1. Remove Network Manager from startup Services.
chkconfig NetworkManager off
2. Add Default Net Manager
chkconfig network on
3.Stop NetworkManager first
service NetworkManager stop
4.and then start Default Manager
service network start
显示RTNETLINK answers: File exists
Error:Connection activation failed:Device not managed by NetworkManager
原文：http://www.cnblogs.com/LinuxSuDa/p/4325697.html