net: dns-sd: support service instance query

[toonst]

There are three DNS‑SD query types in mDNS:

1) Service Instance Query

The query includes an instance name along with the service type, protocol, and domain (e.g. zephyr._http._tcp.local.). This query asks for all records associated with that specific service instance, including SRV, TXT, and address records (A/AAAA).

On Mac:
dns-sd -i <your_nic> -L zephyr _http._tcp local
In Linux (not sure if this works):
avahi-browse -i <your_nic> -rt zephyr._http._tcp -d local

Note: I'm using a _http service as example here, the mdns_responder sample registers a _zephyr service.

2) Service Enumeration Query

In this case, the query omits the instance name and only includes the service type, protocol, and domain (e.g. _http._tcp.local.). This lets the client discover all devices on the network that have this service type. The response is typically a set of PTR records listing the individual service instance names.

On Mac:
dns-sd -i <your_nic> -B _http._tcp local
On Linux:
avahi-browse -i <your_nic> -rt _http._tcp -d local

3) Service Type Enumeration Query

This is a special meta-query using the reserved service name _services._dns-sd._udp.local.. When a client sends a query for this name, it receives a list of all service types currently advertised on the local network. (More info RFC 6763: DNS-Based Service Discovery)

On Mac:
dns-sd -i <your_nic> -B _services._dns-sd._udp local
On Linux:
avahi-browse -i <your_nic> -rt _services._dns-sd._udp -d local
or
avahi-browse -i <your_nic> -a -d local

Describe the bug

When an app sends a service type enumeration query or service enumeration query, zephyr responds correctly:

[00:29:26.639,345] <dbg> net_mdns_responder: dns_read: (net_socket_service): Received 1 query from 169.254.76.202:5353
[00:29:26.639,399] <dbg> net_mdns_responder: dns_read: (net_socket_service): [1] query PTR/IN label ._zephyr._tcp.local (19 bytes)
[00:29:26.639,587] <dbg> net_mdns_responder: send_sd_response: (net_socket_service): matched query: zephyr._zephyr._tcp.local port: 50515

However, we send a regular service instance query, zephyr does not respond:

[00:32:08.827,275] <dbg> net_mdns_responder: dns_read: (net_socket_service): Received 1 query from 169.254.76.202:5353
[00:32:08.827,329] <dbg> net_mdns_responder: dns_read: (net_socket_service): [1] query SRV/IN label .zephyr._zephyr._tcp.local (26 bytes)

As you can also see, the mdns_responder correctly receives these service enumeration queries, but does not reply to them. This is a missing feature/bug in the Zephyr implementation.

To Reproduce

west build samples/net/mdns_responder -- -DCONFIG_MDNS_RESPONDER_LOG_LEVEL_DBG=y

Environment (please complete the following information):

• OS: MacOS
• Commit sha: a29c0af

[jukkar]

This looks more like an enhancement than a bug so changing labels.