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# D-Bus

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Reason: Mention disabling of dbus services through use of `systemctl mask` and overrides in `/etc/dbus-1/services` (Discuss in Talk:D-Bus)

D-Bus is a message bus system that provides an easy way for inter-process communication. It consists of a daemon, which can be run both system-wide and for each user session, and a set of libraries to allow applications to use D-Bus.

dbus is pulled and installed as a dependency of systemd and user session bus is started automatically for each user.

## Implementations

Arch provides two D-Bus message broker implementations. Initially, the user will be asked to choose a desired dbus-units provider during installation of systemd package. Only one implementation can be installed at a time.

### dbus-broker

dbus-broker is currently the default implementation for Arch [1] [2]. It is a drop-in replacement for the reference implementation, which aims "to provide high performance and reliability, while keeping compatibility to the D-Bus reference implementation".

Select dbus-broker-units when asked for dbus-units provider, or install it explicitly.

Note dbus-broker currently does not have support for AppArmor. However, officially supported kernels do not support AppArmor mediation for D-Bus, either.

### Reference implementation

The reference implementation is still officially supported by Arch.

Select dbus-daemon-units when asked for dbus-units provider, or install it explicitly.

## Tips and tricks

### Override dbus service

This is useful when specifying a particular service among other services providing the same well-known bus name. See KeePass#Autostart and KDE Wallet#Automatic D-Bus activation for example.

D-Bus services can be masked by setting `Exec=/bin/false` in service files of `$XDG_DATA_HOME/dbus-1/services`. For example, to mask gvfsd,

    $ cp /usr/share/dbus-1/services/org.gtk.vfs.Daemon.service ~/.local/share/dbus-1/services
    $ sed -i 's|^Exec=.*|Exec=/bin/false|' ~/.local/share/dbus-1/services/org.gtk.vfs.Daemon.service

If the service is already launched, the override will not work. The existing service's process must be killed, or launched earlier.

## Debugging

- Bustle — Draws sequence diagrams of D-Bus activity. It shows signal emissions, method calls and their corresponding returns, with time stamps for each individual event and the duration of each method call.

  https://apps.gnome.org/Bustle/ || bustle

- D-Feet — Easy to use D-Bus debugger GUI tool. Discontinued.

  https://wiki.gnome.org/Apps/DFeet || d-feetAUR

- D-Spy — Easy to use D-Bus debugger GUI tool. D-Spy can be used to inspect D-Bus interfaces of running programs and invoke methods on those interfaces.

  https://apps.gnome.org/Dspy/ || d-spy

- Qt D-Bus Viewer — GUI D-Bus debugger. Can be used to inspect D-Bus services and invoke methods on them.

  https://doc.qt.io/qt-6/qdbusviewer.html || qt6-tools

You can also use busctl(1) from systemd.

## See also

- https://freedesktop.org/wiki/Software/dbus/
- https://freedesktop.org/wiki/IntroductionToDBus/
- https://uyha.github.io/technical/dbus-systemd.html systemd uses DBus as the mechanism to interact with it. This article introduces just enough DBus concepts and the usage of `busctl` to communicate with systemd.

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