# Xdummy

`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.

## Why use `Xdummy` instead of `Xvfb`?

`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.

## Usage

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 :10

This 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.

## Configuration

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.

## Packaging

### versions required

Most recent distributions now ship compatible packages:

- `Xorg` version 1.12 or later
- `dummy` driver 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

### Other issues

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.