my original question: FTP client/server failing on switching to PASV mode
Based on that, I would like to know if/how I can open a FTP connection from a computer that is inside a ISP's network that uses Dual-Stack Lite to a FTP server that is only reachable via IPv4.
Obviously, passive FTP doesn't work, but would active FTP work (if I added port-forwarding on the owner's router)?
Dual-Stack Lite is a NAT solution. Active mode requires, that the client is directly reachable by the server (e.g. no address translation in between), so it will not work if the client is behind any kind of NAT. Passive mode requires that the server is directly reachable by the client and thus does not work if the server is behind NAT.
Worse, if both sides are behind NAT FTP will not work at all, unless these NAT solutions include helpers which rewrite FTP traffic. A workaround would be to use a protocol which does not use any dynamic ports (like ssh/scp/sftp) or have a VPN between the local and the FTP servers network so that they can reach each other without NAT.
Simple port forwarding might work if you can restrict the range of allocated ports used by the client (active mode) or server (passive mode) and have port forwarding established for the complete range. Additionally client (active mode) or server (passive mode) must not send there own IP address inside the PORT command or response to PASV, but the public IP address visible by the peer. If and how all these can be configured at client/server side depends on the specific client/server.
There is a solution to connect to a home network even with dual stack lite using a raspberry pi and the german service feste-ip.net. Please read the (german) blog entry on my webpage if you want to learn how to do it.
Assuming you mean the server is on the public IPv4 internet and the client is behind DS-Lite.
Passive mode will work fine. The client can connect to any port on the server.
Active mode will only work if the NAT in the AFTR is aware of FTP and makes special provisions for it.
This is no conceptually no different from traditional NAT44. However practically DS-Lite is very new while NAT44 has been arround for many years. So it may be harder to find a AFTR implementation that understands and translates active FTP than it is to find a NAT44 implementation that understands and translates active FTP.