A firewall is a system or group of systems that establishes access control policies between two networks. In principle, a firewall can be thought of as a pair of mechanisms: the first blocks network traffic, the second allows network traffic.
A firewall can be used to protect a network from external attacks, but a firewall cannot protect against attacks that don’t go through the firewall and attacks from someone on the same network, and a firewall cannot protect a network from poorly written application programs.
In general, firewalls usually perform the following functions:
- Packet analysis and filtering.
- Content and protocol blocking.
- Connection authentication and encryption.
In this time we will use iptables as a firewall configured on a PC that acts as a PC Router. For details, see here.