[systemd-devel] proper use of /run/{user/<uid>, }/systemd/private sockets

[Simon McVittie]

On Tue, 14 Nov 2017 at 09:19:08 +0100, Jérémy Rosen wrote:
> That being said, the system socket location is hardcoded not just in systemd,
> but in the dbus specification itself.

The D-Bus Specification does not say anything about the
.../systemd/private sockets. That would be inappropriate: those sockets
are private (not API), and are part of systemd (not D-Bus).

The socket path that is hard-coded in the D-Bus specification is the
well-known system bus socket, /var/run/dbus/system_bus_socket, used by
`dbus-daemon --system` and all D-Bus client implementations (dbus, GDBus,
sd-bus and others). This is present in the D-Bus Specification because
every D-Bus implementation needs to agree on it: otherwise they cannot
interoperate.

If you have dbus-daemon installed and available, then you can
communicate with the system instance of systemd (pid 1) via the D-Bus
system bus. This is a public API that offers the same functionality as
/run/systemd/private, except that it does not work without the system
dbus-daemon.

Similarly, if you have dbus-daemon installed and available and dbus was
configured with the --enable-user-session option (or on Debian derivatives
and similar, if the dbus-user-session package is installed), then you
can communicate with the per-user instance of systemd (systemd --user)
via the D-Bus session bus (which is a "user bus" in this configuration),
$XDG_RUNTIME_DIR/bus. This is a public API that offers the same
functionality as $XDG_RUNTIME_DIR/systemd/private, except that it does
not work without the per-user dbus-daemon.

(Or strictly speaking, you don't *need* dbus-daemon; you could be using
a compatible reimplementation instead.)

Regards,
    smcv