The security of network traffic is critical, and OpenStack supports two mechanisms to secure network traffic. Security Groups allow traffic within a tenant's network to be secured. Linux iptables on the compute nodes are used to implement OpenStack security groups.
The traffic that goes outside of a tenant's network, to another network or the Internet, is secured using the OpenStack firewall service functionality. Like routing, firewall is a service with Neutron. The firewall service also uses iptables, but the scope of iptables is limited to the OpenStack router used as part of the firewall service.
The following diagram describes at a high level how iptables are used to secure network traffic:
In this network ...