Continuously listen to tcp port via terminal

Question

Is it possible to listen to a port continuously?

I listen for incoming tcp notifications with following command

sudo nc -l -p 999

But as soon as notification arrives I have to restart listen with same command. Is it possible to listen to port without having to restart command when notifications arrives until user decides to abort listen?

Plenish

Exactly, so whenever a notification arrives it should be printed in terminal without the need to manually restart command. – Plenish

Does this work: while true; do nc -l -p 999; done – Piperine

I get a syntax error sudo while true; do nc -l -p 999; done bash: syntax error near unexpected token `do' – Plenish

If you want to use it with sudo, you have to use sudo sh -c 'while true; do nc -l -p 999; done' – Piperine

Answer 1

Solved with a simple bash script

#!/bin/bash

#Make Sure Script Is Ran As Root
if [ $(id -u) != 0 ]; then
    echo; echo -e "\e[1;31mScript must be run as sudo. Please Type \"sudo\" To Run As Root \e[0m"; echo    
exit 1
fi

echo "Enter port to listen"
read portL

while true;
do
    nc -l -p $portL
done
exit 0

Thanks dreamlax for the tip!

Plenish

I would avoid putting sudo in the loop like that ... unless it is configured not to prompt for a password, it may cause issues if nothing is received for 15 minutes (or whatever the sudo timeout is configured on your system) and sudo is run again. – Piperine

Better to remove sudo from script then run script as sudo? Ex: sudo tcpListen.sh – Plenish

Yeah, that would be better I think! – Piperine

I would emphasize: just run this as the root user and have it check if real UID is 0. Alternatively, run it as a user with sudo configured as NOPASSWD ALL. Two choices. You won't get far automating things if there's a prompt to deal with. (Don't try what some recommend, trying to "answer" the password prompt, ugh) – Opponent

Answer 2

Sorta outdated question, but came up first on my Google search.

In order for netcat not to shutdown as soon as the first connection is received, you can add the -k option.

From the man:

-k Forces nc to stay listening for another connection after its current connection is completed. It is an error to use this option without the -l option.

Src: https://superuser.com/a/708133/410908

Stunt

On Debian -k is present only in nc.openbsd – Cirenaica