[avahi] Documentation about avahi works

Trent Lloyd lathiat at bur.st
Wed Oct 8 16:19:54 PDT 2008

Hi Nathan,

On 09/10/2008, at 4:48 AM, Nathan Huesken wrote:

> Hi,
> Thanks for the reply. I read through some documents. Some questions  
> I am unsure about:

I can't really remember the exact breakdown and how far into detail I  
went on this anymore.. but theres a couple slideset & videos of my  
talks on Avahi here: http://lathiat.net/talks

> 1. When a service registers with the avahi-deamon (telling it which  
> service it provies on which port), is this information imidiatly  
> published through the network or does it just stay on the local  
> machine as long as no dns-sd queries come?
> 2. How does the browsing for services work? Is it as simple as this,  
> or is there more to it:
> - The browsing client sends a "give me your services" request to  
> - All other clients reply with a list of services.

mDNS sort-of operates in 2 ways at once.

When a service is first published it sends out an "announcement",  
advertising to the network that this new service is now available

On top of that.. when clients want to search for a service.. it sends  
out a 'query'.. to which all the mDNS responders that have a result  
for it respond.. however because it may already have a list of  
services it knwos about from the announcements (i.e. the client has  
been around for a while.. it's also possilble avahi just started and  
it knows of none yet).. it sends out a list of 'known answers' in the  
query.. to prevent unnecessary network traffic in the responses  
(particularly as the query is sent multiple times) [responses are sent  
over multicast, as well as the queries]

This is quite simple explanation of this part of the protocol but I  
hope it helps?

If you have any further questions let me know.

Trent Lloyd

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