You appear to have a fundamental misunderstanding of the difference between the TCP/IP transport layer and the application layer. SYN flood attacks are not HTTP traffic and do not get passed to Apache. You're most likely confusing a TCP SYN flood with some other form of attack.
Assuming you are in fact correct in your assertion, you should contact your ISP, as they probably have a way to do this at the router level, or they should at the very least provide some primitive sort of firewalling to their customers. If not, we'll need to know what operating system you're running in order to develop any kind of workable solution for you.
Most routers will block these types of attacks automatically, without user intervention. What makes you believe you're the victim of a SYN attack?
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