CentOS7 执行yum 命令出错

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 

解决方式： 转载：http://blog.csdn.net/zhousenshan/article/details/53140979#

第一步：vi /etc/resolv.conf，添加下面一行就可以了

nameserver 114.114.114.114

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。