In the case of an attack, the attacker continues to send SYN requests without completing the connection, until the server is out of resources and unable to accept any additional traffic. Normally, the other server would respond with ACK, starting the connection. When an attacker sends large numbers of SYN packets to a machine, the server will allocate resources to this request and send a SYN ACK packet back – assuming that it is the beginning of a connection request. A SYN flood exploits the way a three-way handshake works. Protocol DDoS attacks find a weakness in how a protocol operates (such as a SYN flood). In this way, an attacker can send many, smaller packets and the response packets will use up the resources of the target.

For example, the response to a DNS query can be between 28 to 54 times larger than the original request. The advantage to going through an intermediate server rather than attacking the target directly is that response packets are typically much larger than the packet sent. Then, the responses to the spoofed packet will be sent to the target, rather than the attacker. Volumetric DDoS attacks aim to fill up a victim's bandwidth (such as UDP reflection attacks).Ī UDP reflection attack sends packets with the target's IP address spoofed as a the source.

There are many different types of DDoS attacks, but we can broadly group them into three categories – volumetric, protocol, and application attacks.

The difficult part of defending against DDoS attacks is that the hosts are distributed – if it were a single host or small group, you could easily block the traffic with a firewall rule. Distributed Denial of Service (DDoS) attacks aim to take an organization or service offline and originate from multiple, distributed hosts.

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