There are two types of firewalls available in Linux, a packet filtering firewall and a proxy-based firewall. Most Linux users use a packet filtering firewall to setup a basic firewall for their system because Linux already came with firewall package by default.

Linux kernel came with a module called netfilter. Netfilter is used to manipulate incoming and outgoing traffic in Linux system. You can use locate command to find netfilter in your Linux system like in the example below: