> Some FTP servers return wrong IP for PASV command (private instead 
> of public). In such cases, obviously, FTP component can't connect to 
> the server.

This is not really an FTP server issue, but a poorly designed NAT router
that has not replaced the private IP address with a public IP. 
> Filezilla is smart enough to detect this and switch to public IP, 
> instead:

Detecting the wrong address is easy, but whether replacing it with a
public address will do anything useful is more debatable since the NAT
router may not know which private IP address is the intended destination
of the incoming TCP connection.  

NAT routers have to be designed to understand the FTP protocol, and parse
the control channel for private IP addresses and forward connections
appropriately.  Normally this is all hidden and you never know it's

Do you have a specific example of a live public server returning a
private IP that we can test?  It will be very difficult to set-up, since
it needs a crappy NAT router. 


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