If you add a ethernet port to a Linux system and ifconfig doesn't show it, (ex: A usb ethernet device is plugged in ), this is how to create it by hand.
1) Connect the ethernet device.
2) In the CLI type lspci and look for your new device. you can use grep to narrow the list of devices
EX: # lspci | grep -i Ethernet
3) After you confirm its listed, cd to /etc/sysconfig/network-scripts and do a ls
4) cp ifcfg-ethXX (where XX is the number of a network connect you want to clone) to ifcfg-eth?? (where ?? is the next number higher in the list of ifcg-eth's)
5) vi to your newly created ifcfg-eth?? file and remove the MAC and UUID lines.
6) Next rename the name of the port to match the file name then type :wq to write and quit.
7) at this point you can restart or type service network restart, then you are done.