The default firewall that comes along with Linux is “iptables” and you can use iptables to block Ftp access/port on your server.
Completely block Ftp access on the server:
# iptables -A INPUT -p tcp --dport 21 -j DROP
Block Ftp access for a specific IP address, say 11.12.13.14
# iptables -A INPUT -p tcp -s 11.12.13.14 --dport 21 -j DROP
Block Ftp access for a specific subnet
# iptables -I INPUT -p tcp -s 11.12.13.0/24 --dport 21 -j DROP
Make sure you save the iptable rules else they will be erased after a iptable/server restart:
# service iptables save
CSF firewall use iptables in the background to apply it’s rules. Edit the csf configuration file,
# pico /etc/csf/csf.conf
Remove port 21 from the TCP_IN list and restart the csf firewall
# csf -r
Block Ftp access for a specific IP address, edit the csf.deny file
# pico /etc/csf/csf.deny
and place the following line
tcp:in:d=21:s=11.12.13.14
Save the file and restart the csf firewall.
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