Xdummy is used with seamless and desktop servers sessions on posix platforms.
Xdummy was originally developed by Karl Runge as a script to allow a standard X11 server to be used by non-root users with the dummy video driver
Since then, the X11 server gained the ability to run without those LD_SO_PRELOAD hacks and this is now available for most distributions.
Xvfb lacks the ability to simulate arbitrary DPI values and add or remove virtual monitors at runtime.
This affects some X11 application's geometry and font rendering, and prevents the use of the monitor subcommand.
Xdummy standalone
You can start a new display using the dummy driver without needing any special privileges (no root, no suid), you should specify your own log and config files:
Xorg -noreset +extension GLX +extension RANDR +extension RENDER \
-logfile ./10.log -config /etc/xpra/xorg.conf :10This is roughly equivallent to running Xvfb :10.
You can find a sample configuration file for dummy here: xorg.conf.
Xdummy with Xpra
With Xpra, this should have been configured automatically for you when installing - this is not enabled on Debian or Ubuntu.
You choose at build time whether or not to use Xdummy using the --with[out]-Xdummy build switch.
If your packages do not enable Xdummy by default, you may be able to switch to it by modifying the xvfb value in /etc/xpra/conf.d/55_server_x11.conf, something like:
xvfb=Xorg -dpi 96 -noreset -nolisten tcp \
+extension GLX +extension RANDR +extension RENDER \
-logfile ${HOME}/.xpra/Xvfb-10.log -config ${HOME}/xorg.conf
The -noreset option is only needed if the window manager is not the first application started on the display, for example if you use the --start-child= option, or if you want the display to survive once the window manager exits - generally, this is a good idea since xpra could crash and when it exits cleanly via xpra stop it already takes care of shutting down the X11 server.
By default, the configuration file shipped with xpra allocates 768MB of memory, and a maximum virtual size of 11520 6318.
You may want to increase these values to use very high resolutions or many virtual monitors.
Most recent distributions now ship compatible packages:
Xorgversion 1.12 or laterdummydriver version 0.3.5 or later
Starting with dummy version 0.4.0, only one optional patch is added to the version found in the xpra repositories: https://github.com/Xpra-org/xpra/blob/master/packaging/rpm/patches/0006-Dummy-Disconnect.patch
libGL Driver Conflicts
With older distributions that do not use libglvnd, proprietary drivers usually install their own copy of libGL which conflicts with the use of software OpenGL rendering. You cannot use this GL library to render directly on Xdummy (or Xvfb).
The best way to deal with this is to use VirtualGL to take advantage of the OpenGL acceleration provided by the graphics card, just run: vglrun yourapplication.
To make vglrun work properly with Nvidia proprietary drivers make sure to create /etc/X11/xorg.conf using sudo nvidia-xconfig.
The alternative is often to disable OpenGL altogether, more information here: #580
Debian and Ubuntu
Debian and Ubuntu do weird things with their Xorg server which prevents it from running Xdummy (tty permission issues).
Warning: this may also interfere with other sessions running on the same server when they should be completely isolated from each other.
Crashing other X11 sessions is a serious security issue, caused by Debian's packaging and still left unsolved after many years.
non-suid binary
If you distribution ships the newer version but only installs a suid Xorg binary, Xpra should have installed the xpra_Xdummy wrapper script and configured xpra.conf to use it instead of the regular Xorg binary.
This script executes Xorg via ld-linux.so, which takes care of stripping the suid bit.
Some more exotic distributions have issues with non world-readable binaries which prevent this from working.