参考网页
https://blog.csdn.net/xiaojin21cen/article/details/84726193
（1）、首先备份 CentOS-Base.repo
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.bak
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（2）、之后启用 TUNA 软件仓库， 将清华大学镜像仓库信息写入 /etc/yum.repos.d/CentOS-Base.repo
vim /etc/yum.repos.d/CentOS-Base.repo
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将 CentOS-Base.repo 中的内容 更新为 下面的内容：
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-$releasever - Base
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/os/$basearch/
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
released updates
[updates]
name=CentOS-$releasever - Updates
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/updates/$basearch/
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/extras/$basearch/
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/centosplus/$basearch/
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
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（3）、清除缓存
yum clean all # 清除系统所有的yum缓存
yum makecache # 生成yum缓存
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3、epel源 安装和配置
（1）、查看可用的epel源
[java@localhost yum.repos.d]$ yum list | grep epel-release
epel-release.noarch 7-11 extras
[java@localhost yum.repos.d]$
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（2）、安装 epel
[java@localhost yum.repos.d]$ yum install -y epel-release
......省略.....
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（3）、配置阿里镜像提供的epel源
wget -O /etc/yum.repos.d/epel-7.repo http://mirrors.aliyun.com/repo/epel-7.repo
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（4）、清除缓存
yum clean all # 清除系统所有的yum缓存
yum makecache # 生成yum缓存
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3.1、查看yum源
查看所有的yum源：
yum repolist all
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查看可用的yum源：
yum repolist enabled