During connecting to a SSH server, the client stores the few details like Server’s Hostname, IP Address and Host key in a file name known_hosts. This file will be located in you ~/.ssh directory as shown below :

This file contains list of all servers to which you connect in a plaintext. It poses a small security risk if the host is shared or your client gets compromised. This can be avoided by Hashing the known_hosts file. Hashing known_hosts file is easy, you just use the ssh-keygen command as shown below :

 $ ssh-keygen -H -f known_hosts
 known_hosts updated.
 Original contents retained as known_hosts.old
 WARNING: known_hosts.old contains unhashed entries
 Delete this file to ensure privacy of hostnames

Note : You need to delete the backup file known_hosts.old