A denial-of-service attack (DoS attack) is a cyber-attack in which the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host connected to the Internet. Denial of service is typically accomplished by flooding the targeted machine or resource with superfluous requests in an attempt to overload systems and prevent some or all legitimate requests from being fulfilled.
In a distributed denial-of-service attack (DDoS attack), the incoming traffic flooding the victim originates from many different sources. This effectively makes it impossible to stop the attack simply by blocking a single source.
A DoS or DDoS attack is analogous to a group of people crowding the entry door of a shop, making it hard for legitimate customers to enter, disrupting trade.
Criminal perpetrators of DoS attacks often target sites or services hosted on high-profile web servers such as banks or credit card payment gateways. Revenge, blackmail and activism can motivate these attacks.
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F.A.Q about DDoS Protection
What are the Different Types of DDoS Attacks?
Distributed Denial of Service attacks vary significantly, and there are thousands of different ways an attack can be carried out (attack vectors), but an attack vector will generally fall into one of three broad categories:
Volumetric attacks attempt to consume the bandwidth either within the target network/service or between the target network/service and the rest of the Internet. These attacks are simply about causing congestion.
TCP State-Exhaustion Attacks:
TCP State-Exhaustion attacks attempt to consume the connection state tables which are present in many infrastructure components such as load-balancers, firewalls and the application servers themselves. Even high capacity devices capable of maintaining state on millions of connections can be taken down by these attacks.
Application Layer Attacks:
Application Layer attacks target some aspect of an application or service at Layer-7. These are the deadliest kind of attacks as they can be very effective with as few as one attacking machine generating a low traffic rate (this makes these attacks very difficult to proactively detect and mitigate). Application layer attacks have come to prevalence over the past three or four years and simple application layer flood attacks (HTTP GET flood etc.) have been some of the most common denials of service attacks seen in the wild.