← SOA-C02·Amazon Web Services

Question 144 of 1,546

Networking and Content Delivery →mediumMultiple ChoiceObjective-mapped

SOA-C02 Networking and Content Delivery Practice Question

This SOA-C02 practice question tests your understanding of networking and content delivery. 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 has a VPC with multiple subnets. An EC2 instance in a public subnet needs to communicate with an RDS database in a private subnet. The RDS security group allows inbound traffic from the EC2 instance's security group. However, the EC2 instance cannot connect. 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
The VPC does not have DNS resolution enabled, so the RDS endpoint cannot be resolved.
Correct because RDS endpoints are DNS names; without DNS resolution, the instance cannot connect.
B
The network ACL for the private subnet blocks inbound traffic from the public subnet.
Why wrong: Incorrect because default NACLs allow all traffic.
C
The security group of the RDS database does not allow outbound traffic.
Why wrong: Incorrect because security groups are stateful; outbound is not needed for inbound replies.
D
The EC2 instance does not have a public IP address.
Why wrong: Incorrect because the instance is in a public subnet and likely has a public IP, but internal communication uses private IPs.

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 VPC does not have DNS resolution enabled, so the RDS endpoint cannot be resolved.

Option A is correct because the RDS database is in a private subnet and its endpoint is a DNS name. If DNS resolution is disabled on the VPC, the EC2 instance cannot resolve the RDS endpoint's hostname to an IP address, preventing the TCP connection from being established even though security group rules are correctly configured.

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 VPC does not have DNS resolution enabled, so the RDS endpoint cannot be resolved.
Why this is correct
Correct because RDS endpoints are DNS names; without DNS resolution, the instance cannot connect.
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 network ACL for the private subnet blocks inbound traffic from the public subnet.
Why it's wrong here
Incorrect because default NACLs allow all traffic.
✗
The security group of the RDS database does not allow outbound traffic.
Why it's wrong here
Incorrect because security groups are stateful; outbound is not needed for inbound replies.
✗
The EC2 instance does not have a public IP address.
Why it's wrong here
Incorrect because the instance is in a public subnet and likely has a public IP, but internal communication uses private IPs.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates focus on security group or NACL misconfigurations, overlooking that DNS resolution is a prerequisite for connecting to any service using a DNS endpoint, especially when the database is in a private subnet without a direct route to a public resolver.

Detailed technical explanation

How to think about this question

VPC DNS resolution (the `enableDnsSupport` attribute) controls whether instances can resolve public DNS hostnames via the Amazon-provided DNS server (at 169.254.169.253). Without it, DNS queries for RDS endpoints (which are private hosted zone records) fail, causing connection timeouts. This is distinct from `enableDnsHostnames`, which assigns public DNS names to instances with public IPs.

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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.

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 SOA-C02 question test?

Networking and Content Delivery — This question tests Networking and Content Delivery — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The VPC does not have DNS resolution enabled, so the RDS endpoint cannot be resolved. — Option A is correct because the RDS database is in a private subnet and its endpoint is a DNS name. If DNS resolution is disabled on the VPC, the EC2 instance cannot resolve the RDS endpoint's hostname to an IP address, preventing the TCP connection from being established even though security group rules are correctly configured.

What should I do if I get this SOA-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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This SOA-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 SOA-C02 exam.