What is passive and active FTP?

Active FTP

Unlike many other protocols, FTP uses two communication ports. To start with, the FTP client, such as FileZilla or WS-FTP (FrontPage also has an FTP function), connects to an FTP server on port 21. This is the command port, the port to which commands are sent. The client makes this connection from a port of its own with a number above 1024, for example 1027 (port numbers up to and including 1024 are reserved, numbers 20 and 21 for FTP). The client then sends the PORT command with the number of the port on which the client will listen. This is the data port, so it is not the same as the command port. That is why this data port has a different number too, for example 1028. The server then connects from port 20 to the client on this port. This means the data connection between client and server is initiated by the server. And that is exactly what a firewall on the client side normally prevents, as it is a connection from outside. As a result, no data connection is established and no files can be uploaded or downloaded. With problems like these, a passive connection is better.

Passive FTP

To make sure there is a data connection that is not blocked by the firewall, the data connection must be initiated by the client. This works as follows. First the client connects from, for example, port 1027 to the server on port 21. The PASV command is sent instead of the PORT command. The server replies over this connection with PORT plus the number of the server port (also above 1024) on which the client can make a data connection. The client then does so. The result is a command connection and a data connection, both initiated by the client. The firewall will now not block anything.

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