ConfigServer Security & Firewall (CSF) is a stateful packet inspection firewall (SPI), login/intrusion detection, and security application for Linux servers. It is a very popular security suite.
Run the command on server to install/ activate CSF -
Check if CSF is running : service csf status
Disabling CSF: csf -x
Enabling CSF: csf -e
Restarting CSF: csf -r