How Denial of Service Attack Works?

How a "denial of service" attack works

     In a typical connection, the user sends a message asking the server to authenticate it. The server returns the authentication approval to the user. The user acknowledges this approval and then is allowed onto the server.

    

         In a denial of service attack, the user sends several authentication requests to the server, filling it up. All requests have false return addresses, so the server can't find the user when it tries to send the authentication approval. The server waits, sometimes more than a minute, before closing the connection. When it does close the connection, the attacker sends a new batch of forged requests, and the process begins again--tying up the service indefinitely.

Typical connection

"Denial of service" attack

How to block a "denial of service" attack

    One of the more common methods of blocking a "denial of service" attack is to set up a filter, or "sniffer," on a network before a stream of information reaches a site's Web servers. The filter can look for attacks by noticing patterns or identifiers contained in the information. If a pattern comes in frequently, the filter can be instructed to block messages containing that pattern, protecting the Web servers from having their lines tied up.

Distributed Denial of Service Attacks

       Sometimes a  attacker uses a network of zombie computers to sabotage a specific Web site or server. The idea is pretty simple -- the attacker tells all the computers on his botnet to contact a specific server or Web site repeatedly. The sudden increase in traffic can cause the site to load very slowly for legitimate users. Sometimes the traffic is enough to shut the site down completely. We call this kind of an attack a Distributed Denial of Service (DDoS) attack.

   Some particularly tricky botnets use un-corrupted computers as part of the attack. 

Here's how it works:

• The attacker sends the command to initiate the attack to his zombie army. 
• Each computer within the army sends an electronic connection request to an innocent computer called a reflector. 
• When the reflector receives the request, it looks like it originates not from the zombies, but from the ultimate victim of the attack. 
• The reflectors send information to the victim system, and eventually the system's performance suffers or it shuts down completely as it is inundated with multiple unsolicited responses from several computers at once.

   From the perspective of the victim, it looks like the reflectors attacked the system. From the perspective of the reflectors, it seems like the victimized system requested the packets. The zombie computers remain hidden, and even more out of sight is the cracker himself.

The list of DDoS attack victims includes some pretty major names. Microsoft suffered an attack from a DDoS called MyDoom. Crackers have targeted other major Internet players like Amazon, CNN, Yahoo and eBay. The DDoS names range from mildly amusing to disturbing:

• Ping of Death - bots create huge electronic packets and sends them on to victims
• Mailbomb - bots send a massive amount of e-mail, crashing e-mail servers
• Smurf Attack - bots send Internet Control Message Protocol (ICMP) messages to reflectors, see above illustration
• Teardrop - bots send pieces of an illegitimate packet; the victim system tries to recombine the pieces into a packet and crashes as a result

     Once an army begins a DDoS attack against a victim system, there are few things the system administrator can do to prevent catastrophe. He could choose to limit the amount of traffic allowed on his server, but this restricts legitimate Internet connections and zombies alike. If the administrator can determine the origin of the attacks, he can filter the traffic. Unfortunately, since many zombie computers disguise (or spoof) their addresses, this isn't always easy to do.

Demonstration of DDOS Attack