Netcat est une commande Unix très utile que nous utilisons pour effectuer diverses tâches de mise en réseau, et elle est très utile pour déboguer et aussi apprendre comment les choses fonctionnent.
Il est disponible via lenc
commander.
Connectez-vous à n'importe quel serveur réseau en utilisant cette syntaxe:
nc DOMAIN PORT
nc localhost 8000
Une fois qu'il est connecté au serveur, vous pouvez envoyer n'importe quel message en le tapant, et vous verrez toute réponse renvoyée par le serveur.
Comme un simple chat 1-to-1, sur lequel vous pouvez travailler sur des fonctionnalités au niveau de l'application sous la forme de protocoles (HTTP, FTP, SMTP, etc.).
En utilisant Netcat, vous pouvez simuler tous les protocoles de niveau application qui alimentent Internet.
Vous pouvez vous connecter à un serveur Web et lui envoyer les instructions du protocole HTTP. Je peux me connecter à mon site Web avecnc flaviocopes.com 80
, et je peux l'envoyer
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
Download my free Linux Commands Handbook
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