https://documentation.cpanel.net/display/CKB/How+to+Enable+FTP+Passive+Mode

Overview</h2>
Use either the active </em>or passive</em> mode to connect to a File Transfer Protocol (FTP) server.</em>

Active mode vs. passive mode</h2>
FTP utilizes two ports, a data port and a command port, to transfer information from a client to a server. Typically, the command port uses port 21 and the data port uses port 20. When you use a different mode, however, the data port does not always use port 20.

 

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Active</h3>
In active</em> mode, the FTP server responds to the connection attempt and returns a connection request from a different port to the FTP client. Network Address Translation (NAT) configurations block this connection request.
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Passive</h3>
In passive</em> mode, the FTP client initiates both connection attempts. NAT configurations do not</strong> block this connection request.
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Note:</p>

If FTP users exist on the private network side of a NAT configuration, you must</strong> enable FTP's passive</em> mode and open the passive port range in your FTP server's configuration file. You may also need to open the passive port range on your firewall.

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