All posts tagged DNS

Always On VPN and Interface Metrics

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.

Get-NetIpInterface

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)

7 Comments

by Richard M. Hicks on September 25, 2023 • Permalink

Posted by Richard M. Hicks on September 25, 2023

https://directaccess.richardhicks.com/2023/09/25/always-on-vpn-and-interface-metrics/

Always On VPN and Split DNS

Cloudflare Public DNS Resolver Now Available

Split DNS, sometimes called ‘split brain’ DNS, is when an organization uses the same DNS namespace internally and externally. For example, the internal Active Directory domain name is example.com, so internal resources are accessed using a fully qualified domain name (FQDN) like dc1.example.com. Additionally, external properties such as mail and web services use the same namespace so that a public web server might have a name like www.example.com. Internal resources will resolve to internal, private IP addresses, whereas public services resolve to external, public IP addresses.

Complications

Things get complicated when the same resource (FQDN) is available internally and externally, especially for Always On VPN clients. For example, accessing app.example.com on the internal network resolves to a private address, but accessing the same resource on the Internet resolves to a public IP address. Often there are different authentication requirements for internal and external resources, which can yield unexpected results.

Name Resolution

Always On VPN administrators might prefer app.example.com to be accessed via the Internet when connected with Always On VPN. However, VPN clients will attempt to connect via the internal network using their default configuration. Solving this challenge requires internal DNS server changes.

NRPT?

It might be tempting for administrators to use the Name Resolution Policy Table (NRPT) to solve name resolution issues for Always On VPN. However, the NRPT has some limitations and may not always produce the desired results. For example, the NRPT only directs DNS queries. It does not define which resource records are returned by DNS. Also, some applications ignore the NRPT, which limits its usefulness. A better solution is to use DNS Policies in Windows Server.

DNS Policies

Microsoft introduced DNS policies with Windows Server 2016. DNS policies are a powerful tool administrators can use to fine-tune name resolution based on many factors. In the case of split DNS, administrators can configure internal DNS to return an IP address for a resource based on the source IP address of the name resolution query. VPN clients receive one IP address for a given DNS query, while all other clients receive a different IP address. DNS policies ensure that remote clients connected to the VPN will receive the proper IP address for the resource requested, as defined by the administrator.

Caveats

DNS policies are powerful and flexible, but there are some potential drawbacks. All enterprise DNS servers used by Always On VPN clients must be running Windows Server 2016 or later. Also, administrators must use PowerShell to configure DNS policies exclusively. There is no GUI interface to configure DNS policies. DNS policies do not appear in the DNS management interface, which could confuse an administrator unaware that DNS policies are in place. In addition, DNS client subnets and query resolution policies do not replicate across DNS servers. Administrators must manually configure these on each DNS server used by Always On VPN clients. However, zone scopes and resource records in those scopes do replicate automatically.

Scenario

For demonstration purposes, let’s assume that an Always On VPN client needs to access foo.example.com. It resolves to a private IP address on the internal network and a public IP address on the Internet. By default, foo.example.com will resolve to the internal private IP address of the server when connected with Always On VPN. However, the desire is to have foo.example.com resolve to the public IP address when connected with Always On VPN. To accomplish this, we’ll create a DNS policy to ensure that connected Always On VPN clients can resolve foo.example.com to the public IP address when resolving this name over the VPN tunnel.

DNS Policy Configuration

Open an elevated PowerShell command on a DNS server and perform the following steps to create a DNS policy for VPN clients.

Client Subnet

Run the Add-DnsServerClientSubnet PowerShell command to create a client subnet in DNS that includes all IP networks assigned to VPN clients. Summarize IP prefixes if there are multiple VPN servers in the organization.

Add-DnsServerClientSubnet -Name VPN -IPv4Subnet ‘172.16.100.0/22’ -IPv6Subnet ‘2001:db8:fcd2:1000::/60’

If summarizing IP prefixes for multiple servers isn’t possible, multiple subnets can be added to a DNS client subnet using the following command.

Add-DnsServerClientSubnet -Name VPN -IPv4Subnet @(‘172.16.100.0/24’, ‘172.16.101.0/24’, ‘172.16.102.0/24’, ‘172.16.103.0/24’) -IPv6Subnet @(‘2001:db8:fcd2:1001::/64’, ‘2001:db8:fcd2:1002::/64’, ‘2001:db8:fcd2:1003::/64’)

To make changes to an existing DNS client subnet, use the Set-DnsServerClientSubnet PowerShell command.

Note: Client Subnets do not replicate across domain controllers. Run the command above on all DNS servers or each DNS server used by Always On VPN clients.

Zone Scope

Create a Zone Scope that includes the DNS records to be returned to VPN clients. The default zone scope is the DNS zone itself. Configure an additional zone scope for the DNS zone by using the Add-DnsServerZoneScope PowerShell command.

Add-DnsServerZoneScope -ZoneName example.com -Name VPN

Resource Records

Next, add DNS records to the new zone scope using the Add-DnsServerResourceRecord PowerShell command.

Add-DnsServerResourceRecord -ZoneName example.com -A -Name foo -IPv4Address 203.0.113.12 -ZoneScope VPN

Add-DnsServerResourceRecord -ZoneName example.com -AAAA -Name foo -IPv6Address 2001:db8:21::12 -ZoneScope VPN

DNS Policy

Finally, create a DNS query resolution policy that ties everything together. Run the Add-DnsServerQueryResolutionPolicy command to create the DNS query resolution policy. Once configured, when the DNS server receives a DNS query, the policy will recognize that the query originates from a VPN client subnet and will return the resource record from the VPN zone scope with the public IP address defined previously.

Add-DnsServerQueryResolutionPolicy -Name VPN -Action ALLOW -ClientSubnet ‘EQ,VPN’ -FQDN ‘EQ,foo.example.com’ -ZoneScope ‘VPN,1’ -ZoneName example.com

Note: DNS query resolution policies do not replicate across domain controllers. Run the command above on all DNS servers or each DNS server used by Always On VPN clients.

Results

Once complete, the hostname ‘foo’ in our example above resolves to different IP addresses based on the client’s IP address.

DNS query for ‘foo’ from internal client.

DNS query for ‘foo’ from VPN client.

Summary

There are many scenarios where Windows Server DNS policies can be used to fine-tune name resolution for Always On VPN clients. Hopefully, this example gives you an idea of how DNS policies work, and you can use them to solve your unique name resolution challenges with Always On VPN.