Anssi,

Thanks for your help. I am not using any proxy, and this problem does not happen on other CentOS mirror server.

After yum update with enable URLGRABBER_DEBUG, I found that they are using nginx as the web server of mirror, I am guessing if they are using nginx as a proxy in front of the real server (e.g. Apache), so that everyone connects via their proxy in result.

You may check my debug log here:
yumdebug.log: https://file.io/ERNhoZ
yum command output: https://file.io/QJ48q6

Reproduce the problem can be by below 2 commands easily:
========================================================
cat << _EOF_ > /etc/yum.repos.d/CentOS-Base.repo
[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
_EOF_

URLGRABBER_DEBUG=1,yumdebug.log yum -y update
========================================================

Ming