test connection to a remote IP / port

nc -z <IP> <TCP port> OR nc -zu <IP> <UDP port>
-z: Specifies that nc should just scan for listening daemons, without sending any data to them -u: Use UDP instead of the default option of TCP.
Sample Output
# nc -zu 10.162.155.17 4670
Connection to 10.162.155.17 4670 port [udp/*] succeeded!

0
By: frank514
2011-04-01 04:08:53

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  • Ever needed to test firewalls but didn't have netcat, telnet or even FTP? Enter /dev/tcp, your new best friend. /dev/tcp/(hostname)/(port) is a bash builtin that bash can use to open connections to TCP and UDP ports. This one-liner opens a connection on a port to a server and lets you read and write to it from the terminal. How it works: First, exec sets up a redirect for /dev/tcp/$server/$port to file descriptor 5. Then, as per some excellent feedback from @flatcap, we launch a redirect from file descriptor 5 to STDOUT and send that to the background (which is what causes the PID to be printed when the commands are run), and then redirect STDIN to file descriptor 5 with the second cat. Finally, when the second cat dies (the connection is closed), we clean up the file descriptor with 'exec 5>&-'. It can be used to test FTP, HTTP, NTP, or can connect to netcat listening on a port (makes for a simple chat client!) Replace /tcp/ with /udp/ to use UDP instead.


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    dragonauta · 2012-10-26 10:53:47 0
  • Requires software found at: http://lpccomp.bc.ca/remserial/ Remote [A] (with physical serial port connected to device) ./remserial -d -p 23000 -s "115200 raw" /dev/ttyS0 & Local [B] (running the program that needs to connect to serial device) Create a SSH tunnel to the remote server: ssh -N -L 23000:localhost:23000 user@hostwithphysicalserialport Use the locally tunnelled port to connect the local virtual serial port to the remote real physical port: ./remserial -d -r localhost -p 23000 -l /dev/remser1 /dev/ptmx & Example: Running minicom on machine B using serial /dev/remser1 will actually connect you to whatever device is plugged into machine A's serial port /dev/ttyS0.


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What Others Think

The man page for nc states that UDP tests will always succeed and that -zu is useless.
DEinspanjer · 355 weeks and 2 days ago

What do you think?

Any thoughts on this command? Does it work on your machine? Can you do the same thing with only 14 characters?

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