进入yum源配置文件

​cd /etc/yum.repos.d​

备份一下当前的源，以防出错后可以还原回来

​​mv CentOS-Base.repo CentOS-Base-repo.bak​​

下载163源 

​​​​​wget ​​http://mirrors.163.com/.help/CentOS7-Base-163.repo​​

重点来了 Linux服务器空空wget不支持，要想wget支持又必须通过yum下载，这不是进入死循环了吗，还好

curl -O http://mirrors.163.com/.help/CentOS7-Base-163.repo

是可以下载的。

下载完后执行

yum clean all

mv CentOS7-Base-163.repo CentOS-Base.repo（设置默认源）

yum makecache 

执行yum makecache后报错了无法访问到​​http://mirrors.163.com​​但是ping是可以ping通的。

版本也是没有问题的​​http://mirrors.163.com/.help/centos.html​​

找到文件中的一个地址 ​​​http://mirrors.163.com/centos/$releasever​​根本无法访问改为​​http://mirrors.163.com/centos/7​​ 美美访问也就是说$releasever根本未被定义或者说未接受到系统传参，别人家的服务器不敢随便动了喂，vi CentOS-Base.repo 修改文件如下，搞定了。

# 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-$releasever - Base - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
baseurl=http://mirrors.163.com/centos/7/os/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7

#released updates
[updates]
name=CentOS-$releasever - Updates - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
baseurl=http://mirrors.163.com/centos/7/updates/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
baseurl=http://mirrors.163.com/centos/7/extras/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus - 163.com
baseurl=http://mirrors.163.com/centos/7/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7