The X Window System is a client-server graphics system:
- the server manages the display hardware, each typically including one or more video cards and monitors, one or more keyboards, and one or more mice or other pointing devices.
- the client software connects to the server through one of several network transports to draw on the display, get status information, and receive input from the keyboard and mouse.
In a typical Linux system, the server and clients run on the same computer, although one or more clients may also be remotely connected.
X servers are identified by a display specification or displayspec:
- server is the hostname of the server: blank for local connections via the fastest avilable means;
localhostfor local connections via TCP/IP; or a hostname for remote connections via TCP/IP.
- display is the display number. These are numbered starting at zero. The port number used for TCP/IP connections is (6000+display).
- screen is the screen number if the display supports multiple screens (this is mostly obsolete; most multi-screen X servers now merge them into a single display, using the Xinerama protocol extension).
- the default local display number is
- display 0 on the host
Displayspecs are communicated to client programs through the DISPLAY environment variable (or, less often, through command-line arguments).
Remote Access to an X Server with SSH
To run a client in a remote location from the server, you can run the raw X protocol over tcp/ip or other network transports. However, this is very insecure and is not recommended. Instead, current best practice is to use an ssh session to tunnel the X traffic.
Most standard ssh clients can set up the tunnel, X authentication, and other details necessary by simply including the -X or -Y command line parameters.
In these examples:
- red is the local computer, running an X server
- blue is the remote computer on which the client program will be run
- user is the name of the user on blue
To connect to blue and start a shell, from which any program may be run:
ssh -XC user@blue
-X argument sets up the X tunnel, while the
-C argument turns on data compression for improved performance.
To connect to blue and start the program gnome-calculator:
ssh -XC user@blue gnome-calculator
VNC is a remote display software that originated at Olivetti Research Labs in Cambridge UK. It is cross-platform and very lightweight. The underlying protocol is called rfb (for remote frame buffer). There are many different software packages that support various aspects of the vnc/rfb system.
When used with the X Window System, it is necessary to have a dual server that functions as both an X server and a VNC server. The easiest way to do this is to run the
vncserver script from the
tigervnc-server package, which configures and then start the
Xvnc server. To connect X client, set and export the
DISPLAY environment variable; to connect a VNC client, use this command:
vncviewer -via blue localhost:display
Xvfb is a dummy X server that does not display output on any device. This is useful where a program needs to connect to an X server, perhaps for testing purposes, but the actual image drawn does not have to be viewed anywhere. Xvnb would typically be invoked with a command such as:
Xvfb :0 -ac
:0 is the display number and
-ac disables access control to allow any local client to connect. You can then connect a client by setting and exporting the
DISPLAY variable and starting the client:
export DISPLAY=:0 gnome-calculator
- X Power Tools, a book by Chris Tyler published by O'Reilly Media and available through the Seneca Libraries on-line services.