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选题: 10 useful ncat (nc) Command Examples for Linux Systems

sources/tech/20171206 10 useful ncat (nc) Command Examples for Linux Systems.md

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+translating by lujun9972
+10 useful ncat (nc) Command Examples for Linux Systems
+======
+ [![nc-ncat-command-examples-Linux-Systems](https://www.linuxtechi.com/wp-content/uploads/2017/12/nc-ncat-command-examples-Linux-Systems.jpg)][1] 
+
+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.
+
+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.
+
+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.
+
+```
+[root@linuxtechi ~]# yum install nmap-ncat -y
+```
+
+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.
+
+We can also cause it to capture data being sent by client to understand what they are upto.
+
+In this tutorial, we are going to learn about how to use ‘nc’ command with 10 examples,
+
+#### Example: 1) Listen to inbound connections
+
+Ncat can work in listen mode & we can listen for inbound connections on port number with option ‘l’. Complete command is,
+
+$ ncat -l port_number
+
+For example,
+
+```
+$ ncat -l 8080
+```
+
+Server will now start listening to port 8080 for inbound connections.
+
+#### Example: 2) Connect to a remote system
+
+To connect to a remote system with nc, we can use the following command,
+
+$ ncat IP_address port_number
+
+Let’s take an example,
+
+```
+$ ncat 192.168.1.100 80
+```
+
+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
+
+GET / HTTP/1.1
+
+or get the page name,
+
+GET / HTTP/1.1
+
+or we can get banner for OS fingerprinting with the following,
+
+HEAD / HTTP/1.1
+
+This will tell what software is being used to run the web Server.
+
+#### Example: 3) Connecting to UDP ports
+
+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’,
+
+```
+$ ncat -l -u 1234
+```
+
+Now our system will start listening a udp port ‘1234’, we can verify this using below netstat command,
+
+```
+$ netstat -tunlp | grep 1234
+udp           0          0 0.0.0.0:1234                 0.0.0.0:*               17341/nc
+udp6          0          0 :::1234                      :::*                    17341/nc
+```
+
+Let’s assume we want to send or test UDP port connectivity to a specific remote host, then use the following command,
+
+$ ncat -v -u {host-ip} {udp-port}
+
+example:
+
+```
+[root@localhost ~]# ncat -v -u 192.168.105.150 53
+Ncat: Version 6.40 ( http://nmap.org/ncat )
+Ncat: Connected to 192.168.105.150:53.
+```
+
+#### Example: 4) NC as chat tool
+
+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
+
+```
+$ ncat -l 8080
+```
+
+On remote client machine, run
+
+```
+$ ncat 192.168.1.100 8080
+```
+
+Than start sending messages & they will be displayed on server terminal.
+
+#### Example: 5) NC as a proxy
+
+NC can also be used as a proxy with a simple command. Let’s take an example,
+
+```
+$ ncat -l 8080 | ncat 192.168.1.200 80
+```
+
+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,
+
+```
+$ mkfifo 2way
+$ ncat -l 8080 0<2way | ncat 192.168.1.200 80 1>2way
+```
+
+Now you will be able to send & receive data over nc proxy.
+
+#### Example: 6) Copying Files using nc/ncat
+
+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.
+
+Start with machine on which data is to be received & start nc is listener mode,
+
+```
+$ ncat -l  8080 > file.txt
+```
+
+Now on the machine from where data is to be copied, run the following command,
+
+```
+$ ncat 192.168.1.100 8080 --send-only < data.txt
+```
+
+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.
+
+We can also copy entire disk partitions using this method, but it should be done with caution.
+
+#### Example: 7) Create a backdoor via nc/nact
+
+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,
+
+```
+$ ncat -l 10000 -e /bin/bash
+```
+
+‘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,
+
+```
+$ ncat 192.168.1.100 1000
+```
+
+#### Example: 8) Port forwarding via nc/ncat
+
+We can also use NC for port forwarding with the help of option ‘c’ , syntax for accomplishing port forwarding is,
+
+```
+$ ncat -u -l  80 -c  'ncat -u -l 8080'
+```
+
+Now all the connections for port 80 will be forwarded to port 8080.
+
+#### Example: 9) Set Connection timeouts
+
+Listener mode in ncat will continue to run & would have to be terminated manually. But we can configure timeouts with option ‘w’,
+
+```
+$ ncat -w 10 192.168.1.100 8080
+```
+
+This will cause connection to be terminated in 10 seconds, but it can only be used on client side & not on server side.
+
+#### Example: 10) Force server to stay up using -k option in ncat
+
+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,
+
+```
+$ ncat -l -k 8080
+```
+
+Now server will stay up, even if a connection from client is broken.
+
+With this we end our tutorial, please feel free to ask any question regarding this article using the comment box below.
+
+--------------------------------------------------------------------------------
+
+via: https://www.linuxtechi.com/nc-ncat-command-examples-linux-systems/
+
+作者：[Pradeep Kumar][a]
+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