Denial-of-Service (DoS) is an attack that renders a system, service, or network unavailable to legitimate users through overload. In a Distributed Denial-of-Service (DDoS) attack, the assault is launched simultaneously from thousands of compromised devices (a botnet).
Attack methods include volumetric floods (UDP, ICMP), protocol attacks (SYN flood), and application-layer attacks (HTTP flood, Slowloris). Countermeasures include CDNs with DDoS protection, rate limiting, geo-blocking, and anycast routing. In risk assessments, DoS/DDoS scenarios rank among the most common threats to availability.