The warning came after a DDoS attack on the White House Web site on 4 May. The attack lasted for over three hours and overloaded the servers with automated requests for information.
In a related note, the National Infrastructure Protection Centre (NIPC) warned last month of an expected upswing in attacks against US servers by Chinese hackers, given an increase in political tensions between the two countries.
The attackers are transmitting data via User Datagram Protocol (UDP) packets, which, according to the NIPC, bypass standard port protocol blocking techniques.
Network administrators are being advised to inspect firewall logs for evidence of fragmented UDP packets directed at Port 80, the most commonly used portal. Inbound packets could signify that a DoS attack is under way and outbound packets could mean that hacker DDoS software has already compromised the network.
A special utility to detect DDoS software is available from the NIPC.
Most private business sites probably have Web servers equipped with firewalls that are capable of halting these types of attacks, said Ric Steinberger, technology director at security portal Atomic Tangerine. Government sites, however, are usually more vulnerable because employees do not have access to the latest software and hardware defences, he said.