mDNS: always set the "cache flush" bit

[lucasvr]

Always broadcast mDNS messages with the "cache flush" bit set for TXT, SRV, A, and AAAA records. This is useful when the Matter device reboots and gets assigned a different IP address, for example. In such a case, hosts with an outdated cache may need to wait until the original TTL expires (up to 60 seconds, per the TTL definitions on mdns/builtin.rs).

[ivmarkov]

UPDATE: GH links to lines in PR do not work as I expected. I'll update with inline-comments

@lucasvr Sorry for replying so late, but after implementing and submitting this, I think I'm finally somewhat qualified to assess what you are doing here.

Yes, my interpretation matches yours now. Setting the cache-flush bits unconditionally might help (or at least not hurt) for all but those PTR records where we are not the authority (as in https://datatracker.ietf.org/doc/html/rfc6762#section-10.2).

One question though: I think we should also cache-flush those PTR answers where we redirect ("point") a service-type or a service subtype to its host record?

However, in your PR, I see two things which worry me a bit:

• Contrary to what the PR title says, you are cache-flushing also PTR records. Is this by accident or by design?
• You are also cache-flushing PTR records where we are not the authority (i.e. "what DNS-SD service-types and sub-types you have?") (I'll comment on these lines in the PR itself shortly.)
  My interpretation of the spec is that we should definitely not cache-flush non-authority answers as I interpret you are doing in this PR. Specifically where the spec says:
  "
  The cache-flush bit MUST NOT ever be set in any shared resource
  record. To do so would cause all the other shared versions of this
  resource record with different rdata from different responders to be
  immediately deleted from all the caches on the network.
  "

Please let me know if I misunderstood your changes, or the spec. In any case, once #186 is in, I plan to apply a variation of the changes you have suggested here. Thanks again for noticing this and sorry for the late reply! I should've at least left a note why I'm not replying immediately (as in - not being qualified enough to comment).

[ivmarkov]

As per my other comment: #154 (comment)

[lucasvr]

• Contrary to what the PR title says, you are cache-flushing also PTR records. Is this by accident or by design?
• You are also cache-flushing PTR records where we are not the authority (i.e. "what DNS-SD service-types and sub-types you have?") (I'll comment on these lines in the PR itself shortly.)

That was not by design. While it wouldn't hurt to set the cache-flush bit on those cases, it's indeed more sane to leave them out from PTR records. Thanks for the careful code review.

I've updated the PR to address your suggestions. Feel free to use this as a reference only (in case you want to apply a variation of this PR at a later point in time).