An OpenStack security group is a set of firewall rules that determine the allowed incoming and outgoing network traffic for a virtual machine or instance. You can create different security groups for different types of instances (such as web server, database server, etc.) and define what kind of traffic you want to allow based on factors like IP address, protocol, and port.
A security group is a form of a network-level firewall. It controls traffic at the network level, even before it reaches the actual instance.
In contrast, the firewall within the instance's operating system (OS) operates at the operating system level. It examines traffic after it has passed through the network layer and reached the instance.
Having both security groups and an OS firewall is both useful and a best practice in cloud security. Some of the reasons for this include:
Defense in depth: By having multiple layers of security, you increase the likelihood of stopping an attack. If an attacker manages to bypass one security layer, they can still be stopped by another layer.
Different levels of control: Security groups and OS firewalls offer control at different levels. Security groups are well-suited for managing high-level access, while OS firewalls enable finer-grained control.
Redundancy: If one of the security systems fails or is misconfigured, the other can still provide protection.
