On October 21, the east coast of the United States woke up to find a significant portion of the Internet wasn’t working. Twitter, Etsy, Tumblr, Reddit, PayPal, SoundCloud, Spotify, Amazon, and even the New York Times were among the sites users were having trouble reaching. The culprit was a distributed denial of service (DDoS) attack on Dyn, a New Hampshire-based Internet infrastructure company. The incident was an unusually large attack, and fortunately it was resolved by the end of the day. However, it illustrates why DDoS is one of the biggest threats to Internet security today.