Much of the web struggled to stay on its feet today, with outages bringing down U.S.-based services and sites such as Amazon, Twitter and Netflix. A massive distributed denial of service (DDOS) attack on Dyn DNS is reportedly the cause: as a popular provider of domain-name lookup services, it falling over means that browsers simply don't know where to find websites.
Services have been restored to normal as of 13:20 UTC. Posted about 1 hour ago. Oct 21, 2016 – 13:36 UTC
This attack is mainly impacting US East and is impacting Managed DNS customers in this region. Our Engineers are continuing to work on mitigating this issue. Posted about 2 hours ago. Oct 21, 2016 – 12:45 UTC
Starting at 11:10 UTC on October 21st-Friday 2016 we began monitoring and mitigating a DDoS attack against our Dyn Managed DNS infrastructure. Some customers may experience increased DNS query latency and delayed zone propagation during this time. Updates will be posted as information becomes available.
This is "probably why half the internet is shut down today," reports Gizmodo, which offers the following list of sites that were down for the count.
ActBlue, Basecamp, Big cartel, Box, Business Insider, CNN, Cleveland.com, Esty, Github, Grubhub, Guardian.co.uk, HBO Now, Iheart.com (iHeartRadio), Imgur, Intercom, Intercom.com, Okta, PayPal, People.com, Pinterest, Playstation Network, Recode, Reddit, Spotify, Squarespace Customer Sites, Starbucks rewards/gift cards, Storify.com, The Verge, Twillo, Twitter, Urbandictionary.com (lol), Weebly, Wired.com, Wix Customer Sites, Yammer, Yelp, Zendesk.com, Zoho CRM,
Update: The Amazon AWS status page is a useful bookmark, as the service backends many websites and services.