Test that spoofed DNAME is not accepted via spoofable transport

Test that spoofed DNAME is not accepted via spoofable transport

A single spoofed DNAME answer can impact many names, and because of the
nature of DNAME, the attacker can use randomized query names to get
unlimited number of tries to spoof the answer.  To limit impact, we
should not be accepting DNAME over insecure transport, like UDP without
cookies etc.

In short, the attacker tries to spoof at least one answer that has the
following form:

    opcode QUERY
    rcode NOERROR
    flags QR AA
    ;QUESTION
    trigger$RANDOM.test. IN A
    ;ANSWER
    trigger$RANDOM.test. 3600 IN CNAME trigger$RANDOM.attacker.net.
    test. 3600 IN DNAME attacker.net.
    ;AUTHORITY
    ;ADDITIONAL

This has been discovered internally.

Co-authored-by: Michał Kępień <michal@isc.org>