DoS Attack

A DoS attack (Denial of Service) is an attack aimed at making a web service unavailable by exhausting resources. For web systems this can target network/transport layers (e.g., SYN floods) or the application layer (e.g., HTTP floods that tie up server processes and databases). DoS can originate from a single source, while DDoS involves many distributed machines; regardless, the effect on web developers and operators is the same: reduced availability, potential data issues and reputational damage. Mitigation strategies include rate limiting, CDNs and load balancers, Web Application Firewalls, and traffic monitoring to detect and block malicious patterns.

Example: an online store launches a flash sale and at the same time the site receives a huge number of HTTP requests from one or more sources, causing checkouts to hang and visitors to see errors. The developer and sysadmin notice spikes in CPU and concurrent connections in the logs; they quickly apply rate limiting, increase CDN usage and add a WAF rule to block suspicious patterns. Because some attacks are sophisticated (e.g., traffic that looks legitimate or targeted API calls), the team must also use monitoring and traffic analysis to distinguish real users from malicious requests without blocking legitimate customers.