CentOS7.x以上版本配置DNS失效解决办法

这2周做实验，centos7.x经常出现yum安装软件包的时候找不到解析地址，提示如下错误

正在尝试其它镜像。

Error downloading packages:
pam-devel-1.1.8-22.el7.x86_64: [Errno 256] No more mirrors to try.
libcom_err-1.42.9-12.el7_5.x86_64: [Errno 256] No more mirrors to try.
keyutils-libs-devel-1.5.8-3.el7.x86_64: [Errno 256] No more mirrors to try.
libverto-devel-0.2.5-4.el7.x86_64: [Errno 256] No more mirrors to try.
e2fsprogs-1.42.9-12.el7_5.x86_64: [Errno 256] No more mirrors to try.
pcre-devel-8.32-17.el7.x86_64: [Errno 256] No more mirrors to try.
krb5-devel-1.15.1-19.el7.x86_64: [Errno 256] No more mirrors to try.
libcom_err-devel-1.42.9-12.el7_5.x86_64: [Errno 256] No more mirrors to try.
lzo-devel-2.06-8.el7.x86_64: [Errno 256] No more mirrors to try.
krb5-libs-1.15.1-19.el7.x86_64: [Errno 256] No more mirrors to try.
zlib-devel-1.2.7-17.el7.x86_64: [Errno 256] No more mirrors to try.
libsepol-devel-2.5-8.1.el7.x86_64: [Errno 256] No more mirrors to try.
▽libss-1.42.9-12.el7_5.x86_64: [Errno 256] No more mirrors to try.
libselinux-devel-2.5-12.el7.x86_64: [Errno 256] No more mirrors to try.
1:openssl-devel-1.0.2k-12.el7.x86_64: [Errno 256] No more mirrors to try.
e2fsprogs-libs-1.42.9-12.el7_5.x86_64: [Errno 256] No more mirrors to try.
libkadm5-1.15.1-19.el7.x86_64: [Errno 256] No more mirrors to try.
lzo-minilzo-2.06-8.el7.x86_64: [Errno 256] No more mirrors to try.

查看 /etc/resolv.conf 中DNS配置，竟然为空，之前明明配置好了的，真是奇怪，难怪不行

[root@opvnserver ~]# cat /etc/resolv.conf    
# Generated by NetworkManager        #<==啥也没有

百度和Google了一下，NetworkManager与network可能会出现冲突的情况，由于做实验要添加网卡，重启服务，于是重新配置好DNS文件

[root@opvnserver ~]# cat /etc/resolv.conf
# Generated by NetworkManager
nameserver 202.96.128.86
nameserver 202.96.128.166

然后关闭NetworkManager，这样就不会出现之前的情况了，重新yum安装软件也正常了

[root@opvnserver ~]# systemctl stop NetworkManager
[root@opvnserver ~]# systemctl disable NetworkManager
[root@opvnserver ~]# systemctl restart network

更好的方法：在网卡配置文件中添加DNS地址，格式如下：

[root@opvnserver ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
TYPE=Ethernet
BOOTPROTO=static
DEFROUTE=yes
IPV4_FAILURE_FATAL=no
NAME=eth0
DEVICE=eth0
ONBOOT=yes
IPADDR=10.0.0.3
GATEWAY=10.0.0.254
NETMASK=255.255.255.0
DNS1=202.96.128.86
DNS2=202.96.128.166

注意：在centos7.x 中，DNS不在 /etc/resolv.conf 中配置了，而是直接在网卡配置文件中配置DNS，这和centos6.x版本有点区别，真是无语，改来改去，呵呵！