Hello, my friends.
Help me please.
I have oracle linux server 6.5
Command 'ifconfig -a' shows me good network settings.
Command 'netstat -rn' shows me good gateway.
Network was workable before 'yum update'.
And now in gui I can see: 'No network connection', 'Wired network - disconnected'.
I tried to start my eth0 with 'system-config-network', and result is:
Failed to run:
/sbin/ifip eth0 up
Error: Connection activation failed:
Device not managed by NetworkManager
or unavailable

In file /proc/net/dev after 'eth0:' I can see only zeroes.

How can I resolve my problem?

Look in /etc/sysconfig/network-scrips/ifcfg-eth0
and notice that NetworkManager is set to No.

Set it to yes, and then it will be managed by NetworkManager, which you will be able to control from the GUI.

Hope that helps.

Reinstallation of Linux helped me.