Default outbound access in Azure

In Azure, virtual machines created in a virtual network without explicit outbound connectivity defined are assigned a default outbound public IP address. This IP address enables outbound connectivity from the resources to the Internet. This access is referred to as default outbound access.

Examples of explicit outbound connectivity for virtual machines are:

• Created within a subnet associated to a NAT gateway.

• In the backend pool of a standard load balancer with outbound rules defined.

• In the backend pool of a basic public load balancer.

• Virtual machines with public IP addresses explicitly associated to them.

How is default outbound access provided?

The public IPv4 address used for the access is called the default outbound access IP. This IP is implicit and belongs to Microsoft. This IP address is subject to change and it's not recommended to depend on it for production workloads.

When is default outbound access provided?

If you deploy a virtual machine in Azure and it doesn't have explicit outbound connectivity, it's assigned a default outbound access IP.

Why is disabling default outbound access recommended?

• Secure by default

  • It's not recommended to open a virtual network to the Internet by default using the zero trust network security principle.

• Explicit vs. implicit

  • It's recommended to have explicit methods of connectivity instead of implicit when granting access to resources in your virtual network.

• Loss of IP address

  • Customers don't own the default outbound access IP. This IP might change, and any dependency on it could cause issues in the future.

Some examples of configurations that won't work when using default outbound access:

• When you have multiple NICs on the same VM, note that default outbound IPs won't consistently be the same across all NICs.
• When scaling up/down Virtual Machine Scale sets, default outbound IPs assigned to individual instances can and will often change.
• Similarly, default outbound IPs aren't consistent or contiguous across VM instances in a Virtual Machine Scale Set.

How can I transition to an explicit method of public connectivity (and disable default outbound access)?

There are multiple ways to turn off default outbound access. The following sections describe the options available to you.

Utilize the Private Subnet parameter

• Creating a subnet to be Private prevents any virtual machines on the subnet from utilizing default outbound access to connect to public endpoints.

• The parameter to create a Private subnet can only be set during the creation of a subnet.

• VMs on a Private subnet can still access the Internet using explicit outbound connectivity.

  Note

  Certain services won't function on a virtual machine in a Private Subnet without an explicit method of egress (examples are Windows Activation and Windows Updates).

Add the Private subnet feature

• From the Azure portal, ensure the option to enable Private subnet is selected when creating a subnet as part of the Virtual Network create experience as shown below:

• Using PowerShell, when creating a subnet with New-AzVirtualNetworkSubnetConfig, use the DefaultOutboundAccess option and choose "$false"

• Using CLI, when creating a subnet with az network vnet subnet create, use the --default-outbound option and choose "false"

• Using an Azure Resource Manager template, set the value of defaultOutboundAccess parameter to be "false"

Private subnet limitations

• In order to utilize to activate/update virtual machine operation systems, including Windows, it's a requirement to have an explicit outbound connectivity method.

• Delegated subnets can't be marked as Private.

• Existing subnets can't currently be converted to Private.

• In configurations using a User Defined Route (UDR) with a default route (0/0) that sends traffic to an upstream firewall/network virtual appliance, any traffic that bypasses this route (e.g. to Service Tagged destinations) will break in a Private subnet.

Add an explicit outbound connectivity method

• Associate a NAT gateway to the subnet of your virtual machine.

• Associate a standard load balancer configured with outbound rules.

• Associate a Standard public IP to any of the virtual machine's network interfaces (if there are multiple network interfaces, having a single NIC with a standard public IP prevents default outbound access for the virtual machine).

Use Flexible orchestration mode for Virtual Machine Scale Sets

• Flexible scale sets are secure by default. Any instances created via Flexible scale sets don't have the default outbound access IP associated with them, so an explicit outbound method is required. For more information, see Flexible orchestration mode for Virtual Machine Scale Sets

If I need outbound access, what is the recommended way?

NAT gateway is the recommended approach to have explicit outbound connectivity. A firewall can also be used to provide this access.

Constraints

• Public connectivity is required for Windows Activation and Windows Updates. It's recommended to set up an explicit form of public outbound connectivity.

• Default outbound access IP doesn't support fragmented packets.

• Default outbound access IP doesn't support ICMP pings.

Next steps

For more information on outbound connections in Azure and Azure NAT Gateway, see:

• Source Network Address Translation (SNAT) for outbound connections.

• What is Azure NAT Gateway?

• Azure NAT Gateway FAQ