--- name: "Xdummy" section: "8" platform: "linux" package: "x11vnc" version: "0.9.17-1" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/Xdummy" --- # Xdummy(8) A hack to run Xorg or XFree86 X server with the "dummy" video driver ## NAME Xdummy - A hack to run Xorg or XFree86 X server with the "dummy" video driver ## SYNOPSIS Xdummy \ \ ## DESCRIPTION A hack to run a stock [Xorg(1)](/Xorg/1.md) 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)](/Xvfb/1.md). 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,\ e.g: /usr/lib/xorg/modules/drivers/dummy\_drv.so some have it in a package named xserver-xorg-video-dummy you that\ need to install. ## OPTIONS Xdummy-args: \-install Compile the LD\_PRELOAD shared object and install it\ next to the Xdummy script file as: /usr/bin/Xdummy.so 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: /usr/bin/Xdummy.so 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 Xdummy.so\ 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)](/su/1.md) 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:\ /usr/bin/Xorg\ /usr/X11R6/bin/Xorg\ /usr/X11R6/bin/XFree86 \-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\ started. \-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)](/strace/1.md) the Xserver process (for troubleshooting.)\ \-ltrace [ltrace(1)](/ltrace/1.md) instead of strace (can be slow.) \-h, -help Print out this help. Xserver-args: 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. ## EXAMPLES: 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)](/su/1.md) 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: TBD. 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 \ #include \ #include \ #include \ int main (int argc, char \*argv\[\]) {\ extern char \*\*environ;\ char str\[100\];\ sprintf(str, "XDUMMY\_UID=%d", (int) getuid());\ putenv(str);\ setuid(0); \ setgid(0);\ execv("/usr/local/bin/Xdummy", argv); \ [exit(1)](/exit/1.md);\ return 1; } ## NOTES 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.