Who is responsible for configuring security group rules to determine which ports are open to an EC2 Linux instance?

The responsibility for configuring security group rules for an EC2 Linux instance falls on the customer. In the context of AWS, customers have control over their environment, allowing them to specify inbound and outbound traffic rules by configuring security groups. This customization enables customers to tailor their security settings to meet their specific application requirements and security policies.

Security groups act as virtual firewalls for EC2 instances, and it is the customer's responsibility to define which ports are open, thereby managing access to their instances. This level of control is a key aspect of AWS's shared responsibility model, where AWS manages the security of the cloud infrastructure, while customers manage security in the cloud, including instances and the applications running on them.