[avahi] Receive mDNS annoucnements

Mon Jan 11 12:49:20 PST 2016

My Qt zeroconf / avahi wrapper keeps a list of discovered / resolved 
services...

https://github.com/jbagg/QtZeroConf

QMap<QString, QZeroConfService *> services; is private, but you could 
move it to the public part of the class for your project.

On 16-01-08 06:57 PM, Jon Fitt wrote:
> Hi Trent, or any other interested parties,
>
> I don't know if you're watching this mailing list any more, but I'm 
> trying to pick this up again.
>
> What I'm looking to do is query the avahi service to get it to tell me 
> all hosts that it has seen make any service announcements, and ideally 
> when. I'd like to have my Raspberry Pi keep a passive list of all 
> hosts it has seen and when they were last seen.
>
> I'm ultimately looking for it to be able to tell when I have returned 
> home because my iphone will make service announcements as soon as it 
> rejoins the wifi.
>
> I'd guess I can use a service browser to look for "ANY" service and 
> then use a service resolver to resolve all of them to their 
> hostname/IP, but that seems inefficient.
>
> Is there a way to query the avahi service based on hostname, instead 
> of service type?
>
> Regards,
>
> Jonathan
>
>
>
>
> On Sunday, 30 November 2014, 21:58, Trent Lloyd <lathiat at bur.st> wrote:
>
>
> 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 
>> <mailto: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 
>> <mailto: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
>>
>>     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) 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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>
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-- 
Jonathan Bagg
Embedded Systems Developer
NAD Electronics | Lenbrook Industries Limited
633 Granite Court, Pickering, Ontario, Canada L1W 3K1 | 905-831-0799 ext 4478 | http://www.nadelectronics.com

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