yum-install 下载文件报错，yum文件不可用

首先看报错信息如下图：
已加载插件：fastestmirror
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.

这里出现这个报错很大的原因在于我们的镜像文件有点问题，以至于yum源文件出现了问题，博主自己认为是因为yum文件太老的问题，下面是解决yum报错的方法。
第一步检查一下我们的网卡是否可用
1.查看网络接口 ip addr
2.设置我们的dns文件，位置在/etc/resolv.conf

nameserver 8.8.8.8
nameserver 8.8.4.4

3.关闭防火墙
查看防火墙 systemctl status firewalld
关闭防火墙systemctl stop firewalld
第二步，上面那些检查完应该还是无法解决yum报错
下面是重新配置yum操作
1.打开CentOS-Base.repo文件（我这里是管理员用户操作所以不需要加sudo）

vi /etc/yum.repos.d/CentOS-Base.repo

2.将CentOS-Base.repo文件里的内容全部替换为：

# CentOS-Base.repo  
#  
# 这里配置了CentOS的yum源，使用了阿里云和清华大学的镜像点。  
#  
  
[base]  
name=CentOS-$releasever - Base  
baseurl=http://mirrors.aliyun.com/centos/$releasever/os/$basearch/  
        http://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/os/$basearch/  
gpgcheck=1  
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7  
  
#released updates   
[updates]  
name=CentOS-$releasever - Updates  
baseurl=http://mirrors.aliyun.com/centos/$releasever/updates/$basearch/  
        http://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/updates/$basearch/  
gpgcheck=1  
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7  
  
#additional packages that may be useful  
[extras]  
name=CentOS-$releasever - Extras  
baseurl=http://mirrors.aliyun.com/centos/$releasever/extras/$basearch/  
        http://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/extras/$basearch/  
gpgcheck=1  
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7  
  
#additional packages that extend functionality of existing packages  
[centosplus]  
name=CentOS-$releasever - Plus  
baseurl=http://mirrors.aliyun.com/centos/$releasever/centosplus/$basearch/  
        http://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/centosplus/$basearch/  
gpgcheck=1  
enabled=0  
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7  
  
#contrib - packages by Centos Users  
[contrib]  
name=CentOS-$releasever - Contrib  
baseurl=http://mirrors.aliyun.com/centos/$releasever/contrib/$basearch/  
        http://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/contrib/$basearch/  
gpgcheck=1  
enabled=0  
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7  
  
# 使用failovermethod指定优先级，这里设置为随机（roundrobin），也可以改为priority并指定每个URL的优先级  
failovermethod=roundrobin

3.清理yum缓存并且更新

sudo yum clean all

sudo yum makecache

sudo yum update