What is PASV mode?

The FTP protocol has 2 modes, PORT (also called "regular" or "normal" mode) and PASV ("passive" mode).

The FTP client determines the mode that will be used. If the client issues a PORT command, it is attempting "PORT" mode. If the client issues a PASV command, it is attempting "PASV" mode.

PASV mode forces the client-server data connection to be established by the client, rather than the server (which is the default). PASV mode is required for users who are behind a router-based firewall.

Details:
In PORT and PASV mode the FTP session uses two port numbers. The first port number is allocated on the server (default: port 21). This is referred to as the "Control or Command Channel".

If the client sends a PORT command, which contains the client IP address and (the second) allocated port number. Then the FTP server connects to this port to send the data (referred to as the "Data Channel").

Client: PORT 127,0,0,1,12,82

Server: 200 Port command successful.

If the client sends a PASV command, then the FTP server responds back to the client with a port number that it has allocated. In this case the "Data Channel" connection is created by the client to the IP Address and port number provided by the FTP server.

Client: PASV

Server: 227 Entering Passive Mode (127,0,0,1,12,251).