Useful Uses Of netcat
Version 1.0
Author: Falko Timme
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.
8 Links
- netcat: http://netcat.sourceforge.net/
