What is tcp syn cookie?

TCP SYN cookies are a technique used in the TCP/IP protocol to help prevent denial-of-service attacks. When a client wants to establish a TCP connection with a server, it sends a SYN (synchronize) packet to the server.

In a typical scenario, the server responds with a SYN/ACK (synchronize/acknowledgment) packet to acknowledge the request and establish a connection with the client. The client then sends an ACK (acknowledgment) packet back to the server, and the connection is established.

However, in a TCP SYN flood attack, the attacker sends a flood of SYN packets to the server, overwhelming it with requests for new connections. The server may eventually become unresponsive or crash due to the high volume of requests.

TCP SYN cookies are designed to protect against SYN flood attacks by allowing the server to keep track of valid connection requests and temporarily store them until the client sends an ACK packet. When the server receives a SYN packet, it generates a TCP SYN cookie and sends it back to the client instead of a SYN/ACK packet. The client then sends an ACK packet with the cookie, which the server uses to verify the connection request.

TCP SYN cookies are a useful tool for protecting against denial-of-service attacks, but they may also lead to decreased performance or compatibility issues with some systems. Additionally, they may be less effective against more sophisticated attacks that can bypass the cookie verification process.