Is ServiceResolved supposed to catch SRV and TXT advertisements?

[dorfsmay]

I'm running this simple code. It is printing Service Type advertisements (ServiceFound and ServiceRemoved) but not Service Instance advertisements (SRV, TXT). Is ServiceResolved supposed to do this?

In different windows I'm running:

• cargo run to run this code
• tshark -i vmbr0 -f "udp port 5353" -Y mdns - on the same laptop
• avahi-publish -s blah _blah._tcp 4443
• avahi-browse -r _ipp._tcp
• tshark -i vmbr0 -f "udp port 5353" -Y mdns - on a different machine on the same network

When I run the service (publish -s blah), I can see it being added and removed. When I query _ipp, my printer answers and I see the SRV and TXT advertisement in tshark, on both my laptop and the other machine, but this code is not printing anything. I do not see any ServiceResolved printed, ever.

use mdns_sd::{ServiceDaemon, ServiceEvent};

fn main() -> anyhow::Result<()> {
    let worker = ServiceDaemon::new()?;
    let _accept_unscolicited = worker.accept_unsolicited(true);
    //let cloned_worker = worker.clone();

    let receiver = worker.browse("_services._dns-sd._udp.local.")?;

    while let Ok(event) = receiver.recv() {
        match event {
            ServiceEvent::ServiceFound(service_type, fullname) => {
                println!(
                    "Found service type/instance: {} / {}",
                    service_type, fullname
                );
                //let _service_instance_query = cloned_worker.browse(&service_type);
                //let _service_instance_query = worker.browse(&service_type);
            }   
            ServiceEvent::ServiceRemoved(service_type, fullname) => {
                println!("Service removed: {}/{}", service_type, fullname);
            }   
            ServiceEvent::ServiceResolved(resolved) => {
                println!("Resolved a new service: {}", resolved.fullname);
            }
            ServiceEvent::SearchStarted(_service) | ServiceEvent::SearchStopped(_service) => (), 
            other_event => {
                // ServiceEvent is non-exhaustive
                println!("Received other event: {:?}", &other_event);
            }   
        }   
    }   
    Ok(())
}

[keepsimple1]

let receiver = worker.browse("_services._dns-sd._udp.local.")?;

Looks like you are doing a META_QUERY (RFC) which mdns-sd only returns ServiceFound(query_string, fullname), where fullname is a concrete service type, for example _ipp._tcp.local. .

Then you can use browse again targeting that concrete service type, which should return ServiceResolved with SRV, TXT, etc. (if no other issues)

Btw, for a quick test, you can run cargo run --example query to try out things without writing your own code.

[dorfsmay]

Yes, I am doing a meta query, but I have set undisclosed to true so I expect to listen to everything, and when I run a specific query for _ipp._tcp I see the answers on the network (with tshark) but mdns-sd receiver does not print it.

I have tried to use browse again, to add queries for each service found, see the lines that are commented out, but that seems to completely kill the receiver, it stops printing anything.

[keepsimple1]

Yes, I am doing a meta query, but I have set undisclosed to true so I expect to listen to everything, and when I run a specific query for _ipp._tcp I see the answers on the network (with tshark) but mdns-sd receiver does not print it.

Did you mean the accept_unsolicited to be true? It makes the daemon cache all the incoming records but itself does not deliver ServiceResolved.

I have tried to use browse again, to add queries for each service found, see the lines that are commented out,
//let _service_instance_query = worker.browse(&service_type);

You should browse for fullname of ServiceEvent::ServiceFound(service_type, fullname) because fullname is the concrete type. (service_type is still the META_QUERY)

but that seems to completely kill the receiver, it stops printing anything.

A new browse returns a new channel receiver. And because you used service_type still being the META_QUERY, it overwrites the original sender as well so the original receiver is now orphaned, and won't receiver anything more.

[dorfsmay]

Thank you, I see my mistake using service_type instead of fullname.

So there is no way to have a single receiver to browse everything? I have to create a new daemon and a new receiver for each service I want to monitor?

[keepsimple1]

So there is no way to have a single receiver to browse everything? I have to create a new daemon and a new receiver for each service I want to monitor?

browse always returns a new receiver. But you don't need to create a new daemon.

[dorfsmay]

If I create a new receiver from the same daemon, the original receiver will be orphaned, so now I am going to get the the advertisements for the new subject only, correct?

I am trying to listen to everything, PRT, SRV, TXT for all subjects, the same way avahi-browse -ar functions.

[keepsimple1]

If I create a new receiver from the same daemon, the original receiver will be orphaned, so now I am going to get the the advertisements for the new subject only, correct?

No. The original receiver will be orphaned only if you browse again for the exact same ty_domain (service type + domain). If you are browsing for a different service, you can use both receivers at the same time.

I am trying to listen to everything, PRT, SRV, TXT for all subjects, the same way avahi-browse -ar functions.

I'll take a look at avahi-browse -ar and see what we could do.

[keepsimple1]

I checked out avahi-browse -ar and I think we can try to provide the same function as well. I am thinking of a new API like:

fn browse_with(ty_domain: &str, opts: BrowseOption)

pub struct BrowseOption {
  pub all_services: bool, // auto browse discovered service types.
}

and you might do:

let opts = BrowseOption { all_services: true, ..Default::default() };
let receiver = daemon.browse_with("_services._dns-sd._udp.local.", opts)?;

Thoughts?

[dorfsmay]

That could work, but would it not be simpler, and more intuitive, to trigger ServiceResolved regardless of the query, or at least on a meta query?

I've changed my code and I feel for ServiceResolved not getting triggered here. I listen to the wire with sudo tshark -i wlp3s0 -f "udp port 5353" -Y mdns and can clearly see my code sending the QM queries for each service found as well as the devices replying with SRV and TXT, yet, no ServiceResolved.

Note that for debugging I find switching chatty devices off and use avahi-publish -s blab _blah._tcp 4443 'TEST TEST TEST' to control when a service gets published/removed makes it easier.

use mdns_sd::{ServiceDaemon, ServiceEvent};

fn main() -> anyhow::Result<()> {
    let worker = ServiceDaemon::new()?;
    let _accept_unsolicited = worker.accept_unsolicited(true);

    let receiver = worker.browse("_services._dns-sd._udp.local.")?;

    while let Ok(event) = receiver.recv() {
        match event {
            ServiceEvent::ServiceFound(service_type, fullname) => {
                println!(
                    "\nFound service type/instance: {} / {}",
                    &service_type, &fullname
                );
                println!("sending query for {}", &fullname);
                let _service_instance_query = worker.browse(&fullname);
                std::thread::sleep(std::time::Duration::from_secs(1));
                println!("query for {} sent.\n", fullname);
            }
            ServiceEvent::ServiceRemoved(service_type, fullname) => {
                println!("Service removed: {}/{}", &service_type, &fullname);
            }
            ServiceEvent::ServiceResolved(resolved) => {
                println!("==> Resolved a new service: {}", &resolved.fullname);
            }
            ServiceEvent::SearchStarted(_service) | ServiceEvent::SearchStopped(_service) => (), 
            other_event => {
                // ServiceEvent is non-exhaustive
                println!("Received other event: {:?}", &other_event);
            }
        }
    }   
    Ok(())
}

[keepsimple1]

let _accept_unsolicited = worker.accept_unsolicited(true);

This line is not needed, even though it's not harmful either.

            let _service_instance_query = worker.browse(&fullname);

You need to use this new receiver _service_instance_query to get ServiceResolved for this concrete service (i.e. fullname) .

How exactly to use the new receiver depends on your design. If just for testing, you could break the loop first, and then start a new loop waiting for the new receiver, or store the new receiver(s) in a data structure and poll them later...

[dorfsmay]

Ok, that's what I was afraid of. I don't think this crate will work for my use case, capturing all mDNS traffic. Thanks for the help and clarifications.