在Centos7上安装C++时，一直网络不可达，提示如下：
[root@localhost ~]# yum install gcc-c++ -y
已加载插件：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#7 - "Failed to connect to 2600:1f16:c1:5e01:4180:6610:5482:c1c0: 网络不可达"
One of the configured repositories failed (未知),
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
解决方案：更换yum源
1.下载wget
[root@bd01 ~]# yum -y install wget
2.进入yum源目录下
[root@bd01 ~]# cd /etc/yum.repos.d/
3.重命名yum源
mv CentOS-Base.repo CentOS-Base.repo.bak
4.下载yum源
- 阿里-centos7：http://mirrors.aliyun.com/repo/Centos-7.repo
- 网易-centos7：http://mirrors.163.com/.help/CentOS7-Base-163.repo
这里提供两种yum源的下载地址，本下载需要虚拟机联网。
当前采用网易的yum源，阿里的操作步骤类同。
wget http://mirrors.163.com/.help/CentOS7-Base-163.repo
5.修改yum源名称
[root@bd01 yum.repos.d]# mv CentOS7-Base-163.repo CentOS-Base.repo
6.生成yum缓存
[root@bd01 yum.repos.d]# yum makecache
7.更新yum源包
[root@bd01 yum.repos.d]# yum -y update