DBS-C01 Network ACL (NACL) Practice Question
A developer reports that an application is unable to connect to an Amazon RDS for MySQL DB instance. The security group for the DB instance allows inbound traffic on port 3306 from the application server's security group. The DB instance is in a VPC with both public and private subnets. The application server is in a private subnet. What is the most likely cause of the connection failure?
⚠ Common exam trap
Candidates often overlook that network ACLs are stateless and can block traffic even when security groups allow it. Always check both layers.
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 network ACL for the private subnet is blocking outbound traffic to the DB instance.
The most likely cause is that the network ACL (NACL) for the private subnet is blocking outbound traffic to the DB instance. Security groups are stateful and allow return traffic automatically, but NACLs are stateless and require explicit rules for both inbound and outbound. If the private subnet's NACL does not allow outbound traffic to the DB instance's subnet on port 3306, the application server cannot initiate the connection. Options A and B are incorrect because the security group already allows inbound traffic, and instances in public and private subnets within the same VPC can communicate via private IPs. Option D is incorrect; while a DB subnet group is required for RDS, it does not directly affect connectivity once the instance is running.
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 DB instance is in a public subnet and the application server is in a private subnet, so they cannot communicate.
Why it's wrong here
Incorrect. Instances in public and private subnets within the same VPC can communicate using private IPs; subnet type does not prevent connectivity.
- ✗
The security group for the DB instance does not allow inbound traffic from the application server's security group.
Why it's wrong here
Incorrect. The stem explicitly states that the security group allows inbound traffic from the application server's security group, so this is not the cause.
- ✓
The network ACL for the private subnet is blocking outbound traffic to the DB instance.
Why this is correct
Correct. Network ACLs are stateless and can block outbound traffic from the private subnet to the DB instance, even when security groups permit inbound traffic.
- ✗
The DB instance is not part of a DB subnet group that includes the private subnet.
Why it's wrong here
Incorrect. A DB subnet group defines which subnets the DB instance can be placed in, but it does not affect connectivity after the instance is launched.
Visual reference
Go deeper
Related to this question
About these practice questions
This DBS-C01 question is part of Courseiva's 1,663-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.