'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.'

def ftp_PASV(self):

    if (self.dtpFactory is not None):
        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))
