Turn on Listener Logging

Enable loggin in listener:
In order to know, which listener commands were executing, one should enable the logging by the following:

LSNRCTL> set log_directory …/log
LSNRCTL> set log_file Listener_.log
LSNRCTL> set log_status on LSNRCTL> save_config

Set the Listener Password :
Setting Listener Password is strongly recommended in order to prevent the distinct attacks. There are two ways to do the same. Either by setting PASSWORDS_listenername parameter in listener.ora file (Password will be in a Clear Text – Not Recommended)Or By using lsnrctl set password command (Highly Recommended).