在lwn.net上，已经有人提供了SYNPROXY target这个补丁，让内核可以支持syn 代理了。

补丁代码包含了完整的发送自定义数据包的方法，可以参照借鉴。

The SYNPROXY target allows you to intercept TCP connections andestablish them using syncookies before they are passed on to theserver. This allows to avoid conntrack and server resource usageduring SYN-flood attacks.

http://lwn.net/Articles/564824/