The most common implementation is on load balancer and DDoS appliances, with dedicated CPU and OS that can process huge volumes of TCP sequence calculations without loss of performance. The CPU impact may result in servers not able to deliver applications or, at best, to work much more slowly in every circumstance. The CPU requirement to deliver the mathematics for the function calculation is beyond the capacity of x86 servers (and their OS’s) to reliably compute on a real time basis ((although a MSWin / Linux server certainly could compute the functions, its overall performance would be severely impacted)). In general terms, implementing this type of code on servers is a bad idea. The effect is that SYN floods will no longer consume resources on servers or load balancers/ This is especially true in high bandwidth environments such as Data Centres. If the ACK response is not correct the TCP session is not created. This is part of the functions of a PIX/ASA firewall, it will improve the randomness of the ISN to ensure The TCP sequence is what NMAP uses to identify the OS since it ‘knows’ the some OS’s do not have high quality randomisation and NMAP uses algorithms to analyse the ISN to ‘guess’ the OS. The TCP sequence number at the commencement of a TCP sequence is normally a randomised choice. If the check is successful, then the server will create the TCP session and the user connection will proceed as normal. On receipt of the ACK from the Client, the TCP sequence number is checked against the function to determine if this is a legitimate reply. This is highly leveraged attack since a very small amount of bandwidth and CPU can exhaust the resources on a large number of servers.īy specifically calculating the TCP sequence number with a specific, secret math function in the SYN-ACK response, the server does not need to maintain this state table. And since this state table is finite the server will no longer accept new TCP connections and thus fail or deny service to the user ((or worse, buffer overflows or system memory exhaustion has occurred, not so much a problem today)). A simple SYN flood (using suitable software) will generate SYN packets which would consume all available TCP memory as the server must maintain state for all half-open connections. In normal operation, a Client sends a SYN and the Server responds with a SYN+ACK message, the server will then hold state information in the TCP stack while waiting for Client ACK message. What is a SYN Cookie and Why do I want them ?Ī SYN cookie is a specific choice of initial TCP sequence number by TCP software and is used as a defence against SYN Flood attacks. Lets take a quick dive through the technology. A TCP SYN Cookie is typically used in DDoS engines and load balancers to create another level of protocol security for Denial of Service attacks.
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