Azure Firewall forced tunneling
When you configure a new Azure Firewall, you can route all Internet-bound traffic to a designated next hop instead of going directly to the Internet. For example, you may have an on-premises edge firewall or other network virtual appliance (NVA) to process network traffic before it's passed to the Internet. However, you can't configure an existing firewall for forced tunneling.
By default, forced tunneling isn't allowed on Azure Firewall to ensure all its outbound Azure dependencies are met. User Defined Route (UDR) configurations on the AzureFirewallSubnet that have a default route not going directly to the Internet are disabled.
Forced tunneling configuration
To support forced tunneling, Service Management traffic is separated from customer traffic. An additional dedicated subnet named AzureFirewallManagementSubnet (minimum subnet size /26) is required with its own associated public IP address. The only route allowed on this subnet is a default route to the Internet, and BGP route propagation must be disabled.
If you have a default route advertised via BGP to force traffic to on-premises, you must create the AzureFirewallSubnet and AzureFirewallManagementSubnet before deploying your firewall and have a UDR with a default route to the Internet, and Propagate gateway routes disabled.
Within this configuration, the AzureFirewallSubnet can now include routes to any on-premises firewall or NVA to process traffic before it's passed to the Internet. You can also publish these routes via BGP to AzureFirewallSubnet if Propagate gateway routes is enabled on this subnet.
For example, you can create a default route on the AzureFirewallSubnet with your VPN gateway as the next hop to get to your on-premises device. Or you can enable Propagate gateway routes to get the appropriate routes to the on-premises network.
If you enable forced tunneling, Internet-bound traffic is SNATed to one of the firewall private IP addresses in AzureFirewallSubnet, hiding the source from your on-premises firewall.
If your organization uses a public IP address range for private networks, Azure Firewall SNATs the traffic to one of the firewall private IP addresses in AzureFirewallSubnet. However, you can configure Azure Firewall to not SNAT your public IP address range. For more information, see Azure Firewall SNAT private IP address ranges.
Once you configure Azure Firewall to support forced tunneling, you can't undo the configuration. If you remove all other IP configurations on your firewall, the management IP configuration is removed as well and the firewall is deallocated. The public IP address assigned to the management IP configuration can't be removed, but you can assign a different public IP address.