X server in Linux: facts and tips

Some facts:

• X service accepts connections on port number 6000+display number, for example fourth display will be on 6004/TCP port.


• You can start X using startx script or XDMCP(X Display Manager Control Protocol) server, such as kdm, xdm or gdm.


• startx - bash script, is a front end to xinit that provides a somewhat nicer user interface for running a single session of the X Window System. It is often run with no arguments. It uses user's .xinitrc file.


• xinit - is used to start the X Window System server and a first client program.


• xterm - terminal for X enviroment


• When X application is started, it connects to X display(specified in DISPLAY environment variable). Display consists of three parts: host:display number:screen number, for example: osiris:0.0 If display number is zero, then application will connect to 6000 TCP port, if display number is 1, then to 6001 TCP port and so on....

Launch X applications remotely via network

Suppose, we want to launch gcalctool on computer A from computer B. First, wee need to allow connections to X server on computer B from network. To accomplish this, X server must be launched without "notcp" option. Usually X is started via display manager. For example, consider GDM. In GDM configuration file change DisallowTCP=true to false.

Next add computer A to acl on computer B:

xhost +A

Then login via telnet or ssh to computer A from computer B and change DISPLAY environmental variable:

export DISPLAY=B:0.0

After this launch gcalctool on computer A and you will see calculator on computer B :)

Encrypt X connections with SSH

SSH protocol can tunnel other protocols. It can be used to encrypt and compress X connections:

ssh -C -X user@server

After login, you can launch X applications. Using SSH is the most preferable way to launch X applications remotely. First it is secure. Second it is more comfortable: you don't need to use xhost, allow connections to X server and edit DISPLAY variable.

Tips

• Start another X server in virtual terminal 8

startx -- :1 vt8

• Reconfigure X server. In Debian:

/etc/init.d/kdm stop

dpkg-reconfigure xserver-xorg

• In other distributions:

# cd /etc/X11/ && Xorg -configure

• Display information about X:

xdpyinfo

• Obtain detailed technical information about a specific window:

xwininfo

• Read all the fonts in the current directory and creates a fonts.scale file:

mkfontscale

• Combine the fonts.scale file with the fonts.dir file, creating it if it doesn’t already exist

mkfontdir

• Add the font path to a running system:

$ xset fp+ /your/font/directory

$ xset fp rehash

• Select an X core font for display

xfontsel

• Cause Xft to run through its font directories and create index files:

fc-cache

• Select what XDMCP server to use, in Fedora: /etc/sysconfig/desktop file, in openSUSE: /etc/sysconfig/displaymanager, in Debian and Ubuntu via SysV startup script


• XDM’s main configuration file is /etc/X11/xdm/xdm-config


• Make screenshot of remote X server:

xwd -display 192.168.0.1:0 -root -out 192.168.0.1.xpm

Most graphic viewers can open xpm files.