I can not fint anywhere how I set up my Readynas 202 to use FTPS and always requary FTPS. I find the setting, but wich port should I use? Wich port should I open in my firewall? Anything else to think about?
Use explicit ftps signaling in your ftp client - there should be a setting for it.
For passive ports, you should allocate about the 4 x number of expected simultaneous users. So if you expect 3 users, then allocate 12. Take these from the range 49152-65535 - for instance 61001-61012. Set this in the service configuration in the NAS and forward them in the router.
You also need to forward the control port. The default port is 21, and you can use that. Most FTP clients will let you use non-standard port, which is what I do. For instance, 61000.
To access over the internet, you will want a ddns name for your router (since it's IP address changes).
Some clients need the masquerade set the ddns name. If you do set it, you won't be able to use FTP on your local network - so its best to pick a client that doesn't need the masquerade. Filezilla is a good free one.
