[root@localhost yum]# yum update
Loaded plugins: fastestmirror, refresh-packagekit, security
Repository base is listed more than once in the configuration
Repository updates is listed more than once in the configuration
Repository extras is listed more than once in the configuration
Repository centosplus is listed more than once in the configuration
Repository contrib is listed more than once in the configuration
Determining fastest mirrors
epel/metalink                                            | 4.6 kB     00:00
 * epel: mirrors.yun-idc.com
 * rpmforge: mirrors.neusoft.edu.cn
http://mirrors.163.com/centos/$releasever/os/i386/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404 Not Found"
Trying other mirror.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: base. Please verify its path and try again
[root@localhost yum]# vi /etc/yum.conf
[root@localhost yum]# yum update -y
Loaded plugins: fastestmirror, refresh-packagekit, security
Repository base is listed more than once in the configuration
Repository updates is listed more than once in the configuration
Repository extras is listed more than once in the configuration
Repository centosplus is listed more than once in the configuration
Repository contrib is listed more than once in the configuration
Loading mirror speeds from cached hostfile
epel/metalink                                            | 4.6 kB     00:00
 * epel: mirrors.yun-idc.com
 * rpmforge: mirrors.neusoft.edu.cn
base                                                     | 3.7 kB     00:00
extras                                                   | 3.5 kB     00:00
rpmforge


输入下面的命令即可解决问题：

yum clean all
yum makecache

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