On 6th February 2000, Yahoo portal was shut down for 3 hours. Then retailer Buy.com Inc. (BUYX) was hit the next day, hours after going public. By that evening, eBay (EBAY), Amazon.com (AMZN), and CNN (TWX) had gone dark. And in the morning, the mayhem continued with online broker E*Trade (EGRP) and others having traffic to their sites virtually choked off. (Business Week Online, 12 February 2000)

On 6th August 2009, twitter was the victim. Lot of people had trouble on logging on twitter,It was brought down by denial of service attack. 
                                                                              -Reported by CNN’s Don Lemon

 
? Denial of service attack is an attack in which attacker try to crash the system or to make the computer resource unavailable to its legal users, if an attacker is unable to gain access into a server, the attacker most probably will just crash it.
? Denial of service (DOS) attack is the process of sending a large number of packets to the target that interfere with its operation, or just crash it.

Common DOS attacks are
Smurf
? Smurf attack some time refer as Amplification attack, ICMP(Internet Control Message Protocol) flood attack or Fraggle Attack.
? A smurf attack uses spoofing and broadcast addressing to amplify a single packet to multiple packets.
? Three major parties of this attack are.
The attacker
The intermediary (a router) also known as smurf amplifier
The victim

 
? In Smurf attack an attacker first spoof the source IP address of the victim, generates a large amount of ICMP echo traffic toward IP broadcast addresses. The intermediary receives an ICMP echo request packet directed to the IP broadcast address of their network.
 
? Each host that receives the broadcast responds back to the real source IP with an echo reply, this attack can be done with both ICMP packets and UDP echo packets(Fraggle attack).
 
Solution
? Turn off the forwarding of directed broadcast on all router ports.
? Configure each hosts and routers not to respond or reply to ping request
? Use network Ingress filtering.

SYN Flooding? SYN flooding is an another type of DOS attack in which an attacker generates Transmission Control Protocol(TCP) request faster of the system processing capabilities, in SYN flooding an attacker take advantages of TCP handshaking process.
 
? TCP handshaking process involve three steps to establish a connection.
1. In first step client send a TCP Synchronize(SYN) packet to the server.
2. Server receives a SYN packet and send a Synchronize-Acknowledgement (SYNACK)
packet back to the client(host).
3. Client receives SYN-ACK packet and sends Acknowledge(ACK), and the connection is established.

 To Be Continued on the next part we will share some codes to launch a SYN Flooding Attack.