The root user in Ubuntu is disabled by default because the password is not set.

To enable root user:

1. Set root password

customer@ubuntu:~$ sudo passwd root
Enter new UNIX password: # set password
Retype new UNIX password: # confirm password
passwd: password updated successfully
customer@ubuntu:~$ su -
Password: # enter root password
root@ubuntu:~$

2. Limit users to switch to root

root@ubuntu:~$ vi /etc/pam.d/su
(Find line): #auth required pam_wheel.so

(Change to): auth required pam_wheel.so group=adm

root@ubuntu:~$ usermod -G adm customer

3. Enable root login for OpenSSH

root@ubuntu:~$ vi /etc/ssh/sshd_config
(Find line): #PermitRootLogin no

(Change to): PermitRootLogin yes

(Find line): #PasswordAuthentication no

(Change to): PasswordAuthentication yes

4. Restarting OpenSSH

root@ubuntu:~$ /etc/init.d/ssh restart
root@ubuntu:~$ /etc/init.d/ssh status
Notes:

Red text = input commands

customer@ubuntu:~$ = example of what may appear on screen depending on hostname assigned etc.