In trying to diagnose why I couldn’t FTP to my Synology server, I was doing so from a PC running a VPN, with the purpose being to ensure that I was truly isolated from my home network. Well, in addition to some things that appeared wrong in my FTP config., I also found out that a number of the VPN provider’s endpoints apparently have some configuration that actually prevents you from connecting to an FTP server. I’m sure there’s a logic or technical reason why, somewhere, but it escapes me.
I tried using both passive mode (the default) and active mode, but every time, it hung up at the point getting the directory listing. I naturally assumed it was my server’s FTP connection set up, but after conferring with Synology Tech Support, I began to suspect it was not my (or nor entirely my) problem after all.
I tried connecting to several of the endpoints (outside the US) they suggested and, then ‘magically”, FTP connections to my server then worked. If you use PrivateInternetAccess.com (great service, aside from this unexpected result), give them a try. I found these posts related to the problem.