FTP uses one TCP connection for control (such as login, changing directory, selecting the file to transfer), and another TCP connection for transferring data (including directory listings).

So there are two issues here.

1. If you cannot even establish the control connection, then there's a problem with forwarding of the ftp control port (21). This is where the CGN is one possible problem, of many.

2. If you are able to login to the FTP server, but not get a file listing, then the control port is OK but the data ports are not. It then gets more complicated because FTP has two modes, 'active' and 'passive'. 'Passive' is what you want here. The FTP server will allocate a second port from a range, and then wait for another incoming connection from the client.

You will need to configure your FTP server to use a fixed range of data ports (let's say 10000-10999) and also configure your router to port-forward all those ports. Or some routers have a concept of a "DMZ host" which forwards *all* ports to that host.

Given the complexities of FTP, it would be better if you could run an SFTP server instead of an FTP server. With SFTP you only need to forward one port (22). Plus, SFTP is encrypted so people can't sniff your password.

When choosing software, beware: SFTP is not the same as FTPS. FTPS is just normal FTP over SSL, so has the same port problems as normal FTP.

I never touch Windows, so I'm afraid I can't recommend SFTP software for Windows.

In reply to a post by jimasap:

Sorry, yes, I am setting up a local FTP server on Windows 10 . I have configured the firewall and router to forward the port, but unable to connect to the server through the public IP.

There are 5 IPv4 routes listed on the router (not sure what this means), so I got my public IP from a web tool.

Googling solutions raised the issue that if you're behind a NAT444/CGNAT then it is not possible to configure a local server because the port forwarding would need to be done on the ISPs network?

The public IP I have never changes, even when disconnecting, so I'm hoping it is a static IP?

It's been many years since I last set up a local server.

FTP is an old, antiquated protocol that struggles with NAT, and you will be using NAT on your router even without CGNAT.

To be honest, rather than trying to diagnose this, I would consider using SCP or SFTP (which is FTP over an SSH tunnel).

Both are much much much simpler and more reliable to make work.

https://www.solarwinds.com/free-tools/free-sftp-server

Edited by nemeth782 (Mon 19-Mar-18 12:09:49)

In reply to a post by philjohn:

Sadly not going to be unusual for long - with Virgin announcing they'll be going to IPV6 using DS-Lite, so native IPV6 and CGNAT for IPV4

That's the rumour. If they do I will terminate. Fast speeds are not as great when you don't have a proper connection.