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
Ubuntu 20.04 Install SSH, Change SSH Port, Enable root
原文：https://www.cnblogs.com/rusking/p/13420060.html