If you are using PXE to liveboot Ubuntu OS for rescue or for other purpose than installation, you’ll never face this issue as Ubuntu works great over PXE.
But if you Install Ubuntu using PXE, you may not get network connectivity on freshly installed Ubuntu OS.
In network settings, you’ll find Wired Unmanaged
To resolve this, all you need is to remove all other interface except lo from /etc/network/interfaces
sudo cat /etc/network/interfaces #working config
auto lo
iface lo inet loopback
sudo cat /etc/network/interfaces #previous non-working config
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet manual
Once you are done with necessary changes, restart netwok-manager
sudo service network-manager restart
