Holiday shoppers who were hoping to purchase last minute gifts at some of the top e-commerce retailers in the country were greeted with a surprise Wednesday evening as a denial of service attack slowed down sites such as Amazon.com, Walmart.com, Expedia and others. Cnet's Tom Krazit has details on what went wrong, noting that the attack hit the UltraDNS data centers in Palo Alto and San Jose, Calif.

Amazon.com's S3 and EC2 Web services were also hit, causing additional outages and lag times for some smaller Web sites.

Amazon.com's Jeff Barr acknowledged the problems via Twitter, noting that the company was investigating "reports of DNS resolution errors." Later Wednesday, things appeared to have returned to normal at Amazon Web Services, according to its Service Health Dashboard.

Most of the Internet connectivity problems appeared to have been limited to the West Coast, with Krazit reporting that the source of the attack was unknown.

Denial-of-Service attacks have brought down big Internet sites in the past, essentially flooding a company's servers with unwanted Internet traffic.

UltraDNS also was hit with a denial of service attack back in April, one which knocked Amazon.com, Salesforce.com and other sites offline.

It appears that the latest attack was for a limited time and only impacted a small geographic area, so the impact on Amazon and other retailers during this crucial holiday selling period could be minimal.