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Question 799 of 1,750

SDLC Automation →hardMultiple ChoiceObjective-mapped

CodeBuild VPC RDS Connection Timeout — Security Group Outbound Rules

This DOP-C02 practice question tests your understanding of sdlc automation. 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 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?

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.

A
CodeBuild cannot access resources in a private subnet unless it uses a NAT gateway.
Why wrong: CodeBuild in VPC can access private subnet resources directly if security groups allow.
B
The CodeBuild service role does not have rds:Connect permission.
Why wrong: RDS connections are not authorized via IAM; IAM is for API actions.
C
The CodeBuild project's security group outbound rules do not allow traffic to the RDS security group on port 3306 (or appropriate port).
Security groups control network traffic; missing outbound rule causes timeout.
D
The RDS instance is in a different subnet CIDR than CodeBuild's subnet.
Why wrong: Subnet CIDR doesn't matter as long as routing and security groups allow traffic.

Full breakdown with real-world context →

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

✓

The CodeBuild project's security group outbound rules do not allow traffic to the RDS security group on port 3306 (or appropriate port).

Option C is correct because CodeBuild's security group outbound rules must explicitly allow traffic to the RDS security group on the database port (e.g., 3306 for MySQL). Even though CodeBuild runs in a VPC, if the outbound rules are too restrictive, the build agent cannot establish a TCP connection to the RDS instance, causing a timeout. The security group acts as a virtual firewall for the CodeBuild ENI, and without a matching outbound rule, packets are dropped.

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.

✗
CodeBuild cannot access resources in a private subnet unless it uses a NAT gateway.
Why it's wrong here
CodeBuild in VPC can access private subnet resources directly if security groups allow.
✗
The CodeBuild service role does not have rds:Connect permission.
Why it's wrong here
RDS connections are not authorized via IAM; IAM is for API actions.
✓
The CodeBuild project's security group outbound rules do not allow traffic to the RDS security group on port 3306 (or appropriate port).
Why this is correct
Security groups control network traffic; missing outbound rule causes timeout.
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 RDS instance is in a different subnet CIDR than CodeBuild's subnet.
Why it's wrong here
Subnet CIDR doesn't matter as long as routing and security groups allow traffic.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often assume IAM permissions (like rds:Connect) control network access, but RDS network access is governed solely by security groups and VPC routing, not IAM.

Detailed technical explanation

How to think about this question

When CodeBuild is configured with a VPC, it creates an elastic network interface (ENI) in the specified subnet. The ENI's security group must have outbound rules allowing traffic to the RDS security group on the database port, and the RDS security group must have inbound rules from the CodeBuild security group. This is a two-way security group rule requirement; missing either direction causes a timeout. In practice, the default outbound rule for a security group allows all traffic, but if a custom restrictive outbound rule is applied, it can block the connection.

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.

Visual reference

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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FAQ

Questions learners often ask

What does this DOP-C02 question test?

SDLC Automation — This question tests SDLC Automation — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The CodeBuild project's security group outbound rules do not allow traffic to the RDS security group on port 3306 (or appropriate port). — Option C is correct because CodeBuild's security group outbound rules must explicitly allow traffic to the RDS security group on the database port (e.g., 3306 for MySQL). Even though CodeBuild runs in a VPC, if the outbound rules are too restrictive, the build agent cannot establish a TCP connection to the RDS instance, causing a timeout. The security group acts as a virtual firewall for the CodeBuild ENI, and without a matching outbound rule, packets are dropped.

What should I do if I get this DOP-C02 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.

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Same concept, more angles

1 more ways this is tested on DOP-C02

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 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?

medium

✓ A.The security group attached to the RDS instance does not allow inbound traffic from the CodeBuild security group.
B.The CodeBuild project does not have internet access to download packages.
C.The CodeBuild project is not associated with a VPC.
D.The RDS instance is not publicly accessible and requires a NAT gateway.

Why A: The most likely cause is that the security group attached to the RDS instance does not allow inbound traffic from the CodeBuild security group. CodeBuild runs inside the VPC using the specified security group, so it sends traffic to the RDS instance on port 3306 (or the appropriate database port). If the RDS security group's inbound rules do not explicitly permit traffic from the CodeBuild security group (or its CIDR), the connection is dropped, resulting in a timeout.

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