Centos yum安装/更新报错

在使用yum update的时候遇到报错，报错信息如下:

Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
epel/x86_64/metalink                                                                               | 6.1 kB  00:00:00     
 * epel: mirrors.yun-idc.com
http://mirrors.cloud.aliyuncs.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.cloud.aliyuncs.com; Unknown error"
Trying other mirror.


 One of the configured repositories failed (CentOS-7),
 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.cloud.aliyuncs.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.cloud.aliyuncs.com; Unknown error"

原因是 提示中的http://mirrors.cloud.aliyuncs.com/地址并不能访问
使用命令vim /etc/yum.repos.d/CentOS-Base.repo 修改CentOS-Base.repo文件中的相关URL
我是修改mirrors.cloud.aliyuncs.com为mirror.centos.org,然后一切正常.
修改后的CentOS-Base.repo文件

name=CentOS-$releasever                                                                                                   
enabled=1                                                                                                                 
failovermethod=priority                                                                                                   
baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/                                                     
#baseurl=http://mirrors.cloud.aliyuncs.com/centos/$releasever/os/$basearch/                                               
gpgcheck=1                                                                                                                
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-7                                                               
#gpgkey=http://mirrors.cloud.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-7                                                     
                                                                                                                          
[updates]                                                                                                                 
name=CentOS-$releasever                                                                                                   
enabled=1                                                                                                                 
failovermethod=priority                                                                                                   
baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/                                                     
#baseurl=http://mirrors.cloud.aliyuncs.com/centos/$releasever/updates/$basearch/                                          
gpgcheck=1                                                                                                                
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-7                                                               
#gpgkey=http://mirrors.cloud.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-7                                                     
                                                                                                                          
[extras]                                                                                                                  
name=CentOS-$releasever                                                                                                   
enabled=1                                                                                                                 
failovermethod=priority                                                                                                   
baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/                                                     
#baseurl=http://mirrors.cloud.aliyuncs.com/centos/$releasever/extras/$basearch/                                           
gpgcheck=1                                                                                                                
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-7                                                               
#gpgkey=http://mirrors.cloud.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-7

其他问题
搜索的资料发现 有些可能是DNS的问题 可以通过修改host 或在/etc/resolv.conf文件中加上

nameserver 114.114.114.114

或

nameserver 8.8.8.8

参考资料