A DNS service provider and a domain registrar apparently fell victim to major distributed denial-of-service (DDoS) attacks this week.

The attack against DNS provider NeuStar on Tuesday morning disrupted Amazon's S3 cloud computing service, along with the Amazon.com store, according to a blog written by competitor Dynamic Network Services. Other sites affected included Salesforce.com, IMDB.com and Petco.com.

Then on Wednesday night and again Thursday afternoon EDT, Register.com, a major domain registrar and web hosting company, was out of service for several hours, according to reports, because of a suspected DDoS attack affecting its DNS name servers -- domains hosted by Register.com did not resolve and the company's home page was unavailable.

During the Register.com outage, the company provided little information to its customers, other than a notice on Twitter that said: “Register.com is having intermittent service issues -- we have everyone working on it. Will provide an update soon."

A Register.com spokesman in New York did not respond to a request for comment.

In the NeuStar attack, John Schneidawind, a company spokesman, denied that its service was shut down.

“Contrary to previous press reports you may have seen, at no time was our UltraDNS service shut down, offline or anything of the sort,” he told SCMagazineUS.com in an email.

The UltraDNS service was “hit by a huge volume of completely legitimate-looking DNS queries, which all appeared to come from legitimate DNS servers, all asking for data on the true attack targets,” wrote Tier1 Research Vice President Dan Golding in a report. “NeuStar couldn't block the apparent source without causing an entirely different sort of outage.”

UltraDNS was returned to normal service after only a few hours, but the outage made some websites impossible to reach. NeuStar later confirmed the DoS attacks as massive, according to reports.