Because my sever computer is running behind a NAT, I have to assign ProFTPd to a range of passive ports, and then configure my NAT to forward the passive ports to that computer. That's no problem, it works very well. Although, I would like to know how big SHOULD my range be? Can I use ports 1024 to 1025, for instance, or do I have to have a larger amount of ports forwarded? Does it matter at all, performance-wise, either? The ftp on that computer will only be used for certain people uploading things to their web accounts, so there really won't be a big use of the service, if that matters.

If you say it's hardly ever used then I guess two ports should be enough. The passive ports are only used if the person connecting to your FTP server has enabled passive mode because they themselves are behind a firewall. For performance it won't make a difference except the amount limitation. Having only 2 ports will allow only 2 files to be transferred in passive mode similtaneously. If the users connect in direct mode then these ports will not be used. So I would forward as many ports as you think passive users would connect at the same time. Having those ports forwarded to one computer means the other computers behind the NAT server can't listen on the same ports. So it's generally wise to not forward more ports then you really need. Mik