code: func BackendError(b ServiceBackend, zone string, rcode int, state request.Request, err error, opt Options) (int, error) {
  	m := new(dns.Msg)
  	m.SetRcode(state.Req, rcode)
  	m.Authoritative, m.RecursionAvailable, m.Compress = true, true, true
  	m.Ns, _ = SOA(b, zone, state, opt)
  
  	state.SizeAndDo(m)
  	state.W.WriteMsg(m)
  	// Return success as the rcode to signal we have written to the client.
  	return dns.RcodeSuccess, err
  }
docs: // BackendError writes an error response to the client.