In the event of data transfer between two FTP servers (referred to as first server and proxy server below) the FTP client sends either a PASV or EPSV command to the proxy server, depending on whether the proxy server can be reached on the control connection via an IPv4 or IPv6 address. The client sends the address information contained in the response to the first server using a PORT or EPRT command.
The following problem can occur here:
The proxy server has an IPv6 address.
The first server, however, is a pure IPv4 server which cannot be reached via IPv6 addresses and is consequently still operating with IPv4 FTP.
In order to solve this problem, the proxy server must have at least one IPv4 address in addition to the IPv6 addresses. You must specify this IPv4 address as an IPv4-MAPPED address when you set up the control connection. The PASV command then returns an IPv4 address under which the proxy server can be reached and which the first server can also process.