Your best friend for file transfer
Fetch
Fetch Help > Messages > Host Authenticity ("The authenticity of host can't be established" warning)
When Fetch warns you that "The authenticity of host 'ftp.example.com' can't be established, are you sure you want to continue connecting?", all it's really saying is that you're trying to connect using SFTP to a server that you have not connected to previously.
Each SFTP server has an SSH host key that enables secure transfers. When you connect to an SFTP server, Mac OS X stores that server's key for future use. When that happens, you have accepted the host key, saying that you trust the server.
Normally Fetch doesn't notify you when it accepts SSH host keys for servers you have not previously connected to, because accepting a key isn't generally regarded as a security risk. Also, you must accept the key if you want to transfer files from or to that server! However, if you want to confirm that you trust a server before connecting to it, you can check the SFTP: Ask before accepting unknown host keys preference in the Security Preferences pane. When that preference is checked, you will be see the "The authenticity of host 'ftp.example.com' can't be established, are you sure you want to continue connecting?" warnings.
If there is a more serious problem with the SFTP connection — for instance, the SSH host key you have stored for a server does not match the key the server is now using, or the IP address of a server has changed but the key has not — Fetch will refuse the connection to the server. If you are having trouble connection to a server that used to work, check to make sure you've entered the correct information in the New Connection dialog, and then contact the server administrator.
Related topics: