Assuming you have external router with firewall.. not just firewall on Amiga...
Problem is, that ftp uses different connections for control and data transfers and getting data transfers (filelistings, files) work on ftp server, which is behind a router/firewall, is quite hard. At least if ftp server doesn't particularly support it. Easiest way to go is to have ftp server directly on inet instead of being behind the router.
Data connection can be established by two ways, active and passive. By enabling only the port 21 you can have server working with active mode. Then control connection is made from client to server's port 21 and data connection is made from server's port 20 to client's port >=1024. Then the problem is, that if client is behind firewall, connection can't be made. And nowadays clients usually are behind firewall.
In passive mode, the data connection is made by client to server's port >=1024. So if the server is behind firewall, all ports bigger than 1023 should be enabled to server. This would be enough, if the server would be on internet with own ip and own firewall, but when having it behind router on LAN an anohter problem arises. In passive mode server tells to client to which address and port it should establish the data connection. Normally server tells it's own ip address and on LAN it naturally tells the 192... or 10... address, which can't be accessed outside of LAN.
Advanced ftp servers have option to use passive mode behind router/firewall. You can define the port range for passive mode, which limits the amount of ports needed to open. They also tell the client correct ip address for making the data connection.
Only server I know doing this on Amiga is proftpd. If someone knows other servers, please tell me too
I have been using wu-ftpd myself, but Amiga's version is that old that it doesn't have this feature yet. But it came with sources and I modified it to tell the client right ip address, but still have to forward whole port range from router. It works, but would be nice to limit port range
For more info about active and passive modes in ftp protocol:
http://slacksite.com/other/ftp.html
