Ok, I should have been more clear. Modulating SYN packets is stupid. Port knocking by sending a secret to the server, in a very simple protocol (to the point

Ok, I should have been more clear. Modulating SYN packets is stupid. Port knocking by sending a secret to the server, in a very simple protocol (to the point of being obviously correct), is good. Once it gets too complex, the implementation is more likely to have a bug than OpenSSH. So just send that secret in a UDP packet. Modulating SYN packets is like sending your password (or other secret) in morse code rather than ASCII, for no reason. I elaborated more in https://news.ycombinator.com/item?id=41389167 https://news.ycombinator.com/item?id=41389167