Add limit to ioutil.ReadAll for request body (#5224)

This PR adds limit to ioutil.ReadAll for DoH request body
so that it will not be subject to large requests.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/pkg/doh/doh.go

@@ -92,7 +92,7 @@ func requestToMsgGet(req *http.Request) (*dns.Msg, error) {
 }
 
 func toMsg(r io.ReadCloser) (*dns.Msg, error) {
-	buf, err := io.ReadAll(r)
+	buf, err := io.ReadAll(io.LimitReader(r, 65536))
 	if err != nil {
 		return nil, err
 	}