ANS-C01 Network Management and Operations Practice Question
A company has a VPC with a CIDR of 172.16.0.0/16. The VPC contains an Amazon RDS for MySQL database in a private subnet. The database is accessed by EC2 instances in the same VPC and by on-premises servers via a Site-to-Site VPN. The network team recently enabled VPC Flow Logs and noticed that the database is receiving a high number of SYN packets from an IP address that is not part of the VPC or on-premises network. The security group for the database only allows inbound traffic on port 3306 from the EC2 instances' security group and the on-premises CIDR (10.0.0.0/8). The network ACL for the database subnet allows inbound and outbound traffic on all ports from all sources. What is the most likely cause of the unexpected traffic?
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 allows all inbound traffic, so packets from the internet reach the database subnet's network ACL before being evaluated by the security group.
The network ACL (NACL) is stateless and allows all inbound traffic from any source, including the internet. While the security group (SG) denies the unexpected traffic (since it only allows traffic from the EC2 security group and on-premises CIDR), the NACL permits it. VPC Flow Logs capture traffic at the network interface level after NACL evaluation but before SG evaluation; thus, even though the SG drops the SYN packets, they are logged by Flow Logs because they pass through the NACL. Option A is incorrect because the security group explicitly restricts inbound traffic to specific sources, not 0.0.0.0/0. Option B is incorrect because the RDS database is in a private subnet and does not have a public endpoint; a public endpoint would have been configured explicitly. Option C is incorrect because the Site-to-Site VPN is configured for the on-premises network only, and the unexpected IP is not from that CIDR; the VPN itself does not route internet traffic into the VPC.
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 security group for the database has an inbound rule that allows traffic from 0.0.0.0/0.
Why it's wrong here
The security group only allows traffic from specific sources.
- ✗
The RDS database has a public endpoint that is accessible from the internet.
Why it's wrong here
The database is in a private subnet and not publicly accessible.
- ✗
The Site-to-Site VPN is misconfigured and routing internet traffic into the VPC.
Why it's wrong here
VPN routes only on-premises traffic.
- ✓
The network ACL for the database subnet allows all inbound traffic, so packets from the internet reach the database subnet's network ACL before being evaluated by the security group.
Why this is correct
NACL is stateless and allows all traffic, so packets enter the subnet and are then evaluated by the security group, which drops them.
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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.