If the Ethernet network connection on your Mac running OS X 10.11 El Capitan stopped working in the last week, you are not alone. Apple released an update to the kernel extension blacklist that is used by the System Integrity Protection feature. This update installs automatically in the background, so there was nothing to approve or help people be aware of the cause of the problem.
The System Integrity Protection feature utilizes the blacklist to protect from harmful or problematic kernel extensions. This problematic update included the kernel extension information for Apple’s Ethernet drivers, meaning you can no longer connect to the Internet using Ethernet. Wifi should continue to work since it has separate drivers, and with its connectivity, the blocklist will update and resolve the issue. If you received the bad update but did not restart before the fix was released, your Ethernet would continue working until your next reboot, so you might receive the fix as well.
By going to About this Mac… under the Apple menu, and choosing System Report, you can scroll down to Software, Installations. Look for the entry ‘Incompatible Kernel Extension Configuration Data’ and its version number. If it is version 3.28.1, you have the problematic version. 3.28.2 is the fixed release.
If you can get an Internet connection, you can force an update with the command and root privileges:
sudo softwareupdate --background
If you cannot get an Internet connection (you do not have WiFi), Apple has steps to resolve the problem from OS X Recovery in the support article for this issue. You can also choose to reinstall OS X on the drive without reformatting to fix the issue.