stNicolaas/x11docker

Run GUI applications and desktops in docker. Focus on security.

x11docker: Run GUI applications in Docker

Avoid X security leaks and enhance container security

Introduction

x11docker allows to run graphical desktop applications (and entire desktops) in Docker Linux containers.

• Docker allows to run applications in an isolated container environment. Containers need much less resources than virtual machines for similar tasks.
• Docker does not provide a display server that would allow to run applications with a graphical user interface.
• x11docker fills the gap. It runs an X display server on the host system and provides it to Docker containers.
• Additionally x11docker does some security setup to enhance container isolation and to avoid X security leaks. This allows a sandbox environment that fairly well protects the host system from possibly malicious or buggy software.

Software can be installed in a deployable Docker image with a rudimentary Linux system inside. This can help to run or deploy software that is difficult to install on several systems due to dependency issues. It is possible to run outdated versions or latest development versions side by side. Files to work on can be shared between host and container.

x11docker runs on Linux and (with some setup and limitations) on MS Windows. x11docker is not adapted to run on macOS except in a Linux VM.

x11docker wiki provides some how-to's for basic setups without x11docker.

Features

• Focus on security:
  • Avoids X security leaks by running additional X servers.
  • Restricts container capabilities to bare minimum.
  • Container user is same as host user to avoid root in container.
• Low dependencies:
  • No obliging dependencies on host beside X and Docker. Recommended: xpra and Xephyr.
  • No dependencies inside of Docker images except for some optional features.
• Several optional features like GPU, sound, webcam and printer support.
• Remote access with SSH, VNC or HTML5 possible.
• Easy to use. Examples:
  • x11docker jess/cathode
  • x11docker --desktop --size 320x240 x11docker/lxde (needs nested X server Xephyr)

Table of contents

• GUI for x11docker
• Terminal usage
• Options
  • Choice of X servers and Wayland compositors
  • Desktop or seamless mode
  • Shared folders and HOME in container
  • Hardware acceleration
  • Clipboard
  • Sound
  • Webcam
  • Printer
  • Language locales
  • Wayland
  • Init system
  • DBus
  • Container runtime
• Security
  • Options degrading container isolation
  • Sandbox
  • Security and feature check
• Installation
  • Installation options
  • Shortest way for first installation
  • Minimal installation
  • Installation on MS Windows
• Dependencies
• Troubleshooting
• Contact
  • Issues
  • Contributing
  • Support
• Examples
  • Adjust images for your needs
  • Screenshots

GUI for x11docker

x11docker-gui is an optional graphical frontend for x11docker. It runs from console, too.

• x11docker-gui needs package kaptain. If your distribution misses it, look at kaptain repository.
• If kaptain is not installed on your system, x11docker-gui uses image x11docker/kaptain.

Terminal usage

Just type x11docker IMAGENAME [COMMAND].

• Get an overview of options with x11docker --help.
  • For desktop environments in image add option -d, --desktop.
  • To run without X at all use option -t, --tty.
  • Get an interactive TTY with option -i, --interactive.
  • See generated docker command (and further infos) with option --debug.
• If startup fails, look at chapter Troubleshooting.

General syntax:

To run a Docker image with new X server:
  x11docker [OPTIONS] IMAGE [COMMAND]
  x11docker [OPTIONS] -- IMAGE [COMMAND [ARG1 ARG2 ...]]
  x11docker [OPTIONS] -- DOCKER_RUN_OPTIONS -- IMAGE [COMMAND [ARG1 ARG2 ...]]
To run a host application on a new X server:
  x11docker [OPTIONS] --exe COMMAND
  x11docker [OPTIONS] --exe -- COMMAND [ARG1 ARG2 ...]
To run only a new empty X server:
  x11docker [OPTIONS] --xonly

DOCKER_RUN_OPTIONS are just added to docker run command without a check by x11docker.

Options

Description of some commonly used feature options.

• Some of these options have dependencies on host and/or in image. Compare wiki: feature dependencies.

Choice of X servers and Wayland compositors

If no X server option is specified, x11docker automatically chooses one depending on installed dependencies and on given or missing options --desktop, --gpu and --wayland. Most recommended are xpra and Xephyr.

• Overview of all possible X server and Wayland options.
  • Hints to use option --xorg within X.
  • Use option -t, --tty to run without X at all.

Desktop or seamless mode

x11docker assumes that you want to run a single application in seamless mode, i.e. a single window on your regular desktop. If you want to run a desktop environment in image, add option --desktop.

• Seamless mode is supported with options --xpra and --nxagent. As a fallback insecure option --hostdisplay is possible.
  • If neither xpra nor nxagent are installed, but x11docker finds a desktop capable X server like Xephyr, it avoids insecure option --hostdisplay and runs Xephyr with a host window manager.
    • You can specify a host window manager with option --wm=WINDOWMANAGER, for example --wm=openbox.
• Desktop mode with --desktop is supported with all X server options except --hostdisplay. If available, x11docker prefers --xephyr and --nxagent.

Shared folders and HOME in container

Changes in a running Docker container system will be lost, the created Docker container will be discarded. For persistent data storage you can share host directories:

• Option -m, --home creates a host directory in ~/.local/share/x11docker/IMAGENAME that is shared with the container and mounted as its HOME directory. Files in container home and configuration changes will persist. x11docker creates a softlink from ~/.local/share/x11docker to ~/x11docker.
• Option --share PATH mounts a host file or folder at the same location in container. --share PATH:ro restricts to read-only access. Device files in /dev are supported, too.
• Option --homedir DIR is similar to --home but allows you to specify a custom host directory for data storage.
• Special cases for $HOME:
  • --homedir $HOME will use your host home as container home. Discouraged, use with care.
  • --share $HOME will symlink your host home as a subfolder of container home.

Note that x11docker copies files from /etc/skel in container to HOME if HOME is empty. That allows to provide customized user settings.

Hardware acceleration

Hardware acceleration for OpenGL is possible with option -g, --gpu.

• This will work out of the box in most cases with open source drivers on host. Otherwise have a look at wiki: feature dependencies.
• Closed source NVIDIA drivers need some setup and support less x11docker X server options.

Clipboard

Clipboard sharing is possible with option -c, --clipboard.

• Image clips are possible with --xpra and --hostdisplay.
• Some X server options need package xclip on host.

Sound

Sound is possible with options -p, --pulseaudio and --alsa.

• For pulseaudio sound with --pulseaudio you need pulseaudio on host and pulseaudio libraries in image. Compare wiki: feature dependencies.
• For ALSA sound with --alsa you might need to specify a sound card with e.g. --alsa=Generic. Get a list of available sound cards with aplay -l.

Webcam

Webcams on host can be shared with option --webcam.

• If webcam application in image fails, install --gpu dependencies in image. Compare wiki: feature dependencies.
• guvcview needs --pulseaudio or --alsa.
• cheese and gnome-ring need --init=systemd or --dbus-system.

Printer

Printers on host can be provided to container with option --printer.

• It needs cups on host, the default printer server for most linux distributions.
• The container needs cups client libraries in image. Compare wiki: feature dependencies.

Language locales

x11docker provides option --lang for flexible language locale settings.

• --lang without an argument sets LANG in container to same as on host. Same as --lang=$LANG
• x11docker will check on container startup if the desired locale is already present in image and enable it.
• If x11docker does not find the locale, it creates it on container startup. This needs some locale packages in image. Compare wiki: feature dependencies.
• Examples: --lang=de for German, --lang=zh_CN for Chinese, --lang=ru for Russian, --lang=$LANG for your host locale.
• For support of chinese, japanese and korean characters install a font like fonts-arphic-uming in image.

Wayland

To run Wayland instead of an X server x11docker provides options --wayland, --weston, --kwin and --hostwayland. For further description loot at wiki: Description of Wayland options.

• Option --wayland automatically sets up a Wayland environment with some related environment variables.
• Options --kwin and --weston run Wayland compositors kwin_wayland or weston.
  • For QT5 applications without option --wayland add options --dbus and --env QT_QPA_PLATFORM=wayland.
• Option --hostwayland can run single applications on host Wayland desktops like Gnome 3, KDE 5 and Sway.
• Example: xfce4-terminal on Wayland: x11docker --wayland x11docker/xfce xfce4-terminal

Init system

x11docker supports several init systems as PID 1 in container with option --init. Init in container solves the zombie reaping issue. As default x11docker uses tini in/usr/bin/docker-init. Also available are systemd, SysVinit, runit, OpenRC and s6-overlay. elogind is supported, too. Look at wiki: Init systems in Docker.

DBus

Some desktop environments and applications need a running DBus daemon and/or DBus user session. DBus options need dbus in image.

• use --dbus to run a DBus user session daemon.
• use --dbus-system to run DBus system daemon. This includes option --dbus.
  • If startup fails or takes about 90s, install an init system and use that one to run DBus. E.g. install systemd in image and run with --init=systemd.
• use --hostdbus to connect to host DBus user session.
• use --share /run/dbus/system_bus_socket to share host DBus system socket.
• DBus will be started automatically with init systems systemd, openrc, runit and sysvinit (option --init).

Container runtime

It is possible to run containers with different backends following the OCI runtime specification. Docker's default runtime is runc. You can specify another one with option --runtime=RUNTIME. Container runtimes known and supported by x11docker are:

• runc: Docker default.
• kata-runtime: Sets up a virtual machine with its own Linux kernel to run the container. kata aims to combine the security advantages of containers and virtual machines.
  • Some x11docker options are not possible with --runtime=kata-runtime. Most important: --hostdisplay, --gpu, --printer, --webcam and all Wayland related options.
• nvidia: Specialized fork of runc to support nvidia/nvidia-docker images.
• crun: Fast and lightweight alternative to runc with same functionality.

Possible runtime configuration in /etc/docker/daemon.json:

{
  "default-runtime": "runc",
  "runtimes": {
    "kata-runtime": {
      "path": "/opt/kata/bin/kata-runtime",
      "runtimeArgs": [
        "--kata-config /opt/kata/share/defaults/kata-containers/configuration.toml"
      ]
    },
    "crun": {
      "path": "/usr/local/bin/crun",
      "runtimeArgs": []
    },
    "nvidia": {
      "path": "nvidia-container-runtime",
      "runtimeArgs": []
    }
  }
}

Security

Scope of x11docker is to run containerized GUI applications while preserving and improving container isolation. Core concept is:

• Runs a second X server to avoid X security leaks.
  • This in opposite to widespread solutions that share host X socket of display :0, thus breaking container isolation, allowing keylogging and remote host control. (However, x11docker provides this with fallback option --hostdisplay).
  • Authentication is done with MIT-MAGIC-COOKIE, stored separate from file ~/.Xauthority.
• Creates container user similar to host user to avoid root in container.
  • You can also specify another user with --user=USERNAME or a non-existing one with --user=UID:GID.
  • Disables possible root password and deletes entries in /etc/sudoers.
    • If you want root permissions in container, use option --sudouser that allows su and sudo with password x11docker. Alternatively you can run with --user=root.
  • If you want to use USER specified in image instead, set option --user=RETAIN. x11docker won't change container's /etc/passwd or /etc/sudoers in that case. Option --home won't be available.
• Reduces container capabilities to bare minimum.
  • Sets docker run option --cap-drop=ALL to drop all capabilities. Most applications don't need them.
  • Sets docker run option --security-opt=no-new-privileges.
  • These restrictions can be disabled with x11docker option --cap-default or reduced with --sudouser or --user=root.

That being said, Docker's default capabilities and its seccomp profile are not bad. I am not aware of an escape from a container without an additional isolation degrading option or configuration. However, x11docker follows the principle of least privilege. Docker containers should not have capabilities or privileges that they don't need for their job.

Docker's default runtime runc uses Linux namespaces to isolate container applications, but shares the kernel from host. If you are concerned about container access to host kernel, consider to use container runtime kata-runtime instead.

Weaknesses:

• Possible SELinux restrictions are degraded for x11docker containers with docker run option --security-opt label=type:container_runtime_t to allow access to new X unix socket. A more restrictive solution is desirable. Compare: SELinux and Docker: allow access to X unix socket in /tmp/.X11-unix
• A possible user namespace remapping setup is disabled to allow options --home, --homedir and --share without file ownership issues.
  • This is less an issue because x11docker already avoids root in container.
  • Exception: User namespace remapping is not disabled for --user=RETAIN.
• x11docker provides several different X server options. Each X server involved might have its individual vulnerabilities. x11docker only covers well-known X security leaks that result from X11 protocol design.

Options degrading container isolation

x11docker shows warning messages in terminal if chosen options degrade container isolation. Note that x11docker does not check custom DOCKER_RUN_OPTIONS.

Most important:

• --hostdisplay shares host X socket of display :0 instead of running a second X server.
  • Danger of abuse is reduced providing so-called untrusted cookies, but do not rely on this.
  • If additionally using --gpu or --clipboard, option --hostipc and trusted cookies are enabled and no protection against X security leaks is left.
  • If you don't care about container isolation, x11docker --hostdisplay --gpu is an insecure but quite fast setup without any overhead.
• --gpu allows access to GPU hardware. This can be abused to get window content from host (palinopsia bug) and makes GPU rootkits possible.
• --pulseaudio and --alsa allow catching audio output and microphone input from host.

Rather special options reducing security, but not needed for regular use:

• --sudouser and --user=root allow su and sudo with password x11dockerfor container user. If an application somehow breaks out of container, it can harm your host system. Allows many container capabilties that x11docker would drop otherwise.
• --cap-default disables x11docker's container security hardening and falls back to default Docker container capabilities. If an application somehow breaks out of container, it can harm your host system.
• --dbus-system and --init=systemd|sysvinit|openrc|runit allow some container capabilities that x11docker would drop otherwise. --init=systemd also shares access to /sys/fs/cgroup. Some processes will run as root in container. If a root process somehow breaks out of container, it can harm your host system. Allows many container capabilties that x11docker would drop otherwise.
• --hostipc sets docker run option --ipc=host. Allows MIT-SHM / shared memory. Disables IPC namespacing.
• --hostnet sets docker run option --net=host. Shares host network stack. Disables network namespacing. Container can spy on an maybe manipulate host network traffic.
• --hostdbus allows communication over DBus with host applications.

Sandbox

Container isolation enhanced with x11docker allows to use containers as a sandbox that fairly well protects the host system from possibly malicious or buggy software. Though, no sandbox solution in the wild can provide a perfect secure protection, and Docker even with enhanced security settings from x11docker is no exception.

Using Docker with x11docker as a sandbox is not intended to run obviously evil software. Rather use it as:

• Compatibility environment to run software that is hard or impossible to install on host due to dependency issues.
• Development environment to collect libraries, compiler and so on to keep the host clean.
• Development environment to mitigate damage caused by unexpected/buggy behaviour.
• Security layer for software that may be malicious in worst case. Examples: Internet browser with Javascript enabled, or wine with Windows applications.

x11docker already restricts process capabilities. You can additionally restrict access to CPU and RAM with option --limit. As default --limit restricts to 50% of available CPUs and 50% of currently free RAM. Another amount can be specified with --limit=FACTOR with a FACTOR greater than zero and less than or equal one.

For more custom fine tuning have a look at Docker documentation: Limit a container's resources.

NOTE: Internet access is allowed per default. You can disable internet access with --no-internet.

WARNING: There is no restriction that can prevent the container from flooding the hard disk in Docker's container partition or in shared folders.

Security and feature check

To check container isolation and some feature options use image x11docker/check and try out with several options.

• An insecure setup is x11docker --hostdisplay --gpu x11docker/check. It fairly well demonstrates common X security leaks.
• Add options like --pulseaudio --alsa --webcam --clipboard to check their functionality.

Installation

Note that x11docker is just a bash script without library dependencies. Basically it is a wrapper for X servers and Docker. To allow advanced usage of x11docker abilities look at chapter Dependencies.

Installation options

As root you can install, update and remove x11docker to system directories to be available system-wide:

• x11docker --install : install x11docker and x11docker-gui from current directory. (Useful to install from an extracted zip file or a cloned git repository.)
• x11docker --update : download and install latest release from github.
• x11docker --update-master : download and install latest master version from github.
• x11docker --remove : remove all files installed by x11docker.

Copies x11docker and x11docker-gui to /usr/bin. Creates an icon in /usr/share/icons. Creates x11docker.desktop in /usr/share/applications. Copies README.md, CHANGELOG.md and LICENSE.txt to /usr/share/doc/x11docker.

Shortest way for first installation:

• For systems using sudo:

  curl -fsSL https://raw.githubusercontent.com/mviereck/x11docker/master/x11docker | sudo bash -s -- --update
  

• Directly as root:

  curl -fsSL https://raw.githubusercontent.com/mviereck/x11docker/master/x11docker | bash -s -- --update
  

Minimal installation

You can run x11docker from an arbitray location with bash x11docker. For minimal system-wide installation make x11docker executable with chmod +x x11docker and move it to /usr/bin (or another location in PATH). Other files than x11docker script itself are not essential.

Installation on MS Windows

x11docker can run natively on MS Windows electively in one of:

• MSYS2
• Cygwin
• WSL (Windows subsystem for Linux)

It needs X server VcXsrv.exe or Xwin.exe. Further informations at wiki: x11docker on MS Windows.

Dependencies

x11docker can run with standard system utilities without additional dependencies on host or in image.

• As a core it only needs bash, an X server and docker to run Docker containers on X.
• x11docker checks dependencies for chosen options on startup and shows terminal messages if some are missing.

For advanced usage of x11docker it is recommended to install some additional packages. The recommended base commands are: xpra Xephyr weston Xwayland xdotool xauth xinit xclip xhost xrandr xdpyinfo. Some of them are probably already installed.

• To provide these base commands see wiki: Dependencies - Recommended base for a package list matching your distribution.

Some feature options have additional dependencies on host and/or in image. This affects especially options --gpu, --printer and --pulseaudio. Compare wiki: feature dependencies.

Troubleshooting

For troubleshooting, run x11docker or x11docker-gui in a terminal.

• x11docker shows warnings if something is insecure, missing or going wrong.
  • Use option -v, --verbose to see full logfile output.
    • Option -D, --debug gives a less verbose output.
    • You can find the latest dispatched logfile at ~/.cache/x11docker/x11docker.log.
• Some applications fail with fallback option --hostdisplay. Add --clipboard to disable some security restrictions. If that does not help, install additional X servers.
• Make sure your x11docker version is up to date with x11docker --update (latest release) or x11docker --update-master (latest beta).
• The image may have a USER specification and be designed for this user. x11docker sets up a container user that can mismatch this container user setup.
  • Check for a USER specification in image with docker inspect --format '{{.Config.User}}' IMAGENAME.
  • If yes, try with --user=RETAIN to run with the USER specified in image.
• Some applications need more privileges or capabilities than x11docker provides as default.
  • Reduce container isolation with e.g.:
    • x11docker options: --cap-default --hostipc --hostnet. (Try --cap-default first).
    • docker run options: --cap-add ALL --security-opt seccomp=unconfined --privileged
    • Example: x11docker --cap-default --hostipc --hostnet -- --cap-add ALL --security-opt seccomp=unconfined --privileged -- IMAGENAME
    • Try with reduced container isolation. If it works, drop options one by one until the needed one(s) are left.
    • If --cap-add ALL helps, find the capability you really need and add only that one.
    • If --privileged helps, your application probably needs a device in /dev. Find out which one and share it with e.g. --share /dev/vboxdrv. Try also --share /run/udev/data:ro.
      • Please, don't use --privileged as a solution. It allows too much access to host and fatally breaks container isolation. Investigate the permissions your container needs indeed.
  • You can run container applications as root with --user=root.
• A few applications need DBus. Install dbus in image and try option --dbus. If that does not help, try option --dbus-system.
• A few applications need systemd. Install systemd in image and try option --init=systemd.

Contact

Feel free to open a ticket if you have a question or encounter an issue.

Issues

If reporting an issue:

• Have a look at chapter Troubleshooting.
• Most times it makes sense to store the --verbose output (or ~/.cache/x11docker/x11docker.log) at pastebin.com.

Contributing

If you want to contribute to x11docker, please open a ticket before creating a pull request. Often it is possible to accomplish desired tasks with already available options.

Support

Please open a ticket if you need support. Please note that x11docker is a non-commercial project maintained in free time. I'll help where I can, but there is no organisation behind x11docker that can provide large scale support.

Examples

x11docker image examples with desktop environments can be found on docker hub. A special one to check features and container isolation is x11docker/check.

Application	x11docker command
Xfce4 Terminal	x11docker x11docker/xfce xfce4-terminal
GLXgears with hardware acceleration	x11docker --gpu x11docker/xfce glxgears
Kodi media center with hardware acceleration, Pulseaudio sound and shared Videos folder. For setup look at ehough/docker-kodi.	x11docker --gpu --pulseaudio --share ~/Videos erichough/kodi.
XaoS fractal generator	x11docker patricknw/xaos
Telegram messenger with persistent HOME for configuration storage	x11docker --home xorilog/telegram
Firefox with shared Download folder.	x11docker --share $HOME/Downloads jess/firefox
Tor browser	x11docker jess/tor-browser
Chromium browser	x11docker -- jess/chromium --no-sandbox
VLC media player with shared Videos folder and Pulseaudio sound	x11docker --pulseaudio --share=$HOME/Videos jess/vlc

Desktop environment (most based on Debian)	x11docker command
FVWM (based on Alpine, 22.5 MB)	x11docker --desktop x11docker/fvwm
Fluxbox (based on Debian, 87 MB)	x11docker --desktop x11docker/fluxbox
Lumina (based on Void Linux)	x11docker --desktop x11docker/lumina
LXDE	x11docker --desktop x11docker/lxde
LXQt	x11docker --desktop x11docker/lxqt
Xfce	x11docker --desktop x11docker/xfce
CDE Common Desktop Environment	x11docker --desktop --init=systemd --cap-default x11docker/cde
Mate	x11docker --desktop x11docker/mate
Enlightenment (based on Void Linux)	x11docker --desktop --gpu --runit x11docker/enlightenment
Trinity (successor of KDE 3)	x11docker --desktop x11docker/trinity
Cinnamon	x11docker --desktop --gpu --dbus-system x11docker/cinnamon
deepin (3D desktop from China)	x11docker --desktop --gpu --init=systemd x11docker/deepin
LiriOS (needs at least docker 18.06 or this xcb bugfix.) (based on Fedora)	x11docker --desktop --gpu lirios/unstable
KDE Plasma	x11docker --desktop --gpu x11docker/plasma
KDE Plasma as nested Wayland compositor	x11docker --gpu x11docker/plasma startplasmacompositor
LXDE with wine and PlayOnLinux and a persistent HOME folder to preserve installed Windows applications, and with Pulseaudio sound.	x11docker --desktop --home --pulseaudio x11docker/lxde-wine

Adjust images for your needs

For persistent changes of image system adjust Dockerfile and rebuild. To add custom applications to x11docker example images you can create a new Dockerfile based on them. Example:

# xfce desktop with VLC media player
FROM x11docker/xfce
RUN apt-get update && apt-get install -y vlc

Screenshots

Sample screenshots are stored in screenshot branch

x11docker --desktop x11docker/lxde-wine

x11docker --desktop --gpu x11docker/plasma

x11docker --desktop x11docker/lxqt

x11docker --desktop --init=systemd --gpu x11docker/deepin