主机网络ping: unknown host baidu.com问题解决

本机环境：

系统：Centos 网络：NAT

虚拟机之前一直都可以连外网，但最近不能连了，现状如下：

[root@vhost03 ~]# ping baidu.com
ping: unknown host baidu.com

查看了下本机的网卡配置：

[root@vhost03 ~]# more /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
BOOTPROTO="static"
HWADDR="00:0C:29:D7:D2:49"
IPV6INIT="yes"
NM_CONTROLLED="yes"
ONBOOT="yes"
TYPE="Ethernet"
UUID="f08dcf57-d670-46b3-8298-240cd0b76bc2"
IPADDR=192.168.196.129
BROADCAST=192.168.196.255
NETMASK=255.255.255.0
GATEWAY=192.168.1.1

新装的虚拟机默认都是dhcp方式获取IP。为了保证IP固定，自己更新了网卡信息，使用了static模式。其中最后四行是按照dhcp模式时，使用ifconfig命令获取的信息。

查看DNS信息：

[root@vhost03 ~]# more /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

上述结果表示并没有配置DNS。

正确的解决方式：

[root@vhost03 ~]# cd /etc/sysconfig/network-scripts/

[root@vhost03 network-scripts]# cp ifcfg-eth0 ifcfg-eth0.bak
[root@vhost03 network-scripts]# vi ifcfg-eth0

DEVICE="eth0"
BOOTPROTO="dhcp"
HWADDR="00:0C:29:D7:D2:49"
IPV6INIT="yes"
NM_CONTROLLED="yes"
ONBOOT="yes"
TYPE="Ethernet"

[root@vhost03 Desktop]# service network restart

[root@vhost03 Desktop]# ifconfig
eth0      Link encap:Ethernet  HWaddr :0C::D7:D2:
          inet addr:192.168.196.135  Bcast:192.168.196.255  Mask:255.255.255.0
          inet6 addr: fe80::20c:29ff:fed7:d249/ Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:  Metric:
          RX packets: errors: dropped: overruns: frame:
          TX packets: errors: dropped: overruns: carrier:
          collisions: txqueuelen:
          RX bytes: (140.3 MiB)  TX bytes: (3.4 MiB)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::/ Scope:Host
          UP LOOPBACK RUNNING  MTU:  Metric:
          RX packets: errors: dropped: overruns: frame:
          TX packets: errors: dropped: overruns: carrier:
          collisions: txqueuelen:
          RX bytes: (1.0 KiB)  TX bytes: (1.0 KiB)

[root@vhost02 ~]#  netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
192.168.196.0   0.0.0.0         255.255.255.0   U                     eth0
169.254.0.0     0.0.0.0         255.255.0.0     U                     eth0
0.0.0.0         192.168.196.2   0.0.0.0         UG                    eth0

[root@vhost03 Desktop]# more /etc/resolv.conf
# Generated by NetworkManager
domain localdomain
search localdomain
nameserver 192.168.196.2

根据上面的信息重新配置ifcfg-eth0文件

[root@vhost03 ~]# cd /etc/sysconfig/network-scripts/

[root@vhost03 network-scripts]# vi ifcfg-eth0

DEVICE="eth0"
BOOTPROTO="static"
HWADDR="00:0C:29:D7:D2:49"
IPV6INIT="yes"
NM_CONTROLLED="yes"

ONBOOT="yes"
TYPE="Ethernet"
UUID="f08dcf57-d670-46b3-8298-240cd0b76bc2"
IPADDR=192.168.196.129
BROADCAST=192.168.196.255
NETMASK=255.255.255.0
GATEWAY=192.168.196.2

[root@vhost03 ~]# cd /etc/sysconfig/network-scripts/

[root@vhost03 network-scripts]# service network restart

[root@vhost03 network-scripts]# vi /etc/resolv.conf 

# Generated by NetworkManager
domain localdomain
search localdomain
nameserver 192.168.196.2

[root@vhost03 ~]# ping www.baidu.com
PING www.a.shifen.com (115.239.210.27) () bytes of data.
 bytes from 115.239.210.27: icmp_seq= ttl= time=6.46 ms
 bytes from 115.239.210.27: icmp_seq= ttl= time=7.00 ms

ok,问题解决。分析下来，看来是之前获取的历史网卡信息变了，导致不能ping外网。

之前有在其它网络环境下重启过宿主机，应该和这个有关。解决问题的时候发现，只要重启网卡就会出现/etc/resolv.conf 被重置的现象。这是因为

[root@vhost02 ~]# chkconfig --list |grep -i network
NetworkManager  0:off   1:off   2:on    3:on    4:on    5:on    6:off
network         0:off   1:off   2:on    3:on    4:on    5:on    6:off

[root@vhost02 ~]# service NetworkManager stop

NetworkManager进程会实时修改/etc/resolv.conf 中的信息