[systemd-devel] The dependencies between systemd-resolved and dbus.socket

[Mantas Mikulėnas]

On Fri, Apr 11, 2025 at 4:14 PM Jeremy Su <os369510 at gmail.com> wrote:

> Hi,
>
> I meet a problem that systemd-resolved is not able to resolve the DNS.
>
> systemd-resolved[1237]: sd-bus: starting bus bus-api-resolve by connecting
> to /run/dbus/system_bus_socket...
> systemd-resolved[1237]: Added inotify watch for /run on bus
> bus-api-resolve: 2
> systemd-resolved[1237]: Added inotify watch for /run/dbus on bus
> bus-api-resolve: 3
> systemd-resolved[1237]: Added inotify watch for
> /run/dbus/system_bus_socket on bus bus-api-resolve: -1
> systemd-resolved[1237]: sd-bus: starting bus bus-api-resolve by connecting
> to /run/dbus/system_bus_socket...
>
> It seems to me that the systemd-resolved doesn't able to access
> /run/dbus/system_bus_socket. FWIU, the /run/dbus/system_bus_socket is
> created by dbus.socket.
>
> However, the dependencies seem like:
>
> systemd-resolved -> sysinit.target -> dbus.socket.
>
> according to:
>
> $ systemctl show dbus.socket | grep -i after
> After=-.mount sysinit.target system.slice
>
> $ systemctl show systemd-resolved | grep -i before
> Before=initrd-switch-root.target shutdown.target nss-lookup.target
> network.target sysinit.target
>
> If I set "Wants=dbus.socket" "Before=dbus.socket" in systemd-resolved,
> then it'll introduce circular dependency.
>

It's a known issue with D-Bus being the way it is, and that's the whole
purpose of the inotify watch: sd-bus is *watching for the socket to appear*
so that resolved could start before the system bus does, and then connect
to the bus as soon as the socket is created.

Systemd-resolved still offers services either through DNS at 127.0.0.53, or
through a direct Unix socket – in current versions, libnss_resolve
communicates with resolved through a direct socket instead of using the
system bus, so the dependency on dbus.socket isn't as important.
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