Could not resolve host: mirrors.tencentyun.com

yum安装报错，无法拉取源镜像文件
Delta RPMs disabled because /usr/bin/applydeltarpm not installed.
ruby-2.0.0.648-36.el7.x86_64.r FAILED
http://mirrors.tencentyun.com/centos/7/os/x86_64/Packages/ruby-2.0.0.648-36.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: mirrors.tencentyun.com; Unknown error"ETA
Trying other mirror.
ruby-irb-2.0.0.648-36.el7.noar FAILED
http://mirrors.tencentyun.com/centos/7/os/x86_64/Packages/ruby-irb-2.0.0.648-36.el7.noarch.rpm: [Errno 14] curl#6 - “Could not resolve host: mirrors.tencentyun.com; Unknown error”
Trying other mirror.
ruby-libs-2.0.0.648-36.el7.x86 FAILED
http://mirrors.tencentyun.com/centos/7/os/x86_64/Packages/ruby-libs-2.0.0.648-36.el7.x86_64.rpm: [Errno 14] curl#6 - “Could not resolve host: mirrors.tencentyun.com; Unknown error”
Trying other mirror.
rubygem-bigdecimal-1.2.0-36.el FAILED
http://mirrors.tencentyun.com/centos/7/os/x86_64/Packages/rubygem-bigdecimal-1.2.0-36.el7.x86_64.rpm: [Errno 14] curl#6 - “Could not resolve host: mirrors.tencentyun.com; Unknown error”

原因：腾讯云服务器内网yum源的域名 mirrors.tencentyun.com 需要有内网的DNS才能访问，但是实际情况下，我们会根据需要修改DNS，为了使用腾讯云内网快速稳定的内网源，我们需要把DNS恢复为内网DNS，下面为各地区服务器DNS地址
解决办法：
（1）修改服务器的DNS配置文件：/etc/resolv.conf ，添加对应地区的内网DNS服务器
（2）重启网络服务

# 重启方式1：
/etc/init.d/network restart
#重启方式2：
systemctl restart network

广州服务器：
nameserver 10.225.30.181
nameserver 10.225.30.223

北京服务器：
nameserver 10.53.216.182
nameserver 10.53.216.198

上海服务器：
nameserver 10.236.158.114
nameserver 10.236.158.106

上海金融服务器：
nameserver 10.48.46.77
nameserver 10.48.46.27

深圳金融服务器：
nameserver 100.83.224.91
nameserver 100.83.224.88

香港服务器：
nameserver 10.243.28.52
nameserver 10.225.30.178

新加坡服务器：
nameserver 100.78.90.19
nameserver 100.78.90.8

多伦多服务器：
nameserver 10.116.19.185
nameserver 10.116.19.188

硅谷服务器：
nameserver 100.102.22.21
nameserver 100.102.22.30