Ubuntu never shutdown, reboot sorf of process when ftp is connected.

[Jung-Kyu Park]

Installed vsftpd.
Connecting via iOS ftpmanager pro app.

Whenever I attmept to shutdown, or reboot the system with ftp connected,
Ubuntu never proceed the process of shutdown or reboot.
It seems to be awaiting with circling cursor.

It proceed the shutdown process after close the app or close the ftp.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ftp 0.17-33
ProcVersionSignature: Ubuntu 4.4.0-62.83-generic 4.4.40
Uname: Linux 4.4.0-62-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Feb 13 12:13:31 2017
InstallationDate: Installed on 2017-02-13 (0 days ago)
InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
ProcEnviron:
 LANGUAGE=ko
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=ko_KR.UTF-8
 SHELL=/bin/bash
SourcePackage: netkit-ftp
UpgradeStatus: No upgrade log present (probably fresh install)