Apple issues a fix for ethernet diabled by a recent OS X update
On the desktop, though, Apple still includes an ethernet port. Recently, Apple issued a minor kernel extension update for OS X, but the update had a bug that disabled ethernet. Apple fixed the offending update, but if you installed the update and still see problems, Apple has a support article for troubleshooting and fixing your disabled ethernet.
If you have an ethernet and Wi-Fi working at the same time, you may not have noticed your ethernet connection not working. So it’s worth a few minutes of your time to check your connection.
Follow these steps to see if the update was installed on your Mac.
If the version is 3.28.1, you have the bad update and will need to get the latest version.
Wi-Fi still works, and you’ll use it to get the update. Get connected over Wi-Fi and follow these steps.
This will update Incompatible Kernel Extension Configuration Data to version 3.28.2, which will correct the problem.
If you can’t use Wi-Fi, the fix is more complicated. It involves booting into Recovery Mode, using Disk Utility to mount your Mac’s internal drive, running Terminal, entering a command to fix ethernet, restarting, and then using an ethernet connection to get the fixed update. Apple has the complete instructions.