As a first responder to a suspected DoS incident, the initial step is to make an assessment of the situation. This involves analyzing the network traffic using tools like Wireshark to confirm the nature of the traffic and determine if it is indeed a DoS attack. The assessment will help in understanding the scope and impact of the incident and is crucial for deciding the subsequent steps in the response process123.

References: The importance of making an initial assessment is highlighted in various cybersecurity incident response guidelines and best practices, which recommend starting with an evaluation of the situation before proceeding with any other actions123.