When your server is under attack, "we're being DDoSed" is not actionable; "SYN flood, four million packets per second, spoofed sources, targeting port 443" is. The difference between the two is a thirty-second packet capture and a structured look at it. This guide covers capturing safely on a machine that's already saturated (tcpdump, not the Wireshark GUI), the triage workflow in Wireshark (Protocol Hierarchy, Conversations, I/O Graphs) and the display-filter signatures of the common attack vectors, ending with how to turn findings into mitigation.