def ftp_PASV(self):
        """
        Request for a passive connection

        from the rfc::

            This command requests the server-DTP to \"listen\" on a data port
            (which is not its default data port) and to wait for a connection
            rather than initiate one upon receipt of a transfer command.  The
            response to this command includes the host and port address this
            server is listening on.
        """
        # if we have a DTP port set up, lose it.
        if self.dtpFactory is not None:
            # cleanupDTP sets dtpFactory to none.  Later we'll do
            # cleanup here or something.
            self.cleanupDTP()
        self.dtpFactory = DTPFactory(pi=self)
        self.dtpFactory.setTimeout(self.dtpTimeout)
        self.dtpPort = self.getDTPPort(self.dtpFactory)

        host = self.transport.getHost().host
        port = self.dtpPort.getHost().port
        self.reply(ENTERING_PASV_MODE, encodeHostPort(host, port))
        return self.dtpFactory.deferred.addCallback(lambda ign: None)

        Request for a passive connection

        from the rfc::

            This command requests the server-DTP to "listen" on a data port
            (which is not its default data port) and to wait for a connection
            rather than initiate one upon receipt of a transfer command.  The
            response to this command includes the host and port address this
            server is listening on.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/protocols/ftp.py