The answer is to check the network ACL associated with the subnet of the RDS instance to ensure it allows inbound traffic on port 3306 and outbound traffic on ephemeral ports. This is correct because, while the security group permits inbound MySQL traffic from the EC2 instance, a network ACL is a stateless firewall that must explicitly allow both inbound traffic on the database port and outbound response traffic on ephemeral ports (typically 1024-65535). If the outbound rule is missing, the response packets from RDS are dropped, causing a connection timeout despite correct security group settings. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of the layered security model—specifically the difference between stateful security groups and stateless network ACLs. A common trap is focusing only on security groups or misdiagnosing the timeout as an authentication or internet gateway issue. Memory tip: think of network ACLs as a bouncer who checks both who enters (inbound) and who leaves (outbound), whereas a security group only checks who enters.
DBS-C01 Monitoring and Troubleshooting Practice Question
This DBS-C01 practice question tests your understanding of monitoring and troubleshooting. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 database specialist is trying to connect to an Amazon RDS for MySQL instance from an EC2 instance but receives a 'Connection timed out' error. The security group for the RDS instance allows inbound traffic on port 3306 from the security group of the EC2 instance. What should the specialist check next?
Check the network ACL associated with the subnet of the RDS instance to ensure it allows inbound traffic on port 3306 and outbound traffic on ephemeral ports.
Network ACLs are stateless and must allow both inbound and outbound traffic.
B
Check that the RDS instance has a public DNS name and the EC2 instance can resolve it.
Why wrong: The error is a timeout, not a DNS resolution issue.
C
Ensure that the VPC has an internet gateway attached and the route table has a route to it.
Why wrong: If both instances are in the same VPC, internet gateway is not needed.
D
Verify that the security group for the EC2 instance allows outbound traffic on port 3306.
Why wrong: Outbound traffic is typically allowed by default; the issue is likely network ACL.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the network ACL associated with the subnet of the RDS instance to ensure it allows inbound traffic on port 3306 and outbound traffic on ephemeral ports.
Option D is correct because the network ACL must allow inbound traffic on ephemeral ports for the response. Option A is wrong because the security group already allows inbound on 3306. Option B is wrong because the error is 'Connection timed out', not authentication. Option C is wrong because if the VPC had no internet gateway, the error would be different for private connections.
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.
✓
Check the network ACL associated with the subnet of the RDS instance to ensure it allows inbound traffic on port 3306 and outbound traffic on ephemeral ports.
Why this is correct
Network ACLs are stateless and must allow both inbound and outbound traffic.
Check that the RDS instance has a public DNS name and the EC2 instance can resolve it.
Why it's wrong here
The error is a timeout, not a DNS resolution issue.
✗
Ensure that the VPC has an internet gateway attached and the route table has a route to it.
Why it's wrong here
If both instances are in the same VPC, internet gateway is not needed.
✗
Verify that the security group for the EC2 instance allows outbound traffic on port 3306.
Why it's wrong here
Outbound traffic is typically allowed by default; the issue is likely network ACL.
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 DBS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Monitoring and Troubleshooting — This question tests Monitoring and Troubleshooting — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Check the network ACL associated with the subnet of the RDS instance to ensure it allows inbound traffic on port 3306 and outbound traffic on ephemeral ports. — Option D is correct because the network ACL must allow inbound traffic on ephemeral ports for the response. Option A is wrong because the security group already allows inbound on 3306. Option B is wrong because the error is 'Connection timed out', not authentication. Option C is wrong because if the VPC had no internet gateway, the error would be different for private connections.
What should I do if I get this DBS-C01 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 DBS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This DBS-C01 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 DBS-C01 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.