Support outgoing zone transfers

These can be enabled by adding "transfer out" to the Corefile. Without
it no AXFR is allowed.

For now only AXFR and no IXFR. No TSIG and no ACLs.

middleware/recorder.go

@@ -37,7 +37,9 @@ func NewResponseRecorder(w dns.ResponseWriter) *ResponseRecorder {
 // underlying ResponseWriter's WriteMsg method.
 func (r *ResponseRecorder) WriteMsg(res *dns.Msg) error {
 	r.rcode = res.Rcode
-	r.size = res.Len()
+	// We may get called multiple times (axfr for instance).
+	// Save the last message, but add the sizes.
+	r.size += res.Len()
 	r.msg = res
 	return r.ResponseWriter.WriteMsg(res)
 }