Firewall Incompatibilities
Because FTP utilizes a dynamic secondary port (for data channels), many firewalls were designed to snoop FTP protocol control messages in order to determine which secondary data connections they need to allow. However, if the FTP control connection is encrypted using TLS/SSL, the firewall cannot determine the TCP port number of a data connection negotiated between the client and FTP server. Therefore, in many firewalled networks, an FTPS deployment will fail when an unencrypted FTP deployment will work. This problem can be solved with the use of a limited range of ports for data and configuring the firewall to open these ports.
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