VM instances cannot connect to the external network or the Internet

If the virtual machine instances in OpenStack that were created using Heat are not able to communicate with the outside world, then there is a chance that the following configuration is missing in the Nova configuration file:

flat_interface = eth0
public_interface = eth0

These parameters should be set properly according to the physical interfaces on the host machine. In this example, it is assumed that eth0 is the public facing interface on a physical host where all-in-one OpenStack is installed.

It is necessary to adjust the name of the physical interface according to the actual configuration on the physical host machine.

If these configuration parameters are not set properly, then Nova will not be able to make entries for the IPTable rules, which might be required in order for the newly created instance to be able communicate to the Internet. Secondly, it is important to enable IP forwarding and making it persistent by using...