如何使用Netcat

Netcat是一个非常有用的Unix命令,我们可以用来执行各种联网任务,它对调试以及学习工作原理也非常有用。

可通过nc命令。

使用以下语法连接到任何网络服务器:

nc DOMAIN PORT

nc localhost 8000

将其连接到服务器后,您可以通过键入任何消息来发送任何消息,并且您将看到服务器发回的任何答复。

就像简单的一对一聊天一样,您可以在其上以协议(HTTP,FTP,SMTP等)的形式来进行应用程序级功能。

使用Netcat,您可以模拟支持Internet的所有那些应用程序级别协议。

您可以连接到Web服务器并向其发送HTTP协议说明。我可以通过以下方式连接到我的网站nc flaviocopes.com 80,我可以寄出

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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