Popular sites were laid low by a domain snafu.
That awful feeling when you can’t get your Twitter fix was back early Friday morning eastern time for many users.
Twitter users reported getting a DNS error message (see below). But the DNS snafu was much bigger than Twitter. On its status page, Amazon amzn Web Services said it was looking into “elevated errors resolving DNS host names used to access some AWS services” in its massive U.S. east region operating out of data centers in Northern Virginia.
DNS stands for Domain Name System, which acts as a sort of phone book or directory for Internet sites. DNS converts human-recognizable Internet domain names (for example, Twitter.com) into numbers that are readable by the computers that run the Internet. If the DNS system goes south, sites can become inaccessible, which appears to be what’s happening here.
It looks like the outages are the result of a distributed denial of service (DDOS) attack on Dyn, a major DNS host based in New Hampshire. Some sites said the attacks started at 7 a.m. Friday.
Popular tech site Hacker News reported many other sites were affected including Etsy, Spotify, Github, Soundcloud, and Heroku. Fortune’s parent company Time Inc., was also affected
Twitter twtr , the popular social network which serves as a de facto news feed for many people, was not accessible to many users in Boston, parts of the New York Metro area, New Hampshire, and San Francisco, from approximately 8:15 a.m. through 8:38 a.m. EDT, according to an informal and completely unscientific survey of friends and family conducted on (gasp!) Facebook fb and Slack. (Some users still had no access as of 9:15 a.m. EST.)
Zendesk also reported DNS problems was affecting its performance, according to (ironically) a tweet from the company’s service account, which was only viewable via a Google goog news preview because Twitter was still inaccessible for many.
Per Google, the Zendesk message read: “We are still working on mitigating current issues with external DNS providers impacting access to Zendesk services.” Zendesk’s status site could not be reached.
Fortune reached out to Twitter, Amazon, and other sites for comment, and will update this story as needed.
Note: This story was updated to list other affected sites.