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This article mainly introduces "the working mode and principle of pure-ftpd under Ubuntu". In the daily operation, I believe that many people have doubts about the working mode and principle of pure-ftpd under Ubuntu. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts about "the working mode and principle of pure-ftpd under Ubuntu". Next, please follow the editor to study!

The way FTP works

FTP supports two modes, one is called Standard (that is, PORT, active), and the other is Passive (that is, PASV, passive). The client of the Standard mode FTP sends a PORT command to the FTP server. The client of the Passive mode FTP sends a PASV command to FTP Server.

Here is a description of how these two ways work:

The Port mode FTP client first establishes a connection with the TCP 21 port of the FTP server, sends commands through this channel, and sends PORT commands on this channel when the client needs to receive data. The PORT command contains what port the client uses to receive data. When transmitting data, the server connects to the designated port of the client through its own TCP 20 port to send data. FTP server must establish a new connection with the client to transfer data.

Passive mode is similar to Standard mode when establishing a control channel, but instead of a Port command, a Pasv command is sent after the connection is established. After the FTP server receives the Pasv command, it randomly opens a temporary port (also called free port, port number is greater than 1023 and less than 65535) and notifies the client to send data on this port. The client connects to the FTP server this port, and then the FTP server will transmit data through this port. At this time, FTP server no longer needs to establish a new connection with the client.

Many firewalls are not allowed to accept externally initiated connections when they are set up, so many FTP servers located behind firewalls or intranets do not support PASV mode because clients cannot open high-end ports of FTP servers through firewalls. However, many clients of the intranet can not log in to the PORT server because the TCP 20 of the server cannot establish a new connection with the client of the internal network, which makes it unable to work.

At this point, the study of "how and how pure-ftpd works under Ubuntu" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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