here's my config.
1. First, I created a password for authentication.
x11vnc -storepasswd /home/user/.vnc/passwd
2. Now ready to run x11vnc
x11vnc -rfbauth /home/user/.vnc/passwd -display :0 -forever -bg
that's it. I am now able to access via vnc. Of course, make sure iptables allowed you to access it.