Purpose: Help users deal with the problem if launching a Tk application reports:
Xlib: connection to "host.domain:0.0" refused by server
Xlib: Client is not authorized to connect to Server
Application initialization failed: couldn't connect to display "host.domain:0.0"Well, at least you're not getting the dreaded message
X server insecure (must use xauth-style authorization)
You've successfully locked down your X server, and discovered that you've locked yourself out.
The most likely explanation is that you're not logged in as the same user as when you entered your name and password to the display manager or ran x11start (startx, whatever your command is that starts the X server). This isn't a catastrophe, as long as you can become that user. That user has a file called .xauth in the home directory that contains the required magic cookie that you need to access the display.
This is one of the few cases where there's actually an application for the xauth command. On the machine where you originally logged in, as the same user, do
xauth extract - $DISPLAY
and pipe the result of that into
xauth merge -
as the user you now are on the machine where you now are. Often the correct magic is
ssh -l USERID HOST xauth extract - $DISPLAY | xauth merge -
where
If you don't have ssh, try rsh or whatever you use to execute a remote command.