1.CentOS yum出现无法使用的情况
1 问题产生
新安装好的虚拟机无法使用yum命令来安装wget，一直提示报错。
已加载插件：fastestmirror
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. 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
如果这是用是用ping命令来ping公网发现网络通常，一般就是插件或者镜像源配置错误，如果不通则是网络配置错误。
2 解决方案
2.1 禁用插件
一开始搜索网络博客和询问大佬得知是yum的配置问题，在/etc/yum/pluginconf.d的目录下，可能有subscription-manager .conf或者fastestmirror.conf这两个插件，vi编辑这两个插件，将其中的enable属性改为0来禁用插件。
禁用插件之后依然使用不了yum命令，并且出现新的报错。即可往下查看解决方案。
cd /etc/yum/pluginconf.d
vi fastestmirror.conf
# 修改后展示
enabled=0
verbose=0
always_print_best_host = true
socket_timeout=3
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 2a05:d012:8b5:6503:9efb:5cad:348f:e826: 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. 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
2.2 修改网络配置是否
找到/etc/sysconfig/network-scripts/ifcfg-你的网卡名字，可以通过ip addr命令来进行查看，将onboot改为yes。
2.3 配置/etc/resolv.conf
vi /etc/resolv.conf
在文件中加入
nameserver 8.8.8.8
nameserver 8.8.4.4
2.4 配置epel.repo中的镜像源（更新yum源）
2.4.1 阿里
备份当前镜像源。
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
下载新的CentOS-Base.repo 到/etc/yum.repos.d/
CentOS 5
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-5.repo
或者
curl -o /etc/yum.repos.d/CentOS-Base.repo
CentOS 6
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo
或者
curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo
CentOS 7
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
或者
curl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
除了下载方式还可以进行修改文件内容。
1、打开 /etc/yum.repos.d/epel.repo；
vim /etc/yum.repos.d/epel.repo
2、注释掉mirrorlist，取消注释baseurl；
将
[epel]
name=Extra Packages for Enterprise Linux 7 - $basearch
#baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch
mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=$basearch
修改为
[epel]
name=Extra Packages for Enterprise Linux 7 - $basearch
baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch
#mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=$basearch
修改完成之后再次使用yum命令即可。
yum clean all
yum makecache
yum update
2.4.2 网易
备份当前镜像源。
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
下载对应版本的repo文件。
http://mirrors.163.com/.help/CentOS7-Base-163.repo
http://mirrors.163.com/.help/CentOS6-Base-163.repo
http://mirrors.163.com/.help/CentOS5-Base-163.repo
运行yum命令。
yum clean all
yum makecache
yum update