GoKawiilI recently bought a Bambu A1 Mini 3D printer to try out 3D printing. This printer offers a FTP server, allowing users to use FTP clients like FileZilla or WinSCP to upload model files for printing, and download timelapse videos.
However, when I tried connecting to the printer with FileZilla, I found that although the username and password were correct and login was successful, I couldn't retrieve the file list:
Some users on the Bambu official forum have also reported this issue, such as this reply and this reply.
Some users mentioned that WinSCP works, but I use Linux daily and don't want to switch to Windows just to connect to the printer's FTP service. So I investigated the cause of the problem and found a solution for Linux.
Introduction to FTP Protocol
To understand this problem, we first need to understand how the FTP protocol works. FTP (File Transfer Protocol) is an ancient file transfer protocol, born in 1971. It uses multiple TCP connections to separate control commands and data transfer:
Control connection: The client actively connects to the server (usually on port 21), establishing a persistent TCP connection. All commands (such as login, change directory, list files) and server responses are transmitted through this connection.
Data connection: Whenever file content needs to be transferred or file lists need to be retrieved, the client and server establish a new TCP connection. After the transfer is complete, this connection is closed.
Based on how the data connection is established, FTP can be divided into Active Mode and Passive Mode:
Active Mode
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