yum是centos下更新、管理软件的命令，也有相应的图像界面版本；

yum源是yum命令去哪里取安装包的地图；

yum命令查询repo上配置的地址去取相应的rpm包进行安装；

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[root@localhost ~]# uname -r
3.10.0-693.el7.x86_64
[root@localhost ~]# cat /etc/redhat-release
CentOS Linux release 7.4.1708 (Core)
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1、备份系统原来的yum源

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[root@localhost ~]# mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.bak
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2、下载阿里云的CentOS-Base.repo 到/etc/yum.repos.d/ {#2、下载阿里云的CentOS-Base-repo-到-etc-yum-repos-d}

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[root@localhost ~]# wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
或者
[root@localhost ~]# curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
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3、添加阿里云公共DNS {#3、添加阿里云公共DNS}

为了防止不能寻找yun源地址，生成cache，先添加阿里云的DNS

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[root@localhost ~]# vim /etc/resolv.conf
添加
nameserver 223.5.5.5
nameserver 223.6.6.6
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4、清理缓存，并生成新缓存 {#4、清理缓存}

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[root@localhost ~]# yum clean all && yum makecache
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