Mastering NAT Rules for Your Azure Virtual WAN VPN Gateway

As organizations increasingly adopt cloud infrastructure, they often encounter the challenge of interconnecting their on-premises networks with Azure virtual networks. One common scenario is when branch offices have overlapping IP address spaces, making it difficult to establish direct connectivity. Fortunately, Azure Virtual WAN’s site-to-site VPN gateway offers a powerful solution - NAT (Network Address Translation) rules.

In this comprehensive guide, we’ll explore the ins and outs of configuring NAT rules for your Azure Virtual WAN VPN gateway, empowering you to seamlessly interconnect your on-premises networks with Azure resources.

Understanding NAT Types: Static and Dynamic

The Azure Virtual WAN VPN gateway supports two types of NAT rules: static and dynamic.

Static NAT defines a fixed, one-to-one mapping between an internal IP address and an external IP address. This ensures a predictable translation, making it ideal for scenarios where you need to maintain a consistent mapping, such as when connecting to on-premises resources.

Dynamic NAT, on the other hand, assigns IP addresses and ports from a pool based on availability. This is useful when the target address pool is smaller than the original address pool, as it can accommodate the differences.

Configuring NAT Rules

To configure NAT rules for your Azure Virtual WAN VPN gateway, follow these steps:

  1. Navigate to your virtual hub in the Azure portal.
  2. Select the ‘VPN (Site to site)’ option.
  3. Choose ‘NAT rules (Edit)’ to manage your NAT rules.
  4. On the ‘Edit NAT Rule’ page, you can add, edit, or delete NAT rules, specifying details such as the name, type (static or dynamic), mode (IngressSnat or EgressSnat), internal and external mapping, and the associated link connection.

Remember, if you want the site-to-site VPN gateway to advertise the translated address prefixes via BGP, be sure to enable the ‘Enable BGP Translation’ option.

Ingress SNAT for BGP-Enabled VPN Sites

When configuring Ingress SNAT rules for VPN sites that are connected via BGP, the process involves a few additional steps:

  1. Create a static NAT rule to translate the overlapping address spaces between the VPN sites.
  2. Enable BGP Route Translation to ensure the post-NAT address ranges are automatically advertised.
  3. Update the VPN site’s Link Connection BGP address to reflect the NAT-translated address.

This setup allows the site-to-site VPN gateway to distinguish between the VPN sites and ensures seamless communication between the on-premises networks and Azure resources.

Ingress SNAT for Statically Configured VPN Sites

For VPN sites that are not connected via BGP, the configuration process differs slightly:

  1. Create a static NAT rule to translate the overlapping address spaces.
  2. Edit the ‘Private Address Space’ field of the VPN site to include the post-NAT address range.

This ensures the site-to-site VPN gateway can properly route traffic to the correct destination, even when dealing with overlapping IP address spaces.

Verification and Troubleshooting

To ensure your NAT configuration is working as expected, perform the following verification checks:

  • Validate the dynamic NAT rules and ensure the on-premises BGP peer IP is not part of the pre-NAT address range.
  • Inspect the DefaultRouteTable, rules, and routes to ensure the NAT-translated prefixes are properly learned.
  • Verify the effective routes on the network interfaces of spoke virtual machines and the on-premises devices to confirm the correct address prefixes are in place.

By following these best practices and understanding the nuances of static and dynamic NAT configurations, you’ll be well-equipped to seamlessly integrate your on-premises networks with Azure Virtual WAN, unlocking the full potential of your hybrid cloud infrastructure.

For more information on configuring site-to-site connections in Azure Virtual WAN, be sure to check out the official documentation.