One of the configured repositories failed

One of the configured repositories failed (Unknown),  
 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 <repoid>  
  
  
     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=<repoid>.skip_if_unavailable=true  
  
  
Cannot find a valid baseurl for repo: base/7/x86_64 

去修改它大致需要两步

第一步：vi /etc/resolv.conf，添加下面一行就可以了

1. nameserver 114.114.114.114（这是国内的dns服务器系统，还是比较好用的。谷歌的可以使用8.8.8.8）  

修改完后service network restart进行重启，试一下yum update命令，应该就可以了。如果还不可以，可以再进行下一步。

第二步：进入 /etc/yum.repos.d ，编辑vi CentOS-Base.repo，修改下面黄色标识的部分（只需要注释掉第一行，取消注释第二行就可以了）

# 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


~                                                                                                                                                                      

"CentOS-Base.repo" 44L, 1664C

完成上面两步，再重新执行yum update等其它安装命令就可以正常运行了。