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Passive mode

The server configuration is very similar to client configuration for active mode. In passive mode, the server opens a socket and waits for the client to connect to it. By default, FileZilla Server asks the operating system for the machine's IP address, and for a free port number.

EPSV FTP command - Extended Passive Mode

This allows users behind routers/firewalls to connect over FTP when they might not be able to connect over an Active (PORT/EPRT) FTP session. EPSV mode provides information where the client can connect for the data port on the server.

“Passive mode” means that the server will be “passive” and accept data connections from the client, instead of requiring the client be able to accept connections back from the server. ... Almost all modern network client environments only permit outbound connection requests, for security reasons.Jan 26, 2018

In passive mode FTP, the FTP client initiates both connections to the server. ... This method of FTP is insecure, as a random unprivileged port is opened on the Server. This is a potential security issue and it isn't advisable to use the Passive mode of FTP.

In Passive Mode, the FTP server waits for the FTP client to send it a port and IP address to connect to. In Active mode, the server assigns a port and the IP address will be the same as the FTP client making the request.Feb 8, 2021

Active FTP provides security to the FTP server. Passive FTP does not provide security to the FTP server. Active FTP may cause problems because of firewalls. Passive FTP does not have connection issues from firewalls.Apr 22, 2020

FTP may operate in an active or a passive mode, which determines how a data connection is established. ... In the active mode, the client starts listening on a random port for incoming data connections from the server (the client sends the FTP command PORT to inform the server on which port it is listening).Jan 8, 2021

FileZilla has two network modes: active and passive. The active mode opens a specific port in a given range; the passive mode allows the server to determine which port is used for the data connection. The passive mode is the recommended mode for most instances; it is enabled by default when FileZilla is installed.

In general, FTP is a security risk because of insecure logon mechanisms. That statement aside, you need to know about one other problem. In my description of the passive FTP event sequence, note that in Step 4, the server sends the client a random port number greater than 1024.