Another Way To Check UDP Port to a Linux Server

In a previous post, I have shared a way to check for udp port allowance to a linux server using netcat and ngrep.

I have found out an even easier way to accomplish this, just by using netcat, without the need to install additional software like ngrep.

To do this, first we need to setup a netcat to listen to the udp port, in the target machine. For example, we wanted to test udp port 10000 allowance, just run below command on the target machine

$ nc -klu 10000

The command will hang there, waiting for a connection to be sent to it.

In the client machine, just use netcat to send some text over to the target machine, like below (assuming the ip address of the target machine is 10.10.10.10)

$ echo "testing udp" | nc -u 10.10.10.10.10000

If the udp port is not blocked, we will see the "testing udp" text printed on the terminal in the target machine, where we listen for 10000 udp, like example below

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