http://www.rfc-editor.org/rfc/rfc6384.txt
RFC 6384: An FTP Application Layer Gateway (ALG) for IPv6-to-IPv4 Translation, I. van Beijnum2011-10-01T23:00:00-00:00FTP has an active and a passive mode, both as original commands that are IPv4-specific and as extended, IP version agnostic commands. The only FTP mode that works without changes through an IPv6-to-IPv4 translator is extended passive. However, many existing FTP servers do not support this mode, and some clients do not ask for it. This document specifies a middlebox that may solve this mismatch. [STANDARDS-TRACK]