Netcat command in Linux

[Introduction] Netcat is the Swiss Army Knife of network tools. It can read and write data in the network through TCP and UDP. You can use it in a variety of ways within your scripts by combining it with other tools and redirecting it. It's amazing what you can accomplish with the Netcat command. What Netcat does is to establish a connection between two computers. You can use it in a variety of ways within your scripts by combining it with other tools and redirecting it. It's amazing what you can accomplish with the Netcat command.

All Netcat does is establish a link between two computers and return two data streams. What you can do after that depends on your imagination. You can set up a server, transfer files, chat with friends, stream media or use it as a standalone client for other protocols.

The following are some examples of using Netcat.

[A(172.31.100.7) B(172.31.100.23)]

Linux Netcat command example:

1. Port scanning

Port scanning is often used by system administrators and hackers to find open ports on some machines and help them identify vulnerabilities in the system.

$nc -z -v -n 172.31.100.7 21-25

Can run in TCP or UDP mode, the default is TCP, the -u parameter is adjusted to udp.

z parameter tells Netcat to use 0 IO, which means that once The connection is closed and no data is exchanged. : That is, detailed output)

• n parameter tells Netcat not to use DNS to reverse query the domain name of the IP address

• This command will print 21 to 25 All open ports. Banner is a text. Banner is a text message sent to you by a service you are connected to. Banner information is very useful when you are trying to identify a vulnerability or the type and version of a service. However, not all services send banners.

Once you find open ports, you can easily use the Netcat connection service to grab their banners.

• $ nc -v 172.31.100.7 21

  Copy after login

  The Netcat command will connect to open port 21 and print the banner information of the service running on this port.

Chat Server

If you want to chat with your friends, there are many software and information services available for you to use. However, if you don't have such a luxurious configuration, for example, if you are in a computer lab and all external connections are restricted, how do you communicate with your friends who sit in the next room all day? Don't be depressed, Netcat provides such a method, you only need to create a Chat server, a predetermined port, so that he can contact you.

Server

$nc -l 1567

The Netcat command starts a tcp server on port 1567, and all standard output and input will be output to this port. Both output and input are displayed in this shell.

Client

$nc 172.31.100.7 1567

Whatever you type on machine B will appear on machine A.

3. File transfer

Most of the time, we are trying to transfer files through the network or other tools. There are many methods, such as FTP, SCP, SMB, etc., but when you only need to transfer files temporarily or once, it is really worth wasting time to install and configure a software on your machine. Suppose, you want to transfer a file file.txt from A to B. Either A or B can serve as the server or client. Below, let A serve as the server and B as the client.

Server

$nc -l 1567 
Client
$nc -n 172.31.100.7 1567 > file.txt
        

         Copy after login
        
       

Here we create a server on A and redirect Netcat's input to the file file.txt, then when any successful connection to the Port, Netcat will send the file contents of file.

On the client side we redirect the output to file.txt. When B connects to A, A sends the file content and B saves the file content to file.txt.

There is no need to create a file source as Server, we can also use it in the opposite way. Like below we are sending files from B to A, but the server is created on A. This time we only need to redirect the output of Netcat and redirect the input files of B.

BAs Server

Server

$nc -l 1567 > file.txt

Client

nc 172.31.100.23 1567 
4. Directory transfer
Sending a file is very simple, but if we If you want to send multiple files or an entire directory, it is very simple. You only need to use the compression tool tar to compress and send the compressed package. 
If you want to transfer a directory from A to B over the network. 
Server
$tar -cvf – dir_name | nc -l 1567
        

         Copy after login
        
       

Client

$nc -n 172.31.100.7 1567 | tar -xvf -

Here on server A, we create a tar archive and redirect it via - in the console, then use pipe, redirect to Netcat, and Netcat can send it over the network.

On the client, we download the compressed package through the Netcat pipe and then open the file.

If we want to save bandwidth and transmit compressed packages, we can use bzip2 or other tools for compression.

Server

$tar -cvf – dir_name| bzip2 -z | nc -l 1567

Compression via bzip2

Client

$nc -n 172.31.100.7 1567 | bzip2 -d |tar -xvf -

Decompression using bzip2

5. Encrypt the data you send over the network

If you are worried about the security of the data you send over the network, you can encrypt it with a tool such as mcrypt before sending your data.

Server

$nc localhost 1567 | mcrypt –flush –bare -F -q -d -m ecb > file.txt

Use the mcrypt tool to encrypt data.

Client

$mcrypt –flush –bare -F -q -m ecb 
Use the mcrypt tool to decrypt the data. 
以上两个命令会提示需要密码，确保两端使用相同的密码。
这里我们是使用mcrypt用来加密，使用其它任意加密工具都可以。
6. 流视频
虽然不是生成流视频的最好方法，但如果服务器上没有特定的工具，使用Netcat，我们仍然有希望做成这件事。
服务端
$cat video.avi | nc -l 1567
        

         Copy after login
        
       

这里我们只是从一个视频文件中读入并重定向输出到Netcat客户端

$nc 172.31.100.7 1567 | mplayer -vo x11 -cache 3000 -

这里我们从socket中读入数据并重定向到mplayer。

7、克隆一个设备

如果你已经安装配置一台Linux机器并且需要重复同样的操作对其他的机器，而你不想在重复配置一遍。不在需要重复配置安装的过程，只启动另一台机器的一些引导可以随身碟和克隆你的机器。

克隆Linux PC很简单，假如你的系统在磁盘/dev/sda上

Server

$dd if=/dev/sda | nc -l 1567

Client

$nc -n 172.31.100.7 1567 | dd of=/dev/sda

dd是一个从磁盘读取原始数据的工具，我通过Netcat服务器重定向它的输出流到其他机器并且写入到磁盘中，它会随着分区表拷贝所有的信息。但是如果我们已经做过分区并且只需要克隆root分区，我们可以根据我们系统root分区的位置，更改sda 为sda1，sda2.等等。

8、打开一个shell

我们已经用过远程shell-使用telnet和ssh，但是如果这两个命令没有安装并且我们没有权限安装他们，我们也可以使用Netcat创建远程shell。

假设你的Netcat支持 -c -e 参数(默认 Netcat)

Server

$nc -l 1567 -e /bin/bash -i

Client

$nc 172.31.100.7 1567

这里我们已经创建了一个Netcat服务器并且表示当它连接成功时执行/bin/bash

假如Netcat 不支持-c 或者 -e 参数（openbsd Netcat）,我们仍然能够创建远程shell

Server

$mkfifo /tmp/tmp_fifo$cat /tmp/tmp_fifo | /bin/sh -i 2>&1 | nc -l 1567 > /tmp/tmp_fifo

这里我们创建了一个fifo文件，然后使用管道命令把这个fifo文件内容定向到shell 2>&1中。是用来重定向标准错误输出和标准输出，然后管道到Netcat 运行的端口1567上。至此，我们已经把Netcat的输出重定向到fifo文件中。

说明：从网络收到的输入写到fifo文件中

• cat 命令读取fifo文件并且其内容发送给sh命令

• sh命令进程受到输入并把它写回到Netcat。

• Netcat 通过网络发送输出到client

至于为什么会成功是因为管道使命令平行执行，fifo文件用来替代正常文件，因为fifo使读取等待而如果是一个普通文件，cat命令会尽快结束并开始读取空文件。

9、在客户端仅仅简单连接到服务器

Client

$nc -n 172.31.100.7 1567

你会得到一个shell提示符在客户端

反向shell

反向shell是人曾经在客户端打开的shell。反向shell这样命名是因为不同于其他配置，这里服务器使用的是由客户提供的服务。

服务端

$nc -l 1567

在客户端，简单地告诉Netcat在连接完成后，执行shell。

客户端

$nc 172.31.100.7 1567 -e /bin/bash

现在，什么是反向shell的特别之处呢

反向shell经常被用来绕过防火墙的限制，如阻止入站连接。例如，我有一个专用IP地址为172.31.100.7，我使用代理服务器连接到外部网络。如果我想从网络外部访问 这台机器如1.2.3.4的shell，那么我会用反向外壳用于这一目的。

10. 指定源端口

假设你的防火墙过滤除25端口外其它所有端口，你需要使用-p选项指定源端口。

服务器端

$nc -l 1567

客户端

$nc 172.31.100.7 1567 -p 25

使用1024以内的端口需要root权限。

该命令将在客户端开启25端口用于通讯，否则将使用随机端口。

11、指定源地址

假设你的机器有多个地址，希望明确指定使用哪个地址用于外部数据通讯。我们可以在Netcat中使用-s选项指定ip地址。

服务器端

$nc -u -l 1567 
客户端
$nc -u 172.31.100.7 1567 -s 172.31.100.5 > file.txt
        

         Copy after login
        
       

该命令将绑定地址172.31.100.5。

这仅仅是使用Netcat的一些示例。

其它用途有：

• 使用-t选项模拟Telnet客户端，

• HTTP客户端用于下载文件，

• 连接到邮件服务器，使用SMTP协议检查邮件，

• 使用ffmpeg截取屏幕并通过流式传输分享，等等。其它更多用途。

简单来说，只要你了解协议就可以使用Netcat作为网络通讯媒介，实现各种客户端。

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