How to listen on an arbitrary port and print the data which is coming using netcat

We are well known about  telnet which is pretty much handy tool to debug and open a TCP connection to remote server:port. But we want the tool which will listen on given port for connections unlike telnet. Yeah, there is tool called netcat. Usually it comes with many linux distributions by default, also called nc. There is also a windows version . According to the scope of the article we will see how to open a TCP or UDP connection and print the data sent by connected clients, but this tool  is more than capable of just listening on port for a connection.

Synopsis

nc [-46DdhklnrStUuvzC] [-i interval] [-p source_port] [-s source_ip_address] [-T ToS] [-wtimeout] [-X proxy_protocol] [ -x proxy_address[ :port]] [hostname] [port[s]]

 

Listen for UDP on specific host and port using netcat

Listen for the connections and print the data sent by the clients

nc -lvu [IP Address] 5060

nc is the command alias for netcat.

The break down of above command

l        Forcing netcat to listen on given host and port instead of opening/making  connection with remote host and port

v          Give verbose output

u          Use UDP. That is, listen for UDP on give port, by default it listens for TCP unless we give this option.

Ip Address   It is optional, if it is not specified localhost will be used

Port                You must specify the port on which nc should listen on.

Listen for TCP on specific host and port using netcat

This is same as UDP mentioned above except we haven’t specified the option -u . Where netcat will listen on TCP port by default with out that option.

Now netcat is listening on the given port on localhost. You can connect to this TCP port from the other  tab(terminal)   using either netcat or telnet

Connect Using Telnet

After  connected, type “Hello”, you should see that text in the tab where netcat is listening

Connect Using NetCat

After   connected,  we sent some text. It should be displayed on tab where nc is listening.

You can quit the client by sending interrupt  CTRL + D. If you quit the client, the server(nc command) will also stop listening. We can use the option -k     to 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.

I hope, these examples gave you some good understanding. As you can see, we can use nc as both server and client. Practically it is more capable than you think. If you are networking geek or dealing with network application or protocols. You can find this handy for your debugging purpose.

Case Studies

Let’s say, I have web hook URL configured on other 3rd party website. I want to check whether that server is sending events or not. If sending, I would like to see how is API and message format and I don’t have my program ready yet. In such a case, you simply listen on host and port which are configured on 3rd party web site to receive events and simply you can check the data received by nc 

Something like,

The tool netcat (nc) is very powerful tool, it is very simple tool but there are lot of possibilities. Anti-viruses on windows will detect this program as malware sometimes. Where it can also be used as a backdoor.

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