Test UDP Port to Linux Server

To test if a udp port is allowed to a linux server, and not blocked by any firewall, we need ngrep on the server side, and nc (netcat) on the client side.

First, install ngrep on the server

$ sudo apt install ngrep -y

And start to watch the udp 10000 traffic (for example)

$ ngrep -q "accessible" udp port 10000

In the client side, we need to install netcat-openbsd 

$ sudo apt install netcat-openbsd

We are now going to test port 10000 udp in the server, from the client (the "yes, accessible" message could be anything, as long as it contains accessible keyword)

$ echo "yes, accesible" | nc -u server-ip 10000

If the port is opened (not blocked by any firewall), you will get message like below in the server terminal

U client-ip:39062 -> server-ip:443 #1

  yes, accessible...    

If the port is blocked, you won't get any output on the server terminal