Issue with netcat timeout

Question

Why does the following netcat command not time out if the attempt to connect takes longer than 3 seconds (ie: when the port isn't open)? I assumed that the -w flag would be what I needed. OS is OSX 10.9.

nc -v -z -w 3 127.0.0.1 5050

Assuming that worked, I planned to implement like this (unsure if this will work, total bash noob)

nc -v -z -w 3 127.0.0.1 5050 | /dev/null && echo "Online" || echo "Offline"

Answer 1

You need to redirect to /dev/null, not pipe to it. Try the following:

nc -v -z -w 3 127.0.0.1 5050 &> /dev/null && echo "Online" || echo "Offline"

On my machine, port 5050 isn't open, and I get the following:

$ nc -v -z -w 3 localhost 5050 &> /dev/null && echo "Online" || echo "Offline"
Offline

Answer 2

Since Mac OS X 10.8.x, nc has used the -G option to set the timeout for initiating a connection. This is separate from the -w option, which sets the timeout for a connection that has been made but has gone idle.

If you are trying to use nc for port scanning, i.e. nc -v -z 10.0.1.1 1-1023, it will spend over a minute trying to connect to each non-responding port unless you add a -G timeout value:

nc -v -z -G 1 10.0.1.1 1-1023

That's one second per port scanned — much more useful.