olemus
FTP üks tööviise: käsukanali ja andmekanali loob klient
ning server teatab, millist porti kasutada andmekanali jaoks;
rakendatakse siis, kui server ei saa luua andmekanalit,
eeskätt tulemüüri tõttu
= in FTP passive mode the command channel and data channel are created by the client, and the server informs which port to use for the data channel; is applied when the server cannot establish a data channel, primarily due to a firewall

ülevaateid
http://slacksite.com/other/ftp.html

http://stackoverflow.com/questions/1699145/what-is-the-difference-between-active-and-passive-ftp

http://www.serv-u.com/kb/1138/active-and-passive-ftp-transfers-defined

turvalisus
https://securitywing.com/active-vs-passive-ftp-mode/

https://www.itprotoday.com/compute-engines/risks-passive-ftp