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Useful Uses Of netcat


This short article shows some useful netcat commands. netcat is known as the TCP/IP swiss army knife. From the netcat man page: netcat is a simple unix utility which reads and writes data across network connections, using TCP or UDP protocol. It is designed to be a reliable “back-end” tool 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 exploration tool, since it can create almost any kind of connection you would need and has several interesting built-in capabilities.

I do not issue any guarantee that this will work for you!

 

1 Preliminary Note

I’m using two systems in this article:

  • server1.example.com: IP address 192.168.0.100
  • server2.example.com: IP address 192.168.0.101

netcat should already be installed on your system – you can check with

which nc

To learn more about netcat, take a look at its man page:

man nc

 

2 Copying A File From One System To The Other

Let’s say we want to copy the file ISPConfig-2.2.27.tar.gz from server1 to server2. To do this, run

server2:

nc -lp 1234 > ISPConfig-2.2.27.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 ISPConfig-2.2.27.tar.gz on port 1234.

On server1, run

server1:

nc -w 1 server2.example.com 1234 < ISPConfig-2.2.27.tar.gz

to start the file transfer.

 

3 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 192.168.0.12 instead of 192.168.0.101.

On server2, run

server2:

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

Afterwards, on server1, run

server1:

dd if=/dev/sda | nc 192.168.0.12 1234

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

 

4 Port Scanning

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

server1:

nc -v -w 1 server2.example.com -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

 

5 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.

 

6 Spoofing HTTP Headers

You can use netcat to request web pages:

nc ispconfig.org 80

You can then type in headers as follows:

GET / HTTP/1.1
Host: ispconfig.org
Referrer: mypage.com
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 exampple.com 80
GET / HTTP/1.1
Host: example.com
Referrer: mypage.com
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

[…]

 

7 Chatting

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

Type

server2:

nc -lp 1234

on server2. server2 will then wait until server1 connects on port 1234.

On server1, run

server1:

nc server2.example.com 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.

 

  • netcat: http://netcat.sourceforge.net/

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