Steps
1, Create RSA key pair
ssh-keygen -t rsa
Answer the question and enter passphrase.
By default, the public key is now located in /home/user/.ssh/id_rsa.pub and the private key is now located in /home/user/.ssh/id_rsa
2, Copy public key to server
cat ~/.ssh/id_rsa.pub | ssh user@123.45.56.78 "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"
3, Disable password for root login
vim /etc/ssh/sshd_config
Change the line PermitRootLogin yes to:
PermitRootLogin without-password
Save and quit, then reload ssh:
reload ssh
Now you can't login via ssh root@123.45.56.78 by using root's password.
