code: public void setActive(HostPort hostPort)
          throws IOException, ServerException {
          Command cmd = new Command((controlChannel.isIPv6()) ? "EPRT" : "PORT",
                                    hostPort.toFtpCmdArgument());
          try {
              controlChannel.execute(cmd);
          } catch (UnexpectedReplyCodeException urce) {
              throw ServerException.embedUnexpectedReplyCodeException(urce);
          } catch (FTPReplyParseException rpe) {
              throw ServerException.embedFTPReplyParseException(rpe);
          }
  
          this.session.serverMode = Session.SERVER_ACTIVE;
      }
docs: Sets remote server active, telling it to connect to the given
  address.
  @param hostPort the address to which the server should connect