解决办法
删除yum.repos.d目录下所有文件
rm -f /etc/yum.repos.d/*
然后执行下边的命令，重新下载阿里的这个文件（需要外网）
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
执行下边的命令，清理缓存
yum clean all
完成。
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方法二，以上如果未解决问题可以尝试用另一种办法：
直接修改CenOS-Base.repo文件内容
修改/etc/yum.repos.d/CenOS-Base.repo文件内容如下：
# 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-$releasever - Base mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra #baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/ gpgcheck=1 gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7 #released updates [updates] name=CentOS-$releasever - Updates mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra #baseurl=http://mirror.centos.org/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-$releasever - Extras mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra #baseurl=http://mirror.centos.org/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-$releasever - Plus mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra #baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/ gpgcheck=1 enabled=0 gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
编辑命令如下：
vi /etc/yum.repos.d/CentOS-Base.repo
按ESC键 保存命令 :wq
执行下边的命令，清理缓存
yum clean all
完成。