ipt_pkd is an iptables extension implementing port knock detection. This project provides 3 parts: the kernel module ipt_pkd, the iptables user space module libipt_pkd.so, and a user space client knock program. For the knock packet, it uses a UDP packet sent to a random port that contains a SHA-256 of a timestamp, small header, random bytes, and a shared key. ipt_pkd checks the time window of the packet and does the SHA-256 to verify the packet. The shared key is never sent.
You can download it from the following link: https://packetstormsecurity.com/files/download/74854/pkd-1.3.tgz
Source: https://packetstormsecurity.com/files/74854/IPT_PKD-Iptables-Port-Knocking-Detection.html