4

Here's the setup:

  • Windows 10 1803 clients
  • Server 2012R2 RRAS server
  • Always On VPN device tunnel setup per these instructions, with split tunneling.
  • Device VPN only has routes to 1 DC/DNS server, and our configuration manager server, so it can be managed and new users can authenticate when away from the office. When users need full access to the office network, there is a separate user VPN they can connect to. This works well, except for DNS.
  • AD domain name is example.local
  • Public domain name is example.com

The problem:

  • We use split DNS for our public domain name - so mail.example.com resolves to an internal IP address when using our internal DNS servers, and our public address from the outside world.
  • I need VPN clients to resolve it to the public address. The device VPN doesn't let them talk to the internal address for mail.example.com, so they can't get their email.
  • I'd also like (but not a must have) DNS resolution for local services at other locations to work properly - eg. when I'm in the offices of Other Corp, DNS for othercorp.local works even with the VPN connected.

What I want to happen:

  • Queries for example.local go over the VPN to our internal DNS servers
  • Everything else, including example.com, use the DNS servers provided by the LAN/Wifi connection the laptop is connected to.
  • The other user based VPN (which routes all traffic over the VPN) continues to use our internal DNS servers for everything.

What I've tried:

  • Setting Name Resolution Policy Table rules for example.local pointing at our internal DNS servers. This does seem to work, queries for example.local go over the VPN...but so does everything else.
  • Setting NRPT rules for example.com, with blank DnsServers field, which should make sure they are excluded. Seems to have no effect.
  • Setting NRPT rules for example.com, with public resolvers for the DnsServer. This does work, but breaks at remote locations that block anything but their own dns resolvers (which many of the sites my users travel to do), and doesn't solve the local services problem.
  • Setting "Use the following DNS server addresses" on the VPN connection in network connections, and leaving it blank. No effect, the VPN connection still gets set to use our internal servers.
  • Setting "Use the following DNS server addresses", and putting in a public DNS server like 8.8.8.8. When connected, I end up with 3 DNS servers on that interface, with our internal ones at the top and 8.8.8.8 at the bottom of the list.

I suspect that if I could get the VPN to not list any DNS servers at all, the NRPT rules would kick in just for example.local, and everything would work properly. But I can't find a way to make it not use the ones provided by the RRAS server.

2
+500

A potential workaround you could try is standing up a Server 2016 DNS server and implementing a DNS policy to do split-dns with geolocation awareness. This would allow you to tell DNS queries from the Device VPN subnet to use the external IP instead of the internal.

The powershell commands would look something like below.

Device VPN subnet

Add-DnsServerClientSubnet -Name "DeviceVPNSubnet" -IPv4Subnet "192.168.1.0/24"  

Device VPN Zone Scope

Add-DnsServerZoneScope -ZoneName "example.com" -Name "DeviceVPNZoneScope"  

Default A record (should already exist)

Add-DnsServerResourceRecord -ZoneName "example.com" -A -Name "mail" -IPv4Address "192.168.0.5"

Device VPN A record

Add-DnsServerResourceRecord -ZoneName "example.com" -A -Name "mail" -IPv4Address "203.0.113.5" -ZoneScope "DeviceVPNZoneScope" 

Device VPN Resolution Policy

Add-DnsServerQueryResolutionPolicy -Name "Device VPN Policy" -Action ALLOW -ClientSubnet "eq,DeviceVPNSubnet" -ZoneScope "DeviceVPNZoneScope,1" -ZoneName "example.com"  

See: Use DNS Policy for Geo-Location Based Traffic Management with Primary Servers

  • Oh, I like this. That's a handy new feature. Unfortunately I don't have licenses for Server 2016 yet. – Grant Aug 15 '18 at 16:48
  • I'm awarding you the bounty. While yours wasn't the solution I went with, it was the best alternative solution anyone posted. – Grant Aug 20 '18 at 1:28
0

Just a thought, but maybe set a connection script .bat that adds the correct Public IP address of the mail server upon connection to the hosts file, and then removes it from the client's hosts file at disconnection? Theoretically I guess you could always have it in the hosts file, and then they would just access the server from it's public ip even in the office. Regardless that will override any DNS from the RRAS server.

The hosts file is located at C:\Windows\System32\Drivers\etc\hosts .

  • So, the connection script have to run under an administrator/system account, because the host file is protected, a standard user can't change its content. – Swisstone Aug 14 '18 at 19:09
  • That is correct. Hosts is an administrator protected file. – David Eisen Aug 23 '18 at 17:43
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You can define internal DNS servers for any namespace using the DomainNameInformation element in your ProfileXML. If you are using split DNS, there may be some cases where "internal" namespaces should be routed externally and not over the VPN tunnel. In these scenarios you'll have to create "exclusions", which are essentially namespaces defined NOT to use the internal DNS servers. To do this you don't simply leave the DnsServers field blank, but you leave the DnsServers out of the element entirely.

You can find more information here: https://directaccess.richardhicks.com/2018/04/23/always-on-vpn-and-the-name-resolution-policy-table-nrpt/

Hope that helps!

  • Thanks, but I already tried that. In fact I linked to that very article in the question. It doesn't solve the problem. – Grant Aug 15 '18 at 4:17
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I figured out the solution.

Adjusting the interface metric to be a larger number than the existing ethernet/wifi adapters will make it prefer the DNS servers on the local network, but NRPT rules will still work to send DNS queries for my own domain to my servers over the VPN.

However, there's no way to adjust the interface metric for a VPN connection in powershell, nor in VBscript, nor .NET, nor VPNv2 CSP.

If the VPN is connected the metric can be changed in powershell with the Set-NetIPInterface cmdlet, but when the VPN is not connected, it doesn't show up there at all. And changes there will not persist across reboots.

The only way to change it seems to be editing the C:\ProgramData\Microsoft\Network\Connections\Pbk\rasphone.pbk file and changing the lines IpInterfaceMetric and Ipv6InterfaceMetric to higher numbers. I changed them both to 100.

After that, and in combination with the NRPT rules, DNS works as desired - all queries for example.local get sent to my AD DNS servers. Anything else (including example.com) gets sent to the DNS servers of their local network.

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