Firewalls

NSGs provides external security perimeters for requests. However, it does not mean that virtual machines should not implement additional security measures. It is always better to implement security both internally and externally. Virtual machines, whether in Linux or Windows, provide a mechanism to filter requests at the operating system level. This is known as a firewall in both Windows and Linux.

It is advisable to implement firewalls for operating systems. They help build a virtual security wall that allows only those requests that are considered trusted. Any untrusted requests are denied access. There are even physical firewall devices, but on the cloud,  operating system firewalls are used. The following screenshot shows the firewall configuration in Windows operating system:

Firewalls filter network packets, and identify incoming ports and IP addresses. Using the information from these packets, the firewall evaluates the rules and decides whether it should allow or deny access.