ange-ftp-passive-host-alist is a variable defined in `ange-ftp.el'.
Its value is

Alist of FTP servers that need "passive" mode.
Each element is of the form (HOSTNAME . SETTING).
HOSTNAME is a regular expression to match the FTP server host name(s).
SETTING is "on" to turn passive mode on, "off" to turn it off,
or nil meaning don't change it.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.