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Question 1,473 of 1,616

Troubleshooting and Optimization →mediumMultiple SelectObjective-mapped

Quick Answer

The correct answer involves checking the network ACLs for the subnets to ensure they allow inbound and outbound traffic on ephemeral ports. While the RDS security group is correctly configured to allow inbound traffic from the ECS task security group, network ACLs operate at the subnet level and can silently block return traffic if they don't permit ephemeral ports (typically 1024-65535) for the response. This is a common oversight because security groups are stateful—they automatically allow return traffic—but NACLs are stateless, requiring explicit rules for both directions. On the AWS Certified Developer Associate DVA-C02 exam, this question tests your understanding of the layered security model and the distinction between stateful security groups and stateless NACLs. A frequent trap is assuming that security group rules alone guarantee connectivity, forgetting that NACLs can override them. Memory tip: "Security groups are smart, NACLs are dumb—you must write both directions for NACLs."

DVA-C02 Troubleshooting and Optimization Practice Question

This DVA-C02 practice question tests your understanding of troubleshooting and optimization. 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.

An application running on Amazon ECS with Fargate is experiencing intermittent failures when connecting to an Amazon RDS database. The security group for the RDS instance allows inbound traffic from the security group attached to the ECS tasks. Which TWO additional checks should the developer perform to troubleshoot the connectivity issue?

Question 1mediummulti select

Full question →

A
Enable VPC Flow Logs to capture dropped packets.
Why wrong: Flow logs are useful but not a direct first check; the question asks for two additional checks.
B
Check that the database credentials are correctly stored in AWS Secrets Manager.
Why wrong: Credentials affect authentication, not network connectivity.
C
Verify that the security group for the RDS instance has an inbound rule allowing traffic from the security group used by the ECS tasks.
The security group rule must reference the correct source security group.
D
Check the network ACLs for the subnets to ensure they allow inbound and outbound traffic on ephemeral ports.
NACLs can block return traffic if not configured correctly.
E
Ensure that the ECS tasks have a route to the internet through an Internet Gateway.
Why wrong: ECS tasks in private subnets should use a NAT Gateway or VPC endpoints for RDS, not Internet Gateway.

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

✓

Verify that the security group for the RDS instance has an inbound rule allowing traffic from the security group used by the ECS tasks.

Options A and D are correct. Option A: The RDS security group must allow inbound traffic from the ECS task security group; this is already stated, but verifying the exact configuration is important. Option D: The NACL for the subnets must allow ephemeral ports for the response traffic. Option B is wrong because the ECS tasks are in a private subnet, so an Internet Gateway is not used for RDS connectivity. Option C is wrong because the database credentials are not related to network connectivity. Option E is wrong because VPC Flow Logs are useful for analyzing traffic but not a first check; the question asks for additional checks after security group.

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.

✗
Enable VPC Flow Logs to capture dropped packets.
Why it's wrong here
Flow logs are useful but not a direct first check; the question asks for two additional checks.
✗
Check that the database credentials are correctly stored in AWS Secrets Manager.
Why it's wrong here
Credentials affect authentication, not network connectivity.
✓
Verify that the security group for the RDS instance has an inbound rule allowing traffic from the security group used by the ECS tasks.
Why this is correct
The security group rule must reference the correct source security group.
Related concept
CIDR notation defines the prefix length.
✓
Check the network ACLs for the subnets to ensure they allow inbound and outbound traffic on ephemeral ports.
Why this is correct
NACLs can block return traffic if not configured correctly.
Related concept
CIDR notation defines the prefix length.
✗
Ensure that the ECS tasks have a route to the internet through an Internet Gateway.
Why it's wrong here
ECS tasks in private subnets should use a NAT Gateway or VPC endpoints for RDS, not Internet Gateway.

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 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.

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 DVA-C02 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this DVA-C02 question test?

Troubleshooting and Optimization — This question tests Troubleshooting and Optimization — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: Verify that the security group for the RDS instance has an inbound rule allowing traffic from the security group used by the ECS tasks. — Options A and D are correct. Option A: The RDS security group must allow inbound traffic from the ECS task security group; this is already stated, but verifying the exact configuration is important. Option D: The NACL for the subnets must allow ephemeral ports for the response traffic. Option B is wrong because the ECS tasks are in a private subnet, so an Internet Gateway is not used for RDS connectivity. Option C is wrong because the database credentials are not related to network connectivity. Option E is wrong because VPC Flow Logs are useful for analyzing traffic but not a first check; the question asks for additional checks after security group.

What should I do if I get this DVA-C02 question wrong?

What is the key concept behind this question?

CIDR notation defines the prefix length.

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This DVA-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 DVA-C02 exam.