I have a problem with the following setup: A friendly local business has a time stamp clock on site A. This clock provides data via an integrated ftp server. Also on site A: a Raspberry Pi 3 running Debian Wheezy acting as OpenVPN server. Port forwarding on the router was set to enable VPN and SSH connections from the internet via a dynamic DNS address. This enabled users from site B, where the payroll accounting magic is happening, to first establish a tunnel to site A's network and then fetch time stamp clock data with the help of a proprietary Windows software using the (active) FTP protocol.
It all worked fine until they switched their internet provider. The new router (to which I don't have access) was set up identical to the old one with all clients on the network having the same IPs – and the same port forwarding settings as well. After the switch, I still can SSH into the raspberry, open a tunnel to site A's network, and ping the time stamp clock. But whenever I try to fetch the data via FTP (no matter if I try it with the proprietary software from the manufacturer or with a standard FTP client), I errors like "LIST requested action not taken" or something similar.
My guess is that this somehow is an active FTP protocol problem where the answers from the time stamp clock are not routed back through the tunnel properly. But this is just a guess – and I have no idea how to debug and fix this. Maybe anyone here does? Anything that hints me in the right direction is highly appreciated!
Thank you and greetings from Germany!