In this paper, we propose a systematic design framework for output-based dynamic event-triggered control (ETC) systems under Denial-of-Service (DoS) attacks. These malicious DoS attacks are intended to interfere with the communication channel causing periods in time at which transmission of measurement data is impossible. We show that the proposed ETC scheme, if well designed, can tolerate a class of DoS signals characterized by frequency and duration properties without jeopardizing the stability, performance and Zeno-freeness of the ETC system. In fact, the design procedure of the ETC condition allows trade-offs between performance, robustness to DoS attacks and utilization of communication resources. The main results will be illustrated by means of a numerical example.