It’s never easy to set up an FTP server the moment firewalls get involved. But it gets even more difficult once you start using the secure version of FTP, known as FTPS. In this post, we’ll talk about the problem you’ll usually encounter when your FTPS server is behind a firewall and your client is attempting to perform a file transfer using passive mode or PASV.
[blog post] Setting Up An FTPS Server Behind A Firewall or NAT For PASV Mode Data Transfers
Active v.s. Passive FTP Simplified – Understanding FTP Ports
Understanding Key Differences Between FTP, FTPS and SFTP
Securing Trading Partner File Transfers w/ Auto PGP Encryption & FTPS