TCP / UDP connectivity using Netcat

What is Netcat

Netcat (nc) is a computer networking service for reading from and writing to network connections using TCP or UDP.
it is a feature-rich network debugging and investigation tool

• Its list of features includes
• port scanning
• port binding
• transferring files
• port listening
• it can be used as a backdoor
• Ability to use any local source port

Netcat Command Flags

Command Flags	Description
-u	UDP (User Datagram Protocol)
-z	Don't send any Data, just emit a packet without payload(scanning)
-v	be verbose : print out messages on standard information
-n	do not perform DNS lookup on name of system on the other side
-l	Listen mode
-L	Listen harder

How To Use Netcat for Port Scanning

Here, we can scan all ports up to 1000 by issuing this command: 

netcat -z -v targetdomain.com 1-1000

Output

Scan will go much faster if you know the IP address that you need.

netcat -z -n -v 127.0.0.1 1-1000

output 

How To Communicate through Netcat

Netcat is not restricted to sending TCP and UDP packets. It also can listen on a port for connections and packets. This gives us the opportunity to connect two instances of netcat in a client-server relationship. 
one system, you can tell netcat to listen to a specific port for connections. 

netcat -l 1234

This will tell netcat to listen for TCP connections on port 1234 
second server, we can connect to the first machine on the port number we choose

netcat targetdomain.com 1234

Type a message and press ENTER. It will appear on both the local and remote screen. This works in the opposite direction as well. When you are finished passing messages, you can press CTRL-D to close the TCP connection