There are several inspirations for this software that are acknowledged here.
The SYN cookie implementation is inspired by the excellent FreeBSD implementation. It is believed that since ideas cannot be copyrighted, this does not make the independent implementation copyrighted by FreeBSD kernel copyright holders.
The SYN proxy is inspired by other similar SYN proxy features, such as OpenBSD's PF SYN proxy and Linux kernel iptables SYN proxy. The same is true here: ideas cannot be copyrighted, and therefore, this is not a derivative work.