deny rule at the end of the evaluation chain. By writing only one allow rule for the bastion’s source IP, you block all other SSH traffic from the internet without needing an explicit deny rule, making this the most efficient configuration. On the Google Professional Cloud Security Engineer exam, this scenario tests your understanding of VPC firewall rule evaluation and the principle of least privilege. A common trap is thinking you need a separate deny rule for SSH, which adds unnecessary complexity and can create priority conflicts. Remember the memory tip: “One allow, implicit deny—no extra rules, no extra worry.”
PCSE Configuring network security Practice Question
This PCSE practice question tests your understanding of configuring network security. 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 engineer wants to block all SSH access from the internet to a VPC network, except for a specific bastion host. What is the most efficient way to configure this?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a firewall rule allowing SSH only from the bastion host's IP and rely on the implicit deny
Option A is correct because VPC firewall rules are implicitly deny-all at the end of the evaluation order. By creating a single allow rule for SSH from the bastion host's IP, all other SSH traffic is implicitly denied by the default deny rule, without needing an explicit deny rule. This is the most efficient approach as it minimizes rule count and avoids potential priority conflicts.
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.
✓
Create a firewall rule allowing SSH only from the bastion host's IP and rely on the implicit deny
Why this is correct
Since the implicit deny blocks all other traffic, this single rule is sufficient.
Related concept
Read the scenario before looking for a memorised answer.
✗
Create a firewall rule with deny SSH from 0.0.0.0/0 and a higher priority rule allowing SSH from the bastion host's IP
Why it's wrong here
This approach works but requires two rules and careful priority management. The simpler method is to rely on the implicit deny.
✗
Use a service perimeter
Why it's wrong here
Service perimeters are for VPC Service Controls, not for SSH access control.
✗
Create a firewall rule denying SSH from all IPs except the bastion host using the 'except' sources
Why it's wrong here
GCP firewall rules do not support 'except' sources.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that you need an explicit deny rule to block traffic, when in fact the implicit deny at the end of the firewall rule evaluation order already blocks all traffic not explicitly allowed.
Detailed technical explanation
How to think about this question
VPC firewall rules are stateful and evaluated in order of priority (lower number = higher priority), with an implicit deny at the end of the rule chain. The implicit deny applies to all ingress traffic that does not match any allow rule, so a single allow rule for the bastion host's IP effectively blocks all other SSH traffic without an explicit deny. In practice, this means the security engineer can simply create one rule with priority 1000 allowing TCP port 22 from the bastion's IP, and all other SSH attempts are dropped by the implicit deny.
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.
Configuring network security — This question tests Configuring network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Create a firewall rule allowing SSH only from the bastion host's IP and rely on the implicit deny — Option A is correct because VPC firewall rules are implicitly deny-all at the end of the evaluation order. By creating a single allow rule for SSH from the bastion host's IP, all other SSH traffic is implicitly denied by the default deny rule, without needing an explicit deny rule. This is the most efficient approach as it minimizes rule count and avoids potential priority conflicts.
What should I do if I get this PCSE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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