The firewall allows you to define filter rules for incoming traffic. Rules relating to IPv4 source addresses or the TCP / UDP source port can be set. Defining firewall rules such as these prevents unauthorized access to customer servers.
CSF (configserver security & firewall)
Csf is a more advanced firewall system , which in addition to the mere firewalling function via iptables, such as Apf, has integrated some very useful features, such as blocking an IP after total unsuccessful attempts to access the service pop3, the analysis of system logs, an email alert if a server user is modified / deleted / added and above all an excellent anti dos / ddos system. In fact, CSF is a Stateful Packet Inspection (SPI) firewall, Login / Intrusion Detection and Security Application for Linux servers.
In addition to the firewall, there is also the mod_security module directly on Apache which verifies all queries made to the server to evaluate whether they are part of common attacks carried out by attackers.