执行  yum update  报错：

yum install -y yum-utils \ > device-mapper-persistent-data \ > lvm2 --skip-broken 已加载插件：fastestmirror, langpacks 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; 未知的错误" 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 

2024年6月30日，CentOS 7正式结束生命周期（EOL），官方停止维护支持。原本依赖的默认镜像源（如mirrorlist.centos.org）已无法访问，导致yum安装失败并报错

使用第三方镜像源

（如阿里云、腾讯云）仍维护CentOS 7的镜像仓库

以阿里云镜像为例

        1.备份原来的源：

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

        2.下载阿里云的源：

sudo curl -o /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo

        3.更新缓存：

sudo yum clean all
sudo yum makecache

 这样就可以了。如果提示找不到 curl 命令，先安装它：

sudo yum install -y curl

验证镜像源可用性

执行以下命令测试新配置是否生效：

yum repolist                 # 查看已启用的仓库
yum search nginx             # 测试软件包搜索
yum install -y telnet        # 测试安装功能