Configuring Hitron GCN3 modem for DDNS and port forwarding to ftp server

[chank]

This weekend my modem got itself in a state and needed a factory reset. (I should add at this point I had a great phone call with a tech support guy in NB that was really helpful and knowledgeable). I had been using it in bridged mode with a TP Link router, and was configured to use No IP DDNS and forward a port to an internal port of 21 to a Raspberry Pi. It has been running happily for a long time via the TP Link router with no probs. When the modem was reset, I thought I would see if I could remove the TP Link router from the setup as it is a bit old and I don't tend to use the features, that caused me to buy it in the first place, very often. Running the modem as the router has been fine and WiFi access is certainly quicker. 

However, I cannot get it configured correctly for my FTP server. I have the DDNS setup OK and I am using an external port of 21021 to forward to internal of 21. When I use a virtual machine hosted in the UK to see if I can see the contents of the server, it makes a connection, but I think the FTP server is running in passive mode as the FTP client complains with "500 Illegal PORT command", and doesn't display anything.  

What I don't understand is that I have changed nothing in the FTP server and that used to work via the TP Link router. Is there anything else I need to do to get this to work? I don't have a lot of control over the FTP server as it is a service provided by an installation of OSMC.

[RogersTim]

Hey @chank,

It can sometimes be a house of cards by removing one piece of hardware.

I’m wondering if your installation of OSMC has any diagnostics add-ons or dev tools you can tap into and check if port 21021 is showing as open from the Pi.

We might need to nudge @Gdkitty and other members of the Community to give you a hand. 

RogersTim

[chank]

Thanks for the reply. OSMC might have some tools as after all it is a Linux distro, but I am not totally familiar with Linux. I can SSH into the Pi and run commands, but not sure what to run. To be clear though, the port that is open on the Pi is the standard FTP port 21. I am opening 21021 externally and then forwarding from 21021 to port 21 on the Pi.

[chank]

I have just set the external port to be the standard FTP port 21, and that works. I can write/delete files on the server as expected, but it doesn't work when forwarding from port 21021, or another port I tried, 211. I think the modem must do something special in the case of the standard FTP port. I was using a non-FTP port to try to obfuscate it a bit, but looks like I can't do that.