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Problems establishing data connection with FTPS
FTP (and FTPS) require two connections to the server:
- Control Channel - for sending commands and receiving response
- Data Channel - for sending and receiving data (including directory listings)
Sometimes it is possible for Robo-FTP to be able to successfully establish the control connection but not the data connection.
When this happens, the FTPLOGON command will fail. This is because part of the logon process includes retrieving a directory listing. In other words, Robo-FTP is able to log on (send user name and password over the control channel) but fails to retrieve a directory listing (which needs to happen over the data channel) so FTPLOGON returns an error of *Connection to FTP site failed. [1152].
To see more detail, look at the trace log (see the Help file for details on enabling the trace log).
Common error messages in the trace log include Error: LIST command failed: connection to the FTP site has timed out [10060] or Error: LIST command failed: Access denied [40530].
This happens when an FTPS server is deployed behind a firewall because it is not possible for firewalls to perform address translation on FTPS traffic. The result is that the server is unable to communicate the correct IP address for data connections to the client.
The solution is to configure Robo-FTP to automatically use the IP address used for the control connection for the data connection as well.
There are two ways to do this.
If you are using the site manager (Start | All Programs | Robo-FTP | Configure Robo-FTP | Manage Sites) to store connection details, simply enable the Restrict IP Address option. In Robo-FTP 3.5.x and earlier this option is called Use Ctrl For Data.
If you are using the FTPLOGON command to set the connection parameters, add the /restrictipaddr option.
If you still can not connect, another common source of this error with FTPS is having the server type configured wrong. Try each of the four variations for server type (implicit, control + data, control only, data only).
Article last updated: 2021-11-10
Tags: connection, Configuration, Firewall, FTPLOGON, FTPS