PASV FTP, also known as Passive FTP, is an alternative mode for establishing File Transfer Protocol (FTP) connections. In short, it solves the problem of an FTP client's firewall blocking incoming connections. "PASV" is the name of the command that the FTP client uses to tell the server that it is in passive mode. Passive FTP is a preferred FTP mode for FTP clients behind a firewall and is often used for web-based FTP clients and computers connecting to an FTP server within a corporate network.
Active vs. Passive FTP – Understanding FTP Ports
FTP operates over two ports: one for moving data between servers and another for issuing commands. Passive mode works by allowing the FTP client to initiate the sending of both control and data messages.
Normally, it is the FTP server that initiates the data requests, but this kind of setup may not work if the client firewall has blocked the port the server wants to use. For this reason, PASV mode makes FTP "firewall-friendly."
In other words, the client is the one opening the data port and the command port in passive mode, so given that the server-side firewall is open to accept these ports, data can flow between the two. This configuration is ideal because the server most likely has the necessary ports open for the client to communicate with the server.
