I'm running Filezilla FTP server on Windows 7 machine which i access using FTP client running on Virtualbox'ed Slackware guest. According with in the active mode after the successful connection establishment the server connects the client back from it's data port #20. My confusion is that when i'm logged in the server there is a line concerning connection port #21 but there isn't a line concerning data port #20 listed in the CurrPorts or netstat output (on Windows host).

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  • 2
    Unless you’re transferring something no data connection will be active. – Daniel B Apr 30 '17 at 22:36

The diagram on that page shows that your client connects to port 21 to talk about the connection, then the data you download is sent from the server port 20 your your client on port 1027. From your client's point of view, it knows

1) it connects to port 21 from port 1026 (connection 1 in diagram) 2) downloaded data appears on port 1027 (connection 3 in diagram)

The client doesn't necessarily care that the data was sent from the server port 20 so this may not be recorded.

If you upload data to the server, you may see a port 20 reference on the client, because the client will then need to connect to port 20 (connection 4 in diagram).

enter image description here

  • Ok, but I'm monitoring the ports on the server site (not the client!) and as I've written there is no port 20 reference there, only 21. – Mulligan May 1 '17 at 9:27

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