Accessing Avahi through dbus-send

Question

In the process of hacking around with dbus-monitor, I tried connecting to the Avahi daemon:

$ dbus-monitor --system \
  "type='signal',interface='org.freedesktop.Avahi.Server',member='StateChanged'"

This will respond to changes in the state of the Avahi daemon.

$ sudo restart avahi-daemon

signal sender=org.freedesktop.DBus -> 
         dest=:1.315 
         serial=2
         path=/org/freedesktop/DBus;
         interface=org.freedesktop.DBus;
         member=NameAcquired
    string ":1.315"

signal sender=:1.318 ->
         dest=(null destination)
         serial=4
         path=/;
         interface=org.freedesktop.Avahi.Server; 
         member=StateChanged
    int32 1
    string "org.freedesktop.Avahi.Success"

signal sender=:1.318 -> 
         dest=(null destination)
         serial=13
         path=/;
         interface=org.freedesktop.Avahi.Server;
         member=StateChanged
    int32 2
    string "org.freedesktop.Avahi.Success"

However, I cannot browse services like this.

$ dbus-send --system --dest=org.freedesktop.Avahi --print-reply \
    / org.freedesktop.Avahi.Server.ServiceBrowserNew \
    int32:-1 int32:-1 string:'_http._tcp' string:'local' uint32:0

method return sender=:1.345 -> 
         dest=:1.354
         reply_serial=2
    object path "/Client5/ServiceBrowser1"

$ dbus-monitor --system \
    "type='signal',interface='org.freedesktop.Avahi.ServiceBrowser'"

As far as I understand, this should respond to new/removed http service instances, but it does not.

Why?

Update:

Executing the equivalent of the above shell commands with the python and c APIs worked as expected. However, they could not connect to a ServiceBrowser which I had constructed using dbus-send and I could not use dbus-monitor to listen to a ServiceBrowser they constructed.

At this point, my best guesses are:

Once the connection is closed (by the termination of the dbus-send command), the ServiceBrowser I created is destroyed. However this does not explain why dbus-monitor is unable to connect to a ServiceBrowser which is in active communication with my python script.
The signals from the ServiceBrowser are emitted only to the connection which instantiated them.

What happens if you create browser from python and then do `dbus-monitor --system \ "type='signal',interface='org.freedesktop.Avahi.ServiceBrowser'"`? As far as I know, avahi does not know when client closed connection (And it's client's responsibility to free browser). Also, signal message is sent to bus, ind dbus-daemon routs it to subscribers - so as far as I know 1 and 2 are incorrect — Andrey Sidorov, Feb 14 '14 at 00:49
I have node.js wrapper around avahi - might be useful to debug your problem - https://github.com/sidorares/node-gday/blob/master/examples/discover-rfb.js — Andrey Sidorov, Feb 14 '14 at 00:50
Browser created in Python or C is inaccessible with `dbu-monitor`. I already have working code in those languages, so this question is more about curiosity than fixing a bug. I expected shell hackery to be an easy first step, but my understanding of DBus is incomplete. How could I use `node-gday` to answer my question? — Kietz, Feb 16 '14 at 04:06

score 0 · Answer 1 · answered May 11 '22 at 17:04

I believe the problem is caused by the dbus policy. Try to change the avahi dbus policy in /etc/dbus-1/system.d/avahi-dbus.conf like this (remember to restart avahi daemon):

<busconfig>

  <policy context="default">
    <!-- All messages may be received by default -->
    <allow receive_requested_reply="false" receive_type="method_call" eavesdrop="true"/>
    <allow receive_requested_reply="false" receive_type="method_return" eavesdrop="true"/>
    <allow receive_requested_reply="false" receive_type="error" eavesdrop="true"/>
    <allow receive_requested_reply="false" receive_type="signal" eavesdrop="true"/>
    <allow eavesdrop="true"/>
  </policy>
  <policy user="root">
      <allow send_destination="*" eavesdrop="true"/>
      <allow receive_sender="*" eavesdrop="true"/>
  </policy>
</busconfig>

And use root privilege to run dbus-monitor:

sudo dbus-monitor --system \
    "type='signal',interface='org.freedesktop.Avahi.ServiceBrowser'"

reference: http://blog.nutsfactory.net/2011/03/08/test-and-debug-dbus/

Accessing Avahi through dbus-send

Update:

1 Answers1