What exactly mean refused in Oracle listener?
lsnrctl does show the number of connections refused since the listener was started. Connections can be refused for several reasons, usually because some part of the connect string is incorrect, missing, or malformed, like a bad service_name setting. A connection might also be rejected if the server has Valid Node Checking enabled and the client IP address isn't white-listed. Here is a list of common errors and some troubleshooting tips: https://docs.oracle.com/en/database/oracle/oracle-database/12.2/netag/troubleshooting-oracle-net-services.html#GUID-F521AB0A-B67C-432B-B899-8F856EB51CAE


TNS-12500/ORA-12500: TNS: listener failed to start a dedicated server process 
ORA-12514: TNS:listener does not currently know of service
requested in connect descriptor 
ORA-12520: TNS:listener could not find available handler for requested type of server 
ORA-12521: TNS:listener does not currently know of instance requested in connect
descriptor 
ORA-12525: TNS:listener has not received client's request in time allowed 
ORA-12533: TNS:illegal ADDRESS parameters
TNS-12540/ORA-12540: TNS:internal limit restriction exceeded and TNS-00510: Internal limit restriction exceeded 
TNS-12541/ORA-12541: TNS:no listener 
TNS-12549/ORA-12549: TNS:operating system resource quota exceeded and TNS-00519: Operating system resource quota exceeded 
TNS-12560/ORA-12560: TNS:protocol adapter error occurred