Scroll to navigation

man(8) Xdummy man page man(8)


Xdummy - A hack to run Xorg or XFree86 X server with the "dummy" video driver


Xdummy <Xdummy-args> <Xserver-args>


A hack to run a stock Xorg(1) or XFree86(1) X server with the "dummy"
(RAM-only framebuffer) video driver such that it AVOIDS the Linux VT
switching, opening device files in /dev, keyboard and mouse conflicts,
and other problems associated with the normal use of "dummy".

In other words, it tries to make Xorg/XFree86 with the "dummy"
device driver act more like Xvfb(1).

The primary motivation for the Xdummy script is to provide a virtual X
server for x11vnc but with more features than Xvfb (or Xvnc); however
it could be used for other reasons (e.g. better automated testing
than with Xvfb.) One nice thing is the dummy server supports RANDR
dynamic resizing while Xvfb does not.

So, for example, x11vnc+Xdummy terminal services are a little better
than x11vnc+Xvfb.

To achieve this, while running the real Xserver Xdummy intercepts
system and library calls via the LD_PRELOAD method and modifies
the behavior to make it work correctly (e.g. avoid the VT stuff.)
LD_PRELOAD tricks are usually "clever hacks" and so might not work
in all situations or break when something changes.

WARNING: Take care in using Xdummy, although it never has it is
possible that it could damage hardware. One can use the -prconf
option to have it print out the xorg.conf config that it would use
and then inspect it carefully before actually using it.

This program no longer needs to be run as root as of 12/2009.
However, if there are problems for certain situations (usually older
servers) it may perform better if run as root (use the -root option.)
When running as root remember the previous paragraph and that Xdummy
comes without any warranty.

gcc/cc and other build tools are required for this script to be able
to compile the LD_PRELOAD shared object. Be sure they are installed
on the system. See -install and -uninstall described below.

Your Linux distribution may not install the dummy driver by default,


some have it in a package named xserver-xorg-video-dummy you that
need to install.



-install Compile the LD_PRELOAD shared object and install it
next to the Xdummy script file as:


When that file exists it is used as the LD_PRELOAD
shared object without recompiling. Otherwise,
each time Xdummy is run the LD_PRELOAD shared
object is compiled as a file in /tmp (or -tmpdir)

If you set the environment variable
INTERPOSE_GETUID=1 when building, then when
Xdummy is run as an ordinary user, the shared
object will interpose getuid() calls and pretend
to be root. Otherwise it doesn't pretend to
be root.

You can also set the CFLAGS environment variable
to anything else you want on the compile cmdline.

-uninstall Remove the file:


The LD_PRELOAD shared object will then be compiled
each time this program is run.

The X server is not started under -install, -uninstall, or -prconf.

:N The DISPLAY (e.g. :15) is often the first
argument. It is passed to the real X server and
also used by the Xdummy script as an identifier.

-geom geom1[,geom2...] Take the geometry (e.g. 1024x768) or list
of geometries and insert them into the Screen
section of the tweaked X server config file.
Use this to have a different geometry than the
one(s) in the system config file.

The option -geometry can be used instead of -geom;
x11vnc calls Xdummy and Xvfb this way.

-nomodelines When you specify -geom/-geometry, Xdummy will
create Modelines for each geometry and put them
in the Monitor section. If you do not want this
then supply -nomodelines.

-depth n Use pixel color depth n (e.g. 8, 16, or 24). This
makes sure the X config file has a Screen.Display
subsection of this depth. Note this option is
ALSO passed to the X server.

-DEPTH n Same as -depth, except not passed to X server.

-tmpdir dir Specify a temporary directory, owned by you and
only writable by you. This is used in place of
/tmp/Xdummy.$USER/.. to place the
shared object, tweaked config files, etc.

-nonroot Run in non-root mode (working 12/2009, now default)

-root Run as root (may still be needed in some
environments.) Same as XDUMMY_RUN_AS_ROOT=1.

-nosudo Do not try to use sudo(1) when re-running as root,
use su(1) instead.

-xserver path Specify the path to the Xserver to use. Default
is to try "Xorg" first and then "XFree86". If
those are not in $PATH, it tries these locations:

-n Do not run the command to start the X server,
just show the command that Xdummy would run.
The LD_PRELOAD shared object will be built,
if needed. Also note any XDUMMY* environment
variables that need to be set.

-prconf Print, to stdout, the tweaked Xorg/XFree86
config file (-config and -xf86config server
options, respectively.) The Xserver is not

-notweak Do not tweak (modify) the Xorg/XFree86 config file
(system or server command line) at all. The -geom
and similar config file modifications are ignored.

It is up to you to make sure it is a working
config file (e.g. "dummy" driver, etc.)
Perhaps you want to use a file based on the
-prconf output.

-debug Extra debugging output.

-strace strace(1) the Xserver process (for troubleshooting.)
-ltrace ltrace(1) instead of strace (can be slow.)

-h, -help Print out this help.


Most of the Xorg and XFree86 options will work and are simply
passed along if you supply them. Important ones that may be
supplied if missing:

:N X Display number for server to use.

vtNN Linux virtual terminal (VT) to use (a VT is currently
still used, just not switched to and from.)

-config file Driver "dummy" tweaked config file, a
-xf86config file number of settings are tweaked besides Driver.

If -config/-xf86config is not given, the system one
(e.g. /etc/X11/xorg.conf) is used. If the system one cannot be
found, a built-in one is used. Any settings in the config file
that are not consistent with "dummy" mode will be overwritten
(unless -notweak is specified.)

Use -config xdummy-builtin to force usage of the builtin config.

If "file" is only a basename (e.g. "xorg.dummy.conf") with no /'s,
then no tweaking of it is done: the X server will look for that
basename via its normal search algorithm. If the found file does
not refer to the "dummy" driver, etc, then the X server will fail.


Xdummy -install

Xdummy :1

Xdummy -debug :1

Xdummy -tmpdir ~/mytmp :1 -nolisten tcp

startx example:

startx -e bash -- Xdummy :2 -depth 16

(if startx needs to be run as root, you can su(1) to a normal
user in the bash shell and then launch ~/.xinitrc or ~/.xsession,
gnome-session, startkde, startxfce4, etc.)

xdm example:

xdm -config /usr/local/dummy/xdm-config -nodaemon

where the xdm-config file has line:

DisplayManager.servers: /usr/local/dummy/Xservers

and /usr/local/dummy/Xservers has lines:

:1 local /usr/local/dummy/Xdummy :1 -debug
:2 local /usr/local/dummy/Xdummy :2 -debug

(-debug is optional)

gdm/kdm example:


Root permission and x11vnc:

Update: as of 12/2009 this program no longer must be run as root.
So try it as non-root before running it as root and/or the
following schemes.

In some circumstances X server program may need to be run as root.
If so, one could run x11vnc as root with -unixpw (it switches
to the user that logs in) and that may be OK, some other ideas:

- add this to sudo via visudo:

ALL ALL = NOPASSWD: /usr/local/bin/Xdummy

- use this little suid wrapper: /*
* xdummy.c
cc -o ./xdummy xdummy.c
sudo cp ./xdummy /usr/local/bin/xdummy
sudo chown root:root /usr/local/bin/xdummy
sudo chmod u+s /usr/local/bin/xdummy
*/ #include <unistd.h> #include <stdlib.h> #include <sys/types.h> #include <stdio.h>

int main (int argc, char *argv[]) {
extern char **environ;
char str[100];
sprintf(str, "XDUMMY_UID=%d", (int) getuid());
execv("/usr/local/bin/Xdummy", argv);
return 1; }


The Xorg/XFree86 "dummy" driver is currently undocumented. It works
well in this mode, but it is evidently not intended for end-users.
So it could be removed or broken at any time.

If the display Xserver-arg (e.g. :1) is not given, or ":" is given
that indicates Xdummy should try to find a free one (based on
tcp ports.)

If the display virtual terminal, VT, (e.g. vt9) is not given that
indicates Xdummy should try to find a free one (or guess a high one.)

This program is not completely secure WRT files in /tmp (but it tries
to a good degree.) Better is to use the -tmpdir option to supply a
directory only writable by you. Even better is to get rid of users
on the local machine you do not trust :-)

Set XDUMMY_SET_XV=1 to turn on debugging output for this script.

19 Mar 2019 1.0