The answer is that the security group attached to the RDS instance does not allow inbound traffic from the CodeBuild security group. This is the most likely cause because when CodeBuild is configured with a VPC, subnet IDs, and security group IDs, it runs inside your VPC and must communicate with RDS via private IP addresses; the RDS security group must explicitly permit inbound traffic on the database port from the CodeBuild project’s security group as the source. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of VPC security group configuration for cross-service access, often appearing as a trap where candidates mistakenly assume a NAT gateway or internet gateway is needed. The key insight is that both resources are within the same VPC, so no internet access is required—only proper security group rules. Memory tip: think “SG to SG” — always check that the target resource’s security group references the source resource’s security group ID, not a CIDR block.
DOP-C02 SDLC Automation Practice Question
This DOP-C02 practice question tests your understanding of sdlc automation. 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 company is using AWS CodeBuild to run integration tests. The tests require access to an Amazon RDS instance in a private subnet. The CodeBuild project is configured with a VPC ID, subnet IDs, and security group IDs. However, the tests fail with a connection timeout. What is the MOST likely cause?
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
✓
The security group attached to the RDS instance does not allow inbound traffic from the CodeBuild security group.
Option D is correct because the security group must allow inbound traffic from the CodeBuild security group. Option A is wrong because the CodeBuild project has VPC configuration. Option B is wrong because NATgateway is not needed for VPC-to-VPC communication within same VPC. Option C is wrong because internet access is not required.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 security group attached to the RDS instance does not allow inbound traffic from the CodeBuild security group.
Why this is correct
Security group rules must allow traffic on the database port from the CodeBuild security group.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The CodeBuild project does not have internet access to download packages.
Why it's wrong here
Integration tests may not need internet.
✗
The CodeBuild project is not associated with a VPC.
Why it's wrong here
The question states it is configured with VPC ID.
✗
The RDS instance is not publicly accessible and requires a NAT gateway.
Why it's wrong here
NATgateway is for outbound internet, not for inbound.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
SDLC Automation — This question tests SDLC Automation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The security group attached to the RDS instance does not allow inbound traffic from the CodeBuild security group. — Option D is correct because the security group must allow inbound traffic from the CodeBuild security group. Option A is wrong because the CodeBuild project has VPC configuration. Option B is wrong because NAT gateway is not needed for VPC-to-VPC communication within same VPC. Option C is wrong because internet access is not required.
What should I do if I get this DOP-C02 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
About these practice questions
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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. A company uses AWS CodeBuild to run integration tests. The tests require access to an RDS database in a private subnet. CodeBuild runs in a VPC but the build times out waiting for the database connection. What is the MOST likely cause?
hard
A.CodeBuild cannot access resources in a private subnet unless it uses a NAT gateway.
B.The CodeBuild service role does not have rds:Connect permission.
✓ C.The CodeBuild project's security group outbound rules do not allow traffic to the RDS security group on port 3306 (or appropriate port).
D.The RDS instance is in a different subnet CIDR than CodeBuild's subnet.
Why C: Option B is correct because CodeBuild must be configured with a security group that allows outbound traffic to the RDS security group on the database port. Option A is wrong because IAM permissions affect API calls, not network connectivity. Option C is wrong because subnet CIDR is not relevant for security group rules. Option D is wrong because CodeBuild can access resources in the same VPC if security groups allow.
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This DOP-C02 practice question is part of Courseiva's free Amazon Web Services 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 DOP-C02 exam.
Question Discussion
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