that the certificate is not valid. The client can choose to accept the certificate or reject the connection. This is in contrast to the SSH File Transfer Protocol (SFTP), which does not present signed certificates, but instead relies on Out-of-band authentication of public keys. Firewall incompatibilities Because FTP uses a dynamic secondary port (for data channels), many firewalls were designed to snoop FTP protocol control messages in order to determine which secondary data connections they need to allow. However, if the FTP control connection is encrypted using TLS/SSL, the firewall cannot determine the TCP port number of a data connection negotiated between the client and FTP server. Therefore, in many firewalled networks, an FTPS deployment will fail when an unencrypted FTP deployment will work. This problem can be solved with the use of a limited range of ports for data and configuring the firewall to open these ports. See also Notes External links Overview of FTPS, and lists of clients, servers Curl-loader - an open-source FTPS loading/testing tool File Transfer Protocol Internet Standards Transport Layer Security