Character Encoding

Overview

FTP is a rather old protocol and things we take for granted now were not even considered 20 years ago. One of these things is support for non-English characters. When the FTP protocol was designed, computers mostly spoke English and were unable to display any non-English characters like umlauts, accented letters or even completely different scripts like for example Chinese. As such, the FTP protocol has been designed to be used with English characters only, namely 7-bit ASCII.

The problem is that many FTP clients and servers purposely violate the FTP specifications in order to support other, non-standard character sets. Which of these character sets are used is not subject to any negotiation and completely arbitrary. For any character set in existence, you can find a server using it with no way of detecting the proper encoding.

To solve this problem, the FTP protocol has been extended in a backwards compatible way to use UTF-8 as character set, which is a strict superset of the previously used character set. Note that this obviously can only be backwards compatible with severs in compliance with the original specifications.

If you have problems with filenames containing any foreign characters, this can have two reasons:

• The server or client follows the original specifications by the letter and rightfully rejects those filenames
• The server or client violates the specifications and uses a custom encoding

Note that both FileZilla Client and Server are fully compliant with the updated specifications and use UTF-8.

If you have problems with other clients or servers, please upgrade to FTP software capable of UTF-8 or refrain from using foreign characters. Anything else is in violation to the FTP specifications and does not work.

Technical details

The FTP protocol is specified in RFC 959, which got published in 1985. The FTP protocol is designed on top of the original Telnet protocol, which is specified in RFC 854. The relevant sections of the Telnet specification regarding FTP are those covering the Network Virtual Terminal (NVT). According to RFC 854, the NVT requires to use 7-bit ASCII as character set with any other character set being subject of explicit negotiation. This character set only contains 127 different characters: English letters and numbers, punctuation characters and a few control characters. Accented letters, umlauts or other scripts are not contained in the ASCII character set.

In order to support non-English characters, the FTP specifications have been extended 1999 in RFC 2640. This extension requires the use of UTF-8 as character set. This character is a strict superset of ASCII, every valid ascii character is also the same character in UTF-8. The UTF-8 character set can display any valid Unicode character. That includes umlauts, accented letters and also different scripts. This extension is fully backwards compatible. As long as you're not using any non-English characters, it doesn't matter if the used software supports RFC 2640 or not. Note that if you used non-English characters before using RFC 2640 compatible software, there will be problems. Problems which are entirely self-made by not obeying the specifications.