This fails when this IP is private and port is not open in client's firewall.Instead, machine "B" informs machine "A" that the result is waiting to be fetched in a given IP and port of machine "B". This time, the result will not be delivered to port N+1 of machine "A".The client then initiates the connection from port N+1 to port P on the server to transfer data. The result of this is that the server then opens a random unprivileged port (P > 1023) and sends P back to the client in response to the PASV command. The first port contacts the server on port 21, but instead of then issuing a PORT command and allowing the server to connect back to its data port, the client will issue the PASV command. When opening an FTP connection, the client opens two random unprivileged ports locally (N > 1023 and N+1). In passive mode FTP, the client initiates both connections to the server, solving the problem of firewalls filtering the incoming data port connection to the client from the server. Even if the IP is public, the ports N+1 must be open in the firewall. This fails because FTP client in machine "A" asks machine "B" to connect to a private IP.Machine "A" sends "ls" to machine "B", but because it is Active Mode, the result of "ls" will be delivered to port N+1 (N>1023) of machine "A".FTP client in machine "A" connects to FTP server in machine "B" on port 21.The server will then connect back to the client's specified data port (.) Then, the client starts listening to port N+1 and sends the FTP command PORT N+1 to the FTP server. In active mode FTP, the client connects from a random unprivileged port (N > 1023) to the FTP server's command port, port 21.
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