nc command examples in linux?

Netcat or nc is a networking utility for debugging and investigating the network.

This utility can be used for creating TCP/UDP connections and investigating them.

The biggest use of this utility is in the scripts where we need to deal with TCP/UDP sockets.

How to Install and Use Netcat in Linux

To install the netcat package on your system, use the default package manager for your

Linux distribution.

[On CentOS/RHEL]

$ yum install nc                  

 [On Fedora 22+ and RHEL 8]

$ dnf install nc                 

 [On Debian/Ubuntu]

$ sudo apt-get install Netcat    

Port Scanning

Netcat can be used for port scanning: to know which ports are open and running services on

a target machine. It can scan a single or multiple or a range of open ports.

 It may be useful to know which ports are open and running services on a target machine.

  The -z flag can be used to tell nc to report open ports, rather than initiate a connection.

Usually it's useful to turn on verbose output to stderr by use this option in conjunction with -

v    option.

Where 

    -v      Have nc give more verbose output.

    -w    timeout      Connections which cannot be established or are idle timeout after timeout

seconds.   The -w flag has no effect on the -l option, i.e. nc will listen forever for a connection, with or without the -w flag.  The default is no timeout.

    -z      Specifies that nc should just scan for listening daemons, without sending any data

to them.  It is an error to use this option in con‐junction with the -l option.

  -n      Do not do any DNS or service lookups on any specified addresses, hostnames or

ports.

  #scan a single port

$ nc -v -w 2 z 192.168.56.156 22   

Example:

    

 nc -v -w 2 -z 192.168.56.156 22 

 Connection to 192.168.56.156 22 port [tcp/ssh] succeeded!

#scan multiple ports

$ nc -v -w 2 z 192.168.56.156 22 80  

 #scan range of ports

$ nc -v -w 2 z 192.168.56.156 20-22 

Example:

nc -v -w 2 -z 192.168.56.156 20-22

nc: connect to 192.168.56.156 port 20 (tcp) failed: Connection refused

nc: connect to 192.168.56.156 port 21 (tcp) failed: Connection refused

Connection to 192.168.56.156 22 port [tcp/ssh] succeeded!

Find a Service Running on Port

You can also use Netcat to obtain port banners. In this case, it will tell you what service is

running behind a certain port. For example to know what type of service is running behind

port 22 on a specific server, run the following command

nc -v -n 192.168.56.156 22

Connection to 192.168.56.156 22 port [tcp/*] succeeded!

SSH-2.0-OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.7