FTP

File Transfer Protocol (FTP) is a standard network protocol used to copy a file from one host to another over a TCP/IP-based network. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server applications which solves the problem of different end host configurations (i.e. Operating System, file names).

The separate FTP connections are utilized on two ports:

• "data" port – used for transmitting data;
• "command" port (also known as "control" port) – used for transmitting commands between the FTP server and client which control the data transfers.

FTP can work in two modes: active and passive mode.

Depending on the mode set, the data and the commands are transmitted on different port numbers.

• In active FTP mode, the client connects to the FTP server's command port (default command port is "21"). After that, the client starts listening on a random port and tells it to the FTP server by sending a command. The server will then connect back to the client from its local data port (default data port is "20").

  The FTP client doesn't make the actual connection to the server's data port – it only tells the server what port it is listening on and the server connects back to the specified port. If there is a firewall on the client side, this would appear as an incoming connection and it could be blocked.

• In passive FTP mode, the client initiates both connections to the server, which solves the problem of firewalls filtering the incoming data port connection on the client. When an FTP connection is opening, the client opens two random ports locally. Using the first port, the client contacts the server on port 21, but instead of sending a PORT command and allowing the server to connect back to its data port, the client will send the PASV command. As a result, the server opens a random unprivileged port (P > 1023) and sends the PORT P command back to the client. Then, the client initiates the connection to port "P" on the server to transfer data.

In MassTransit, you can use both active and passive mode connections. For the passive ones, you can configure a port range that MassTransit will use to find the random port number which will be sent to the client. If MassTransit is behind a firewall or Network Address Translation (NAT), this port range as well as a public IP Address can be specified for the public connections.