Ubuntu 20.04 Install SSH, Change SSH Port, Enable root

1. Install SSH service

$ sudo apt install openssh-server
$ systemctl status sshd
$ sudo systemctl restart ssh
$ sudo ufw allow ssh
$ sudo systemctl enable ssh

2.Change default SSH port 22 to some other port number above 1024. 

To do so edit the /etc/ssh/sshd_config
Port 8888

$ sudo ufw allow 8888/tcp 

$ sudo systemctl restart ssh

3. Enable root access SSH
Open the /etc/ssh/sshd_config file with administrative privileges and change the following line:
FROM:
#PermitRootLogin prohibit-password
TO:
PermitRootLogin yes

$ sudo systemctl restart ssh

4. Enable root account

By default the root's password is not set on Ubuntu 20.04 and the error message Permission denied, please try again will appear when attempting to SSH login as a root user. From this reason we need to set root's password. When prompted enter your current user password followed by new root password:
$ sudo passwd
[sudo] password for rusking:
New password:
Retype new password:
passwd: password updated successfully

 5. Allow GUI root login on Ubuntu 20.04

Edit the /etc/gdm3/custom.conf GDM configuration file to allow root login.

add the AllowRoot=true line.

Next, edit PAM authentication daemon configuration file /etc/pam.d/gdm-password and comment line which denies root access to graphical user interface.

To do so edit file /etc/pam.d/gdm-password and put # in front of the line containing:
auth required pam_succeed_if.so user != root quiet_success

All done. Reboot your Ubuntu system.