[avahi] Receive mDNS annoucnements

Trent Lloyd lathiat at bur.st
Sun Nov 30 20:58:25 PST 2014 

A quick Avahi primer,

A “browser” is a long running process to watch for records coming and going,
A “resolver” is a short running process to resolve a record and get a final result.

Avahi is also a combination of two technologies, first “multicast dns” which implements DNS with special considerations for a shared multicast network — and then “DNS-SD” or DNS based service discovery.  This is a set of DNS records that can be used to discover a service, it can be used over multicast dns, or in fact can also be used over normal DNS.


Generally you “browse” for some set of records, then if you are interested in one, you “resolve” it.

So generally in most Avahi applications, the “Service Browser” queries a list of services over time, e.g. get a list of all _http._tcp services.  It then returns a list of such services, which you can then (optionally) resolve with the “Service Resolver”, that will then fetch the service and resolve it’s additional TXT records, the IP address for the hostname, etc.

A typically use case in an application is that you browse the service list with a ServiceBrowser,  you provide the user with a list of the service names, they then click a service and you then run the ServiceResolver to get the IP and other TXT details — to then connect to it.  Some applications will also run a ServiceResolver on every service because they need the TXT data to show extra information to the user.

The reason these are split is that it saves unnecessary network traffic if the application did not want to resolve the service.  This can be a big issue in large networks, so you want to avoid every application resolving all services if possible.  Or at least, not all the time.



The “Record Browser”  is a more low level tool for browsing a specific DNS record over mDNS.  It is not normally used by most applications,  as they use the ServiceBrowser/ServiceResolver which queries multiple records for you and does extra logic to handle service discovery.  But if you wanted to watch a specific record, this is the tool to use.  For example perhaps you want to see if there is an entry for “my-computer.local” with an A record (IPv4 Address).  

This might actually be ideal for this use case, since you want to know over time if a specific device is on the network or not - regardless of what services it is advertising.


Hope that helps!

- Trent

> On 26 Nov 2014, at 6:24 am, Jon Fitt <jonfitt at yahoo.com> wrote:
> 
> In the avahi-client C-API there is the concept of a Service Browser and a Record Browser. Service Browser sounds like what you are using now:
> 
> AvahiServiceBrowser <http://avahi.org/download/doxygen/lookup_8h.html#ade1392a93ff4d0407c1735374bc0ff7b>* avahi_service_browser_new <http://avahi.org/download/doxygen/lookup_8h.html#a52d55a5156a7943012d03e6700880d2b>	(	AvahiClient <http://avahi.org/download/doxygen/client_8h.html#a3d65e9ea7182c44fa8df04a72f1a56bb> * 	client,
> AvahiIfIndex <http://avahi.org/download/doxygen/address_8h.html#a2fda397a8f430cb537e37bbca0afb535> 	interface,
> AvahiProtocol <http://avahi.org/download/doxygen/address_8h.html#a5342ada86ff1566a7fc6e855b3d46c3d> 	protocol,
> const char * 	type,
> const char * 	domain,
> AvahiLookupFlags <http://avahi.org/download/doxygen/defs_8h.html#afa669747c3ff55190d86673dff960e50> 	flags,
> AvahiServiceBrowserCallback <http://avahi.org/download/doxygen/lookup_8h.html#a666e712e840c612ba24d9e41673a94df> 	callback,
> void * 	userdata 
> )
> 
> The type being for example _http._tcp. Whereas a Record Browser looks like this:
> 
> AvahiRecordBrowser <http://avahi.org/download/doxygen/lookup_8h.html#ad0a38359b6c52d4272acd831410186f2>* avahi_record_browser_new <http://avahi.org/download/doxygen/lookup_8h.html#a21ac2275126fb9e5eaaae8afe3b75532>	(	AvahiClient <http://avahi.org/download/doxygen/client_8h.html#a3d65e9ea7182c44fa8df04a72f1a56bb> * 	client,
> AvahiIfIndex <http://avahi.org/download/doxygen/address_8h.html#a2fda397a8f430cb537e37bbca0afb535> 	interface,
> AvahiProtocol <http://avahi.org/download/doxygen/address_8h.html#a5342ada86ff1566a7fc6e855b3d46c3d> 	protocol,
> const char * 	name,
> uint16_t 	clazz,
> uint16_t 	type,
> AvahiLookupFlags <http://avahi.org/download/doxygen/defs_8h.html#afa669747c3ff55190d86673dff960e50> 	flags,
> AvahiRecordBrowserCallback <http://avahi.org/download/doxygen/lookup_8h.html#a867a24784f27539ed34291e0db8b51b8> 	callback,
> void * 	userdata 
> )
> 
> I wonder if that is a more generic mDNS browser? Unfortunately the parameters, and what a "record" is, is not explained in the online documentation. I'll have a look in the source to see if I can find out more.
> 
> Regards,
> Jonathan
> 
> 
> On Tuesday, 25 November 2014, 13:17, Michael Jones <avahi at jonesmz.com> wrote:
> 
> 
> 
> 
> Hi Michael,
> I'm looking at your ZConfBrowserWidget and I see that it needs a service type. Technically I believe from what I've been reading the mDNS host announcements are different to an DNS-SD service, although they are both announced in a similar way.Is there a "service type" that catches the mDNS hostname announcements?
> Regards,
> Jonathan
> 
>      On Friday, 21 November 2014, 15:08, Jonathan Fitt <jonfitt at yahoo.com <mailto:jonfitt at yahoo.com>> wrote:
> 
> 
>  Thanks! I'll give that a go and let you know how it goes. 
> Jonathan
> On Nov 21, 2014, at 1:25 PM, Michael Jones <avahi at jonesmz.com <mailto:avahi at jonesmz.com>> wrote:
> 
> 
> 
> 
> Hello,I am looking for a way for a Linux machine to tell when I return home. My idea is that my iPhone will always do an mDNS announcement when it joins my home wifi. So I'm looking for a way to watch for mDNS announcements and perform and action if the right one is seen. Obviously the avahi-daemon is watching these all the time and updating its cache, so I was thinking that there might be a good place to start. However the only command line way I can see to get to the avahi-daemon cache is to send a SIGUSR1 to get it to dump to /var/log/syslog. Then I can look at the timestamp to determine the last time it registered and compare with now. That seems quite ugly though, and I don't like the idea of continually dumping to syslog.
> So is there a way to make it more event driven with the avahi-daemon? I think from looking at the D-Bus API that might be the way to go?If it is the right approach, I would appreciate it if someone knew of a related chunk of source that I could use as a starting point.
> Thanks!Jonathan
> 
> 
> 
> Write a small program that uses the Avahi API. A very high level wrapper for the Avahi API is: https://github.com/johanneshilden/qtzeroconf <https://github.com/johanneshilden/qtzeroconf>
> 
> Using that, you can detect your phone joining your home network in under 10 lines of code.
> Create the service browser object
> Connect that new object to a qt slot that'll respond to the new service joining your network, Tell the service browser object which services to listen for.
> 
> >From there, you can do whatever you want with the information. Perhaps you would want to send some kind of DBus signal to let other programs know.
> 
> I'm afraid I haven't investigated the DBus API, so I'm not certain I can give you any advice in that direction, but I have written apps using the high level Qt API that I linked above, and I've never had any trouble with it.
> 
> --Michael Jone
> 
> 
> Hi Jonathan 
> 
> 
> That's a good question. I actually don't know. The qtzeroconf library isn't actually mine, originally. I made a lot of modifications to it (Here: https://github.com/jonesmz/qtzeroconf <https://github.com/jonesmz/qtzeroconf>) that take advantage of Qt5 and C++11, but those changes aren't compatible with older versions of Qt, so I didn't want to recommend it to you.
> 
> If you feel like poking around at the Avahi C API and can demonstrate how to get mDNS host announcements out of the Avahi C api, I'd be willing to add support for that to my version of the qtzeroconf library (though, it'll still not be compatible with Qt4 or older versions of C++, so you might end up needing to backport the changes to the original authors github project.)
> 
> If you want to work just with what the current qtzeroconf library can do, you can just make your phone advertise an avahi service. Perhaps give the protocol name something like "_phone._tcp" or something. 
> 
> Another thing to keep in mind, I don't know whether Avahi will automatically detect your phone on the local network unless your phone's operating system automatically sends that announcement. As far as I know, Android doesn't do that. I don't know about the other phone OS's.
> 
> Best of luck, and let me know if you want to investigate the host announcement support in qtzeroconf.
> 
> --Michael Jones
> 
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