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b84fa122ce
We cannot use DoH for zone transfers. According to RFC8484 a DoH request contains exactly one DNS message (see Section 6: Definition of the "application/dns-message" Media Type, https://datatracker.ietf.org/doc/html/rfc8484#section-6). This makes DoH unsuitable for zone transfers as often (and usually!) these need more than one DNS message, especially for larger zones. As zone transfers over DoH are not (yet) standardised, nor discussed in RFC8484, the best thing we can do is to return "not implemented." Technically DoH can be used to transfer small zones which fit in one message, but that is not enough for the generic case. Also, this commit makes the server-side DoH code ensure that no multiple responses could be attempted to be sent over one HTTP/2 stream. In HTTP/2 one stream is mapped to one request/response transaction. Now the write callback will be called with failure error code in such a case.