[Errno 256] No more mirrors to try. http:

Centos7系统安装yum遇到的问题
问题1：
[root@ar yum.repos.d]# yum makecache
Loaded plugins: fastestmirror
base | 5.6 kB 00:00:00
http://mirrors.163.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno -1] Error importing repomd.xml for base: Damaged repomd.xml file
Trying other mirror.

One of the configured repositories failed (CentOS-7 - Base - 163.com),
and yum doesn't have enough cached data to continue. At this point the only
safe thing yum can do is fail. There are a few ways to work "fix" this:

 1. Contact the upstream for the repository and get them to fix the problem.

 2. Reconfigure the baseurl/etc. for the repository, to point to a working
    upstream. This is most often useful if you are using a newer
    distribution release than is supported by the repository (and the
    packages for the previous distribution release still work).

 3. Disable the repository, so yum won't use it by default. Yum will then
    just ignore the repository until you permanently enable it again or use
    --enablerepo for temporary usage:

        yum-config-manager --disable base

 4. Configure the failing repository to be skipped, if it is unavailable.
    Note that yum will try to contact the repo. when it runs most commands,
    so will have to try and fail each time (and thus. yum will be be much
    slower). If it is a very temporary problem though, this is often a nice
    compromise:

        yum-config-manager --save --setopt=base.skip_if_unavailable=true

failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try.
http://mirrors.163.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno -1] Error importing repomd.xml for base: Damaged repomd.xml file
[root@ar yum.repos.d]#

问题2：
[root@ar]# rpm -ivh yum-*
not an rpm package

问题一解决方法：
[root@ar]# cat /etc/redhat-release #检查自己的系统版本
去官网下载rpm包：http://vault.centos.org/7.5.1804/os/x86_64/Packages/
先卸载repos.d下的所有包：rpm -e 参数
下载完卸载的rpm包再rz上传至： cd /etc/repos.d下 解决第1个问题
此处不要使用weget，否则会出现问题2报错
再yum clean all yum makecache rpm -ivh yum-*#解决第2个问题

CentOS-Base.repo文件丢失或者误删除可直接使用eliyunrepo：
http://mirrors.aliyun.com/repo/centos-7.repo

清华大学yum配置：
http://mirrors.ustc.edu.cn/help/centos.html