Denial of Service attack is a type of attack in which the network is flooded with useless traffic which ultimately leads to its failure. Here, Denial means refusal to satisfy a request. This attack may be caused by single or a group of people. When a single host attacks it is called Denial of Service (Dos) attack and when multiple host attacks it is called as Distributed Denial of Service (DDos) attack. Due to this attack, users cannot use the network and this interruption may be for short or for long time. This attack also leads to server overload and can no longer give response to the user.
The purpose of attack may vary and it may be to shut down site, overload server, extortion purpose, for modification of data or for any social action. First Denial of Service(Dos) attack occurred on November 2,1998 and affected 10% of internet computers over the world. This attack was named as ‘Morris Worm Attack’ as it was caused by person named Morris. This attack affected large amount of network as it was self replicating and self propagating. There are different type of Dos attack such as Buffer Overflow attack, Nuke, Smurf attack, permanent Denial of Service attack, Eavesdropping, viruses, etc.
Buffer Overflow attack– This attack mainly aims to send more traffic to network address than the data buffers in it can handle. This is most common type of attack and attackers are aware of the weakness of targeted system.
Nuke- This attack sends invalid or fragmented ICMP message to the target machine resulting it to slow down or stop completely.
Smurf attack– It involves sending forged request to large number of computers who will give response to this request. All these response are sent to targeted system which will result in flooding at the targeted system.
Permanent Denial of service attack– This attack affects the hardware of system leading to permanent damage of system.
Eavesdropping– Attacker gain access to some network and listens to traffic going in or out of network. Attacker gains control over network.
Viruses– This attack is common attack in which viruses replicate themselves where victim can be any host who is affected by that virus. In this attack victim is not any targeted system.
Denial of Service attack can be blocked through firewall, switches, routers and various other mechanism. Firewall can be used to prevent DoS attack by rejecting messages which are coming from unusual IP address. Switches can be used for traffic shaping, filtering bogus IP address thus protecting system from DoS attack.
Recently DoS attack affected Chinese Internet on Sunday morning 25August at 2 AM. It became more worse at 4 AM Experts says that attack might have been carried out by single individual. The attack affected registry for about four hours. Due to this there was nearly 32% drop in traffic.