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Azure NAT Gateway considerations for multitenancy

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Azure NAT Gateway provides control over outbound network connectivity from your resources that are hosted within an Azure virtual network. In this article, we review how NAT Gateway can mitigate Source Network Address Translation (SNAT) port exhaustion, which can affect multitenant applications. We also review how NAT Gateway assigns static IP addresses to the outbound traffic from your multitenant solution.

Note

Firewalls, like Azure Firewall, enable you to control and log your outbound traffic. Azure Firewall also provides similar SNAT port scale and outbound IP address control to NAT Gateway. NAT Gateway is less costly, but it also has fewer features and is not a security product.

Features of NAT Gateway that support multitenancy

High-scale SNAT ports

SNAT ports are allocated when your application makes multiple concurrent outbound connections to the same public IP address, on the same port. SNAT ports are a finite resource within load balancers. If your application opens large numbers of separate connections to the same host, it can consume all of the available SNAT ports. This situation is called SNAT port exhaustion.

In most applications, SNAT port exhaustion indicates that your application is incorrectly handling HTTP connections or TCP ports. However, some multitenant applications are at particular risk of exceeding SNAT port limits, even if they reuse connections appropriately. For example, this situation can occur when your application connects to many tenant-specific databases behind the same database gateway.

Tip

If you observe SNAT port exhaustion in a multitenant application, you should verify whether your application follows good practices. Ensure you reuse HTTP connections and don't recreate new connections every time you connect to an external service. You might be able to deploy a NAT Gateway to work around the problem, but if your code doesn't follow the best practices, you could encounter the problem again in the future.

The issue is exacerbated when you work with Azure services that share SNAT port allocations between multiple customers, such as Azure App Service and Azure Functions.

If you determine you're experiencing SNAT exhaustion and are sure your application code correctly handles your outbound connections, consider deploying NAT Gateway. This approach is commonly used by customers who deploy multitenant solutions that are built on Azure App Service and Azure Functions.

Each public IP address attached to NAT gateway provides 64,512 SNAT ports for connecting outbound to the internet. NAT gateway can scale to use up to 16 public IP addresses which provides over 1 million SNAT ports. If you need to scale beyond this limit, you can consider deploying multiple NAT Gateway instances across multiple subnets or VNets. Each virtual machine in a subnet can use any of the available SNAT ports, if it needs them.

Outbound IP address control

Outbound IP address control can be useful in multitenant applications, when you have all of the following requirements:

  • You use Azure services that don't automatically provide dedicated static IP addresses for outbound traffic. These services include Azure App Service, Azure Functions, API Management (when running in the consumption tier), and Azure Container Instances.
  • You need to connect to your tenants' networks over the internet.
  • Your tenants need to filter incoming traffic based on the IP address of each request.

When a NAT Gateway instance is applied to a subnet, any outbound traffic from that subnet uses the public IP addresses that's associated with the NAT gateway.

Note

When you associate multiple public IP addresses with a single NAT Gateway, your outbound traffic could come from any of those IP addresses. You might need to configure firewall rules at the destination. You should either allow each IP address, or use a public IP address prefix resource to use a set of public IP addresses in the same range.

Isolation models

If you need to provide different outbound public IP addresses for each tenant, you must deploy individual NAT Gateway resources. Each subnet can be associated with a single NAT Gateway instance. To deploy more NAT gateways, you need to deploy multiple subnets or virtual networks. In turn, you likely need to deploy multiple sets of compute resources.

Review Architectural approaches for networking in multitenant solutions for more information about how to design a multitenant network topology.

Contributors

This article is maintained by Microsoft. It was originally written by the following contributors.

Principal author:

  • John Downs | Principal Customer Engineer, FastTrack for Azure

Other contributors:

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