[ Thanks to Andrew
Weber for this link. ]
“Active connections are safest for the server but at
times may not work for some clients. Passive connections may not be
the safest for the server but usually work well for the clients. In
addition, these factors must be taken into account with the
firewall. This is where connection tracking can be helpful.“Firewall: Enable Active FTP Connections
“If you wanted to create an anonymous FTP server that anyone
could get access to you could set up FTP access with this
command:ufw allow ftp
“This shows you that ftp is allowed for everyone to your server.
However, you can see it only opens port 21.”