Question 629 of 977
hardMultiple ChoiceObjective-mapped
Troubleshoot Database Connectivity in VPC: NACL vs Security Group
A cloud administrator is troubleshooting an application that fails to connect to a database. The database server is in a private subnet, and the application server is in a public subnet. The security group for the database allows inbound traffic on port 3306 from the application's security group. Which of the following is the MOST likely reason the connection fails?
Quick Answer
The answer is the network ACL for the database subnet blocking inbound traffic on port 3306. This is correct because network ACLs are stateless and apply to the entire subnet, meaning they act as a subnet-level firewall that must explicitly allow traffic, regardless of the security group rules on the database instance. While the database’s security group permits inbound traffic from the application’s security group, the NACL is evaluated first at the subnet boundary, and custom NACLs default to denying all inbound traffic. On the CompTIA Cloud+ CV0-004 exam, this scenario tests your understanding of the layered defense model in a VPC, where security groups are stateful instance-level filters and NACLs are stateless subnet-level filters—a common trap is assuming security group rules alone guarantee connectivity. Remember the mnemonic: “NACL blocks the subnet, SG lets the instance in.”
⚠ Common exam trap
CompTIA often tests the difference between stateful security groups and stateless network ACLs, and the trap here is that candidates assume security group rules are sufficient for connectivity, forgetting that NACLs must also permit the traffic at the subnet boundary.
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 database subnet is blocking inbound traffic on port 3306
The network ACL (NACL) for the database subnet is the most likely cause because NACLs are stateless and apply to the entire subnet. Even if the database security group allows inbound traffic from the application's security group, the NACL must also allow inbound traffic on port 3306 (MySQL) from the application subnet. By default, custom NACLs deny all inbound traffic, so if the administrator did not explicitly add a rule for port 3306, the connection will be blocked 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 for the database subnet is blocking inbound traffic on port 3306
Why this is correct
NACLs are stateless and require explicit allow rules.
- ✗
The database server's operating system firewall is blocking the connection
Why it's wrong here
Could be, but security group is stateful and properly configured; OS firewall is less common in cloud.
- ✗
The application's security group does not allow outbound traffic to the database
Why it's wrong here
Outbound is usually allowed by default.
- ✗
The route table for the public subnet does not have a route to the private subnet
Why it's wrong here
Within a VPC, subnets can communicate by default.
Visual reference
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Same concept, more angles
1 more way this is tested on CV0-004
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A cloud administrator is troubleshooting an application that fails to connect to a database. The application and database are in the same VPC. Which THREE steps should the administrator take to diagnose the issue?
hard- A.Check the routing table for a route to the internet.
- ✓ B.Test connectivity to the database using a telnet or netcat command from the application server.
- ✓ C.Verify that the security group associated with the database instance allows inbound traffic from the application's security group on the database port.
- D.Check the DNS resolution of the database endpoint in the application's subnet.
- ✓ E.Verify that the network ACL for the database subnet allows inbound traffic on the database port.
Why B: Telnet or netcat can test basic TCP connectivity to the database port, confirming whether the database is reachable from the application server at the network layer. This step isolates whether the issue is a network connectivity problem versus an authentication or configuration issue within the database itself.
Last reviewed: Jun 30, 2026
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