How to use Netcat

Netcat is a very useful Unix command that we can use to perform various networking tasks. It is also very useful for debugging and learning how it works.

accessiblenccommand.

Use the following syntax to connect to any web server:

nc DOMAIN PORT

nc localhost 8000

After connecting it to the server, you can send any message by typing any message, and you will see any reply sent back by the server.

Just like a simple one-to-one chat, you can perform application-level functions on it in the form of protocols (HTTP, FTP, SMTP, etc.).

Using Netcat, you can simulate all those application-level protocols that support the Internet.

You can connect to the web server and send it an HTTP protocol description. I can connect to my website bync flaviocopes.com 80, I can send

GET / HTTP/1.1
Host: flaviocopes.com
(the third line is an empty line)


and the server will reply back, with an HTTP/1.1 301 Moved Permanently to https://flaviocopes.com/ response because I force HTTPS.




Here’s a simple example to interface with an “echo” server I built separately, that sends back what we send to it:




Netcat can work with TCP, the default protocol, or UDP.


To use UDP, add the -u flag:


nc -u localhost 8000


You can create a server, too. Use the -l (listen) option to listen on a specific port:


nc -l PORT


and Netcat will print every command received.


Try running the server with nc -l 8001 on one terminal window, and the client nc localhost 8001 on another, then send messages to the server by typing them in the client terminal.


Netcat can also be used for network inspection. You can scan the open ports of a server, in a specific range:


nc -v -z localhost 1-10000




Tip: combine with grep to filter the noise: nc -v -z localhost 1-10000 2>&1 | grep succeeded




(if you’re curious which is the process using a port, run lsof -i :PORT)


You can tell a nc server to send the content of a file to the client that connects:


nc -l PORT < FILENAME


The client connecting with nc DOMAIN PORT will get the content of that file printed out, but it can save the content to a file using nc DOMAIN PORT > FILENAME, resulting in a basic file transfer.


After the file has been served, the server will terminate.


You can wrap that command in a simple Bash shell loop:


while true; do nc -l PORT  < FILENAME; done


That’s the simplest implementation of a Web server:


while true; do nc -l 80  < index.html; done


You can tell the client to send to the server the instructions contained in a file:


nc DOMAIN PORT < FILENAME

  

  
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