x11vnc

$ x11vnc -loop -rfbauth ~/.vnc/passwd

-loop (restarting the x11vnc process whenever it terminates)

$ x11vnc -bg -reopen -forever -rfbauth ~/.vnc/passwd

-bg - is running on the background
-reopen - after you will log in and the x11vnc will be terminated this swich renew x11vnc connection
-forever - after you close vnc client the x11vnc server won't close but keep "running"

Authentication
create password file

-storepasswd pass file (default file is ~/.vnc/passwd)

use authentication on RFB protocol

-rfbauth passwd-file

References :
hykang's blog: x11vnc指令教學 
[xubuntu] X11vnc as boot loaded service