Today, the society is increasingly reliant on IT systems and networks connecting them. In most developed countries the critical infrastructures are connected to the Internet, but because of the absence of adequate safety features, these systems are vulnerable to a possible attack.
Criminals get new weapons by the development of the global computer network, and in order to achieve their goals they use the Internet more frequently. These new types of attack are difficult to control, and it is almost impossible to identify the attacker.
The attack is not always making money, or taking over control of the target, but has also been extensive damage when the system is inoperative. Beyond the direct costs the attacks caused, the most feared risk is reputation loss and the loss of customers' confidence. This is the main reason why the attacks - even if - on critical systems will not be published in most cases.
In my thesis I present some well-known denial of service attack methods and defense options against them. I develop an application, which can simulate the best-known attacks in order to test computer systems. In order to simulate larger workloads I expand the capabilities of the application to distributed operation. Finally, I demonstrate some test case with measurement results about how the attacks work.