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I found a prior Q&A (that is now closed preventing me from replying) that says this:

So I want to allow my machine behind NAT to be accessible via FTP.....

The answer posted by a user was this (my emphasis):

Yes, portforwarding should work, provided your router firewall is FTP aware, as there is a second connection established on a random port with ftp, which needs to be opened and forwarded automatically by the router.

I have installed QuickFTP on my Mac Studio. It works well from my local network but fails from outside the network. I have forwarded the port on my router. However, I am not sure what is meant by "there is a second connection established on a random port with ftp which needs to be opened and forwarded..."

I am currently using port 2121 for both the external and internal ports and as the listening port for QuickFTP. Should I instead port forward a range of port numbers? Or, is there a separate router firewall setting that I should change?

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The usual practice is to open also port 20 on firewall and NAT it to FTP server. And when you connect from client set connection to be passive.

As you change the standard port I am not so sure how this will work.

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    Thank you. That worked! I changed QuickFTP to listen to 21 and forwarded both 20 and 21 and changed the client to passive.
    – JKapp
    Aug 26, 2022 at 17:34
  • For anyone who stumbles onto this thread—unfortunately, I was not able to achieve my ultimate goal (but for other reasons). My goal was to wirelessly send photo data from my Sony camera (A73) to my phone as a hotspot to my home computer. I can remotely connect to my ftp server from software on my phone. However, apparently TMobile prevents/closes ports when the phone operates as a hotspot (unless you pay for a separate hotspot data plan). When traveling, apparently I’ll have to connect my camera to other (not T-Mobile hotspot) networks.
    – JKapp
    Aug 27, 2022 at 18:29
  • @JKapp, what you can try is to create a VPN server on you home computer and connect to this VPN from your phone. This may help you to avoid mobile operator restrictions about ports and so on. Also will help you to secure your connection (FTP use plain text communication)
    – Romeo Ninov
    Aug 28, 2022 at 8:18
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    Thanks! It was worth a shot. I setup a vpn tunnel from my phone to the mac with the ftp server using WireGuard vpn. Unfortunately, same results. File transfer success when the camera connects to the server via WiFi, no connection when connection through phone over cellular; phone app ftp connects no problem over cellular w/ vpn enabled…. I read on a T-Mobile board that the glitch is due to conversion btw ip4 and ip6.
    – JKapp
    Aug 29, 2022 at 20:01

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