Symptoms: after changing hardware (motherboard, NIC, virtual machine setup etc.) network connections are no longer available. ifconfig does not list the connections, although the adapters are displayed by ifconfig -a. ifup eth0 returns “Ignoring unknown interface eth0=eth0.”
Problem: udev stores device rules about assigning a device name (e.g. eth0) to an interface with a certain MAC address. You may have an auto eth0 connection defined in /etc/network/interfaces, but it is not activated because a new interface name (not eth0) is generated and assigned to the new device with an unknown MAC address.
Solution: remove previous entries from udev config:
sudo nano /etc/udev/rules.d/70-persistent-net.rules