Refer to the following to set root login:
sudo -s (to become root)
vi /root/.ssh/authorized_keys
Delete the lines at the begining of the file until you get to the words ssh-rsa.
vi /etc/ssh/sshd_config
Set the variable PermitRootLogin to PermitRootLogin without-password (without quotes)
sudo /etc/init.d/sshd restar
https://stackoverflow.com/questions/7407333/amazon-ec2-root-login
原文：https://www.cnblogs.com/xiangnan/p/9434104.html