[CentOS-mirror] HK mirror - repo.virtualhosting.hk sometimes HTTP Error 503 - Service Unavailable

Hi CentOS maintainers,

In Hong Kong (HK) sometimes yum update would get "HTTP Error 503 - Service
Unavailable", finally I found that running "yum -y update" after "Loading
mirror speeds from cached hostfile", while yum selects a mirror "
repo.virtualhosting.hk", 95% chance will get error 503.

How to duplicate problem:
Since yum will select yum server from different mirror servers
automatically, so I updated /etc/yum.repos.d/CentOS-Base.repo as below in
order to force checking on "repo.virtualhosting.hk":

========================================================
[base]
name=CentOS-$releasever - Base
baseurl=http://repo.virtualhosting.hk/centos/7/os/x86_64/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[updates]
name=CentOS-$releasever - Updates
baseurl=http://repo.virtualhosting.hk/centos/7/os/x86_64/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[extras]
name=CentOS-$releasever - Extras
baseurl=http://repo.virtualhosting.hk/centos/7/os/x86_64/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[centosplus]
name=CentOS-$releasever - Plus
baseurl=http://repo.virtualhosting.hk/centos/7/os/x86_64/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
========================================================

# yum -y update
...
(10/366):
avahi-libs-0.6.31-19.el7.x86_64.rpm
|  61 kB  00:00:00
(11/366):
apr-1.4.8-3.el7_4.1.x86_64.rpm
| 103 kB  00:00:00
(12/366):
bash-4.2.46-31.el7.x86_64.rpm
| 1.0 MB  00:00:00
bind-libs-9.9.4-72.el7.x86_64.
FAILED
http://repo.virtualhosting.hk/centos/7/os/x86_64/Packages/bind-libs-9.9.4-72.el7.x86_64.rpm:
[Errno 14] HTTP Error 503 - Service Unavailable
Trying other mirror.
bind-libs-lite-9.9.4-72.el7.x8
FAILED
http://repo.virtualhosting.hk/centos/7/os/x86_64/Packages/bind-libs-lite-9.9.4-72.el7.x86_64.rpm:
[Errno 14] HTTP Error 503 - Service Unavailable
Trying other mirror.
(13/366):
bind-9.9.4-72.el7.x86_64.rpm
| 1.8 MB  00:00:00
(14/366):
MariaDB-10.1.38-centos73-x86_64-server.rpm
| 104 MB  00:00:02
...

However, if I try to manually update the failed package (e.g. bind-libs
like above) by "yum -y update bind-libs" , it will be success. It means the
package exists in mirror server repo.virtualhosting, however somehow the
server limits the HTTP request and return error 503, can CentOS mirror
admin inform this mirror admin to fix the problem?
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