The warning, issued Monday night in the US by the bureau's National Infrastructure Protection Centre, said that the centre had received "credible, but non-specific information that wide-scale hacker attacks against US Web sites and Internet service providers (ISP) are being planned for later tonight, possibly emanating from Western Europe."
Web site and ISP administrators should "heighten their awareness of network traffic during this period" and "report suspected malicious activities to their local FBI office or the NIPC and to other appropriate authorities," the bulletin said.
Denial-of-service (DoS) attacks are intended to disable Web sites and ISPs by barraging addresses with traffic. A wave of attacks crippled major Web sites such as Yahoo two years ago. More recently, the Code Red worm generated a string of attacks earlier this year.
Key companies measuring Internet performance said attacks were limited and had minimal impact on Web functioning. Matrix NetSystems saw an increase in traffic from Middle Eastern countries starting around 1 a.m. PDT Tuesday and tapering off a few hours later, according to Abelardo Gonzalez, senior Internet analyst for the security and Internet monitoring company.
The number of reachable sites monitored by the company dropped about 1 percent during the period, he said.
"The overall effect was pretty minimal," he said. "Everything was back to normal" by 6 a.m. PDT.
Keynote Systems, which measures Web site performance, saw no unusual activity since Monday evening, said Product Manager Roopak Patel. "We have not heard or seen anything that would indicate any significant degradation in performance," he said.
Elad Yoran, executive vice president of security services company Riptech, said there had been no appreciable increase in hacker attacks Tuesday. "We're really not seeing anything that leads us to believe there's any coordinated or systemic attack going on," he said.
Representatives from Microsoft and Yahoo said none of the companies' Web properties or internal networks had been affected by attacks. Representatives of EarthLink and AT&T WorldNet, among the nation's biggest ISPs, said none of their services had been affected.
