0 SYN Attack

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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.