In Windows, each network interface identified by the operating system is assigned a metric value. Interface metrics are settings that determine the priority or preference of network interfaces when there are multiple active network connections. The Windows networking stack uses these metrics to determine which network interface should be used for routing traffic when multiple network interfaces are available. Network interface metrics are critical for Always On VPN administrators to understand because they can impact how name resolution requests are processed when an Always On VPN connection is established.
Metric Values
By default, Windows automatically assigns metric values to network interfaces (including VPN interfaces) based on various factors, including the connection speed, link state, and interface type. It tries to select the most suitable interface for general internet connectivity.
Metrics and DNS
Windows will also use the network interface with the lowest metric value as the preferred interface for sending DNS queries by default. This means that DNS queries will be routed through the network interface with the lowest metric value, assuming it is available and connected. When an Always On VPN connection is established, DNS queries may fail or return unexpected results if the network interface metrics are not configured optimally.
Split DNS and Wired Ethernet
Split DNS (sometimes called ‘split brain DNS’) is when the DNS namespace is the same internally and externally. The most common scenario where interface metric settings interfere with DNS operation is when using split DNS and the endpoint is connected to the Internet with a wired Ethernet connection. In this scenario, the Ethernet interface will be assigned the same or lower interface metric value as the Always On VPN interface, which can yield unexpected results.
Viewing Metrics
Always On VPN administrators can view currently assigned interface metric values by running the following PowerShell command.
Assigning Metrics
Most Always On VPN administrators will never have to change interface metric settings. However, if your implementation uses split DNS and some of your endpoints connect using wired Ethernet connections, you may need to update the interface metric settings to ensure proper DNS operation. Choose a setting for the interface metric value that is lower than the wired Ethernet interface. I’ve used a value of ‘3’ without issue for many years. Use one of the following methods to update the interface metric for Always On VPN connections.
PowerShell
Updating interface metric settings in Windows can be accomplished by running the Set-NetIpInterface PowerShell command.
Set-NetIpInterface -InterfaceAlias <connection name> -InterfaceMetric 3
Note: Using PowerShell to assign the interface metric is not persistent! While this method is suitable for local validation testing, you should use one of the following methods to implement this change permanently.
Rasphone.pbk
To assign the interface metric permanently, Always On VPN administrators can edit the following settings in the rasphone.pbk configuration file.
IpInterfaceMetric=3
Ipv6InterfaceMetric=3
Administrators can automate updating this setting using the Update-Rasphone.ps1 PowerShell script. In addition, the following scripts can be used with Microsoft Intune remediation.
Detect-DeviceIpv4InterfaceMetric.ps1
Remediate-DeviceIpv4InterfaceMetric.ps1
Detect-DeviceIpv6InterfaceMetric.ps1
Remediate-DeviceIpv6InterfaceMetric.ps1
Detect-Ipv4InterfaceMetric.ps1
Remediate-Ipv4InterfaceMetric.ps1
Detect-Ipv6InterfaceMetric.ps1
Remediate-Ipv6InterfaceMetric.ps1
DPC
Organizations using PowerON Platforms’ Dynamic Profile Configurator (DPC) to manage Always On VPN client configuration settings with Active Directory and group policy or Microsoft Intune can enable the VPN Tunnel Metric setting.
Additional Information
Get-NetIpInterface PowerShell Command
Set-NetIpInterface PowerShell Command
Managing Always On VPN Client Settings with DPC
Always On VPN DPC with Microsoft Intune
Always On VPN DPC Advanced Features
Always On VPN DPC Video Demonstration
PowerON Platforms Always On VPN Dynamic Profile Configurator (DPC)
