常见错误1：curl#6 - “Could not resolve host: mirrorlist.centos.org; 未知的错误”

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

常见错误2： [Errno 256] No more mirrors to try

Downloading packages:
Delta RPMs disabled because /usr/bin/applydeltarpm not installed.
lrzsz-0.12.20-36.el7.x86_64.rp FAILED                                          
http://mirror.bit.edu.cn/centos/7.6.1810/os/x86_64/Packages/lrzsz-0.12.20-36.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: mirror.bit.edu.cn; Unknown error"
正在尝试其它镜像。
lrzsz-0.12.20-36.el7.x86_64.rp FAILED                                          
http://mirror.lzu.edu.cn/centos/7.6.1810/os/x86_64/Packages/lrzsz-0.12.20-36.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: mirror.lzu.edu.cn; Unknown error"
正在尝试其它镜像。
lrzsz-0.12.20-36.el7.x86_64.rp FAILED                                          
http://mirrors.163.com/centos/7.6.1810/os/x86_64/Packages/lrzsz-0.12.20-36.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: mirrors.163.com; Unknown error"
正在尝试其它镜像。
lrzsz-0.12.20-36.el7.x86_64.rp FAILED                                          
http://mirrors.cn99.com/centos/7.6.1810/os/x86_64/Packages/lrzsz-0.12.20-36.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: mirrors.cn99.com; Unknown error"
正在尝试其它镜像。
lrzsz-0.12.20-36.el7.x86_64.rp FAILED                                          
http://mirrors.cqu.edu.cn/CentOS/7.6.1810/os/x86_64/Packages/lrzsz-0.12.20-36.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: mirrors.cqu.edu.cn; Unknown error"
正在尝试其它镜像。
lrzsz-0.12.20-36.el7.x86_64.rp FAILED                                          
http://mirrors.huaweicloud.com/centos/7.6.1810/os/x86_64/Packages/lrzsz-0.12.20-36.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: mirrors.huaweicloud.com; Unknown error"
正在尝试其它镜像。
lrzsz-0.12.20-36.el7.x86_64.rp FAILED                                          
http://mirrors.neusoft.edu.cn/centos/7.6.1810/os/x86_64/Packages/lrzsz-0.12.20-36.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: mirrors.neusoft.edu.cn; Unknown error"
正在尝试其它镜像。
lrzsz-0.12.20-36.el7.x86_64.rp FAILED                                          
http://mirrors.njupt.edu.cn/centos/7.6.1810/os/x86_64/Packages/lrzsz-0.12.20-36.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: mirrors.njupt.edu.cn; Unknown error"
正在尝试其它镜像。
lrzsz-0.12.20-36.el7.x86_64.rp FAILED                                          
http://mirrors.aliyun.com/centos/7.6.1810/os/x86_64/Packages/lrzsz-0.12.20-36.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: mirrors.aliyun.com; Unknown error"
正在尝试其它镜像。
lrzsz-0.12.20-36.el7.x86_64.rp FAILED                                          
http://mirrors.zju.edu.cn/centos/7.6.1810/os/x86_64/Packages/lrzsz-0.12.20-36.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: mirrors.zju.edu.cn; Unknown error"
正在尝试其它镜像。

Error downloading packages:
  lrzsz-0.12.20-36.el7.x86_64: [Errno 256] No more mirrors to try.

解决办法

第一步：检查网络

（1）使用命令ping，测试网络与互联网是否联通

ping www.baidu.com

（2）如果（1）不能联通，则进行这一步，很有可能是虚拟机网络配置问题。

ping <网关>

第二步：检查yum文件配置

vi /etc/resolv.conf

检查看一下文件里面有没有对应配置内容，如果没有请加上：

添加以下内容，增加网关IP

nameserver <网关>

或者增加本机IP

nameserver 8.8.8.8
nameserver 8.8.4.4

为什么要增加DNS服务器呢？
8.8.8.8和8.8.4.4是google公司提供的两台DNS服务器，可以很好的解析域名。因为linux自带默认的yum源地址是以域名的形式存在的，所以增加google的域名解析服务器，就可以解决解析的问题了。

第三步重新制作

在设置完以后一定要重置一下yum

yum clean all
yum makecache