Function validate_nameserver_response

fn validate_nameserver_response(
    question: &Question,
    response: &Message,
    current_match_count: usize,
) -> Option<NameserverResponse>

Validate a nameserver response against the question by only keeping valid RRs:

• RRs matching the query domain (or the name it ends up being after following CNAMEs) and type (or CNAME)

• NS RRs for a superdomain of the query domain (if it matches better than our current nameservers).

• A RRs corresponding to a selected NS RR

Then, decide whether:

• This is an answer: it has a possibly-empty sequence of CNAME RRs and a record of the right type at the final name.

• This is a cname to follow: it has a non-empty sequence of CNAME RRs but no final record of the right type.

• This is a delegation to other nameservers: there’s at least one NS RR.

This makes the simplifying assumption that the question message has a single question in it, because that is how this function is used by this module. If that assumption does not hold, a valid answer may be reported as invalid.