Once SSH Forwarding is established on a Unix box, all users of that machine can - and in fact, have to - use the forwarding. If someone is trying to FTP to localhost (perhaps to access files as another user) while you have forwarding set up on that port, they'd get a big surprise, finding themselves connected to another server. This would be a pretty good argument for using a high number non-standard port if you're working on a multi-user system. However, it would occasionally be advantageous to allow other users on other machines to use a forward that you have set up: by default, SSH doesn't allow this, but you can switch it on if you like. The option is "-g". I would suggest using this with extreme caution.