systemd dbussing

content/posts/systemd-monitoring-dbus.md

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+date = "2021-06-01"
+draft = false
+path = "/blog/systemd-monitoring-dbus"
+tags = ["linux", "systemd"]
+title = "Monitoring D-Bus calls from systemd-run"
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+
+Recently, I was [working][cargo-play] on a tool that incidentally makes
+ephemeral units on the systemd user instance to do what one would use `atd` for
+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 `dbus-monitor` or `busctl
+monitor` on the session bus failed to find any traffic.
+
+I investigated this by `strace`ing the execution to find it was using some socket
+`/run/user/1000/systemd/private`. From some helpful folks on irc in `#systemd`,
+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.
+
+[cargo-play]: https://github.com/lf-/dotfiles/tree/main/pwaygwoumd
+