Why does passive FTP work behind router/firewall?

From: Georges Heinesch (void_at_void.com)
Date: 02/17/04 

Date: Tue, 17 Feb 2004 18:34:38 +0100

Hi.

I have a question about an SMC router incl. firewall, NAT, ... (model:
SMC2804WBR V.2).

I use an FTP Server behind this router. Clients can access the server in
passive mode and active mode.

active mode: port 21 has to be forwarded. The router configuration was
changed accordingly by myself and it works! So far, everything is normal.

passive mode: here comes the interesting part. To make passive mode
possible, the FTP server (1.) must know the global IP of the router and
(2.) must have a defined port pool (must be set in the configuration of
the FTP server). On top of this, the router has to be configured to
forward this defined port pool to the computer where the FTP server is
running.

I my case, neither the router, nor the FTP server is configured in any
respect. Hence, passive mode should _not_ work. However it does.

How is that possible?

TIA 

-- 
Georges

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