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The following tutorial describes how to install the CSF firewall on a Debian system 11
We will do everything in the console by executing a few simple commands.

We start the terminal and update the system packages:

sudo apt update

We install the necessary packages:

sudo apt install libyan-socket-inet6-perl libsocket6-perl -y

sudo apt install sendmail dnsutils unzip libio-socket-ssl-perl -y

sudo apt install libcrypt-ssleay-perl git perl iptables libnet-libidn-perl -y

Pobieramy CSF

wget http://download.configserver.com/csf.tgz

unpack

sudo tar -xvzf csf.tgz

We install in the system:

cd csf && sh install.sh

The next step is to run it in the system:

sudo systemctl start csf

The next step is to update the perl rules:

perl /usr/local/csf/bin/csftest.pl

We add csf to the system autostart:

sudo systemctl enable csf

We go to the initial configuration.

edit file:

At section

# Allow incoming TCP ports

And

# Allow outgoing TCP ports

We set the ports we want to have unlocked, for example:

If we want to block an ip address, we add it to the file:

sudo nano /etc/csf/csf.deny

After making the changes, reload the firewall with the command:

sudo csf -r