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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"
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In netcat, Verbose is a mode which can be initiated using [-v] parameter. Now verbose mode generates extended information. Basically, we will connect to a server using netcat two times to see the difference between normal and verbose mode.
By default Netcat uses the TCP protocol for its communications, but it can also UDP using the -u option. As we mentioned at the previous step, Netcat lets you convert your PC in a server. In this case we're going to establish the connection between the server and the client but using UDP.
Remote Administration with netcat Netcat-traditional comes with '-e' option which can be used to bind a program (i.e cmd.exe in Windows or bash in Linux) with a port, that means netcat will act as communicator between the program and the remote PC.
Ctrl+d is the correct way to close netcat in the *Nix world.
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
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.
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