配置yum 源，yum makecache时报错

因为yum安装的一个软件要用到线网源，因此把原来的CentOS-Base.repo又放回yum.repos.d中，结果从新加载出现问题，以下：

[root@host3 yum.repos.d]# mv backup/CentOS-Base.repo CentOS-Base.repo
[root@host3 yum.repos.d]# yum clean all
Loaded plugins: fastestmirror, langpacks
Cleaning repos: base extras updates
Cleaning up list of fastest mirrors
Other repos take up 18 M of disk space (use --verbose for details)
[root@host3 yum.repos.d]# yum makecache
Loaded plugins: fastestmirror, langpacks
Determining fastest mirrors
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. Run the command with the repository temporarily disabled yum --disablerepo=<repoid> ... 4. Disable the repository permanently, 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>
        or
            subscription-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 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

[外链图片转存失败,源站可能有防盗链机制,建议将图片保存下来直接上传(img-z39qzms3-1601277682727)(https://img-blog.csdnim.cn/20200921134733515.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L20wXzQ3NDUyNDA1,size_16,color_FFFFFF,t_70#pic_center)]
排查以下：
由于是线网源，因此先ping www.baidu.com，看看能不能上网

问题出现，不能上网，而后看一下网卡配置，发现没有配DNS，加上DNS

重启

而后再测试一下可否上网

能上网了，再yum makecache 一下，能够了