[BUG] mDNS responses do not conform to RFC 6762

[bjsowa]

Describe the bug
There is a couple of errors in mDNS reply that make Avahi ignore it:

1. "Multicast DNS responses MUST NOT contain any questions in the Question Section." - FreeRTOS+TCP leaves the question from the request in the reply
2. "In response messages for Multicast domains, the Authoritative Answer bit MUST be set to one" - FreeRTOS+TCP leaves this bit as 0

[jasonpcarroll]

Hi @bjsowa,

Thanks for bringing this to our attention. I will reach out to the team to make them aware. Would you like to open a PR addressing this?

Best,

Jason Carroll

[htibosch]

The two issues were already reported on the forum.

@ActoryOu already proposed a change in [PR #1223](Set MDNS AA bit while answering MDNS request).

I already prepared PR #1292 that repairs both items. It is waiting for reparation of the unit-testing, I am working on that.

[htibosch]

But anyway, @bjsowa, thank you for reporting this. Better once too often than never reported and going unnoticed.

I could replicate the problem on my Linux computer.

If you're in a hurry, you can clone my https://github.com/htibosch/FreeRTOS-Plus-TCP
and check out the branch "MDNS_reply_with_answers_only"

Regards

[bjsowa]

@htibosch Apologies I didn't look through existing PRs before submitting the issue. I'll check out your branch.

[htibosch]

Thank you for testing #1292, glad it worked for you.

@htibosch I tried your changes and it fixed mDNS for me. I found, however, two small issues that still persist:

When IPv4 endpoint receives AAAA question, it answers with empty IP (::)
When a single query contains two or more questions (for example one A and one AAAA type), FreeRTOS+TCP will only answer the last one.

I knew that it will only answer a single request. We did that to keep the driver simpler.