While this mitigation effort does lose some information about the TCP connection, it is better than allowing denial-of-service to occur to legitimate users as a result of an attack. If the connection is a legitimate request, and a final ACK packet is sent from the client machine back to the server, the server will then reconstruct (with some limitations) the SYN backlog queue entry. In order to avoid the risk of dropping connections when the backlog has been filled, the server responds to each connection request with a SYN-ACK packet but then drops the SYN request from the backlog, removing the request from memory and leaving the port open and ready to make a new connection. This strategy involves the creation of a cookie by the server. This particular defense fails when the attack volume is increased, or if the backlog size is too small to be practical. SYN cookies that use cryptographic hashing in the ACK packet to verify. This strategy requires that the legitimate connections can be fully established in less time than the backlog can be filled with malicious SYN packets. A SYN flood attack exploits TCP/IP to conduct a distributed denial-of-service. Recycling the Oldest Half-Open TCP connectionĪnother mitigation strategy involves overwriting the oldest half-open connection once the backlog has been filled. If the system does not have enough memory to be able to handle the increased backlog queue size, system performance will be negatively impacted, but that still may be better than denial-of-service. When the hardware SYN cookie feature is active, the system monitors relevant virtual servers, and determines when to enable SYN cookie protection. In order to successfully increase the maximum backlog, the system must reserve additional memory resources to deal with all the new requests. In BIG-IP 11.3.0 and later versions, the SYN cookie feature uses collaborative hardware/software SYN cookie protection for platforms that contain the HSB chip. One response to high volumes of SYN packets is to increase the maximum number of possible half-open connections the operating system will allow. Learn more about how Cloudflare's DDoS Protection works.įrom the same source ( Cloudflare - SYN Flood Attack), mitigation techniques include: Increasing Backlog queueĮach operating system on a targeted device has a certain number of half-open connections that it will allow. This strategy takes the resource cost of maintaining the connections with the bogus SYN packets off the targeted server and places it on Cloudflare’s Anycast network. When the initial SYN request is made, Cloudflare handles the handshake process in the cloud, withholding the connection with the targeted server until the TCP handshake is complete. Per their documentation: How does Cloudflare mitigate SYN Flood attacks?Ĭloudflare mitigates this type of attack in part by standing between the targeted server and the SYN flood. Appears to be a misconfiguration since this traffic is being seen at your server. Cloudflare should terminate all connections and thus prevent these spoofed syns from arriving at your server.
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