Linux-SSH：WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED solution:

ssh 192.168.21.53:
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@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
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IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx.
Please contact your system administrator.
Add correct host key in /root/.ssh/known_hosts to get rid of this message.
Offending key in /root/.ssh/known_hosts:1
RSA host key for 192.168.xxx.xxx has changed and you have requested strict checking.
Host key verification failed.

First input more ~/.ssh/known_hosts
You will see the following content:
… … … …
192.168.21.53 ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDQ25kqKE1uT7wtcaFp3zFSMxpwweif0YcKMNmpp4Alql/ZmPHaS/fI1dmXxyzq77wj8uXJgkh7xF0doyFeBk1c8D7jG/5SkAdpwlh6uixYEI3SlRY96InnQd/zEqRGjMvwoEcSmdMdkRsrK5EkPblhDfsQgc5RwKJpkQ+GWXM7oibHZB/P/G2Husnwb0rSwIRvUkwfamM9wDBtA3nEZLcG6wScwm2sZ8SwL9eJMNJXRHR90vewQ6MVrv+k5kB+BSrJ92TH8uZP3R9oO4X6tvRKNZOubQJqKqPDujVrTCD9XnMDb+Mx0yKMqvoGoVBM7fnZMGWtTHBkacsyYSiXaJrV
vi ~/.ssh/known_hosts, edit this file, delete the content beginning with 192.168.21.53, :wq! save and exit can be.

Cause Analysis:
It is also possible that a host key has just been changed.