FTP (File Transfer Protocol)

The File Transfer Protocol (FTP) is a network protocol that provides file access, file transfer, and file management functionalities.

FTP is built on a client-server architecture and uses separate control and data connections between the client and the server. FTP users may authenticate themselves using a clear-text sign-in protocol, normally in the form of a username and password, but can connect anonymously if the server is configured to allow it. For secure transmission that protects the username and password, and encrypts the content, FTP is often secured with SSL/TLS (FTPS).1

Active and Passive Mode

FTP may run in active or passive mode, which determines how the data connection is established. In both cases, the client creates a TCP control connection to the FTP server command port 21. In active modes, the client starts listening for incoming data connections on random port from the server (the client sends the FTP command PORT to inform the server on which port it is listening). In situations where the client is behind a firewall and unable to accept incoming TCP connections, passive mode may be used. In this mode, the client uses the control connection to send a PASV command to the server and then receives a server IP address and server port number from the server, which the client then uses to open a data connection from an arbitrary client port to the server IP address and server port number received.

You can select between the active and passive mode on Connection page of Advanced site settings dialog.