What is the difference between active and passive FTP?
note: I think this would be useful as a community wiki question. Also, I'm hoping someone can come up with a clear concise answer like this question: How does Subnetting Work?
Active and passive are the two modes that FTP can run in. FTP uses two channels between client and server, the command channel and the data channel, which are actually separate TCP connections. The command channel is for commands and responses, the data channel is for actually transferring files. It's a nifty way of sending commands to the server without having to wait for the current data transfer to finish.
In active mode, the client establishes the command channel (from client port
In passive mode, the client establishes both channels. In that case, the server tells the client which port should be used for the data channel.
Passive mode is generally used in situations where the FTP server is not able to establish the data channel. One of the major reasons for this is network firewalls. While you may have a firewall rule which allows you to open up FTP channels to
Passive mode solves this by opening up both types of channel from the client side. In order to make this hopefully clearer:
At this point, the command and data channels are both open.
(a)Note that the selection of ports on the client side is up to the client, as the selection of the server data channel port in passive mode is up to the server.
(b)Further note that the use of port 20 and 21 is only a convention (although a strong one). There's no absolute requirement that those ports be used although the client and server both have to agree on which ports are being used. I've seen implementations that try to hide from clients by using different ports (futile, in my opinion).
Original answer from stackoverflow