centos7执行yum命令时报：Could not resolve host: mirrorlist.centos.org； Unknown error
项目场景：
打算学习一下docker,所以重新装了一台虚拟机，centos7。安装完成后，通过 yum 命令下载必要软件时报了错误，对此错误进行了分析，解决和记录。
问题描述
通过yum 命令下载软件的时候报错，如下：
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=stock error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"One of the configured repositories failed (Unknown),and yum doesn't have enough cached data to continue. At this point the onlysafe 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 workingupstream. This is most often useful if you are using a newerdistribution release than is supported by the repository (and thepackages for the previous distribution release still work).3. Run the command with the repository temporarily disabledyum --disablerepo=<repoid> ...4. Disable the repository permanently, so yum won't use it by default. Yumwill then just ignore the repository until you permanently enable itagain or use --enablerepo for temporary usage:yum-config-manager --disable <repoid>orsubscription-manager repos --disable=<repoid>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 muchslower). If it is a very temporary problem though, this is often a nicecompromise:yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=trueCannot find a valid baseurl for repo: base/7/x86_64
原因分析：
下载软件导致，第一想法是网络是否通的，使用命令ping 了百度
ping www.baidu.com
返回消息正常，说明网络没有问题。
然后通过查找问题，发现是镜像源的问题，可以通过切换镜像源解决。
解决方案：
切换阿里云镜像源。
1.备份之前镜像源
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
2.下载新的 CentOS-Base.repo 到 /etc/yum.repos.d/
centos7
wget -O /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo
或者
curl -o /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo
3.运行命令生成缓存
yum makecache