Investigating firewalls

In a practical sense, a firewall is the means to ensure only specific data is allowed in or out of a specific context. For Moodle installations, the context is typically as follows:

• The network, which means a web application firewall will filter IP packets before they reach the server. It can also filter IP packets when they leave the server and before they reach the internet.
• The server, which means the firewall is a filter that’s built into the operating system kernel. It will filter incoming and outgoing packets journeying through the kernel.

Usually, one finds both types of firewalls operate in some form in production environments. This is because each type affords different kinds of protection against cyber threats. As the subject of this chapter is building a secure server, we will limit our discussion here to server firewalls only. We will learn more about network firewalls in Chapter 6.

A firewall filters IP packets, which...