CentOS7的yum无法安装，报错could not resolve host: mirrorlist.centos.org

一、问题描述

错误信息如下：

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 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

二、原因分析

根据错误提示“Could not resolve host”，可以推断为网络问题，无法访问yum源，通过nmcli d查看设备状态：

nmcli d

可以看到ens33网卡没有连接：

三、解决方案

1. 进入网卡配置文件所在目录：

cd /etc/sysconfig/network-scripts/

2. 编辑网卡配置文件：

vi ifcfg-ens33

3. 将文件中的最后一行的ONBOOT=no 改为ONBOOT=yes，如下图
   
4. 保存文件并退出vi，重启network：service network restart，然后再执行yum install xxx，正常执行。