Allow SSH root login on Debian and Ubuntu Linux

By default Ubuntu 18.04 installation comes with unset root password. To set root password open up terminal and execute the following linux command:

$ sudo passwd
[sudo] password for user: 
Enter new UNIX password: 
Retype new UNIX password: 
passwd: password updated successfully

Edit SSH configuration file: /etc/ssh/sshd_config and set property PermitRootLogin to yes:

Save configuration file and restart ssh service by command:

sudo service ssh restart

To restart SSH on Debian you can use command:

systemctl restart ssh.service