yum报错如下
[root@java152 ~]# yum makecache
已加载插件：fastestmirror
Loading mirror speeds from cached hostfile
* base: mirrors.cloud.aliyuncs.com
* extras: mirrors.cloud.aliyuncs.com
* updates: mirrors.cloud.aliyuncs.com
http://mirrors.aliyun.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found
正在尝试其它镜像。
To address this issue please refer to the below wiki article
https://wiki.centos.org/yum-errors
If above article doesn't help to resolve this issue please use https://bugs.centos.org/.
base | 1.0 kB 00:00:00
http://mirrors.cloud.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno -1] Error importing repomd.xml for base: Damaged repomd.xml file
正在尝试其它镜像。
http://mirrors.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30000 milliseconds')
正在尝试其它镜像。
http://mirrors.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
正在尝试其它镜像。
http://mirrors.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
正在尝试其它镜像。
http://mirrors.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
正在尝试其它镜像。
http://mirrors.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
正在尝试其它镜像。
http://mirrors.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
正在尝试其它镜像。
One of the configured repositories failed (CentOS-$releasever - Base - mirrors.aliyun.com),
and yum doesn't have enough cached data to continue. At this point the only
safe thing yum can do is fail. There are a few ways to work "fix" this:
1. Contact the upstream for the repository and get them to fix the problem.
2. Reconfigure the baseurl/etc. for the repository, to point to a working
upstream. This is most often useful if you are using a newer
distribution release than is supported by the repository (and the
packages for the previous distribution release still work).
3. Run the command with the repository temporarily disabled
yum --disablerepo=base ...
4. Disable the repository permanently, so yum won't use it by default. Yum
will then just ignore the repository until you permanently enable it
again or use --enablerepo for temporary usage:
yum-config-manager --disable base
or
subscription-manager repos --disable=base
5. Configure the failing repository to be skipped, if it is unavailable.
Note that yum will try to contact the repo. when it runs most commands,
so will have to try and fail each time (and thus. yum will be be much
slower). If it is a very temporary problem though, this is often a nice
compromise:
yum-config-manager --save --setopt=base.skip_if_unavailable=true
failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try.
http://mirrors.aliyun.com/centos/$releasever/os/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found
http://mirrors.cloud.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: [Errno -1] Error importing repomd.xml for base: Damaged repomd.xml file
http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30000 milliseconds')
http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
1.修改DNS，失败告终。（还是不能用）
命令：vim /etc/sysconfig/network-scripts/ifcfg-eth0
vim /etc/sysconfig/network-scripts/ifcfg-eth0
vim /etc/sysconfig/network-scripts/ifcfg-eth0
2.重装yum服务，还是不能用。
yum-3.4.3-161.el7.centos.noarch.rpm
yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
yum-plugin-fastestmirror-1.1.31-50.el7.noarch.rpm
命令：rpm -ivh yum-3.4.3-161.el7.centos.noarch.rpm --nodeps --force //nodeps 强制安装 force 避免依赖
命令：rpm -ivh yum-metadata-parser-1.1.4-10.el7.x86_64.rpm --nodeps --force
命令：rpm -ivh yum-plugin-fastestmirror-1.1.31-50.el7.noarch.rpm --nodeps --force
3.更新yum源，失败告终。
命令：mv /etc/yum.repos.d/* /etc/yum.repos.d/backup.bak
命令：wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo //指向阿里源
命令：yum clean all //清除缓存
命令：yum makecache //生成缓存
最终解决方法
上边一直提示 404 错误，然后我直接访问
http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml
发现这个地址打不开
命令：vim /etc/yum.repos.d/Centos-7.repo
命令：:%s/$releaserver/7/g #文本编辑器中的替换命令
修改完之后执行
命令：yum makecache #OK
知识扩展：
问题分析一下
对比.repo文件和报错信息，可以发现$releasever变量没有被系统识别，在连接网络yum源的时候，变成了%24releasever
查询资料得知，yum中的变量$releasever是由/etc/yum.conf中的distroverpkg定义的。centos-release为一个rpm包，所谓“distroverpkg=centos-release”的意思，其实是将 $releasever设置为centos-release 这个RPM包的版本号
所以打开yum的配置文件
命令：vim /etc/yum.conf
通过rpm可以查询到改软件包并没有被安装
解决思路
由于centos-release依赖关系比较复杂，约40个左右依赖包，我们可以修改yum源文件，把$releasever全部替换为7（上边已操作）
此时yum可以正常使用，通过yum安装centos-release
命令：yum -y install centos-release
重新下载yum源，覆盖修改之后的yum。yum makecache系统可把$releasever解析为当前的CentOS的版本，恢复正常状态
总结：。。。。。。。。。。（此处省略一万字）