Source NAT, or SNAT for short, is the method of changing the source address of a packet as it leaves the interface of a router. When a Neutron router is allocated an IP address from an external network using the
router-gateway-set command, the IP is used in source NAT operations. The source IP of traffic from virtual machine instances to external networks will be translated as the router's address when the instances do not have 1-to-1 floating IPs configured. All routers in Neutron, whether they are standalone, HA, or distributed, support SNAT.
As of the Kilo release of OpenStack, routers that handle SNAT are centralized on a single node and are not highly available. As a workaround, multiple nodes may be configured in