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.
Additional Information
Windows Server DNS Policies Overview
When configuring Always On VPN, administrators have the option to enable DNS registration for VPN clients. When this option is set, VPN clients will register the IP address assigned to their VPN interface in the internal DNS. This allows client devices to be managed using their hostname from the internal network whenever they are connected remotely.
