今天准备搭建一套Docker环境，先安装了一个CentOS 7.2的虚拟机，然后使用【yum】命令来装其他的软件包，出现了如下消息：

  Loaded plugins: fastestmirror, langpacks

  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

  难道是虚拟机没有网络访问权限，先【ping www.baidu.com】试一下

  网络访问权限没有问题，那问题可能是CentOS的yum源的问题，下载一个能访问的yum源放到【/etc/yum.repos.d/】目录下，例如阿里的：

curl -o /etc/yum.repos.d/CentOS-Base-Ali.repo http://mirrors.aliyun.com/repo/Centos-7.repo

  运行【yum makecache】生成缓存

  现在在输入命令【yum search docker】测试下：

  查找镜像正常。
【附】加入163的yum源

curl -o /etc/yum.repos.d/CentOS-Base-163.repo https://mirrors.163.com/.help/CentOS7-Base-163.repo