Nat (network address translation) or Router are devices that allows You to have several computers to share a single internet IP address to go online.

Many people have this kind of device at home but do not really understand how to configure them properly so they can run a FTP server behind a NAT/router.

To run a FTP server (RaidenFTPD ftp server) properly behind a NAT/Router, You have to setup the so called PORT MAPPING or NAT rules, first of all we'll tell You the basic principles:

A. Open the following ports on Your NAT/router for bi-directions (in and out) TCP connections.
1. server port
2. implicit ssl port
3. explicit ssl port
4. data ports (a range of ports)

What will these ports be? You can refer to Your RaidenFTPD server options window like the following:

B. For some of the routers that monitor the FTP packets and setup automatic port mappings for data connections:
If Your router is aware of FTP commands, it maybe able to do this, but in this case You *MAY* have to TURN OFF Auto IP in Your RaidenFTPD, this is because these routers need Your server to return 192.168.x.x private allocated IP addresses in the PASV command, but if Auto IP is enabled, Your RaidenFTPD will return real WAN (Wide area network) IP address and the passive mode won't work.

C. Case study: trying to configure a linksys broadband router (wrt300n)
Connect to Your NAT's web interface, and login. Click Application & Gaming -> Single Port Forwarding:

Add 1 rules for RaidenFTPD ftp server (for the normal server port 21)
external port 21, internal port 21, protocol tcp, to ip address = Your LAN IP-address,enable

And if you are setting up a SSL RaidenFTPD ftp server, add the rules like above but for your implicit ssl port and explicit ssl port.

The setup is not done yet, you need to setup data ports forwarding too.

Click Application & Gaming -> Port Range Forwarding:
Add a rule for your RaidenFTPD data ports, from the start to end of the range, using tcp protocol and forward to your LAN IP-Address.