make copies of RRs before returning them (#4409)

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/file/lookup.go

@@ -171,7 +171,12 @@ func (z *Zone) Lookup(ctx context.Context, state request.Request, qname string)
 			return z.externalLookup(ctx, state, elem, rrs)
 		}
 
-		rrs := elem.Type(qtype)
+		treeRRs := elem.Type(qtype)
+		// make a copy of the element RRs to prevent response writers from mutating the tree
+		rrs := make([]dns.RR, len(treeRRs))
+		for i, rr := range treeRRs {
+			rrs[i] = dns.Copy(rr)
+		}
 
 		// NODATA
 		if len(rrs) == 0 {