一、出现问题

出现报错信息：

[root@xsnq ~] yum -y install wget
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=
x86_64&repo=os&infra=genclo error was 12: Timeout on http://mirrorlist.cento
s.org/?release=7&arch=x86_64&repo=os&infra=genclo:
 (28, 'Resolving timed out after 30553 milliseconds')

 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

二、产生原因

报错信息为解析超时，说明是网络连接原因。

连接不上的原因可能有：

1. DNS解析失败
2. 信号不好
3. 网络中断等

三、解决方法

1.修改resolv.conf

[root@localhost ~] vim /etc/resolv.conf

添加一个nameserver：

最好不要修改nameserver，可以把想先使用的nameserver加在其他nameserver的前面，作为主用DNS。

nameserver 8.8.8.8                              更适合国外以及访问国外网站的用户使用
或者
nameserver 114.114.114.114                      更适合国内用户

至此问题已解决（修改后会立即生效，不需做任何额外处理；如果是修改的ifcfg-eth 的文件需要重启网卡才生效）

注：读取时是按nameserver 在本文件中的顺序进行，当第一个nameserver 没有反应时才查询下面的nameserver

2.查看dns

[root@localhost ~] cat /etc/resolv.conf

扩展

全国各DNS查询：点击此处

参考文献：IP.cn查询网；https://ip.cn/dns.html

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