I’m sorry but I cannot understand: is it necessary or not, once I’ve installed Virtualmin, to install even a third part firewall (as csf for example)?
I mean: with Centos and Virtualmin is already included a firewall pre-installed or it’s necessary to add it?
Nope, it’s not necessary to install a third party firewall.
If you wish to use a firewall, it’s no problem to use the one that’s included with CentOS.
Is it firewalld? Do You have any clue about how to install it from virtualmin?
What CentOS version are you using?