In explicit mode (also known as FTPES), an FTPS client must "explicitly request" security from an FTPS server and then step up to a mutually agreed encryption method. As such, it is considered an earlier, deprecated method of negotiating TLS/SSL for FTP. Note that implicit negotiation was not defined in RFC 4217. This allowed administrators to retain legacy-compatible services on the original 21/TCP FTP control channel. In order to maintain compatibility with existing non-FTPS-aware clients, implicit FTPS was expected to listen on the IANA well known port 990/TCP for the FTPS control channel, and port 989/TCP for the FTPS data channel. If such a message is not received by the FTPS server, the server should drop the connection. A client is immediately expected to challenge the FTPS server with a TLS ClientHello message. Negotiation is not supported with implicit FTPS configurations. While the implicit method requires that a Transport Layer Security is established from the beginning of the connection, which in turn breaks the compatibility with non-FTPS-aware clients and servers, the explicit method uses standard FTP protocol commands and replies in order to upgrade a plain text connection to an encrypted one, allowing a single control port to be used for serving both FTPS-aware and non-FTPS-aware clients. Two separate methods were developed to invoke client security for use with FTP clients: Implicit and Explicit. However, the RFC was not finalized until 2005. An official IANA port was registered shortly thereafter. The SSL protocol was eventually applied to FTP, with a draft Request for Comments (RFC) published in late 1996. While it could add security to any protocol that uses reliable connections, such as TCP, it was most commonly used by Netscape with HTTP to form HTTPS. This protocol enabled applications to communicate across a network in a private and secure fashion, discouraging eavesdropping, tampering, and message forgery. In 1994, the Internet browser company Netscape developed and released the application layer wrapper, Secure Sockets Layer. The opportunity for unauthorized third parties to eavesdrop on data transmissions increased proportionally. Access to the ARPANET during this time was limited to a small number of military sites and universities and a narrow community of users who could operate without data security and privacy requirements within the protocol.Īs the ARPANET gave way to the NSFNET and then the Internet, a broader population potentially had access to the data as it traversed increasingly longer paths from client to server. The File Transfer Protocol was drafted in 1971 for use with the scientific and research network, ARPANET. It is also different from FTP over SSH, which is the practice of tunneling FTP through an SSH connection. RFC 959 (FTP), RFC 8446 (TLS 1.3), RFC 4217 (Explicit FTPS)įTPS (also known as FTP-SSL and FTP Secure) is an extension to the commonly used File Transfer Protocol (FTP) that adds support for the Transport Layer Security (TLS) and, formerly, the Secure Sockets Layer (SSL, which is now prohibited by RFC7568) cryptographic protocols.įTPS should not be confused with the SSH File Transfer Protocol (SFTP), a secure file transfer subsystem for the Secure Shell (SSH) protocol with which it is not compatible. File Transfer Protocol over TLS Communication protocolįor implicit FTPS, 990 for control, 989 for data transfer For FTP Software, the defunct network software company, see FTP Software.
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