Inspired by this answer, I set up a FTP reverse proxy using proftp and mod_proxy. According to the comments, to be able to use the same port regardless of the backend ftp server, the client needs to use the HOST command.
I couldn't manage to make this work with either Filezilla or or bash' command
ftp. But I don't know whether the proxy configuration is too blame, or whether those two clients don't support the HOST command. I wasn't able to find the answer with my usual search engine.
So, is there a list of FTP clients that use the HOST command somewhere ?