vnc 用 gdm 去 login

viewer ---> vncserver --> xdmcp ---> xserver

xinetd

service vnc
{
   disable     = no
   socket_type = stream
   protocol    = tcp
   wait        = no
   user        = nobody
   server      = /usr/bin/Xvnc
   server_args = -inetd -once -query localhost -geometry 1024x800 -depth 16
   type        = UNLISTED
   port        = 5900
}

-query localhost. This option tells the VNC X server to check with the localhost system for XDMCP authentication.

XDMCP support for VNC

/etc/X11/gdm/custom.conf    

[xdmcp]    
enable=true