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translating by lujun9972
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10 useful ncat (nc) Command Examples for Linux Systems
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======
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[][1]
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ncat or nc is networking utility with functionality similar to cat command but for network. It is a general purpose CLI tool for reading, writing, redirecting data across a network. It is designed to be a reliable back-end tool that can be used with scripts or other programs. It’s also a great tool for network debugging, as it can create any kind of connect one can need.
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ncat/nc can be a port scanning tool, or a security tool, or monitoring tool and is also a simple TCP proxy. Since it has so many features, it is known as a network swiss army knife. It’s one of those tools that every System Admin should know & master.
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In most of Debian distributions ‘nc’ is available and its package is automatically installed during installation. But in minimal CentOS 7 / RHEL 7 installation you will not find nc as a default package. You need to install using the following command.
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```
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[root@linuxtechi ~]# yum install nmap-ncat -y
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```
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System admins can use it audit their system security, they can use it find the ports that are opened & than secure them. Admins can also use it as a client for auditing web servers, telnet servers, mail servers etc, with ‘nc’ we can control every character sent & can also view the responses to sent queries.
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We can also cause it to capture data being sent by client to understand what they are upto.
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In this tutorial, we are going to learn about how to use ‘nc’ command with 10 examples,
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#### Example: 1) Listen to inbound connections
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Ncat can work in listen mode & we can listen for inbound connections on port number with option ‘l’. Complete command is,
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$ ncat -l port_number
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For example,
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```
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$ ncat -l 8080
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```
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Server will now start listening to port 8080 for inbound connections.
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#### Example: 2) Connect to a remote system
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To connect to a remote system with nc, we can use the following command,
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$ ncat IP_address port_number
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Let’s take an example,
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```
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$ ncat 192.168.1.100 80
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```
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Now a connection to server with IP address 192.168.1.100 will be made at port 80 & we can now send instructions to server. Like we can get the complete page content with
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GET / HTTP/1.1
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or get the page name,
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GET / HTTP/1.1
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or we can get banner for OS fingerprinting with the following,
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HEAD / HTTP/1.1
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This will tell what software is being used to run the web Server.
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#### Example: 3) Connecting to UDP ports
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By default , the nc utility makes connections only to TCP ports. But we can also make connections to UDP ports, for that we can use option ‘u’,
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```
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$ ncat -l -u 1234
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```
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Now our system will start listening a udp port ‘1234’, we can verify this using below netstat command,
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```
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$ netstat -tunlp | grep 1234
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udp 0 0 0.0.0.0:1234 0.0.0.0:* 17341/nc
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udp6 0 0 :::1234 :::* 17341/nc
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```
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Let’s assume we want to send or test UDP port connectivity to a specific remote host, then use the following command,
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$ ncat -v -u {host-ip} {udp-port}
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example:
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```
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[root@localhost ~]# ncat -v -u 192.168.105.150 53
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Ncat: Version 6.40 ( http://nmap.org/ncat )
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Ncat: Connected to 192.168.105.150:53.
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```
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#### Example: 4) NC as chat tool
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NC can also be used as chat tool, we can configure server to listen to a port & than can make connection to server from a remote machine on same port & start sending message. On server side, run
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```
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$ ncat -l 8080
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```
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On remote client machine, run
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```
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$ ncat 192.168.1.100 8080
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```
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Than start sending messages & they will be displayed on server terminal.
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#### Example: 5) NC as a proxy
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NC can also be used as a proxy with a simple command. Let’s take an example,
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```
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$ ncat -l 8080 | ncat 192.168.1.200 80
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```
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Now all the connections coming to our server on port 8080 will be automatically redirected to 192.168.1.200 server on port 80\. But since we are using a pipe, data can only be transferred & to be able to receive the data back, we need to create a two way pipe. Use the following commands to do so,
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```
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$ mkfifo 2way
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$ ncat -l 8080 0<2way | ncat 192.168.1.200 80 1>2way
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```
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Now you will be able to send & receive data over nc proxy.
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#### Example: 6) Copying Files using nc/ncat
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NC can also be used to copy the files from one system to another, though it is not recommended & mostly all systems have ssh/scp installed by default. But none the less if you have come across a system with no ssh/scp, you can also use nc as last ditch effort.
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Start with machine on which data is to be received & start nc is listener mode,
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```
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$ ncat -l 8080 > file.txt
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```
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Now on the machine from where data is to be copied, run the following command,
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```
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$ ncat 192.168.1.100 8080 --send-only < data.txt
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```
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Here, data.txt is the file that has to be sent. –send-only option will close the connection once the file has been copied. If not using this option, than we will have press ctrl+c to close the connection manually.
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We can also copy entire disk partitions using this method, but it should be done with caution.
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#### Example: 7) Create a backdoor via nc/nact
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NC command can also be used to create backdoor to your systems & this technique is actually used by hackers a lot. We should know how it works in order to secure our system. To create a backdoor, the command is,
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```
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$ ncat -l 10000 -e /bin/bash
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```
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‘e‘ flag attaches a bash to port 10000\. Now a client can connect to port 10000 on server & will have complete access to our system via bash,
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```
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$ ncat 192.168.1.100 1000
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```
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#### Example: 8) Port forwarding via nc/ncat
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We can also use NC for port forwarding with the help of option ‘c’ , syntax for accomplishing port forwarding is,
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```
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$ ncat -u -l 80 -c 'ncat -u -l 8080'
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```
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Now all the connections for port 80 will be forwarded to port 8080.
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#### Example: 9) Set Connection timeouts
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Listener mode in ncat will continue to run & would have to be terminated manually. But we can configure timeouts with option ‘w’,
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```
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$ ncat -w 10 192.168.1.100 8080
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```
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This will cause connection to be terminated in 10 seconds, but it can only be used on client side & not on server side.
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#### Example: 10) Force server to stay up using -k option in ncat
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When client disconnects from server, after sometime server also stops listening. But we can force server to stay connected & continuing port listening with option ‘k’. Run the following command,
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```
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$ ncat -l -k 8080
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```
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Now server will stay up, even if a connection from client is broken.
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With this we end our tutorial, please feel free to ask any question regarding this article using the comment box below.
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--------------------------------------------------------------------------------
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via: https://www.linuxtechi.com/nc-ncat-command-examples-linux-systems/
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作者：[Pradeep Kumar][a]
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译者：[lujun9972](https://github.com/lujun9972)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://www.linuxtechi.com/author/pradeep/
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[1]:https://www.linuxtechi.com/wp-content/uploads/2017/12/nc-ncat-command-examples-Linux-Systems.jpg
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@ -0,0 +1,199 @@
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10 个例子教你学会 ncat (nc) 命令
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======
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[][1]
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ncat 或者说 nc 是一款功能类似 cat 的网络工具。它是一款拥有多种功能的 CLI 工具，可以用来在网络上读，写以及重定向数据。 它被设计成可以被脚本或其他程序调用的可靠后端工具。 同时由于它能创建任意所需的连接，因此也是一个很好的网络调试工具。
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ncat/nc 既是一个端口扫描工具，也是一款安全工具， 还能是一款检测工具，甚至可以做一个简单的 TCP 代理。 由于有这么多的功能，它被誉为是网络界的瑞士军刀。 这是每个系统管理员都应该知道并且掌握它。
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在大多数 Debian 发行版中，`nc` 是默认可用的，它会在安装系统的过程中自动被安装。 但是在 CentOS 7 / RHEL 7 的最小化安装中，`nc` 并不会默认被安装。 你需要用下列命令手工安装。
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```
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[root@linuxtechi ~]# yum install nmap-ncat -y
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```
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系统管理员可以用它来审计系统安全，用它来找出开放的端口然后保护这些端口。 管理员还能用它作为客户端来审计 Web 服务器， 远程登陆服务器， 邮件服务器等， 通过 `nc` 我们可以控制发送的每个字符，也可以查看对方的回应。
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我们还可以 o 用它捕获客户端发送的数据一次来了解这些客户端是做什么的。
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在本文中，我们会通过 10 个例子来学习如何使用 `nc` 命令。
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### 例子: 1) 监听入站连接
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通过 `l` 选项，ncat 可以进入监听模式，使我们可以在指定端口监听入站连接。 完整的命令是这样的
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$ ncat -l port_number
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比如，
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```
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$ ncat -l 8080
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```
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服务器就会开始在 8080 端口监听入站连接。
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### 例子: 2) 连接远程系统
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使用下面命令可以用 nc 来连接远程系统，
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$ ncat IP_address port_number
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让我们来看个例子，
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```
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$ ncat 192。168。1。100 80
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```
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这会创建一个连接，连接到 IP 为 192。168。1。100 的服务器上的 80 端口，然后我们就可以向服务器发送指令了。 比如我们可以输入下面内容来获取完整的网页内容
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GET / HTTP/1.1
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或者获取页面名称，
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GET / HTTP/1.1
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或者我们可以通过以下方式获得操作系统指纹标识，
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HEAD / HTTP/1.1
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这会告诉我们使用的是什么软件来运行这个 web 服务器的。
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### 例子: 3) 连接 UDP 端口
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默认情况下，nc 创建连接时只会连接 TCP 端口。 不过我们可以使用 `u` 选项来连接到 UDP 端口，
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```
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$ ncat -l -u 1234
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```
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现在我们的系统会开始监听 udp 的'1234'端口，我们可以使用下面的 netstat 命令来验证这一点，
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```
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$ netstat -tunlp | grep 1234
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udp 0 0 0。0。0。0:1234 0。0。0。0:* 17341/nc
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udp6 0 0 :::1234 :::* 17341/nc
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```
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假设我们想发送或者说测试某个远程主机 UDP 端口的连通性，我们可以使用下面命令，
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$ ncat -v -u {host-ip} {udp-port}
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比如:
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```
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[root@localhost ~]# ncat -v -u 192。168。105。150 53
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Ncat: Version 6。40 ( http://nmap.org/ncat )
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Ncat: Connected to 192。168。105。150:53。
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```
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#### 例子: 4) 将 NC 作为聊天工具
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NC 也可以作为聊天工具来用，我们可以配置服务器监听某个端口，然后从远程主机上连接到服务器的这个端口，就可以开始发送消息了。 在服务器这端运行:
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```
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$ ncat -l 8080
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```
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在远程客户端主机上运行:
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```
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$ ncat 192。168。1。100 8080
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```
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之后开始发送消息，这些消息会在服务器终端上显示出来。
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#### 例子: 5) 将 NC 作为代理
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NC 也可以用来做代理。比如下面这个例子，
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```
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$ ncat -l 8080 | ncat 192。168。1。200 80
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```
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所有发往我们服务器 8080 端口的连接都会自动转发到 192。168。1。200 上的 80 端口。 不过由于我们使用了管道，数据只能被单向传输。 要同时能够接受返回的数据，我们需要创建一个双向管道。 使用下面命令可以做到这点:
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```
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$ mkfifo 2way
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$ ncat -l 8080 0<2way | ncat 192。168。1。200 80 1>2way
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```
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现在你可以通过 nc 代理来收发数据了。
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#### 例子: 6) 使用 nc/ncat 拷贝文件
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NC 还能用来在系统间拷贝文件，虽然这么做并不推荐，因为绝大多数系统默认都安装了 ssh/scp。 不过如果你恰好遇见个没有 ssh/scp 的系统的话， 你可以用 nc 来作最后的努力。
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在要接受数据的机器上启动 nc 并让它进入监听模式:
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```
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$ ncat -l 8080 > file.txt
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```
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现在去要被拷贝数据的机器上运行下面命令:
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```
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$ ncat 192。168。1。100 8080 --send-only < data.txt
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```
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这里，data.txt 是要发送的文件。 –send-only 选项会在文件拷贝完后立即关闭连接。 如果不加该选项， 我们需要手工按下 ctrl+c to 来关闭连接。
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我们也可以用这种方法拷贝整个磁盘分区，不过请一定要小心。
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#### 例子: 7) 通过 nc/ncat 创建后门
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NC 命令还可以用来在系统中创建后门，并且这种技术也确实被黑客大量使用。 为了保护我们的系统，我们需要知道它是怎么做的。 创建后门的命令为:
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```
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$ ncat -l 10000 -e /bin/bash
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```
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‘e‘ 标志将一个 bash 与端口 10000 相连。现在客户端只要连接到服务器上的 10000 端口就能通过 bash 获取我们系统的完整访问权限:
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```
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$ ncat 192。168。1。100 10000
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```
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#### 例子: 8) 通过 nc/ncat 进行端口转发
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我们通过选项 `c` 来用 NC 进行端口转发，实现端口转发的语法为:
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```
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$ ncat -u -l 80 -c 'ncat -u -l 8080'
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```
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这样，所有连接到 80 端口的连接都会转发到 8080 端口。
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#### 例子: 9) 设置连接超时
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ncat 的监听模式会一直运行，直到手工终止。 不过我们可以通过选项 `w` 设置超时时间:
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```
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$ ncat -w 10 192。168。1。100 8080
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```
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这回导致连接 10 秒后终止，不过这个选项只能用于客户端而不是服务端。
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#### 例子: 10) 使用 -k 选项强制 ncat 待命
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当客户端从服务端断开连接后，过一段时间服务端也会停止监听。 但通过选项 `k` 我们可以强制服务器保持连接并继续监听端口。 命令如下:
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```
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$ ncat -l -k 8080
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```
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现在即使来自客户端的连接断了也依然会处于待命状态。
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自此我们的教程就完了，如有疑问，请在下方留言。
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--------------------------------------------------------------------------------
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||||
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||||
via: https://www.linuxtechi.com/nc-ncat-command-examples-linux-systems/
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|
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作者：[Pradeep Kumar][a]
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译者：[lujun9972](https://github.com/lujun9972)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]:https://www.linuxtechi.com/author/pradeep/
|
||||
[1]:https://www.linuxtechi.com/wp-content/uploads/2017/12/nc-ncat-command-examples-Linux-Systems.jpg
|
||||
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