Monitor remote DNS listener, enable local DNS listener VS when remote listener is unavailable

Unfortunately the reverse setting on monitors only really work when you get a response. For example, using a reverse http monitor that checks for the string "error" will mean the pool member is green/up as long as you get any string NOT containing "error" in it, but if you either get a string with "error" or indeed no response at all the pool member is considered down.

I think your best bet is to assign an external monitor on your secondary VS, using netcat (or something more sophisticated) to check for a response from the primary VS and then setting the pool state based on the opposite of the normal based on the exit-code received, e.g. DOWN if 0 and UP if 1.

If you've never worked with external monitors before you can actually get what you want by slightly tweaking the included example monitor in /usr/bin/monitors/sample_monitor