I'm using proftpd on my server (ubuntu 16.04 x86_64).
Default proftpd use standart 21 port. I can connect from my home notebook to ftp with active mode without problem.
Now I stop proftpd, change port from 21 to 10021, start service again. And now I can not connect with active mode, only with passive mode.
Also I can not understand, why works active mode? I have internet access by router. I do not forwand any ports to my notebook at router. As I now, on connect, my notebook (ftp client) create connection from some port > 1023 to server port 21. My notebook send to server also second (data) port to server and server connect from own port 20 to me with this data port. But how second connection can established, if my ports are closed from wan?