Introduction

Netcat (nc) command is a command-line utility for reading and writing data between two computer networks. It works like a port scanning tool, a security tool, or a network monitoring tool.

Network and system administrators use this tool to quickly identify how their network is performing and what type of activity is occurring. In addition, you can transfer files directly through Netcat or use it as a backdoor into other networked systems.

In this article, we will discuss how to use the Netcat utility. Netcat command can assist you in monitoring, testing, and sending data across network connections.

The command differs depending on the operating system (netcat,nc,ncat, and others).

Below is a detailed explanation of the Netcat command:

nc [<options>] <host> <port>

On Ubuntu, the command is to be usednc whereas, on CentOS, Debian, and RHEL, the command isncat

<host> - can contain a numeric IP address or a hostname.
<port> - can contain a numeric port or service name.
<options> - The table below contains commonly usednccommand options:

Option	Type	Description
`-4`	Protocol	Use IPv4 only.
`-6`	Protocol	Use IPv6 only.
`-u` `--udp`	Protocol	Use for UDP connection.
`-s <host>`	Connect mode	Binds the Netcat host to <host>
`-u`	Protocol	Use for UDP connection.
`-l`	Listen mode	Listens for connections instead of using connect mode.
`-k`	Listen mode	Keeps the connection open for multiple simultaneous connections.
`-v`	Output	Sets verbosity level. Use multiple times to increase verbosity.
`-z`	Output	Report connection status without establishing a connection.

1. How Client-Server Connection works using Netcat

A simple client/server connection is between two machines. On one machine, start nc listening on a specific port for a connection.

For Example:

a. On machine 1, run thenccommand with IP address or hostname in listen mode and provide a port:

nc -lv 66.63.166.76 5000

With the start "-l" parameter, listen mode is activated, which makes machine 1 the server. The "-v" parameter shows the device listening for connections.

b. On machine 2, run thenccommand with the IP address or hostname of machine 1 and the port:

nc -v 66.63.166.76 5000

The output shows the connection is successful.

Also, machine 1 confirms the link and prints the IP address of machine 2.

The client/server connection was established successfully. Anything typed at the second console will be concatenated to the first, and vice-versa. The connection can be terminated using an EOF('^C').

2. Data Transfer using Netcat

Netcat allows you to transfer files between two Linux machines or servers. Below is an example that explains data transfer using the Netcat command:

a. Create a simple file on machine 1 using thetouchcommand:

touch test.txt

The above command creates an empty text file.

b. Create a listening connection on machine 1 with IP address:

nc -lv 66.63.166.76 5000 < test.txt

The output shows machine 1 listening for connections due to the-vparameter.

c. On machine 2, connect to machine 1 and transfer the file:

nc -zv 66.63.166.76 5000 > test.txt

The output displays the 'test.txt' file name is available, indicating the transfer was successful.

3. Ping Specific Port on the Website

Netcat allows you to test a specific port to a website.

For example, If you need to ping google.com

nc -zv google.com 443

The output shows the successful connection message. The-zparameter ensures the connection does not persist.

4. Port Scanning

The commandnc can be used to scan for open ports.

a. Run the following command to check whether port 8080 is open:

For example:

nc -zv 192.168.6.106 8080

If the port is open, the output shows a successful connection message.

b. Alternatively, scan multiple ports by adding a port range.

For example:

nc -zv 192.168.6.106 8076-8082

Above are the examples where the Netcat (nc) command is used for communication in Linux OS. To understand the various options available with Netcat commands Refer to the following link.