AWS Certified Advanced Networking Specialty Practice Exam

Instances cannot access a private authentication service in a consumer VPC. What is a likely cause?

1. The outbound security group does not permit the authentication port

2. The correct IP address is not assigned to the instances

3. The service is not enabled in the region

4. Endpoint policies are misconfigured

The correct answer is: The outbound security group does not permit the authentication port

When instances cannot access a private authentication service in a consumer VPC, one common reason is related to network security configurations, specifically security groups. The correct assertion here is that the outbound security group does not permit the authentication port. Security groups in AWS act as virtual firewalls that control both inbound and outbound traffic for instances. If the outbound rules are restrictive and do not allow traffic on the specific port used by the authentication service, instances will be unable to establish a connection to that service. Ports are crucial for networking, as they allow specific types of traffic to flow. Each service typically listens on a designated port, and if the security group blocks this port, communication is effectively halted. Understanding this aspect of AWS networking is essential because misconfigurations in security group settings are common issues that can lead to access problems. Therefore, ensuring that the outbound rules of the security group associated with the instances allow traffic on the appropriate port for the authentication service is vital in resolving such issues.