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PASV Response Contains Internal IP Address  -  KB Article #1300

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There are two types of ways that data can be transfered in FTP communications, active (PORT) and PASV. To learn more about the difference between these two modes of data transfer, please see Knowledge Base Article #1138.

When issuing a PORT command, FTP Voyager issues the IP address of the machine it is running on since it has no knowledge of any other IP addresses on the network. If this IP address is an internal IP address, the server might not be able to connect back to the client. This is a common problem for thousands of people on the Internet, which is why PASV mode data transfers are used. When doing PASV mode data transfers, the IP address is provided by the server instead. If the FTP server is properly configured from behind a firewall or router, then there will be no problems with the PASV command.

However, if the PASV command response still contains an internal IP address, then the FTP server is not correctly configured for PASV mode data transfers and the server administrator should be notified of this.

In versions of FTP Voyager prior to, there was no way around this until the server administrator corrected their PASV mode configuration. FTP Voyager and greater can correct this server misconfiguration by substituting the known external IP address for an FTP server when it receives an internal IP address in the PASV command response from the server.

This is especially useful for secure/SSL connections where the server's NAT router cannot translate the internal IP address into the proper external IP address since the communication is encrypted.