Recently, I was working on a tool that incidentally makes
ephemeral units on the systemd user instance to do what one would use
on an older system. As part of working on that, I needed to figure out what RPC
calls I had to do to add the unit. Attempts to use
busctl monitor on the session bus failed to find any traffic.
I investigated this by
straceing the execution to find it was using some socket
/run/user/1000/systemd/private. From some helpful folks on irc in
I found out that this is some kind of socket using the D-Bus serialization
stuff but is not a bus, and thus can't be intercepted (I tried).
The solution to this was to use
--machine @.host, which will cause
systemd-run to communicate on the session/system D-Bus instances as it is
going through the code path to connect to a container but connecting to the
host. Thus I got my traffic and could do the same calls myself.