X authorization

[dkklee@MIT.EDU]

How is the X authorization set up in Debian?

I am running xdm and I discovered that only the login user can
start an X window. There are 2 situations when I cannot start a 
X window:
(1) when I try to start a window after 'su' to root. I discovered this
    can be fixed if I do 'xhost +mymachinename'. How to this automatically?
    A line in Xaccess or some /etc/X11/X???.hosts file?
(2) after establishing a PPP connection to my service provider and 
    changing my hostname (I have only a dynamic IP address from my ISP).
    Here I suppose there is nothing I can do except do xhost from my ip-up
    script after fixing (1).

--Derek Lee