# How to use Netcat

> Learn how to use Netcat, the nc command, to connect to servers, send raw HTTP, listen on a port, scan open ports, and even transfer files over the network.

Author: [Flavio Copes](https://flaviocopes.com/about/) | Published: 2020-11-17 | Topics: [CLI](https://flaviocopes.com/tags/cli/) | Canonical: https://flaviocopes.com/netcat/

Netcat is a very useful Unix command we use to perform various networking tasks, and it's very useful to debug and also learn how things work.

It's available through the `nc` command.

Connect to any network server using this syntax:

`nc DOMAIN PORT`

`nc localhost 8000`

Once it's connected to the server, you can send any message by typing them, and you will see any reply sent back by the server.

Like a simple 1-to-1 chat, upon which you can work on application-level functionality in the form of protocols (HTTP, FTP, SMTP, etc).

Using Netcat you can simulate all those application level protocols that power the Internet.

You can connect to a Web server and send it the HTTP protocol instructions. I can connect to my website with `nc flaviocopes.com 80`, and I can send it

```
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.

![Terminal showing netcat HTTP request to flaviocopes.com returning 301 redirect response with headers](https://flaviocopes.com/images/netcat/Screen_Shot_2020-11-16_at_09.23.19.png)

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

![Terminal showing netcat connecting to echo server that responds with received messages hi and test](https://flaviocopes.com/images/netcat/Screen_Shot_2020-11-16_at_09.04.15.png)

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:

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

That's the simplest implementation of a Web server:

```bash
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`