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Port knocking is a security by obscurity technique that allows to open a port when some port are knocked in a defined order.
For example, the port
22 is filtered. If anyone send a
SYN packet on port
4000 then the port
22 will appear open.
for x in 567 356 4000; do nmap -Pn --host_timeout 201 --max-retries 0 -p $x server_ip_address; done