rules = [
  ## Allow MQTT client using username "dashboard"  to subscribe to "$SYS/#" topics
  {"permit": "allow", "username": "dashboard", "action": "subscribe", "topics": ["$SYS/#"]}

  ## Deny "All Users" subscribe to "$SYS/#" "#" Topics
  {"permit": "deny", "username": "#", "action": "subscribe", "topics": ["$SYS/#", "#"]}

  ## Allow any other publish/subscribe operation
  {"permit": "allow"}
]