Monitor System DBus

I’m currently writing a program which needs to listen to signals on the system DBus, and I needed a way to monitor the system bus.  I followed the directions here, but I came across a slight problem with that.  It turns out that the solution posted only allows you to listen for things that root sends out.  When I ran my program as an unprivileged user, I didn’t see any output in dbus-monitor, however I would see output if I ran the same program as root.  From what I can tell, the configuration posted will only allow you to see what root is sending on the bus, given that user=”root”.

Fortunately, there’s a simple fix for this:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE busconfig PUBLIC
  "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
  "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">

<busconfig>
  <policy context="default">
    <!-- Allow everything to be sent -->
    <allow send_destination="*" eavesdrop="true"/>
    <!-- Allow everything to be received -->
    <allow eavesdrop="true"/>
  </policy>
</busconfig>

Simply change the policy user to a policy context, and you can now see everything that’s on the bus.

As always, make sure that you remove this config once you’re done testing, otherwise it will be easy to circumvent security mechanisms that are in place.

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