Nowadays, there are many open-source networking tools available in Linux Based Systems to perform network troubleshooting activities. Netcat is one such tool. Netcat (nc) command in Linux is used for various purposes like checking the status of remote ports, initiating chat services between server and client, listening on some ports for incoming connections etc.

Here, we will only look into the steps to install Netcat as the package does not come by default in RedHat Based systems, so we need to install it manually using the below steps.

To install netcat

We are going to install netcat on Ubuntu 18.04 machine using apt install or compiling it from source code.

Installation using apt is pretty simple, you just need to type the following command in the terminal:

sudo apt install netcat

To install netcat from source code

Compiling netcat from source code is not as easy as installing via apt install, but if you follow the steps below you can install it easily.

Download the source code from netcat website with the following command

wget http://sourceforge.net/projects/netcat/files/netcat/0.7.1/netcat-0.7.1.tar.gz

Extract the newly downloaded archive. To do so you can run:

tar -xzvf netcat-0.7.1.tar.gz

cd to the directory containing the package's source code and type ./configure to configure the package for your system.

cd netcat-0.7.1
./configure

If you are getting error message like this - "no acceptable C compiler found in $PATH" when running ./configure command, make sure you have installed gcc compiler. To install it type the following command:

apt-get install build-essential

Running configure takes awhile.

Once configure has been successfully finished run:

sudo make

and

sudo make install

You can remove the program binaries and object files from the source code directory by typing make clean. To also remove the files that configure created, run make distclean command.

How to use netcat

Before starting to explore some netcat commands it's important to know that if you are binding to well-known ports (0-1023) with nc, you need root privilege. Otherwise, you can run nc as a normal user.

1. Test if a particular TCP port of a remote host is open

nc -vn 184.145.20.128 2424

Output if the 2424 port on remote server is closed

nc: connect to 184.145.20.128 port 2424 (tcp) failed: Connection refused

Output if the port on remote server is opened (e.g. 22 port)

Connection to 184.145.20.128 22 port [tcp/*] succeeded!
SSH-2.0-OpenSSH_7.6p1 Ubuntu-4

2. Perform TCP port scanning against a remote host

The command below will check the ports from 20 to 25 on the remote host and print the result.

nc -vnz -w 1 184.145.20.128 20-25

Output will look like this

nc: connect to 184.145.20.128 port 20 (tcp) failed: Connection refused
nc: connect to 184.145.20.128 port 21 (tcp) failed: Connection refused
Connection to 184.145.20.128 22 port [tcp/*] succeeded!
nc: connect to 184.145.20.128 port 23 (tcp) failed: Connection refused
nc: connect to 184.145.20.128 port 24 (tcp) failed: Connection refused
nc: connect to 184.145.20.128 port 25 (tcp) failed: Connection refused

3. Perform UDP port scanning against a remote host

nc -vnzu 184.145.20.128 1-65535

Output will show only the ports which allow udp connections.

Connection to 184.145.20.128 2424 port [udp/*] succeeded!
Connection to 184.145.20.128 12354 port [udp/*] succeeded!

4. Send a test UDP packet to a remote host

echo -n "udp test" | nc -u -w1 184.145.20.128 2424

The command above will send a test UDP packet with 1 second timeout to a remote host at port 2424

5. Copy a file (e.g., test.txt) from one host to another

On the receiver host (184.145.20.128 in my case) run:

nc -lp 2424 > test.txt

On the sender host (184.145.20.126) run the following command:

nc 184.145.20.128 2424 < test.txt

This will copy test.txt file from sender host to receiver host via 2424 port. make sure to allow incoming connections on 2424 port on the receiver host.

6. Transfer a whole directory (including its content) from one host to another

On the receiver host run:

nc -l 2424 | tar xvf -

On the sender host run the following command:

tar cvf - /path/to/dir | nc 184.145.20.128 2424

7. Create a compressed backup of hard drive (e.g., /dev/sdc) on a remote host

On the remote host run:

nc -lp 2424 | sudo dd of=/path/to/image.img.gz

On the local host run the following command:

dd if=/dev/sdc | gzip -c | nc 184.145.20.128 2424

8. Restore a hard drive (e.g. /dev/sdc) from a compressed disk image stored in a remote host

On the local host run:

nc -lp 2424 | gunzip -c | sudo dd of=/dev/sdc

On the remote host run the following command:

cat /path/to/image.img.gz | nc 184.145.20.126 2424

9. Run insecure online chat between two hosts

On one host (e.g. 184.145.20.126) run the command below:

nc -lp 2424

On another host (e.g. 184.145.20.128) run the following command:

nc 184.145.20.126 2424

After running these commands, anything typed in both terminals will be seen on both host machines.

10. Run a web server with a static web page

Run the command below on local host (e.g. 184.145.20.126) to start a web server that serves test.html on port 80. Note that you must run with sudo privileges as 80 is in range of well known ports (1-1023)

while true; do sudo nc -lp 80 < test.html; done

Now open http://184.145.20.126/test.html from another host to access it.

11. Listen on a TCP port using IPv6 address

You can use the following command to allow nc use IPv6 address when listening on a TCP port.

nc -6 -l 2424

Check if it works with the command below

sudo netstat -nap | grep 2424

Output will look like this

tcp6 0 0 :::2424 :::* LISTEN 15665/nc

12. Stream a video file from a server for client to watch the streamed video using video player (e.g., mplayer)

On a video server (184.145.20.126):

cat sample_video.avi | nc -l 2424

On a client host (184.145.20.128):

nc 184.145.20.126 2424 | mplayer -vo x11 -cache 3000 -

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