Short for denial-of-service attack, a type of attack on a network that is designed to bring the network to its knees by flooding it with useless traffic. Many DoS attacks, such as the Ping of Death and Teardrop attacks, exploit limitations in the TCP/IP protocols. For all known DoS attacks, there are software fixes that system administrators can install to limit the damage caused by the attacks. But, like viruses, new DoS attacks are constantly being dreamed up by hackers.In a denial-of-service (DoS) attack, an attacker attempts to prevent legitimate users from accessing information or services. By targeting your computer and its network connection, or the computers and network of the sites you are trying to use, an attacker may be able to prevent you from accessing email, web sites, online accounts (banking, etc.), or other services that rely on the affected computer.
The most common and obvious type of DoS attack occurs when an attacker "floods" a network with information. When you type a URL for a particular web site into your browser, you are sending a request to that site's computer server to view the page. The server can only process a certain number of requests at once, so if an attacker overloads the server with requests, it can't process your request. This is a "denial of service" because you can't access that site.

What is a distributed denial-of-service (DDoS) attack?

In a distributed denial-of-service (DDoS) attack, an attacker may use your computer to attack another computer. By taking advantage of security vulnerabilities or weaknesses, an attacker could take control of your computer. He or she could then force your computer to send huge amounts of data to a web site or send spam to particular email addresses. The attack is "distributed" because the attacker is using multiple computers, including yours, to launch the denial-of-service attack.

DOS Attacks are of the following different types-: 1. Those that exploit vulnerabilities in the TCP/IP protocols suite. 2. Those that exploit vulnerabilities in the Ipv4 implementation. 3 There are also some brute force attacks, which try to use up all resources of the target system and make the services unusable. Some common vulnerabilities are Ping of Death, Teardrop, SYN attacks and Land Attacks.

Ping of Death

This vulnerability is quite well known and was earlier commonly used to hang remote systems (or even force them to reboot) so that no users can use its services. This exploit no longer works, as almost all system administrators would have upgraded their systems making them safe from such attacks. In this attack, the target system is pinged with a data packet that exceeds the maximum bytes allowed by TCP/IP, which is 65 536. This would have almost always caused the remote system to hang, reboot or crash. This DOS attack could be carried out even through the command line, in the following manner: The following Ping command creates a giant datagram of the size 65540 for Ping. It might hang the victim's computer:

C:\windows>ping -l 65540 .


The Teardrop attack exploits the vulnerability present in the reassembling of data packets. Whenever data is being sent over the Internet, it is broken down into smaller fragments at the source system and put together at the destination system. Say you need to send 4000 bytes of data from one system to the other, then not all of the 4000 bytes is sent at one go. This entire chunk of data is first broken down into smaller parts and divided into a number of packets, with each packet carrying a specified range of data. For Example, say 4000 bytes is divided into 3 packets, then: The first Packet will carry data from 1 byte to 1500 bytes The second Packet will carry data from 1501 bytes to 3000 bytes The third packet will carry data from 3001 bytes to 4000 bytes These packets have an OFFSET field in their TCP header part. This Offset field specifies from which byte to which byte does that particular data packet carries data or the range of data that it is carrying. This along with the sequence numbers helps the destination system to reassemble the data packets in the correct order. Now in this attack, a series of data packets are sent to the target system with overlapping Offset field values. As a result, the target system is not able to reassemble the packets and is forced to crash, hang or reboot. Say for example, consider the following scenario-: Normally a system receives data packets in the following form, with no overlapping Offset values. (1 to 1500 bytes) (1501 to 3000 bytes) (3001 to 4500 bytes) Now in a Teardrop attack, the data packets are sent to the target computer in the following format: (1 to 1500 bytes) (1500 to 3000 bytes) (1001 to 3600 bytes) When the target system receives something like the above, it simply cannot handle it and will crash or hang or reboot.

SYN Attack

The SYN attack exploits TCP/IP's three-way handshake. Thus in order to understand as to how SYN Attacks work, you need to first know how TCP/IP establishes a connection between two systems. Whenever a client wants to establish a connection with a host, then three steps take place. These three steps are referred to as the three-way handshake. In a normal three way handshake, what happens is that, the client sends a SYN packet to the host, the host replies to this packet with a SYN ACK packet. Then the client responds with a ACK (Acknowledgement) packet. This will be clearer after the following depiction of these steps-: 1. Client SYN Packet à Host In the first step the client sends a SYN packet to the host, with whom it wants to establish a three-way connection. The SYN packet requests the remote system for a connection. It also contains the Initial Sequence Number or ISN of the client, which is needed by the host to put back the fragmented data in the correct sequence. 2. Host SYN/ACK Packet à Client In the second step, the host replies to the client with a SYN/ACK packet. This packet acknowledges the SYN packet sent by the client and sends the client its own ISN. 3. Client ACK à Host In the last step the client acknowledges the SYN/ACK packet sent by the host by replying with a ACK packet. These three steps together are known as the 3-way handshake and only when they are completed is a complete TCP/ IP connection established. In a SYN attack, several SYN packets are sent to the server but all these SYN packets have a bad source IP Address. When the target system receives these SYN Packets with Bad IP Addresses, it tries to respond to each one of them with a SYN ACK packet. Now the target system waits for an ACK message to come from the bad IP address. However, as the bad IP does not actually exist, the target system never actually receives the ACK packet. It thus queues up all these requests until it receives an ACK message. The requests are not removed unless and until, the remote target system gets an ACK message. Hence these requests take up or occupy valuable resources of the target machine. To actually affect the target system, a large number of SYN bad IP packets have to be sent. As these packets have a Bad Source IP, they queue up, use up resources and memory or the target system and eventually crash, hang or reboot the system. Land Attacks A Land attack is similar to a SYN attack, the only difference being that instead of a bad IP Address, the IP address of the target system itself is used. This creates an infinite loop between the target system and the target system itself. However, almost all systems have filters or firewalls against such attacks.

UDP Flooding

This kind of flooding is done against two target systems and can be used to stop the services offered by any of the two systems. Both of the target systems are connected to each other, one generating a series of characters for each packet received or in other words, requesting UDP character generating service while the other system, echoes all characters it receives. This creates an infinite non-stopping loop between the two systems, making them useless for any data exchange or service provision.