TLS specifications

The Transport Layer Security (TLS) protocol gets used to cryptographically protect a socket connection. It is the successor of the Secure Sockets Layer (SSL) protocol. TLS gets used for the FTP over TLS/SSL (FTPS) protocol.

Technical Specifications

• SSLv3 draft (obsolete)
• RFC 2246 "TLS 1.0"
• RFC 4346 "TLS 1.1"
• RFC 5246 "TLS 1.2"
• RFC 7465 "Prohibiting RC4 Cipher Suites"

Points of interest

• Section 7.2.1. of RFC 2246: Closure alerts
  

The client and the server must share knowledge that the connection is
ending in order to avoid a truncation attack. Either party may
initiate the exchange of closing messages.
[...]
Each party is required to send a close_notify alert before closing
the write side of the connection.

Lots of FTP servers did not follow the specifications and were vulnerable to truncation attacks. FileZilla will properly fail transfers on those servers.

See also

• FTP specifications
• Other specifications
• SFTP specifications