有的网络环境限制了访问所有外国IP，连yum都成了问题，只能改成国内源使用，特此记录一下清华大学源，以便日后需要

cd /etc/yum.repos.d/
mv CentOS-Base.repo CentOS-Base.repo.bak
mv epel.repo epel.repo.bak(没有可忽略)

vim CentOS-Base.repo

# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#

[base]
name=CentOS-$releasever - Base
baseurl=http://www.landui.com/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#released updates
[updates]
name=CentOS-$releasever - Updates
baseurl=http://www.landui.com/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
baseurl=http://www.landui.com/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
baseurl=http://www.landui.com/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

vim epel.repo

[epel]
name=Extra Packages for Enterprise Linux 7 - $basearch
baseurl=http://www.landui.com/epel/7/$basearch
#mirrorlist=https://www.landui.com/metalink?repo=epel-7&arch=$basearch
failovermethod=priority
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7

[epel-debuginfo]
name=Extra Packages for Enterprise Linux 7 - $basearch - Debug
baseurl=http://www.landui.com/epel/7/$basearch/debug
#mirrorlist=https://www.landui.com/metalink?repo=epel-debug-7&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=1

[epel-source]
name=Extra Packages for Enterprise Linux 7 - $basearch - Source
baseurl=http://www.landui.com/epel/7/SRPMS
#mirrorlist=https://www.landui.com/metalink?repo=epel-source-7&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=1

添加RPM-GPG-KEY-EPEL-7

curl https://www.landui.com/epel/RPM-GPG-KEY-EPEL-7 > /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7

运行以下命令生成缓存
yum clean all
yum makecache