knock [-u] [-d=ms] [-c=ms] [-v] [-h] host port1 port2...
Required:
host host name or address
port port numbers to knock
Optional:
-u knock to UDP ports (Default TCP)
-d=ms delay between knocks (Default 250 ms)
-c=ms connect timeout delay (Default 250 ms)
-v verbose output
knock -u -d=500 -c=500 -v 12.3.4.56 1234 1235 1236 1237
Knocking to UDP ports 1234-1237 of IP 12.3.4.56 with 500 ms delay between knocks and maximum 500 ms timeout on every try to connect and some verbose messages.