CV0-004 Troubleshooting Practice Question
A user reports that they cannot connect to a RDS database instance from their application. The security group for the RDS instance allows inbound traffic on port 3306 from the application server's security group. What should the administrator check NEXT?
⚠ Common exam trap
CV0-004 often tests the difference between stateful security groups and stateless NACLs; candidates may forget that NACLs require explicit outbound rules for return traffic, leading them to overlook NACL checks.
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
✓
Network ACL rules for the RDS subnet
The security group for the RDS instance allows inbound traffic on port 3306 from the application server's security group, which is correct. However, network ACLs (NACLs) are stateless and can block traffic even if security groups allow it. The administrator should check the NACL rules for the RDS subnet to ensure that inbound and outbound rules permit traffic on port 3306 and the ephemeral ports for return traffic. This is the next logical step after verifying security group rules.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
IAM policy attached to the RDS instance
Why it's wrong here
An IAM policy governs API-level actions on the RDS resource, not TCP connectivity to port 3306, so it cannot block the application's connection. It is tempting because IAM controls database authentication and access management, and would be the right check when a user can reach the instance but lacks permission to perform operations.
- ✓
Network ACL rules for the RDS subnet
Why this is correct
Security groups are stateful and already permit port 3306, so the next layer to verify is the stateless network ACL on the RDS subnet, which must allow inbound 3306 and outbound ephemeral return traffic. This satisfies the requirement to check the next likely blocker.
- ✗
Route table entries for the RDS subnet
Why it's wrong here
Route tables direct traffic between subnets and gateways; within a single VPC, the local route already permits communication, so a missing entry rarely explains this failure. Route tables are the correct check when the application server sits in a peered VPC, transit gateway, or on-premises network reaching the RDS subnet.
- ✗
Outbound security group rules on the RDS instance
Why it's wrong here
Outbound rules on an RDS instance's security group govern traffic the database initiates, not the inbound MySQL connection the application server requests on port 3306. It is tempting because security groups are stateful and both directions appear relevant, but outbound rules would matter only if the RDS instance itself needed to initiate traffic, such as calling an external service.
Visual reference
Go deeper
Related to this question
About these practice questions
One of 834 original CV0-004 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official CompTIA exam blueprint
This CV0-004 practice question is part of Courseiva's free CompTIA 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 CV0-004 exam.