Understanding NAT Gateway in Azure

Introduction to NAT Gateway

Azure NAT Gateway is a fully managed service that simplifies outbound connectivity for virtual machines (VMs) within a Virtual Network (VNet) in Azure. It provides a consistent, secure, and highly available method for handling outbound traffic from your VMs to the internet while managing IP address exhaustion and scaling challenges.

Key Features of Azure NAT Gateway

• Outbound Traffic Management: NAT Gateway handles all outbound internet traffic from VMs within a VNet, providing a static public IP address that external services can use to whitelist your traffic.
• Automatic Scaling: NAT Gateway scales automatically to accommodate the traffic needs of your VMs without manual intervention.
• High Availability: NAT Gateway is designed for high availability, ensuring that your outbound traffic is reliably processed.

How NAT Gateway Works

Outbound Connectivity

When VMs in a subnet need to communicate with the internet, NAT Gateway ensures that all outbound traffic is routed through a single public IP address (or a pool of IP addresses). This provides a consistent outbound IP for your VMs, which is crucial when you need to whitelist your IP address on external services.

Example Scenario:

• Subnet: 10.0.0.0/24
• VM Private IP: 10.0.0.5
• NAT Gateway Public IP: 203.0.113.2

When the VM with IP 10.0.0.5 sends a request to an external service, NAT Gateway translates the source IP to 203.0.113.2. The external service sees the request coming from 203.0.113.2, which can be whitelisted for secure access.

Static NAT (SNAT) and Dynamic NAT (DNAT)

Static NAT (SNAT)

SNAT refers to the process of translating a private IP address to a public IP address for outbound traffic. In Azure, NAT Gateway handles SNAT by translating the private IP addresses of VMs in a VNet to the public IP address(es) assigned to the NAT Gateway.

• Purpose: Ensures that outbound traffic from VMs uses a consistent public IP address.
• Configuration: NAT Gateway automatically handles SNAT for all VMs in the subnet it is attached to.

Dynamic NAT (DNAT)

DNAT, or Destination Network Address Translation, is used to map incoming traffic from a public IP address and port to a private IP address and port within a VNet. This allows external users to access services hosted on VMs in a private network.

• Purpose: Allows external users to access internal services by redirecting traffic from a public IP to a private IP.
• Configuration: DNAT is configured using Azure Firewall, not NAT Gateway, as NAT Gateway only handles outbound traffic.

Example of DNAT Using Azure Firewall

Imagine you have a web server running on a VM in Azure with the following details:

• VM Private IP: 10.0.0.4
• Azure Firewall Public IP: 203.0.113.10

To allow external users to access the web server, you can set up a DNAT rule in Azure Firewall:

• DNAT Rule Configuration:Incoming Request: HTTP traffic on port 80.Destination: 10.0.0.4:80 (private IP address and port of the web server).

Steps:

1. Assign Public IP: Azure Firewall is configured with the public IP 203.0.113.10.
2. Create DNAT Rule: Set up a DNAT rule to forward incoming HTTP requests on port 80 to 10.0.0.4:80.
3. Incoming Request: An external user sends an HTTP request to 203.0.113.10:80.
4. Traffic Redirection: Azure Firewall receives the request on its public IP and port 80, and the DNAT rule redirects this traffic to 10.0.0.4:80.
5. Access Internal Service: The request is forwarded to the internal web server at 10.0.0.4:80, and the response is sent back through the firewall to the external user.

Azure NAT Gateway vs. Azure Firewall

• NAT Gateway: Handles only outbound traffic from your VMs, providing SNAT functionality. It ensures all outbound connections from a subnet use the same public IP or a set of public IPs.
• Azure Firewall: Can handle both inbound and outbound traffic, providing both DNAT and SNAT functionalities. It is used to protect and filter traffic flowing into and out of your VNet.

Example Scenario: Using NAT Gateway and Azure Firewall Together

1. Outbound Traffic: You can configure NAT Gateway to manage outbound traffic for your VMs, ensuring that all outgoing requests use a consistent public IP address.
2. Inbound Traffic: For handling inbound requests, such as allowing users to access a web server, you can use Azure Firewall with DNAT rules to route traffic from a public IP to the internal server.

Conclusion

NAT Gateway in Azure is essential for managing outbound traffic for your VMs, offering a simple, scalable solution to provide consistent IP addresses for outbound connections. For inbound traffic, Azure Firewall with DNAT rules is the go-to solution, allowing you to map public IP addresses and ports to internal services securely. Understanding when to use NAT Gateway versus Azure Firewall is crucial for designing robust and secure Azure network architectures.