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What is Virtual Network NAT?

Article
02/02/2023
6 minutes to read

Virtual Network NAT is a fully managed and highly resilient Network Address Translation (NAT) service. Virtual Network NAT simplifies outbound Internet connectivity for virtual networks. When configured on a subnet, all outbound connectivity uses the Virtual Network NAT’s static public IP addresses.

Figure shows a NAT receiving traffic from internal subnets and directing it to a public IP (PIP) and an IP prefix.

Figure: Virtual Network NAT

Virtual Network NAT benefits

Security

With a NAT gateway, individual VMs or other compute resources, don’t need public IP addresses and can remain private. Resources without a public IP address can still reach external sources outside the virtual network with NAT gateway’s static public IP addresses or prefixes. You can associate a public IP prefix to ensure that a contiguous set of IPs will be used for outbound. Destination firewall rules can be configured based on this predictable IP list.

Resiliency

Virtual Network NAT is a fully managed and distributed service. It doesn’t depend on individual compute instances such as VMs or a single physical gateway device. A NAT gateway always has multiple fault domains and can sustain multiple failures without service outage. Software defined networking makes a NAT gateway highly resilient.

Scalability

Virtual Network NAT is scaled out from creation. There isn’t a ramp up or scale-out operation required. Azure manages the operation of Virtual Network NAT for you.

A NAT gateway resource can be associated to a subnet and can be used by all compute resources in that subnet. All subnets in a virtual network can use the same NAT gateway resource. Outbound connectivity can be scaled out by assigning up to 16 IP addresses to NAT gateway. When a NAT gateway is associated to a public IP prefix, it automatically scales to the number of IP addresses needed for outbound.

Performance

Virtual Network NAT is a software defined networking service. A NAT gateway won’t affect the network bandwidth of your compute resources. Learn more about NAT gateway’s performance.

Virtual Network NAT basics

Outbound connectivity

Virtual Network NAT (NAT gateway) is the recommended method for outbound connectivity. NAT gateway doesn’t have the same limitations of SNAT port exhaustion as does default outbound access and outbound rules of a load balancer.
NAT gateway allows flows to be created from the virtual network to the services outside your virtual network. Return traffic from the internet is only allowed in response to an active flow. Services outside your virtual network can’t initiate an inbound connection through NAT gateway.
- To migrate outbound access to a NAT gateway from default outbound access or load balancer outbound rules, see Migrate outbound access to Azure Virtual Network NAT.
NAT gateway takes precedence over other outbound scenarios (including Load balancer and instance-level public IP addresses) and replaces the default Internet destination of a subnet.
When NAT gateway is configured to a virtual network where standard Load balancer with outbound rules already exists, NAT gateway will take over all outbound traffic moving forward. There will be no drops in traffic flow for existing connections on Load balancer. All new connections will use NAT gateway.
Presence of custom UDRs for virtual appliances and ExpressRoute override NAT gateway for directing internet bound traffic (route to the 0.0.0.0/0 address prefix).
The order of operations for outbound connectivity follows this order of precedence:
Virtual appliance UDR / ExpressRoute >> NAT gateway >> Instance-level public IP addresses on virtual machines >> Load balancer outbound rules >> default system
NAT gateway supports TCP and UDP protocols only. ICMP isn’t supported.
NAT gateway will send a TCP Rest (RST) packet to the connection endpoint that attempts to communicate on a connection flow that does not exist. This connection flow may no longer exist if the NAT gateway idle timeout was reached or the connection was closed earlier. When the NAT gateway TCP RST packet is received by the connection endpoint, this signifies that the connection is no longer usable.

NAT gateway configurations

Outbound connectivity can be defined for each subnet with a NAT gateway. All outbound traffic for the subnet is processed by the NAT gateway without any customer configuration.
A NAT gateway can’t span multiple virtual networks.
Multiple subnets within the same virtual network can either use different NAT gateways or the same NAT gateway.
Multiple NAT gateways can’t be attached to a single subnet.
A NAT gateway can’t be deployed in a gateway subnet.
A NAT gateway resource can use up to 16 IP addresses in any combination of:
- Public IP addresses
- Public IP prefixes
- Public IP addresses and prefixes derived from custom IP prefixes (BYOIP), to learn more, see Custom IP address prefix (BYOIP).
NAT gateway can’t be associated to an IPv6 public IP address or IPv6 public IP prefix. It can be associated to a dual stack subnet, but will only be able to direct outbound traffic with an IPv4 address.
NAT gateway can be used to provide outbound connectivity in a hub and spoke model when associated with Azure Firewall. NAT gateway can be associated to an Azure Firewall subnet in a hub virtual network and provide outbound connectivity from spoke virtual networks peered to the hub. To learn more, see Azure Firewall integration with NAT gateway.

Availability zones

A NAT gateway can be created in a specific availability zone or placed in ‘no zone’.
NAT gateway can be isolated in a specific zone when you create zone isolation scenarios. This deployment is called a zonal deployment. After NAT gateway is deployed, the zone selection can’t be changed.
NAT gateway is placed in no zone by default. A non-zonal NAT gateway is placed in a zone for you by Azure.

NAT gateway and basic SKU resources

NAT gateway is compatible with standard SKU public IP addresses or public IP prefix resources or a combination of both. You can use a public IP prefix directly or distribute the public IP addresses of the prefix across multiple NAT gateway resources. The NAT gateway will groom all traffic to the range of IP addresses of the prefix.
Basic resources, such as basic load balancer or basic public IPs aren’t compatible with Virtual Network NAT. Basic resources must be placed on a subnet not associated to a NAT gateway. Basic load balancer and basic public IP can be upgraded to standard to work with a NAT gateway
- Upgrade a load balancer from basic to standard, see Upgrade a public basic Azure Load Balancer.
- Upgrade a public IP from basic to standard, see Upgrade a public IP address.

NAT gateway timers

NAT gateway holds on to SNAT ports after a connection closes before it’s available to reuse to connect to the same destination endpoint over the internet. SNAT port reuse timer durations for TCP traffic vary depending on how the connection closes. To learn more, see Port Reuse Timers.
A default TCP idle timeout of 4 minutes is used and can be increased to up to 120 minutes. Any activity on a flow can also reset the idle timer, including TCP keepalives. To learn more, see Idle Timeout Timers.
UDP traffic has an idle timeout timer of 4 minutes that can’t be changed.
UDP traffic has a port reset timer of 65 seconds for which a port is in hold down before it’s available for reuse to the same destination endpoint.