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In passive mode, the involved routers and firewalls on the server side need to be configured to accept and forward incoming connections. Once established, the connection can be used both for uploads or downloads.Ī common network setup might look like this:

Note that the only difference is how the connection gets established. In passive mode, the connection is outgoing on the client side and incoming on the server side. Generally, establishing outgoing connections requires little or no configuration of the involved routers/firewalls compared to allowing incoming connections. In both cases, the required file or listing gets transferred over the data connection. Once FileZilla Pro issues a command to transfer a file or listing, the server will connect to the provided address. In active mode, FileZilla/FileZilla Pro opens a socket on the local machine and tells its address to the server using the PORT command. FileZilla Pro then issues a command to transfer a file or to get a directory listing, and establishes a secondary connection to the address returned by the server. In passive mode, which is the recommended, FileZilla/FileZilla Pro sends the PASV command to the server, and the server responds with an address. The data connection can be established in two different ways, using active mode or passive mode.

Then, in order to transfer a file or a directory listing, the client sends a particular command over the control connection to establish the data connection. When you connect to an FTP server, you are actually making two connections.įirst, the control connection is established, over which FTP commands and their replies are transferred. What distinguishes FTP from most other protocols is the use of secondary connections for file transfers. Setting up the network components for FTP outside a local area network (LAN) might be non trivial. It is important to understand the basics of the FTP protocol in order to configure FileZilla/FileZilla Pro, firewalls and routers.