If you see persistent DoS attacks from multiple IP addresses, a more systemic solution might be required. A number of ISPs provide distributed denial-of-service (DDoS) mitigation in the cloud (Cisco Systems Inc. Guards or Arbor Networks TMS) as a service to their customers. These services can filter DDoS traffic quite a few hops beyond the enterprise network's border router, thereby protecting the network. Another option for how to stop a DoS attack might be to purchase a traffic anomaly detection appliance and deploy it in front of your border routers. In this case, mitigation will still work, but, as the appliance is much closer to the network, it could potentially be less effective. The reason for this is that bandwidth saturation, due to the DDoS attack, has propagated to the last hop where available bandwidth is limited. Addressing this at the ISP's cleaning centers avoids this limitation.
As to contacting law enforcement, organizations in the U.S. interested in an investigation of a DoS attack can contact their local FBI field office for guidance and information.
This was first published in July 2010