[avahi] synchronous service discovery

Lennart Poettering lennart at poettering.net
Thu May 4 14:13:59 PDT 2006

On Wed, 03.05.06 13:03, Mihai Ibanescu (misa+avahi at redhat.com) wrote:


> I am trying to use zeroconf with unicast dns (using SRV and PTR records for
> describing the services that are available in the network).
> Although I think I can see how I can use the python bindings to do the service
> discovery asynchronously, I don't think I was able to find anything for
> synchronous calls. I am looking for the equivalent of doing a PTR lookup (both
> for browsing the domain and for browsing instances for each service type).
> Is there such a thing in the API?

Nope. Avahi doesn't provide a synchronous DBUS interface for service
browsing. Only resolving can be done synchronously. The reason is that
mDNS/DNS-SD is a "living" system where services appear and disappear
every now and then. Avahi doesn't maintain a list of all currently
available services all the time. Instead, only if a local client
browses for something a list is assembled from the responses recieved
from the LAN. However, we never know when this list is complete. (Just
think of a crowd of a few thousands of people. Tell them to shout
their names, all at the same time. You'll see that you can never make
sure that everyone of them has really followed your order or even that
everyone understood your order in the first place)

Hence there is no clean way to implement a synchronous browsing API:
we just never know when the right time to return has come.

However, the asynchronous browser API has a facility that can emulate
the behaviour for you: an AVAHI_BROWSER_ALL_FOR_NOW event is emitted
if Avahi thinks that it is unlikely that more responses will turn up
in the the near future. (This is used by "avahi-browse -t") 

AVAHI_BROWSER_ALL_FOR_NOW is generated like this:

  1. On mDNS, if there are already local clients querying the same
     record type, return it immediately if the first of the clients
     began to query it more than a second ago. If they began less than
     a second ago, wait until the second elapsed.

  2. On mDNS, if there are no other local clients querying the same
     record type, just wait a second

  3. On unicast DNS, just wait until the server responded, since we
     don't need to aggregate multiple responses in this case.

Keep in mind that this event is unreliable. It might definietly happen
that another service is found just msecs after this event was

I hope this helps,

Lennart Poettering; lennart [at] poettering [dot] net
ICQ# 11060553; GPG 0x1A015CC4; http://0pointer.net/lennart/

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