[SOLVED] Blocked ports even when UFW disable (inactive) on Ubuntu Desktop 20.04 LTS

This problem stumped me for at least 4 hours and was very frustrating.  Basically I installed a brand new Ubuntu 20.04 Desktop LTS with all its drivers and softwares (full install).  The installation process went flawlessly.

One of the first thing I did was to install XRDP.  I prefer RDP over VNC for its speed and simplicity.  However I immediately hit a wall because the I can not connect to my Ubuntu Desktop via RDP even though I already confirmed the port 3389 is open and listening.

Here is the command you need to use to confirm Linux port is listening:

netstat -tuplen
systemctl status xrdp

Here is the command you need to confirm ports are open and answering:

telnet localhost 3389

All this time I made sure already UFW is disabled by issuing the following command:

ufw disable
ufw status verbose

I had a feeling that firewall or something is blocking, so I tested using SSH Tunnel method.  I confirmed I was able to connect to XRDP via SSH Tunnel.  So that confirmed RDP is working.  It is just the issue with network port blocking somehow.  What made it very strange is that Port 22 (SSH) is not blocked.

I even installed NGINX (webserver) on port 80 and it also was blocked! 

If you are experiencing all of the above issue... you are in luck, here is the solution:

SOLUTION! :

iptables -F

That will flush your IP Chains and that is the single command that solved all my problem above.

I have also rebooted the Ubuntu many times and iptable -F seems to be permanent (persistent).

I hope this helped somehow. 

Thanks for reading!