This is really cool feature on Cisco router not usually mentioned until you dig a little deeper inside Cisco IOS. But first a bit of theory…
What is TCP SYN flood attack
TCP 3-way handshake
SYN flood DoS attack happens when many sources start to send a flood of TCP SYN packets usually with fake source IP.
This attack uses TCP 3-way handshake to reserve all server available resources with fake SYN requests not allowing legitimate users to establish connection to the server. SYN packet is the first step in TCP 3-way handshake. This is the step where client sends connection synchronization request to the server. Server receives TCP SYN from client, the server replies back with SYN ACK. SYN ACK acknowledges synchronization request.
In that moment server is waiting the client to complete the handshake by sending an ACK back to server to acknowledge the SYN ACK. With this third step, TCP session is successfully established and communication between server and client begins.
If the ACK is not received from the client side, server will wait for it for some time and then the session will timeout and get dropped. When the server deletes the session, his resources will be released.
TCP SYN flood attack
TCP SYN flood attack sends first packet of 3-way handshake SYN packet to server many times to cause the server to allocate resources for sessions that will never become established. It means that client who is attacking will never respond to server SYN ACK and the session will remain on the second step of 3-way.