We notice that FTP sensors switch from explicit to implicit mode when the sensor doesn't receive data or when the connection time-outs.
There is then made an log entry: Connection established using implicit mode.
We have 2 nodes for checking. This message applies for one of the two nodes, the other node stays using port 21 explicit mode. However the node isn't noted down in the message. When PRTG (or the single node) switches to implicit mode it uses port 990 instead of 21 and keeps using this until the sensor gets paused. So it doesn't seem to reset or detect that port 21 explicit mode becomes functional again. The only way to assure that port 990 isn't used is to choose "Do not use Transport-Level Security" bu this completely disables ssl.
This comes down to these requests:
FIX: Correct logging; noting down the Node that switched mode. IMPROVE: Detection that port 21 explicit is working again and switching back from explicit. ADD: Have the ability to configure the sensor to use implicit or explicit mode. As an alternative; force to always use the configured port.
The last one is the most important for us because it gives us to control how measurements are done and also makes them more predictable (or less unpredictable).