Denial of service or rather distributed Denial of Service is actually the term used for signifying an attack in the networking model and is signified as an attempt for making available resources as unavailable to its users. This is most commonly called the DoS attack or the DDoS attack.
In the distributed networking environment, or the client server model, where there is or are dedicated servers who are in possession of resources needed by clients connected to it in the network, may be over-sent with requests for resource acquisition and this leads of Denial of service. Whenever a client needs some resource from the server, or needs some performance or evaluation from the server, it sends a request message to the server. The message, when received by the server, leads to the reservation of the needed resources for the client to use.
Now, if the server receives quite a number of such requests from other clients, all requiring the use of some resource or other, then all the resource stock at the server side will get used up at the allocation, and thus may lead to the server running out of resources. Thus, any other client requesting for additional resources will be denied of service from the server as it is not in a state to answer to such clients. This may also lead to the crashing of the server as because the server, after allocating resources for all the requests, may not be left with the minimum amount of resource required for its servicing the requests. Thus, the server may even stop functioning.
This state may also be caused by malicious users who do not intend to use any service from the server, but just is of the motive to make the server crash. The malicious user may send a large number of fictitious request messages with different IP addresses, making the server believe that the requests are from the different clients, and thus the server will allocate resources for each of the clients requests, and thus a time will come when the server will run out of resources to allocate and then ultimately crash, thus denying service to the clients. This is what is meant by the Denial of Service Attack. When the Denial of Service attack takes place is a distributed environment where there are multiple servers and client machines, then also the attack can take place and it is called the Distributed Denial of Service attack.
The incidents of such attack can be chronologically shown as follows:
- Register.com attacked by requests for MX records of the AOL.com
- Department of Finance if Irish Govt. attacked by a student campaign
- DNS Backbone DDoS Attacks performed on root servers for DNS.
- RUS, a hacker group had attacked for over 10,000 game servers online such as Return to Castle Wolfenstein, Counter- Strike and Halo.
- Georgian Government sites faced DDoS attacks
- Iran’s Govt. faced DDoS attacks from foreign activists.
- The Pirate Bay being a P2P site was made inaccessible because it faced the DDoS attack.
- Major websites of United States and South Korea were attacked by cyber-attacks.
- Social networking sites as Facebook, Twitter, Livejournal, and even Google pages of blogging got hit from the DDoS attacks.
- Irish Central Application Office faced DDoS attacks
- Wikileaks.org hit by DDoS attack