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Suddenly users were not able to ftp using FileZilla and only ftp connections were made through the command prompt. When I searched for this issue, I found a couple of questions here in serverfault facing this issue and the reply was because command line uses active mode and the solution was to change the transfer mode in filezilla from passive to active, and this actually worked. I need to know the reason why this happened since it was working fine with the passive mode before, is this something to be configured from the server instead of making all clients change their transfer mode. We are using FTP server on IIS 6.

Thanks in advance

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Most likely due to the random high port for the data channel from the client to the server is now blocked by a firewall.

With Passive FTP, the client establishes both the control channel and data channel. The data channel is established from the client to a random high port on the server. If those random high ports are blocked, passive mode will fail.

From the server-side firewall's standpoint, to support passive mode FTP the following communication channels need to be opened:

  • FTP server's port 21 from anywhere (Client initiates connection)
  • FTP server's port 21 to ports > 1023 (Server responds to client's control port)
  • FTP server's ports > 1023 from anywhere (Client initiates data connection to random port specified by server)
  • FTP server's ports > 1023 to remote ports > 1023 (Server sends ACKs (and data) to client's data port)

More information:

https://stackoverflow.com/questions/1699145/what-is-the-difference-between-active-and-passive-ftp

Active FTP vs. Passive FTP, a Definitive Explanation
http://slacksite.com/other/ftp.html#passive

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