1. Test if a particular TCP port of a remote host is open.

$ nc -vn 5000
nc: connect to 5000 (tcp) failed: Connection refused
$ nc -v 22
Connection to 22 port [tcp/ssh] succeeded!
SSH-2.0-OpenSSH_6.0p1 Debian-4

2. Send a test UDP packet to a remote host.

The command below sends a test UDP packet with 1 second timeout to a remote host at port 5000.

$ echo -n “foo” | nc -u -w1 5000

3. Perform TCP port scanning against a remote host.

The command below scans ports in the ranges of [1-1000] and [2000-3000] to check which port(s) are open.

$ nc -vnz -w 1 1-1000 2000-3000

4. Perform UDP port scanning against a remote host.

$ nc -vnzu 1-65535
Connection to 68 port [udp/*] succeeded!
Connection to 5353 port [udp/*] succeeded!
Connection to 16389 port [udp/*] succeeded!
Connection to 38515 port [udp/*] succeeded!
Connection to 45103 port [udp/*] succeeded!

The above command checks which UDP port(s) of a remote host are open and able to receive traffic.

5. Listen on a UDP port and dump received data in text format.

The command below listens on UDP port for incoming messages (lines of text).

$ nc -u localhost 5000

Note that this command dies after receiving one message. If you want to receive multiple messages, use while loop as follows.

$ while true; do nc -u localhost 5000; done

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