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Sending and receiving TCP/UDP packets using Netcat

· 2 min read
Saverio Ferrara
Saverio Ferrara
Software Engineer

This story will show you how to use Netcat to send and receive TCP/UDP packets. We'll focus on a specific example... we'll simulate a Statsd client/server.

What is Netcat? Netcat is a featured networking utility which reads and writes data across network connections, using the TCP/IP protocol. Designed to be a reliable "back-end" tool, Netcat can be used directly with other programs and scripts to send files from a client to a server and back. At the same time, it is a feature-rich network debugging and exploration tool that can specify the network parameters while also establishing a connection to a remote host via a tunnel.

Although Netcat can do many things, its main purpose and most desirable functions are to:

  1. Create an initial socket to establish a connection from the server to the client.
  2. Once connected, Netcat will automatically generate a second socket to transmit files from the server to the client and visa versa.

Netcat version​

There are several, and incompatible, Netcat implementations. I believe Linux has the most advanced implementation but, unfortunately, I am on MacOS Catalina 10.15.6. So I used netcat 0.7.1 installed through homebrew. Actually, MacOS already provides a netcat implementation (i.e. /usr/bin/nc) that I honestly don't like.

Test your Netcat understanding as a client-server​

Open two computer terminals, the first will act as the server and the second will be the client.

Simulate the server​

With Netcat, your computer can be converted into a fake server. Focus to the server terminal, you want to start at port 8125/UDP, so issue:

$ netcat -ulzp 8125

The server is now up and listening.

Simulate the client​

In addition, we can use netcat to connect to the port (8125) recently opened. Switch to the client terminal and send a message to 8125/UDP:

echo "my-app.node.timer:102522|ms" | netcat -nuc 127.0.0.1 8125

Now you should see the fake metric my-app.node.timer:102522|ms sent by the client in the output of the fake server.