If you use the “TCP.VALIDNODE_CHECKING“, “TCP.INVITED_NODES” or “TCP.EXCLUDED_NODES” parameters in your listener’s sqlnet.ora file, you may have already encountered the following error when starting or restarting your listener:

TNS-12560: TNS:protocol adapter error
TNS-00584: Valid node checking configuration error

The problem is easy to fix when you have only a few hostnames referenced, but when the list is long, finding the server that caused the error can be much more time consuming.