You have to make your server or linux safer,u might find such log text like below:
There were xx times failed login until the last successful login
That is because many people use scripts to scan computer’s port,some for learning to hack,some for hacking
your computer for money and etc.(what’s worst,because of the lack of safety consciousness,many people set
their account name as root,and password like ‘admin’,123456`````,or the listening port always is 22)Sound
like a family put their valuables on the front door and hope no one will stole it.
So we should change our login account and some settings