Port Knocking And DNAT Rules

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So I found an excellent port knocking tutorial using ONLY iptables rules that looks to be among the best I’ve ever seen. (warning: techno music, tough to read screen, you don’t need to type it in because I post a link to script below)


It works fabulously for simply opening a port to a locally managed service, but I can’t seem to get it to work for a PREROUTING/DNAT rule. I’ve posted the shell script I’m trying to get to work, it should be self-documented.


I’ve confirmed that the logs correctly show port knocking 2, 3, and 4 in
/var/log/messages so everything seems to be working golden all the way up to the last line. There are no errors reported when I run this script. The result that I get is that it acts as though packets are being dropped for 15 seconds, then I get connection refused.

What am I doing wrong?