The Passive property enables or disables passive FTP file
transfers between the local and server. In passive transfer
mode, the client is responsible for establishing the data connection
between the server and the local system. By default, the
Passive property is set to True, with the client establishing
the data connection with the server.
The majority of FTP servers support passive mode transfers, and in
most cases, passive mode is required when attempting to upload or
download files when the client is behind a firewall or a router that
performs Network Address Translation (NAT). However, if the
Passive property is set to True and the server does not
support passive mode, an error will be returned the next time a file
transfer or directory listing is attempted. In this case, set the
Passive property to False and attempt the transfer again.
Note that setting this property has no effect when uploading or
downloading a file using the Hypertext Transfer Protocol (HTTP).