The FTP-Proxy White Paper: Inbound and Outbound FTP Traffic

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## 5. Inbound and Outbound FTP Traffic

The most common use of FTP-Proxy will probably be "inbound" FTP traffic. This means that clients from an "outside" world seek access to a "local" FTP server specified in the `DestinationAddress` variable.

Nevertheless, FTP-Proxy supports also an "outbound" mode, where clients have more control over the FTP connections. The FTP-Proxy implements two concepts of "outbound" traffic.

- MagicUser

  The first one is based on the increased level of trust that users enjoy. When setting the `AllowMagicUser` config option to "yes" and allowing the "@" (see also `UseMagicChar` option) and ":" characters as part of the USER command argument, users can determine the destination server's address and port with the USER command. All they have to do is to append the host name, separated by the "@" sign (or other set using `UseMagicChar` option), optionally followed by a colon and the port. These components will be removed from the name and evaluated as destination.

- Transparent-Proxy

  The second one is based on IP-NAT packet redirections, commonly called transparent proxying. This method is currently implemented for Linux ipchains (2.2 kernel) and iptables (2.4 kernel), as well as for BSD ipnat, tested on OpenBSD 2.9 and FreeBSD 4.4. A description how to setup the redirections is provided in the `TransProxy-Mini-Howto.txt` file.

  When setting the `AllowTransProxy` config option to "yes", the proxy will evaluate the original destination address and port the client wanted connect as destination. If `AllowMagicUser` is enabled as well, the users are still able to provide a different destination using the USER command argument.

If `AllowTransProxy` and `AllowMagicUser` are not used, the FTP-Proxy runs in the "inbound" mode and the `DestinationAddress` is mandatory.

In "outbound" mode, `DestinationAddress` is used as default or fallback destination, that will be used if no other destination is found using Transparent-Proxy or Magic-User mechanisms.

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