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By default, when a gateway is attached to a router using the Neutron L3 extension, Network Address Translation (NAT) is enabled for traffic generated by subnets attached to the router. With this extension, the user will have the option of choosing whether SNAT should be enabled or not on a router basis. This is achieved simply by specifying a boolean attribute, enable_snat, in the external_gateway_info attribute of the router resource. This extension redefines the external_gateway_info attribute as depicted in the following table.

external_gateway_info attributes

Attribute	Type	Required	Default Value	Validation Constraints	Notes
network_id	UUID	Yes	N/A	Must be a valid uuid representative of an external network.
enable_snat	Boolean	No	True	{True|False}	The default setting is True to ensure backward compatibility for plugins supporting this extension.

SNAT can be enabled or disabled at any time on a Neutron router regardless of the current status of floating IPs. Floating IPs will continue working even when SNAT is disabled.

Verb URI Description &GET; /routers Retrieve Neutron routers Success and error response codes are not changed with regards to the operation as introduced by the L3 API extension. When this extension is enabled, this operation will also return the current Source NAT status for configured routers, as shown in the sample below. The response for the show router operation is the same, with the obvious exception that a single router is returned.

Verb URI Description &POST; /routers Create a new Neutron router Success and error response codes are not changed with regards to the operation as introduced by the L3 API extension. Neutron API users can specify whether SNAT should be performed on the network specified as the router's external gateway by setting enable_snat in external_gateway_info to either True or False; the default value is True.

Verb URI Description &PUT; /routers/router_id Create a new Neutron router Success and error response codes are not changed with regards to the operation as introduced by the L3 API extension. Neutron API users can enable or disable SNAT on a router specifying the enable_snat attribute in the external_gateway_info attribute for the router resource. This operation can be either used for updating the SNAT status only, the external network, or both attributes at the same time. In any case, if the enable_snat attribute is not specified, it will default to True. For instance, if the current SNAT status is disabled, and the router's gateway is updated to a different external network without specifying enable_snat, SNAT will be enabled for the new network. It is important to note that whenever updating a router's external gateway information, the network_idparameter must be specified always, even if the final goal is just to enable or disable SNAT for the router on the same external network. The rest of this section provides some samples for updating a router's external gateway info with SNAT mode.