Configuring the Ethernet NIC in CentOS 6 (RHEL 6, Scientific Linux 6)

Take a look at your ifcfg-ethX configuration file under /etc/sysconfig/network-scripts directory (substitute ethX with your network interface, e.g. eth0). In my case, "Minimal" install of CentOS 6 configured the ethernet interface not to start automatically (line ONBOOT="no") and to rely on NetworkManager (NM_Controlled="yes"). This is odd, as NetworkManager is not installed by default. Thus, even though the system is in runlevel 3 and you should have networking (if you NIC is supported), you won’t have it - you can start the network with "service network start", but you won’t be able to do the same for NetworkManager, as no such initialization script is available for it. The easiest solution would be to set the following values in ifcfg-ethX (don’t forget to bring down the network interface first, by issuing "ifdown ethX", or another way):

Else, you will first have to install NetworkManager package, available from OS installation DVD.

Also, as you can note from your ifcfg-ethX file, by default, configuration via DHCP is not enabled. You may need to add the following line to enable it (again, don’t forget to bring down the network interface first, if it’s up):

Kudos to the following CentOS forums thread for tips:
https://www.centos.org/modules/newbb...32163&forum=55