A firewall is a system used to control access to a computer network. Firewalls can isolate risk areas from safe access areas and prevent unpredictable or unauthorized external access from invading the internal network. Learning firewalls under Linux is not only for these reasons. The most important thing is that during the learning stage, through the use of firewalls, you can better master the principles of network communication, become familiar with the operation of Linux, and master network services and network protocols. It is also important for learning Linux operation and maintenance or Cybersecurity is helpful. There are many firewalls under Linux, including iptables, firewalld, selinux, tcp_warppers, etc. This course summarizes these firewall-related technologies.