Comment 44 for bug 306362

(In reply to comment #42)
> avahi-daemon also breaks. It has several dbus interface from which it sends
> signals which are denied with this fix.

What is the interface used in the signals? org.freedesktop.Avahi? Probably adding:

  <allow send_interface="org.freedesktop.Avahi"/>

to the <policy group="avahi"> section should be the easiest fix (well, it may be better to bound it to the owner of the org.freedesktop.Avahi name as a sender, but that can't be done in the send rules). On the receiver side, following already existing context="default" rule should be sufficient:

  <allow receive_sender="org.freedesktop.Avahi"/>

As for the fix proposed here:
  http://lists.freedesktop.org/archives/dbus/2008-December/010706.html
  <allow send_requested_reply="true" send_type="signal"/>

This rule will only be considered for messages of type signal. send_requested_reply attribute will be ignored, so the rule will allow sending any signal, by any service connected to the bus (not only avahi-daemon).

Not quite sure what to look for in the avahi-discover output...