dns resolve to act as a "relay" for specific applications

[Cristib05]

I am currently implementing OpenThread Border Router support in Zephyr and in my current usecase, a DNS upstream resolver functionality is required. An OpenThread node can initiate a DNS query which will arrive on the Border Router. Now, in theory, the Border Router should start querying on the backbone interface for that particular transaction initiated by the Thread node and provide an answer if available.

Looking over what is currently implemented, this platform code that would reside on the Border Router side will duplicate what is currently existing in the dns_resolve.h / resolve.c files from Zephyr. What is making this implementation not 100% percent usable in this case is the fact that the answer will be stored in dns_addrinfo data structure from dns_resolve_cb_t, while the OpenThread node that initiatied that query expects a DNS packet as a response.

Would it be possible to add some new API that will return a DNS packet instead of that data structure? I am thinking to implement something new in order not to break compatibility with previous version. If this is to be agreed, I am willing to start working on this specific feature. Thanks!

[Cristib05]

@jukkar, as I have seen that you've worked on this, can you please let me know what do you think about this? Thanks!

[rlubos]

Maybe we could have an optional packet callback pointer stored in struct dns_resolve_context, registered with a separate API? All responses could then be passed "raw" to the registered callback w/o affecting existing functionalities.

[rlubos]

Or another option to have a packet callback registered per struct dns_pending_query, but that would require a separate API to trigger requests and probably more changes in the resolver itself, I imagine we'd skip packet parsing in such case?

[Cristib05]

I won't do more than checking if the packet is valid, if that's required in this case.
Seems this idea might be accepted; I will start sketching something and come up with a draft PR in the next few days. Thanks!