[avahi] Resolving many ( > 200 ) items seems to lead to apparent congestion in the dbus

Daniel Wynne daniel.wynne at mobotix.com
Mon Jun 29 02:05:56 PDT 2009

Hi Trent!

These are great news for us! :-)
We were seriously concerned about the issue, that the Avahi mDNS 
implementation prevented us from developing Linux camera management 
applications for huge setups.

In the following I provide a short calculation to give you a hint of 
what we need:

    - Right now we have setups with deployments of up to 800 devices We 
do not think that deployments with even more devices in one network make 
sense, as this already can be considered as a individual feasibility 
study. But lets calculate with this worst case scenario and for the sake 
of convenience we even round it up to N = 1000 devices per network.

    - Since every device typically comes up with 2-3 ip addresses, we 
have to reserve cache entries for N*( 3 A's, 1 PTR, 1 SRV, 1 TXT. ) => 
AVAHI_CACHE_ENTRIES_MAX >= 6*N = 6000 entries.

    - As the dbus limitations are concerned the maximum number of dbus 
objects per client will have to be increased, too. Since we have to 
create N*3 AddressRecordBrowsers, N*1 ServiceResolver, 1 Client, 2 
ServiceBrowsers => OBJECTS_PER_CLIENT_MAX >= 4*N + 3 = 4003 ~ 4000 entries

As I do not have that much insight into the Avahi-code there might be 
even more limits that might have to be adapted to achieve a coherent 

If you need any additional information, let me know.

Thank you Guys for the great support so far!

Kind Regards


Trent Lloyd wrote:
> FWIW, I've always felt this was a little restrictive as well but never  
> really ran into it in production.
> I am personally keen on upping them a little - Daniel, can you tell us  
> what values seem to work well for you?
> Thanks,
> Trent
> On 27/06/2009, at 12:04 AM, Daniel Wynne wrote:
>> Hi Lennart!
>> After a short interrupt I am back in avahi-business ;-)
>> PLaying around a little bit with parameters I boiled it down to
>> These Parameters are slightly tight I think when it comes to bigger
>> networks i.e. more than 100 Services.
>> You said these limitations are implemented to avoid misuse, i.e.
>> DOS-Attacks.
>> Do you think it is possible in future releases of Avahi to loosen  
>> these
>> limitations a little bit, to enable more comprehensive usage of Avahi,
>> even for professional applications like ours ;-)
>> DOS Attacks are way above the limitations of 500, even way above  
>> 5000. I
>> think when talking about DOS we are in millions and above.
>> What do you say?
>> Kind Regards
>> Daniel
>> On Sun, 2009-06-07 at 01:47 +0200, Lennart Poettering wrote:
>>> On Thu, 28.05.09 13:43, Daniel Wynne (daniel.wynne at mobotix.com)  
>>> wrote:
>>>> But I think thats exactly the problem in our case. In our  
>>>> Testnetwork
>>>> reside about 250 cameras we want to find and resolve via Avahi. So  
>>>> the
>>>> cache is way big enough.
>>> No. The limit of 500 entries in the cache that Avahi enforces is for
>>> RRs, not services or hosts.
>>> For each service you need (at least) one A, one PTR one SRV and one
>>> TXT RR. Usually even more. I.e. if you register 250 services and want
>>> to browser for them all at the same time you need as absolute minimum
>>> space for 1000 entries, and practically even more.
>>> Lennart
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Daniel Wynne, Softwareentwicklung
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