10.2. X Servers
starts up and "takes over" the bitmapped display on a computer system. This may happen automatically when a user logs in or it may happen when the
Usually at the time the X server starts, one or two
On systems that don't start an X server for you at login time, you can log in and run the
10.2.1. Screen Geometry
The layout of the screen managed by the X server is called
. A bitmapped display has a certain
Screen geometry is specified by referencing either a specific position on the screen (i.e., 500 x 200) or
Figure 10-1 shows the positioning of various geometry specifications.
Figure 10-1. X Window System screen geometry specifications.
10.2.2. Security and Authorization
As you may have guessed by now, the ability to scribble on any computer screen in your network could lead to security problemsnot so much because the act of writing on someone else's screen is anything more than annoying, if the recipient does not want it, but because I/O to an X server is just that, input and output. Write access to an X server also gives you the ability to query that system for a current copy of the display or even keyboard input.
Because of this, the X Window System has a certain amount of security built into the X server. It isn't highly rated security, but it is enough to keep the casual snoop from gaining unauthorized access.
By default, the X server running on any computer system only allows X clients on that same system to talk to it. The X server does not accept connections from "foreign" X clients without knowing who they are. This causes the default configuration of an X server to be very much like a traditional window system, where only applications running on that computer can write to that computer's display. In order to take advantage of the network capabilities of the X Window System, you must allow outside access.
The xhost command (an X client) is used to allow X clients on other systems to display to your system (Figure 10-2).
Figure 10-2. Description of the xhost command.
$ xhost +bluenote
will allow X clients running on the computer called "bluenote" to write to the display on the system where the xhost utility was run. Later, when whatever you needed to run is finished, you can disallow access with the command:
$ xhost -bluenote
In a secure environment where you aren't afraid of other systems writing to your display, you can allow any X client on the network to write to your display with the command:
$ xhost +
You can also take away access to all remote X clients with:
$ xhost -