Denial of service is a very aggressive attack usually launched against websites as a form of protest or activism. The ultimate aim is to bring down or crash the web server. There are many readymade tools for this. Here, I will give instructions on how to use a very simple DoS tool called Low Orbit Ion Cannon (LOIC). LOIC is a free and open source and can be found in Sourceforge.
Please note that the information provides here is only for educational purpose.
First download LOIC from here. The original version requires Microsoft .net to run. If you are using a Linux system, then you can use the java version, Java LOIC.
Once downloaded, you can go ahead and run the tool. The directions are pretty straight forward.
1. Enter the URL of the website you are planning to attack. There is also an option to enter the IP address.
2. Once the URL is entered, click on "Lock On"
3. Select the protocol you want to use. TCP is fine.
4. Enter the no of simultaneous threads. (In other words, the severity of your attack) 500 to 1000 threads will do.
5.Finally, click on the large button labelled as "IMMA CHARGIN MAH LAZER"
That will start the attack. In the bottom, you will see a substantial increase in the no. of requested connections. This means your attack is on. After some time, the website will slow down and eventually stop responding(crash). The best way to observe this is by creating a web server in a virtual lab and attacking it. You can then analyze the web server logs for more information.
Note: Please don't use tool on any website other than your own. It is a very powerful tool and you could get in trouble with the law.