Denial of service attacks flood the target computer with so many requests for information that they become overloaded and are unable to respond to legitimate requests for service.
Multiple computers worldwide are taken over by DDoS attacks, with target systems knocked out from several locations. Such an attack took major Web sites such as Yahoo and Amazon offline for as long as a week in February 2000.
A group of Web site defacers called the Dispatchers have announced their readiness for such actions on 18 September, according to the NIPC. The Dispatchers have already claimed responsibility for defacements of some Web sites and have said they have begun to target infrastructure components such as routers.
The latest advice comes soon after a similar notice issued by the NIPC on 14 September warning of a possible increase in the number of cyberattacks targeted against the "perceived perpetrators of the 11 September 2001 terror attacks".
Some incidents have already occurred, such as a Web site crack which resulted in subscriber addresses to a Muslim fundamentalist e-mail list being published on the Web.
US intelligence officials believe the perpetrators of last week's attacks have links with Islamic extremist groups. But President Bush and other government leaders have stressed that the attackers are not supported by mainstream Islamic doctrine.
The NIPC offers a tool that systems administrators can use to check whether their systems have been infected with common DDoS tools in preparation for an attack. The tool can be downloaded at www.nipc.gov/warnings/advisories/2000/00-055.htm .