/usr/src/xtables-addons-2.12/extensions/pknock/knock.sh is in xtables-addons-dkms 2.12-0.1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | #!/bin/bash
if [ "$#" -ne 4 ]; then
echo "usage: $0 <IP src> <IP dst> <PORT dst> <secret>"
exit 1
fi
python gen_hmac.py "$4" "$1" | socat - "udp-sendto:$2:$3,bind=$1"
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