Overhaul mDNS to allow arbitrary records (supports DNS Service Discovery)

[george-palmsens]

This changes the DNS hook to return a list of records to serve over mDNS. By setting an appropriate set of records, this satisfies DNS-SD (Avahi, Bonjour, etc).

An "appropriate set of records" looks like:

static DNSRecord_t s_records[] = {
    {.usRecordType = dnsTYPE_PTR,
     .pcName       = "_services._dns-sd._udp.local",
     .xData.pcPtrRecord  = "_myservice._udp.local"},

    {.usRecordType = dnsTYPE_PTR,
     .pcName       = "_myservice._udp.local",
     .xData.pcPtrRecord  = "myhostname._myservice._udp.local"},

    {.usRecordType = dnsTYPE_SRV,
     .pcName       = "myhostname._myservice._udp.local",
     .xData.xSrvRecord   = {.pcTarget = "myhostname.local", .usPort = SERVICE_PORT}},

    {.usRecordType = dnsTYPE_TXT,
     .pcName = "myhostname._myservice._udp.local",
     .xData.pcTxtRecord = "Arbitrary service text"},
     
    {.usRecordType = dnsTYPE_A_HOST,
     .pcName = "myhostname.local"},
};

Other things that have been changed to accomodate this:

• DNS responses now check all questions, not just the first one
• DNS responses may contain many answers

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

I have tested my changes locally, and verified the device is discoverable through Avahi. I have not run an application level test suite.

I have added a shim that allows the old hooks to still work via a back-compatibility flag, I hope that is a fine approach.

I have added the necessary unit tests that reach 100% coverage. A few isolated cases had to be ignored from the coverage report since they are defensive checks that are already caught earlier in the logic.

Requests for assistance:

• How do I document this feature?

Related Issue

See #1284

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[george-palmsens]

Apologies for spamming the CI.

I have added a back compatibility shim for the old style DNS hooks. Defining ipconfigDNSQuery_BACKWARD_COMPATIBLE should hopefully mean that old code continues to work.

[george-palmsens]

Added support for "Additional RRs" in the generated response, with a corresponding hook.

This is a SHOULD in the DNS-SD spec, and clients aren't allowed to rely on it. But they still do 🤷 and so this is necessary in order to get everything working reliably with a variety of client libraries.

[htibosch]

Hi @george-palmsens , you're doing a great job. Please allow me time to review and test the changes, most importantly: keeping thing downward compatible.
Thanks so far!