Update (12:04p ET): A second wave of DDoS attacks against Dyn is underway, as of noon Eastern Time today. Dyn is continuing to work on the issue. Our original story follows below; further updates will be added as information becomes available.

A distributed denial of service attack against Dyn, the dynamic DNS service, affected the availability of dozens of major websites and Internet services this morning, including Twitter and Reddit. The attack, which began this morning at 7:10am Eastern Time (12:10pm UK), is apparently focused on Dyn’s US East Coast name servers.

“This morning, Dyn received a global DDoS attack on our Managed DNS infrastructure in the east coast of the United States,” Doug Madory, Director of Internet Analysis at Dyn, said in an e-mail sent to Ars this morning. “DNS traffic resolved from east coast name server locations are experiencing a service interruption during this time.” By 9:20am ET this morning, Dyn had mitigated the attack and services returned to normal.

[Update, 1:20 PM ET] Less than three hours later, the attack began again, and is still in progress.

"As of 15:52 UTC, we have begun monitoring and mitigating a DDoS attack against our Dyn Managed DNS infrastructure," a Dyn spokesperson reported through the company's status page. "Our Engineers are continuing to work on mitigating this issue.This DDoS attack may also be impacting Dyn Managed DNS advanced services with possible delays in monitoring."

Some aspects of Twitter appear to be affected by the latest round, as is the personal website provider Wix.com. The first wave of the attack increased the response time to DNS queries and in some cases delayed the propagation of some address changes. This also meant that some websites and mobile applications, such as Twitter, experienced intermittent outages.

Posts to Twitter from some East Coast locations failed for a brief period this morning, and webpages for Twitter and Reddit have been slow to load as DNS lookups for addresses have added latency to Web requests. The DDoS attack also affected a number of major news sites, as Mikko Hypponen, chief research officer at the information security service company F-Secure noted:

Sites that use Dyn one way or another include Twitter, Reddit, Github, New York Times, Boston Globe... https://t.co/qaueh5U5ti — Mikko Hypponen (@mikko) October 21, 2016

Ars will update this story as more details become available.