Enabling SYN Cookie

A TCP connection is established through a three-way handshake. An attacker can exploit this mechanism to mount SYN Flood attacks. The attacker sends a large number of SYN packets, but does not respond to the SYN ACK packets from the server. As a result, the server establishes a large number of TCP semi-connections and can no longer handle normal services.

SYN Cookie can protect the server from SYN Flood attacks. When the server receives a SYN packet, it responds with a SYN ACK packet without establishing a TCP semi-connection. The server establishes a TCP connection and enters ESTABLISHED state only when it receives an ACK packet from the client.

1. Enter system view.

   system-view

2. Enable SYN Cookie.

   tcp syn-cookie enable

   By default, SYN Cookie is disabled.