I have recently setup vsftpd to listen on port 990 for FTP over explicit SSL. I know from the RFCs that 990 is normally used for implicit. However, we have other FTPS servers accessed from the Internet via a single IP with NAT with static port forwarding. As far as I know this means we need to use different port number for each FTPS server daemon run from a different machine (physical/virtual etc).
Many FTP client programs support manual specification of which port to connect to and which mode to connect with i.e. implicit/versus explicit.
i.e. CuteFTP works fine, ftp-ssl on debian works fine.
However I have come across a couple of FTP clients such as FTP On the Go PRO for iPhone that are set to do implicit mode on ports 990/989 and explicit mode on the default FTP ports 21/20.
FTP On the Go PRO developers have so far refused to build in an option to allow explicit mode FTP on port 990/989 - and do not acknowledge the NAT and port forwarding issue.
This has lead me to ask the question: is there anything fundamentally wrong with an App supporting FTPES on ports 990/989?
My arguement is that one should be able to configure the client to connect to whichever port works.