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

这个错误提示意味着您的系统无法访问 CentOS 官方的软件仓库，可能是由于网络问题或者系统配置问题所导致的。

您可以尝试以下几种方法解决该问题：

1. 检查网络连接是否正常

可以使用 `ping` 命令检查网络是否正常，例如：

ping www.centos.org

如果可以正常 ping 通，则说明网络连接正常。

2. 确认系统配置是否正确

可以检查 `/etc/yum.repos.d/` 目录下的 `.repo` 文件，看看是否有错误或者配置不正确的地方。

3. 临时禁用仓库/源

您可以尝试临时禁用仓库/源，然后再运行命令尝试安装。具体操作可以参考我之前回答的那个问题。

4. 更换软件源

如果以上方法都无法解决问题，您可以尝试更换软件源，例如使用阿里云的软件源。具体操作如下：

备份原有的 CentOS-Base.repo 文件：

sudo mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup

下载阿里云的 CentOS 7 源文件：

sudo wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo

更新缓存：

sudo yum clean all
sudo yum makecache

尝试再次安装软件包：

sudo yum install net-tools

如果仍然无法解决问题，您可以尝试更换其他的软件源，例如网易、搜狐等。