當系統提示：

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

時，首先看看能否ping通路由器，並且路由器能夠連接上外網

[root@localhost ~]# ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=128 time=123 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=128 time=175 ms
64 bytes from 8.8.8.8: icmp_seq=3 ttl=128 time=196 ms
^C
--- 8.8.8.8 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2001ms
rtt min/avg/max/mdev = 123.681/165.315/196.354/30.601 ms

如果網絡安全沒有問題且可以ping通

確定已經設置了域名服務器, 如果沒有, 推薦設置DNS服務,

[root@localhost ~]# cat /etc/resolv.conf
search localdomain

因為我的DNS沒有設置所以導致了ping外網ping不通。

[root@localhost ~]# vi /etc/resolv.conf

nameserver （自己的IP）

或者加入（網關）

確保網關已設置 ：

[root@localhost ~]# grep GATEWAY /etc/sysconfig/network-scripts/ifcfg*
/etc/sysconfig/network-scripts/ifcfg-eno16777736:GATEWAY=192.168.100.2

如果未設置, 可以用以下方式增加網關:

# route add default gw 192.168.100.2

確定可用dns解析

# grep hosts /etc/nsswitch.conf

[root@localhost ~]# grep hosts /etc/nsswitch.conf
#hosts: db files nisplus nis dns
hosts: files dns

修復了以上問題，在嘗試用ping一下www.baidu.com

[root@localhost ~]# ping www.baidu.com
PING www.a.shifen.com (183.232.231.173) 56(84) bytes of data.
64 bytes from 183.232.231.173: icmp_seq=1 ttl=128 time=31.9 ms
64 bytes from 183.232.231.173: icmp_seq=2 ttl=128 time=26.0 ms
64 bytes from 183.232.231.173: icmp_seq=3 ttl=128 time=26.0 ms
64 bytes from 183.232.231.173: icmp_seq=4 ttl=128 time=25.4 ms
64 bytes from 183.232.231.173: icmp_seq=5 ttl=128 time=25.6 ms
64 bytes from 183.232.231.173: icmp_seq=6 ttl=128 time=37.5 ms
64 bytes from 183.232.231.173: icmp_seq=7 ttl=128 time=27.3 ms

LINUX 解決linux ping: unknown host www.baidu.com