CloudFlare's I'm Under Attack Mode (IUAM) is elegantly simple. When a site is under an application layer (Layer 7) distributed denial of service (DDoS) attack, the mode will return a challenge page to a visitor.
Yesterday I posted a post mortem on an outage we had Saturday. The outage was caused when we applied an overly aggressive rate limit to traffic on our network while battling a determined DDoS attacker.
CloudFlare's I'm Under Attack mode allows our customers to, at the click of a button, tell us that they are experiencing an attack and enable automatic protection. It works by slowing down visits to the web site that's under attack and performing extra work to identify malicious visitors.
CloudFlare provides a broad level of protection from a wide range of attacks. We do this while minimizing false positives or annoyances to legitimate customers. CloudFlare didn't begin as a DDoS mitigation service, but we've rapidly found that we are good at it.