centos6.5 yum update 报错Couldn't resolve host 'centos.ustc.edu.cn'

异常信息

[root@localhost ~]# yum -y update
Loaded plugins: fastestmirror, refresh-packagekit, security
Setting up Update Process
Loading mirror speeds from cached hostfile
 * base: centos.ustc.edu.cn
 * extras: ftp.sjtu.edu.cn
 * updates: centos.ustc.edu.cn
http://centos.ustc.edu.cn/centos/6.8/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 6 - "Couldn't resolve host 'centos.ustc.edu.cn'"
Trying other mirror.
http://ftp.sjtu.edu.cn/centos/6.8/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 6 - "Couldn't resolve host 'ftp.sjtu.edu.cn'"
Trying other mirror.
http://mirror.bit.edu.cn/centos/6.8/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 6 - "Couldn't resolve host 'mirror.bit.edu.cn'"
Trying other mirror.
http://mirrors.163.com/centos/6.8/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 6 - "Couldn't resolve host 'mirrors.163.com'"
Trying other mirror.
http://mirrors.aliyun.com/centos/6.8/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 6 - "Couldn't resolve host 'mirrors.aliyun.com'"
Trying other mirror.
http://mirrors.cn99.com/centos/6.8/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 6 - "Couldn't resolve host 'mirrors.cn99.com'"
Trying other mirror.

.....

sssd-krb5-common-1.13.3-22.el6_8.6.x86_64: failure: Packages/sssd-krb5-common-1.13.3-22.el6_8.6.x86_64.rpm from updates: [Errno 256] No more mirrors to try.
  nss-tools-3.21.3-2.el6_8.x86_64: failure: Packages/nss-tools-3.21.3-2.el6_8.x86_64.rpm from updates: [Errno 256] No more mirrors to try.
  iscsi-initiator-utils-6.2.0.873-22.el6_8.x86_64: failure: Packages/iscsi-initiator-utils-6.2.0.873-22.el6_8.x86_64.rpm from updates: [Errno 256] No more mirrors to try.
  db4-4.7.25-20.el6_8.1.x86_64: failure: Packages/db4-4.7.25-20.el6_8.1.x86_64.rpm from updates: [Errno 256] No more mirrors to try.

 1.服务器解析不了域名, ping mirrors.ustc.edu.cn 这个地址,发现 unknown host mirrors.ustc.edu.cn,ping www.baidu.com同样不行

[root@localhost ~]# ping mirrors.ustc.edu.cn
ping: unknown host mirrors.ustc.edu.cn

2.查看 /etc/resolv.conf 文件,发现没有设置域名服务

[root@localhost ~]# cat /etc/resolv.conf
# Generated by NetworkManager


# No nameservers found; try putting DNS servers into your
# ifcfg files in /etc/sysconfig/network-scripts like so:
#
# DNS1=xxx.xxx.xxx.xxx
# DNS2=xxx.xxx.xxx.xxx
# DOMAIN=lab.foo.com bar.foo.com
search localhost

3.查看网络配置文件,发现没有配置默认网关

[root@localhost ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
IPADDR=192.168.200.105
NETMASK=255.0.0.0
NETWORK=127.0.0.0
# If you're having problems with gated making 127.0.0.0/8 a martian,
# you can change this to something else (255.255.255.255, for example)
BROADCAST=127.255.255.255
ONBOOT=yes
NAME=eth0

3.查看当前网关信息,发现默认网关不对

[root@localhost ~]# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
link-local      *               255.255.0.0     U     1002   0        0 eth0
192.0.0.0       *               255.0.0.0       U     0      0        0 eth0

4.添加默认网关（虚拟机选择桥接模式配置下面信息）

vi /etc/sysconfig/network-scripts/ifcfg-eth0

DEVICE=eth0 #网卡名称
#HWADDR=00:0C:29:3B:A3:99 MAC地址可以不写(与 /etc/udev/rules.d/70-persistent-net.rules 文件中的 ATTR{address}值一致就行)
TYPE=Ethernet #网络类型（通常是Ethemet）
UUID=3259a85a-c34c-4ecf-98e8-e1fe495c899f #随机数,唯一标识
ONBOOT=yes #设置为yes,开机自动启用网络连接
NM_CONTROLLED=yes #network mamager的参数 ,是否可以由NNetwork Manager托管
BOOTPROTO=static #设置为none禁止DHCP，设置为static启用静态IP地址，设置为dhcp开启DHCP服务
DNS1=192.168.200.1 #DNS 配置
IPV6INIT=no #是否启用IPv6的接口
USERCTL=no #是否允许非root用户控制该设备,设置为no,只能用root用户更改
IPADDR=192.168.200.200 #指定静态ip地址
NETMASK=255.255.255.0 #子网掩码
GATEWAY=192.168.200.1 #默认网关

windows查看网关

控制面板 -->网络和Internet -->网络和共享中心 -->本地连接 -->详细信息

如图

5.重启网络

service network restart