Added netcat's notes to the knowledge base

knowledge base/nc.md

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+# Netcat
+
+- `-N`: close the network socket when finished; not available in nmap's netcat
+- `-l`: bind to the port and listen for incoming connections (server mode)
+- `-n`: do not resolve hostnames via DNS
+- `-p`: specify the source port to use
+- `-t`: use telnet negotiation
+- `-u`: use UDP
+- `-v`: set verbosity level; can be used several times
+- `-w=SECS`: timeout for connects and final net reads, in seconds
+- `-z`: zero-I/O mode, exit once connected
+
+## TL;DR
+
+```shell
+# Check port 22 on hosts.
+nc -Nnvz 192.168.0.81 22
+parallel -j 0 "nc -Nnvz -w 2 192.168.0.{} 22 2>&1" ::: {2..254} | grep -v "timed out"
+
+# Wait for a host to be up.
+until nc -Nvz -w 3 pi4.lan 22; do sleep 3; done
+```