The wired network suddenly did not work yesterday. I can not update the server. After a detailed checking, I found the DNS that a DHCP server gave was the issue. I can ping 126.96.36.199, but I cannot ping http://www.google.com.
When I modified the DNS to correct ones. vi /etc/resolv.conf
And then restart the network. /etc/init.d/network restart
The DNS server was modified to previous data.
The solution is set PEERDNS=no when vi /etc/sysconfig/network-scripts/ifcfg-em1. The file name of ifcfg-** may be different according to different server. Then restart the network. The modified DNS server would not be changed anymore.
Got help from http://ask.xmodulo.com/configure-static-dns-centos-fedora.html.