When opening the transparent tunnel, accepting new or changed server host keys cannot be prompted from the user. In addition, transparent FTP tunneling uses always the IP address of the Secure Shell server when opening the secure tunnel. This means that the host keys of the Secure Shell tunneling servers must be stored beforehand based on the IP addresses of the servers.
The keys can be stored by connecting to each host individually with the IP address of the host using an interactive shell and accepting the host keys one by one, or by using the ssh-keydist2 key distribution tool. More information and examples on storing remote server keys can be found in Sections Authenticating Remote Server Hosts and Fetching Remote Server Keys.