add closest encloser stuff

middleware/file/closest.go

@@ -0,0 +1,16 @@
+package file
+
+import "github.com/miekg/dns"
+
+// ClosestEncloser returns the closest encloser for rr.
+func (z *Zone) ClosestEncloser(rr dns.RR) string {
+	elem := z.Tree.Prev(rr)
+	if elem == nil {
+		// SOA?
+		return ""
+	}
+	for _, r := range elem.All() {
+		return r.Header().Name
+	}
+	return ""
+}