What do Security Groups act like in AWS?

Security Groups in AWS function as a firewall at the instance level, meaning they are designed to control inbound and outbound traffic specifically to and from Amazon EC2 instances. When you associate a Security Group with an EC2 instance, it provides a set of rules that determine which traffic is allowed to reach that instance and which traffic the instance can send out.

The rules defined in a Security Group can filter traffic based on protocol types (such as TCP, UDP, or ICMP), the port ranges, and the source or destination IP addresses. This fine-grained control over network traffic allows users to bolster their security posture by only allowing necessary traffic to communicate with instances, thereby reducing the attack surface.

By focusing on the instance level, Security Groups complement other networking features, like Network Access Control Lists (NACLs), which operate at the subnet level. This distinction is crucial since it allows for more tailored security measures on a per-instance basis rather than a one-size-fits-all approach across a subnet.

In contrast to other options, a Security Group does not function as a network monitoring tool or a threat detection system, as those roles generally involve different capabilities such as analyzing traffic patterns and identifying potential security breaches rather than directly managing the traffic flow to instances.