Fallback to the server's external ip address if the IP returned by requesting passive mode is not routable.

ftp_pasv_fallback.rb

require 'net/ftp'

module Net
  class FTP
    alias :old_makepasv :makepasv
    def makepasv # :nodoc:
      host, port = old_makepasv
      host = @sock.peeraddr[3] unless is_routeable?(host)
      return host, port
    end 

    def is_routeable?(host) # :nodoc:
      if @sock.peeraddr[0] == "AF_INET"
        # http://snipplr.com/view/31058/ - this is good enough :)
        host.match(/(^10\.)|(^172\.1[6-9]\.)|(^172\.2[0-9]\.)|(^172\.3[0-1]\.)|(^192\.168\.)|(^127\.0\.0\.1)/).nil?
      else
        # TODO: IPv6
        true
      end
    end
  end
end

if $0 == __FILE__
	ftp = Net::FTP.new('yourhost')
	ftp.login('user', 'pass')
	ftp.passive = true
	puts ftp.list
	ftp.close
end