ANS-C01 Network Design Practice Question
A company has a VPC with multiple subnets. They have an Amazon RDS for MySQL database in a private subnet. The application team needs to access the database for maintenance from a bastion host in a public subnet. The security group for the database allows inbound traffic from the security group of the bastion host on port 3306. However, the application team cannot connect. What is the most likely cause?
⚠ Common exam trap
AWS often tests the distinction between stateful security groups and stateless network ACLs, trapping candidates who assume that a security group rule alone is sufficient for connectivity.
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 on the private subnet is blocking inbound traffic on port 3306.
The most likely cause is that the network ACL (NACL) on the private subnet is blocking inbound traffic on port 3306. Even though the security group on the RDS instance allows traffic from the bastion host's security group, NACLs are stateless and must explicitly allow both inbound and outbound traffic. By default, custom NACLs deny all inbound traffic, so unless an inbound rule for port 3306 (MySQL) is added, the connection will be dropped at the subnet boundary.
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 network ACL on the private subnet is blocking inbound traffic on port 3306.
Why this is correct
Network ACLs are stateless and can block traffic even if security groups allow it.
- ✗
The database does not have a route to the internet.
Why it's wrong here
Not required for internal connectivity.
- ✗
The security group rule for the database is incorrectly configured.
Why it's wrong here
It allows inbound from bastion security group.
- ✗
The bastion host does not have a route to the database subnet.
Why it's wrong here
RDS is in the same VPC, routing should work.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-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 ANS-C01 exam.