Here's a write-up on active-vs-passive FTP, with some description of the associated firewall complications:
http://slacksite.com/other/ftp.htmlAs I remember, to have an FTP server behind your SME server, you'll need to:
- configure the internal ftp server to use only a limited range of ports for the data communications
- forward tcp 21 and udp 20 from the SME to the internal FTP server
- forward the configured range of FTP data ports from the SME to the internal FTP server
...then you get to deal with the security problems inherent in FTP.
For simplicity, I would probably use one of the many home NAS devices that includes a built-in FTP server.
If you can find a NAS device that supports ftps (ftp over ssl) or sftp (ftp over ssh), I think your firewall configuration is greatly simplified:
https://www.google.com/search?q=%28netgear+OR+buffalo+OR+Seagate+OR+Maxtor%29+AND+%28+sftp+OR+ftps%29[you're not asking but]
I would tend to solve this problem using Dropbox, Google Drive,
Owncloud,
Webshare or
WebDAV[/you're not asking but]