Internet users in parts of California were hit by DDoS attack preventing them from reach several sites. including Amazon. The sites' DNS provider, UltraDNS,  was targeted by the DDoS attack. 

Amazon Web Services (AWS) was the first to signal something was amiss and began investigating a problem at 5.43 pm Pacific Time,  The problem persisted until 6:38 pm, but in the meantime the S3 service continued to operate, AWS said.

However, staff at Neustar, the owner of Amazon's DNS provider UltraDNS, was aware of the problem around an hour earlier, at 4:45 p.m. Pacific Time.

"At 7:45 p.m. Eastern Time we noticed an abnormal spike in queries and immediately identified it as a DDoS attack," said Allen Goldberg, vice president of corporate communications at Neustar.

The company was able to analyse the attack pattern and take steps to limit its effects within minutes of identifying the problem, he said.

"We had everything under control well under an hour. The attack was limited to Northern California Internet users," he said.

Other AWS cloud services in North America experienced similar problems, including the Virginia and Northern California data centres hosting EC2, the Elastic Compute Cloud. The company's SimpleDB and Simple Queue Service servers, and all AWS servers in Ireland, were unaffected by the DNS problem.

The outage affected the e-commerce servers of parent company Amazon too, and many others: "Tons of sites are offline," wrote Jeff Barr, Amazon Web Services strategist, in a Twitter message.

Others reported that sites including Second Life were also experiencing problems.