The "listen" state lock is encountered not only with SYN packets, but also other initial connection state packets like SYN-ACK and ACK packets (the last three-way handshake (3WHS) packet).
For "established" state connections, it can scale very well. This directly relates to a lock per socket when in the "listen" state. The basic TCP scalability problem for the Linux kernel is related to how many new connections can be created per second. To take advantage of them, you will first need to enable some settings including a few additional iptables rules to your existing firewall ruleset. It's important to note that while the following DDoS defense features are available in RHEL 7 Beta, they're not enabled by default. In this post I will provide a more condensed version of the talk highlighting how you can use these same techniques to protect your servers running Red Hat Enterprise Linux 7 Beta. Recently at 2014, I gave a talk focusing on how you can survive TCP SYN-flooding attacks by implementing some recently developed kernel level Netfilter/iptables defense mechanisms. As a result, your server is unable to properly handle any new incoming connection requests. It is a basic end-host resource attack designed to bring your server to its knees.
One of the most common types of DDoS attacks is the well-known SYN-flood attack. Distributed Denial of Service (DDoS) attacks are becoming increasingly commonplace as business becomes more and more dependent on delivering services over the Internet.
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