How to do ssh login by root on Linux

Brief

It can't be done ssh login by root on Linux default configuration.
The below is the solution.

Environment

CentOS[6-7]

Procedure

1. Put id_rsa under /root/.ssh/ on a client side and authorized_key under /root/.ssh/ on a server side

2. Comment the below portion on /etc/ssh/sshd_config
#PermitRootLogin no

3. Add the below on /etc/ssh/sshd_config
PermitRootLogin without-password

4. Restart sshd
(CentOS7)
sudo systemctl restart sshd

(CentOS6)
sudo service restart sshd