Server sent passive reply with unroutable address in FileZilla

Question

I am trying to browse old plain FTP server with FileZilla and getting the following errors

Server sent passive reply with unroutable address. Using server address instead.

Browsing is nearly impossible.

What does this message means and how to know that "unroutable address"?

Answer 1

To establish a data connection for a file transfer or a directory listing, an FTP client (in the passive FTP mode) sends PASV commands to the server. The server responds with its IP address and port number. The client should connect to that IP/port to initiate the transfer.

It's quite common though that the FTP server is behind a NAT. And it's aware of its local address within its NATted network only. When it reports such IP address to a client outside the NAT, the IP address is obviously of little use. It's clearly a misconfiguration of the FTP server. The FTP server must report its external IP address, that can be used outside the NATted networks.

As this is unfortunately a common issue, many FTP clients try to detect this (by checking that the reported IP address is within a range of addresses reserved for local communications within a private network). FileZilla does too and reports the mentioned message. You should see the offending address in the FileZilla log:

2017-04-05 08:02:00 12064 3 Command: PASV
2017-04-05 08:02:00 12064 3 Response: 227 Entering Passive Mode (10,0,0,1,213,36).

When encountering this, FileZilla tries to connect to the primary address of the FTP server instead, as that is usually the correct address to use. So in most cases you won't notice the problem at all.

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Though as you mention that "Browsing is nearly impossible" (a rather useless problem report), it looks like that it does not help. If that's the case, most usual problem is that some firewall (or the NAT) on the way between you and the server blocks the connection attempt to the data port (as reported in the response to the PASV command).

See my article about Network configuration for FTP passive mode for details.